 |
   |
           |
|
 | |
|
Biochemistry Cell
Biology Computational
Biology Developmental
Biology Ecology Evolution Genetics Microbiology Molecular
Biology Plant
Biology Science
Education Structural
Biology |
Peterson, C.J., and W.P. Carson (2008) Constraints on forest regeneration in abandoned tropical pastures: do temperate paradigms of succession apply to the tropics? Pp in Tropical Forest Community Ecology, Carson, W.P., and S.A. Schnitzer, Ed. Blackwell Publishing, Oxford Carson, W.P., J. Anderson, E. Leigh, and S.A. Schnitzer (2008) Challenges Associated with Testing and Falsifying the Janzen-Connell Hypothesis: A Review and Critique. Pp in Tropical Forest Community Ecology, Carson, W.P., and S.A. Schnitzer, Ed. Blackwell Publishing, Oxford Schnitzer, S.A., J. Mascaro, and W.P. Carson (2008) Treefall gaps and the maintenance of species diversity in tropical forests. Pp in Tropical Forest Community Ecology, Carson, W.P., and S.A. Schnitzer, Ed. Blackwell Publishing, Oxford Paine, C.E., K.E. Harms, S.A. Schnitzer, and W.P. Carson (2008) Weak Competition Among Tropical Tree Seedlings: Implications for Species Coexistence. Biotropica :In Press The intensity of competition among forest tree seedlings is poorly understood, but has important ramifications for their recruitment and for the maintenance of species diversity. Intense competition among seedlings could allow competitively dominant species to exclude subordinate species. Alternatively, the low density and small stature of forest tree seedlings could preclude intense interseedling competition. In this case, other processes, such as size-asymmetric competition with adults, interactions with consumers, or neutral dynamics would prevail as those structuring the forest understory. We tested the intensity of, and potential for, intraspecific competition among tree seedlings of three species (Brosimum alicastrum, Matisia cordata, and Pouteria reticulata) in two Neotropical rain forests. We reduced stem densities by up to 90 percent and three monitored individual growth and survival rates for up to 24 mo. Individual growth and survival rates were generally unrelated to stem density. Contrary to the predicted behavior of intensely competing plant populations, the distribution of individual heights did not become more left-skewed with time for any species, regardless of plot density; i.e., excesses of short, suppressed individuals did not accumulate in high-density plots. We further measured the overlap of zones of influence (ZOIs) to assess the potential for resource competition. Seedling ZOIs overlapped only slightly in extremely dense monodominant plots, and even less in ambient-density plots of mixed composition. Our results thus suggest that interseedling competition was weak. Given the low density of tree seedlings in Neotropical forests, we infer that resource competition among seedlings may be irrelevant to their recruitment. Long, Z.T., T.H. Pendergast IV, and W.P. Carson (2007) The impact of deer on relationships between tree growth and mortality in an old-growth beech-maple forest. Forest Ecol. Manag. 252:230-238 White-tailed deer have been at high levels in the northeastern forests of the United States for decades and have strongly influenced forest dynamics. In this long-term study, we found that the composition of the overstory and understory assemblages of an old-growth beech-maple forest differed significantly. We used exclosures to test the hypothesis that deer contributed to these differences by differentially influencing the relationship between growth and mortality among seedlings of the six most abundant tree species. In the absence of deer, we found that the mortality of the six species decreased with increased growth and that interspecific differences in the relationships between growth and mortality coincided with previously observed shade-tolerance rankings. In the presence of deer, mortality decreased with growth only for the browse tolerant species (American beech, black cherry, and sugar maple). Mortality did not decrease with growth for preferred browse species (oak species, ash species, and red maple), rather, this relationship was eliminated in the presence of deer. The changes in growth and mortality relationships in the presence of browsing generally corresponded to observed changes in seedling density following the removal of deer. Sugar maple, ash, black cherry, and total stem density increased in the absence of deer. Our results suggest that the relationship between survival and growth in the understory, a metric of shade tolerance, is a fairly plastic response that varies depending upon the presence and absence of herbivores. Our results indicate that deer have contributed to the differences between understory and overstory vegetation, with browse tolerant species increasing in abundance at the expense of preferred browse species.
Royo, A., and W.P. Carson (2006) On the formation of dense understory layers in forests worldwide: consequences and implications for forest dynamics, biodiversity, and succession. Can. J. For. Res. 36:1345-1362 The effect of understory herbs, shrubs, and vines on forest regeneration has been virtually ignored in forest successional theory and empirical models. Nonetheless, in some forests, a few opportunistic species can increase dramatically in abundance and form nearly monodominant understory canopies. Following their expansion, this recalcitrant understory layer can strongly alter the rates of tree seedling germination, growth, and survival. Thus, this layer can change the composition and size structure of the advance regeneration layer and, ultimately, influence the rate and direction of secondary forest succession. In this review, we first examine the factors facilitating the establishment of recalcitrant understory layers worldwide. In over half of the examples we examined, canopy disturbance acted synergistically with either increased herbivory or altered fire regimes to generate dramatic increases in understory plant abundance. Second, we explore how this recalcitrant vegetation layer can potentially alter, delay, or even arrest successional trajectories. Third, we critically review the various interference mechanisms exerted by a recalcitrant understory layer. We find that although many putative interference mechanisms are suggested in the literature, only a quarter of the published studies used rigorous manipulative field experiments to substantiate any particular mechanism. Of these, resource competition and allelopathy were the predominant interference mechanisms. Fourth, we link the factors facilitating the formation of recalcitrant understory layers with their specific interference mechanisms and effects on vegetation development representing a conceptual advance over prior efforts. This approach identifies gaps in our knowledge and provides a framework that will aid in directing future research efforts, sustaining forest diversity, developing increasingly realistic forest successional models, and improving forest management practices.
Stark, S.C., D. Bunker, and W.P. Carson (2006) A null model of exotic plant diversity tested with exotic and native species-area relationships. Ecol. Lett. 9:136-141 At large spatial scales, exotic and native plant diversity exhibit a strong positive relationship. This may occur because exotic and native species respond similarly to processes that influence diversity over large geographical areas. To test this hypothesis, we compared exotic and native species#area relationships within six North American ecoregions. We predicted and found that within ecoregions the ratio of exotic to native species richness remains constant with increasing area. Furthermore, we predicted that areas with more native species than predicted by the species#area relationship would have proportionally more exotics as well. We did find that these exotic and native deviations were highly correlated, but areas that were good (or bad) for native plants were even better (or worse) for exotics. Similar processes appear to influence exotic and native plant diversity but the degree of this influence may differ with site quality.
Kurzal, B., S. Schnitzer, and W.P. Carson (2005) Predicting liana crown location from stem diameter in three Panamanian lowland forests. Biotropica 32:262-266 In three forests that differed in annual rainfall and seasonality, the probability of a liana with a stem ¡Y2.0 cm stem diameter reaching the canopy was >50 percent. Lianas reached the canopy at significantly smaller size-classes (1.5 cm) in the wet aseasonal forest, suggesting that this estimate changes with forest type. Nevertheless, as a general rule, we suggest that 2.0 cm is the minium stem diameter to examine the abundance and diversity of canopy lianas or canopy competition between lianas and trees.
Banta, J.A., A.A. Royo, C. Kirschbaum, and W.P. Carson (2005) Plant communities growing on boulders in the allegheny national forest: Evidence for boulders as refugia from deer and as a bioassay of overbrowsing. Nat. Areas J. 25:10-18 Deer have been overabundant throughout much of Pennsylvania since at least the 1940's. We compared plant communities in the Allegheny National Forest (ANF) on boulder tops and the forest floor to test the hypothesis that large boulders serve as refugia for plants threatened by deer herbivory. Five of the ten most common woody species (hemlock, Tsuga canadensis L., mountain maple, Acer spicatum Lam., red maple, A. rubrum L., striped maple, A. pensylvanicum L., and yellow birch, Betula alleghaniensis Britton) occurred at much higher densities on boulders than in randomly selected areas of the same size adjacent to these boulders on the soil surface. We never encountered any individuals of hemlock, mountain maple, or red maple on the forest floor. Total woody species density (excluding root suckers of beech, Fagus grandifolia Ehrh.) was nearly three times higher on the boulders. Woody species richness and evenness, as well as forb and shrub cover and richness, were also much greater on boulders. Our results strongly suggest that overbrowsing by deer can dramatically reduce tree regeneration and diversity as well as reduce forb and shrub abundance in the ANF. Furthermore, understory plant communities are now dominated by species that are known to be unpalatable or tolerant of deer browse, particularly beech, grasses, and ferns (Dryopteris carthusiana Vill. and Thelypteris noveboracensis L.). The primary alternative explanation is that conditions on the surface of boulders are superior for numerous woody and herbaceous species. Although we cannot rule this out, we consider this alternative improbable because of the poor nature of these habitats.
Comisky, L., A.A. Royo, and W.P. Carson (2005) Deer browsing creates rock refugia gardens on large boulders in the Allegheny National Forest, Pennsylvania. Am. Midl. Nat. 154:201-206 We surveyed the vegetation growing on the surface of tall boulders (> 1.5 m), on the surface of short boulders (< 0.5 m) and in areas adjacent to these boulders on the soil surface in the Allegheny National Forest, Pennsylvania. Our purpose was to test the hypothesis that tall boulders serve as refugia from deer browsing for both woody plant species as well as understory herbs. We conducted our surveys during spring when many of these herbs reproduce sexually and their inflorescences are vulnerable to browsing. Deer cannot gain access and browse on these tall boulders, whereas they can easily browse plant species on shorter boulders and on the forest floor. We predicted that the tall boulders would have the highest plant diversity, woody species density and the density of reproductive individuals of understory herbs. We found a total of more than 1300 reproductive individuals of flowering herbs of more than 10 species growing on tall boulders and only 6 reproductive individuals of only a few species growing on the soil surface. Overall, there was a much greater density and diversity of woody species and sexually reproducing herbs on tall boulders or Rock Refugia Gardens than either on short boulders or the soil surface. Because the diversity and abundance of vegetation on the short boulders was nearly identical to that on the forest floor, our findings are unlikely to be explained by tall boulders providing a superior edaphic or abiotic habitat relative to habitats on the forest floor. Species accumulation curves showed that plant species continued to accrue with area on tall boulders, but did not increase with area on the forest floor or on the short boulders. These Rock Refugia Gardens demonstrate the pervasive and deleterious impact that deer are having on plant species diversity and forest regeneration in the Allegheny National Forest and likely elsewhere in the eastern US where these vertebrates are abundant.
Bunker, D.E., and W.P. Carson (2005) Drought stress and tropical forest woody seedlings: effect on community structure and composition. J. Ecol. 93:794-806 We hypothesized that severe drought affects the structure of tropical forests by favouring seedlings of some species or groups at the expense of others. To test this hypothesis, we irrigated naturally occurring woody seedlings during an El Nino-related drought in seasonal moist tropical forest on Barro Colorado Island, Panama. We predicted that irrigated plots would retain greater species richness than control plots, and that the benefits of irrigation would increase with the abundance of: trees relative to lianas, wet-forest seedlings relative to dry-forest seedlings, and rare species relative to common species. We also hypothesized that the strength of this filter would increase with increased moisture limitation, predicting that the benefits of irrigation would increase with seedling density and light availability, and decrease with mean seedling age. Irrigation did reduce species losses, but not by limiting the loss of drought-sensitive species as predicted. Instead, mortality in irrigated plots was density dependent, whereas species losses in control plots were well predicted by random thinning, suggesting that density dependence weakened as abiotic stress increased. Irrigation increased seedling growth, but did not affect seedling mortality. Contrary to our predictions, irrigation increased growth in plots dominated by dry-forest species relative to those dominated by wet-forest species, suggesting that dry-forest seedlings either occur in moisture-limited microsites or are more able to utilize dry-season precipitation. The strength of the filter did increase with potential moisture limitation, as irrigation increased seedling growth more in higher light environments. Annual precipitation has declined over much of the humid tropics during the 20th century. Our results suggest that this trend may reduce tropical forest diversity by weakening density-dependent mechanisms that maintain diversity. In addition, plots dominated by dry-forest species experienced higher growth in response to irrigation and also far lower dry-season mortality relative to plots dominated by wet-forest species. While we cannot disentangle the effects of microsite from species composition, these results suggest that dry-forest species may benefit from any increase in dry season length or severity. Research conducted during 'normal' conditions may overlook the impact of severe events and thus fail to identify critical mechanisms structuring ecological communities.
Royo, A.A., and W.P. Carson (2005) The herb community of a tropical forest in central Panama: dynamics and impact of mammalian herbivores. Oecologia 145:66-75 Mammals are hypothesized to either promote plant diversity by preventing competitive exclusion or limit diversity by reducing the abundance of sensitive plant species through their activities as browsers or disturbance agents. Previous studies of herbivore impacts in plant communities have focused on tree species and ignored the herbaceous community. In an experiment in mature-phase, tropical moist forest sites in central Panama, we studied the impact of excluding ground-dwelling mammals on the richness and abundance of herbs in 16, 30x45-m plots. Within each plot, we censused the herbaceous community in 28, 2x2-m subplots (1,792 m(2) total area sampled). We identified over 54 species of herbs averaging 1.21 ramets m(-2) and covering approximately 4.25% of the forest floor. Excluding mammals for 5 years had no impact on overall species richness. Within exclosures, however, there was a significant two-fold increase in the density of rare species. Overall herbaceous density and percent cover did not differ between exclosures and adjacent control plots, although cover did increase over time. Mammalian exclusion significantly increased the total cover of three-dominant herb species, Pharus latifolius, Calathea inocephala, and Adiantum lucidum, but did not affect their density. This study represents one of the most extensive herbaceous community censuses conducted in tropical forests and is among a few that quantify herbaceous distribution and abundance in terms of both density and cover. Additionally, this work represents the first community level test of mammalian impacts on the herbaceous community in a tropical forest to date. Our results suggest that ground dwelling mammals do not play a key role in altering the relative abundance patterns of tropical herbs in the short term. Furthermore, our results contrast sharply with prior studies on similar temporal and spatial scales that demonstrate mammals strongly alter tree seedling composition and reduce seedling density. Thus, we question the pervasiveness of top-down control on tropical plant communities and the paradigm that defaunation will inexorably lead to widespread, catastrophic shifts in plant communities.
Collins, R.J., and W.P. Carson (2004) The effects of environment and life stage on Quercus abundance in the eastern deciduous forest, USA: are sapling densities most responsive to environmental gradients?. Forest Ecol. Manag. 201:241-258 Most studies that describe tree species distributions across environmental gradients focus only on the abundance of adults. This approach may overlook the response of younger life stages (i.e., seedlings and saplings) and thus may fail to identify the life stage that is most sensitive to a particular environmental gradient (i.e., site characteristics that vary across landscapes such as elevation or aspect). Alternatively, seedling and sapling abundance may be unrelated to environmental gradients and may be entirely dependent on adult abundance. In order to effectively manage eastern hardwood forests to promote species compositions that maximize economic or ecological value, we need to understand the relative roles of site characteristics and adult abundance on all life stages. To address this need, we examined the relationships between five site characteristics (aspect, slope position, site index, stand age and elevation) and the abundance of seedlings, saplings and adults of three species of oak (Quercus rubra, Quercus alba, and Quercus prinus). We assessed these relationships by censusing seedlings, saplings, and adults in large plots (500 m(2)) and quantifying local site characteristics in 21 forest stands in the Monongahela National Forest in North Central West Virginia. Adult white oak densities peaked on southwestern slopes (R-2 = 0.24); adult chestnut oak densities peaked on ridge tops (R-2 = 0.58): and adult northern red oak densities were not correlated with any site characteristics (R-2 = 0.00). Densities of white oak and chestnut oak saplings were highly correlated with a combination of site characteristics and seedling densities (R-2 = 0.75, 0.81, respectively), and northern red oak densities were highly correlated with site characteristics solely (R-2 = 0.70). For seedlings, white oak and chestnut oak densities were correlated only with adult abundance (R-2 = 0.53, 0.56, respectively), and northern red oak densities were correlated solely with site characteristics (R-2 = 0.45). These data suggest that in the absence of disturbance, site characteristics have the strongest effects on abundance patterns of these oaks during the sapling stage. For each of the three species, seedling, sapling and adult densities were correlated with a different subset of factors meaning that different biotic and abiotic conditions promoted peak densities for each life stage. Such a pattern indicates that important "demographic conflicts" exist (sensu [Battaglia, L.L., Fore, S.A., Sharitz, R.R., 2000. Seedling emergence, survival and size in relation to light and water availability in two bottomland hardwood species. J. Ecol. 88, 1041-1050]).
Mascaro, J., S.A. Schnitzer, and W.P. Carson (2004) Liana diversity, abundance, and mortality in a tropical wet forest in Costa Rica. Forest Ecol. Manag. 190:3-14 Lianas can have a large impact on the diversity, structure, and dynamics of tropical forests, yet they remain essentially unknown even in some of the most intensely studied tropical forests, such as La Selva Biological Station in Costa Rica. We quantified the diversity, abundance, and mortality of lianas in primary and selectively logged forest at La Selva for over 3 years, from January 1999 until July 2002. We measured, identified, permanently marked, and mapped all lianas greater than or equal to1.3 m in length and 2 mm in diameter, whether climbing or free-standing, in nine, 24 m x 36 m (864 m(2)) plots. There were no significant differences in density, diversity, or mortality between primary forest and areas that were selectively logged approximately 50 years prior to 2 our study. We found a mean density of 1493 lianas ha(-1) and a mean species richness of 23 species per 864 m(2) plot. Annual mortality was 9.4% over all size-classes, but was the highest for the smallest individuals (<2 cm in diameter). Annual mortality for larger individuals (greater than or equal to5 cm) was much lower over the 3.5-year period (3.2% per year) and the five most abundant species suffered no mortality in this size-class. In contrast to many lowland neotropical forests, where Bignoniaceae and Fabaceae are reported to be the dominant liana families, at La Selva we found that Sapindaceae was the most speciose family and Dilleniaceae the most abundant. Moutabea aculeata (Polygalaceae) was the most abundant species, constituting approximately 17% of the individuals and having the lowest mortality of all 60 species. The 10 most abundant species at La Selva accounted for more than 60% of all individuals. Compared to other lowland sites in the neotropics, including other wet forests, the abundance and diversity of lianas at La Selva are very low.
Stevens, M.H.H., S.A. Schnitzer, D. Bunker, and W.P. Carson (2004) Establishment limitation reduces species recruitment and species richness as soil resources rise. J. Ecol. 92:339-347 1. At local spatial scales, species richness tends to fall as productivity rises. Most explanations have focused on increased extinction, but, instead, we test experimentally whether increased soil fertility reduces recruitment. Specifically, we test whether variation in recruitment is due to source limitation, germination limitation or establishment limitation, and how litter accumulation and seed predation contribute to these processes. 2. We established four crossed experimental treatments in a perennial dominated early successional plant community over three years. We added seed of thirty species, manipulated access by selected seed predators, removed litter and added slow release fertilizer at four levels (0, 8, 16, 32 g N m-2). 3. Species recruitment and richness both decreased with increasing fertility, but, counter to our expectations, we found that neither seed additions nor litter removal could counteract the negative effects of fertilizer. 4. Seed additions increased seedling density at all fertilizer levels, and seed predation appeared to have no influence on seedling densities. In spite of high seedling densities at all fertilizer levels, final stem density declined by 70% as fertilizer increased. A strong stem density ? species richness relationship suggests that declines in final stem density caused more than half of the decline in species richness along this fertility gradient. 5. These results suggest that establishment limitation, i.e. the reduction of growth and survival from seedling to adult, controls species recruitment in highly fertile sites. 6. The high degree of recruitment limitation commonly observed in productive habitats suggests that high productivity causes establishment limitation, thereby isolating these communities from the regional species pool. We suggest that such isolation provides a mechanism to explain why the species composition of productive communities exhibits higher variability than the composition of less productive communities within the same regional source pool.
Carson, W.P., J.P. Cronin, and Z.T. Long (2004) A general rule for predicting when insects will have strong top-down effects on plant communities: on the relationship between insect outbreaks and host concentration. Pp 193-221 in Insects and Ecosystem Functions, Weisser, W.W., and E. Siemann, Ed. Springer-Verlag, Berlin We provide a new and general rule that predicts when native insect herbivores will have a major influence on dominant native plant species in communities and ecosystems worldwide. We argue that native insect herbivores will function as classic keystone species whenever their hosts become abundant and form large, persistent, dense stands. Specifically, our Host Concentration Model predicts that the impact of specialist insect herbivores will be more severe on a per individual basis as host species build-up to form large and dense stands. The impact of these native insect herbivores, while important at non-outbreak levels, will be most important during major bouts of defoliation that occur during periodic insect outbreaks. Our review of the literature suggests that such outbreaks are common from a phytocentric perspective. Consequently, these insect outbreaks will have a major influence on ecosystem function via their ability to regulate and reduce the abundance of host species that may typically be the superior competitor across the landscape. Finally, we believe that this Host Concentration Model will predict when specialist insects will regulate plant communities and ecosystems better than resource supply models that rely on gradients in fertility or productivity.
Long, Z.T., C.L. Mohler, and W.P. Carson (2003) Extending the resource concentration hypothesis to plant communities: Effects of litter and herbivores. Ecology 84:652-665 We extend the resource concentration hypothesis (herbivorous insects are more likely to find and stay in more dense and less diverse patches of their host plants) to plant communities. Specifically, whenever superior plant competitors spread to form dense stands, they will be found and attacked by their specialist insect enemies. This will decrease host plant abundance, causing a reduction in standing crop biomass, which will indirectly increase subordinate competitors and plant species richness. In this study, we found that a native, specialist chrysomelid beetle (Trirhabda virgata) in an old-field community decreased total standing crop biomass, leading to an increase in plant species richness. This reduction in biomass was due solely to a reduction in the biomass of the beetle's host plant, meadow goldenrod (Solidago altissima), which was the dominant plant species in this community. Our results demonstrate that when a superior competitor increases in density, the per-stem impact of herbivores increases due to a buildup of these herbivores in high-density host patches. Specifically, we found that as goldenrod density increased, the per-stem abundance of Trirhabda virgata also increased. In turn, species richness increased as the negative effect of insects on goldenrod biomass increased. Recent research suggests that litter accumulation could negate or cancel the effect of herbivorous insects on plant communities because litter accumulation increases with standing crop biomass, causing a decline in species richness. The litter accumulation hypothesis states that, in productive communities, the increase in the abundance of the superior competitor will lead to a dense accumulation of plant litter, causing a decline in species richness. Consistent with this hypothesis, we found that as the biomass of the dominant plant species increased, litter mass also increased. In turn, species richness decreased as the negative effects of litter on stem density increased. Interestingly, the effect of litter on stem density depended on whether insects were present. Our results suggest the potential for a general rule: specialist insect herbivores will function as classic keystone species in plant communities whenever host species form large, dense aggregations if host plants are dominant species. Schnitzer, S.A., P.B. Reich, B. Bergner, and W.P. Carson (2002) Herbivore and pathogen damage on grassland and woodland plants: a test of the herbivore uncertainty principle. Ecol. Lett. 5:531-539 Researchers can alter the behaviour and ecology of their study organisms by conducting such seemingly benign activities as non-destructive measurements and observations. In plant communities, researcher visitation and measurement of plants may increase herbivore damage in some plant species while decreasing it in others. Simply measuring plants could change their competitive ability by altering the amount of herbivore damage that they suffer. Currently, however, there is only limited empirical evidence to support this `herbivore uncertainty principle' (HUP). We tested the HUP by quantifying the amount of herbivore and pathogen damage in 13 plant species (> 1400 individuals) at four different visitation intensities at Cedar Creek Natural History Area, Minnesota, USA. Altogether, we found very little evidence to support the HUP at any intensity of visitation. Researcher visitation did not alter overall plant herbivore damage or survival and we did not detect a significant visitation effect in any of the 13 species. Pathogen damage also did not significantly vary among visitation treatments, although there was some evidence that high visitation caused slightly higher pathogen damage. Based on our results, we question whether this phenomenon should be considered a `principle' of plant ecology.
Stevens, M.H.H., and W.P. Carson (2002) Resource quantity, not resource heterogeneity, maintains plant diversity. Ecol. Lett. 5:420-426 Resource heterogeneity has often been proposed to explain the maintenance of plant species diversity and patterns of species diversity along productivity gradients. Resource heterogeneity should maintain biodiversity by preventing competitive exclusion because different species are superior competitors in different parts of a heterogeneous environment. In natural systems, however, resource heterogeneity covaries with average resource supply rate, making the effect of heterogeneity difficult to isolate. Using a novel experimental approach, we tested the independent effects of resource heterogeneity and average supply rate on plant species diversity. We show that the average supply rate of the most limiting resource controlled species diversity, whereas heterogeneity of this resource had virtually no effect. These findings also suggest that biodiversity declines with increasing productivity because at high enough levels of productivity one resource may always be driven to sufficiently short supply to exclude many species.
Stevens, M.H.H., and W.P. Carson (2001) Phenological complementarity, species diversity, and ecosystem function. Oikos 92:291-296 Increasing species diversity frequently enhances ecosystem function. Phenological complementarity, the asynchrony of species resource use and growth, may explain how species diversity influences ecosystem function but remains largely untested. We used an early successional plant community containing species with a variety of phenologies to test whether increasing species diversity enhances ecosystem function by increasing phenological complementarity. Over a two-year period, we increased environmental heterogeneity within an abandoned held with variation in disturbance, soil nutrients, water, light availability, and disturbance in 160 permanent plots, and measured percent cover of each plant species three times in each growing season. We did not manipulate species composition directly, and thus diversity and complementarity in each plot were the result of pre-existing conditions and responses of individuals to experimental treatments. Species diversity was measured in two ways, as the total number of species per plot and as the evenness of species abundances. Phenological complementarity was measured as the negative logarithm of the variance ratio. We tested whether the number of plant species per plot, species evenness, and their phenological complementarity in the first year predicted total annual cover in the second year. Total annual cover increased only moderately with number of species and evenness, consistent with studies that randomize species composition among replicate plots. Any effect that species number or evenness had on total annual cover, however, was not due to phenological complementarity. Rather, diversity was unrelated to phenological complementarity. These results indicate that naturally occurring variation in species diversity had little effect on whether phenological complementarity can enhance ecosystem function.
Schnitzer, S., and W.P. Carson (2001) Treefall gaps and the maintenance of species diversity in a tropical forest. Ecology 82:913-919 The maintenance of species diversity by treefall gaps is a long-standing paradigm in forest ecology. Gaps are presumed to provide an environment in which tree species of differing competitive abilities partition heterogeneous resources. The empirical evidence to support this paradigm, however, remains scarce, and some recent studies even suggest that gaps do not maintain the diversity of shade-tolerant species. Although there is evidence that gaps maintain the diversity of pioneer trees, most of this evidence comes from studies that did not make comparisons between gaps and intact forest sites (controls). Further, nearly all studies on the maintenance of diversity by gaps have ignored Iianas, an important component of both old-world and neotropical forests. We tested the hypothesis that treefall gaps maintain shade-tolerant tree, pioneer tree, and liana species diversity in an old-growth forest on Barro Colorado Island (BCI), Panama. We compared the density and species richness of these guilds between paired gap and non-gap sites on both a per-area and a per-individual (per capita) basis. We found no difference in shade-tolerant tree density and species richness between the gap and non-gap sites. Both pioneer tree and liana density and species richness, however, were significantly higher in the gap than in the nongap sites on both a per-area and a per-individual basis. These results suggest that gaps maintain liana species diversity and that this effect is not merely a consequence of increased density. Furthermore, our data confirm the long-held belief that gaps maintain pioneer tree species diversity. Because lianas and pioneer trees combined account for similar to 43% of the woody plant species on BCI, and in other forests, our results are likely to be broadly applicable and suggest that gaps play a strong role in the maintenance of woody species diversity. Schnitzer, S.A., and W.P. Carson (2000) Have we forgotten the forest because of the trees? Trends Ecol. Evol. 15:375-376
Schnitzer, S., J. Dalling, and W.P. Carson (2000) The impact of lianas on tree regeneration in tropical forest canopy gaps: Evidence for an alternative pathway of gap-phase regeneration. J. Ecol. 88:655-666 1. Regeneration in forest canopy gaps is thought to lead invariably to the rapid recruitment and growth of trees and the redevelopment of the canopy. Our observations, however, suggest that an alternate successional pathway is also likely, whereby gap-phase regeneration is dominated by lianas and stalled in a low-canopy state for many years. We investigated gap-phase regeneration in an old-growth tropical forest on Barro Colorado Island (BCI) in Panama to test the following two hypotheses: (i) many gaps follow a pathway in which they remain at a low canopy height and are dominated by lianas and (ii) the paucity of trees in this pathway is a function of liana density. 2. We surveyed a total of 428 gaps of varying ages (c. 5, c. 10, and 13+ years old) and identified those which followed the conventional pathway of regeneration and others that remained stalled in a low-canopy state for many years and were dominated by either lianas or palms. Each of these pathways will likely have different successional trajectories that will favour the growth of a distinct suite of mature species and ultimately result in contrasting species composition. 3. The successional pathway of liana-dominated, stalled gaps is common throughout the forest. We estimate conservatively that 7.5% of the gaps that form each year will follow this pathway, probably due to the suppression of tree regeneration by lianas, and that many of these stalled gaps will persist for much longer than 13 years. Consequently, a high proportion of gaps in the forest at any given time will be stalled. Furthermore, liana tangles, which persist in the tropical forest understorey for extended periods of time, almost certainly originate in these gaps. 4. Liana abundance was positively correlated with pioneer tree abundance and diversity while negatively correlated with non-pioneer tree abundance and diversity. Thus, lianas appear to inhibit non-pioneer tree survival while indirectly enhancing that of pioneer trees. 5. Lianas are abundant in many types of tropical and temperate forests and a successional pathway involving liana-dominated, stalled gaps may therefore be frequent and widespread.
Carson, W.P., and R.B. Root (2000) Herbivory and plant species coexistence: community regulation by an outbreaking phytophagous insect. Ecol. Monogr. 70:73-99
Most general theories proposed to explain the trophic structure of communities ignore the possibility that insect outbreaks can severely damage vegetation and reduce the abundance of dominant plant species over vast areas. Specialist chrysomelid beetles can irrupt and defoliate goldenrods (Solidago spp.), a group of widespread, long-lived, herbaceous perennials. We examined the long-term effects (10 yr.) of suppressing insects using insecticide in replicated plots on the structure and diversity of an old-field dominated by the goldenrod, Solidago altissima. An outbreak of the chrysomelid beetle, Microrhopala vittata, that specializes on S. altissima, occurred during the experiment and persisted several years. Damage caused by this outbreak dramatically reduced the biomass, density, height, survivorship, and reproduction of S. altissima. Herbivore exclusion caused the formation of dense stands of goldenrods with a 2-fold increase in both standing crop biomass and litter. The understory in these dense stands had significantly lower plant abundance, species richness, flowering shoot production and light levels; these conditions persisted for years following the outbreak. Thus, M. vittata functioned as a keystone species. Furthermore, insect herbivory indirectly increased the abundance of invading trees, thereby increasing the rate of succession by speeding the transition of this old-field to a tree dominated stage.
Lawson, D., R.S. Inouye, N. Huntly, and W.P. Carson (1999) Patterns of woody plant abundance, recruitment, mortality, and growth in a 65 year chronosequence of old-fields. Plant Ecol. 145:267-279 We surveyed vegetation along forest margins in a 65-year chronosequence of old-fields at the Cedar Creek Natural History Area in east-central Minnesota, USA, to identify successional patterns of woody plants and to determine if these were correlated with soil nitrogen. We predicted that shrub and tree abundance, size, and distance of occurrence from the forest edge would be correlated with field age or soil nitrogen. Instead we did not find successional trends in the abundance or composition of woody species. Even in the oldest field the abundance of trees and shrubs was low and concentrated in areas close to the forest. Though trees were larger and present further from the forest edges in older fields, average tree height was less than 126 cm in all fields. Since we did not find successional trends we looked at various local factors (local seed sources, deer browsing, and forest edge aspect) and their relation to recruitment, mortality, or growth to explain variation among fields in abundance of trees or shrubs. The three most common tree species (Quercus rubra, Q. macrocarpa, and Populus tremuloides) all had a higher relative abundance of seedlings, and two (Q. rubra and Q. macrocarpa) had a higher relative abundance of large trees adjacent to forests with a high abundance of conspecific adults. Most trees taller than 20 cm were browsed by deer and were shorter in 1995 than they were in 1993. Mortality was higher for trees less than 30 cm indicating that mortality was size-dependent. Forest edge aspect did not significantly influence the abundance or demography of any species. Our results suggest that the patterns of seedling recruitment were largely determined by the proximity of seed sources and that these patterns may persist so that tree communities in old-fields resemble adjacent forests. Deer may be a significant factor in the suppression of tree populations in old-fields through repeated browsing which reduces tree growth and elevates tree mortality by prolonging the period of time trees remain susceptible to size-dependent mortality.
Carson, W.P., and R.B. Root (1999) Top-down effects of insect herbivores during early succession: influence on biomass and plant dominance. Oecologia 121:260-272 We tested the hypothesis that phytophagous insects would have a strong top-down effect on early successional plant communities and thus would alter the course of succession. To test this hypothesis, we suppressed above-ground insects at regular intervals with a broad spectrum insecticide through the first three years of old-field succession at three widely scattered locations in central New York State. Insect herbivory substantially reduced total plant biomass to a similar degree at all three sites by reducing the abundance of meadow goldenrod, Solidago altissima. As a result, Euthamia graminifolia dominated control plots whereas Solidago altissima dominated insecticide treated plots by the third year of succession. S. altissima is the dominant old-field herbaceous species in this region but typically requires at least 5-years to become dominant. Past explanations for this delay have implicated colonization limitation whereas our data demonstrate that insect herbivory is a likely alternative explanation. A widespread, highly polyphagous insect, the xylem-tapping spittlebug, Philaenus spumarius appeared to be the herbivore responsible for the reduction in standing crop biomass at all three sites. Insect herbivory typically caused little direct leaf tissue loss for the 10 plant species we examined including S. altissima. Consequently, the amount of leaf area removed was not a reliable indicator of the influence of insect herbivory on standing crop biomass or on early succession. Overall, we found a strong top-down effect of insect herbivores on biomass at multiple sites; consequently our results may be broadly applicable. These findings run counter to generalizations that top-down effects of herbivores, particularly insects, are weak in terrestrial systems. These generalizations may not apply to insects, such as spittlebugs, that can potentially mount an effective defense against predators (i.e., spittle) and subsequently reach relatively high abundance on common plant species. Our results suggest that insect herbivory may play an important but often overlooked role during early old-field succession.
Stevens, M.H.H., and W.P. Carson (1999) Plant density determines species richness along an experimental productivity gradient. Ecology 80:455-465 A number of authors have suggested that, within areas a few square meters to many square kilometers in size, species diversity appears to peak at moderate levels of productivity, and this pattern is currently unexplained. Among the best examples of this pattern have been description of vegetation in which species richness declines as soil fertility increases. We tested two hypotheses that have been proposed to explain this pattern. The interspecific competitive exclusion hypothesis proposes that dominant species suppress the growth of competitively subordinate species and exclude subordinate species as fertility rises. In contrast, the assemblage-level thinning hypothesis proposes that individuals of all species tend to become larger as fertility rises, and individuals of all species tend to exclude subordinate individuals of each species. Because total density declines, samples of finite numbers of individuals will result in fewer species by chance alone. To test these hypotheses, we established an experimental productivity gradient in a first-year old field using four levels of slow-release NPK fertilizer (0, 8, 16, and 32 g N/m(2)). At the end of the growing season, we sampled aboveground biomass and numbers of stems for each species in 72 20 x 20 cm subplots (18 reps x 4 levels), with an average sample size of 260 individual stems per plot. We observed an 80% decline in stem density with increasing fertility, and a 50% decline in species richness along this fertility gradient. A simulation of random thinning along a fertility gradient showed a nearly identical decline in species richness, supporting the assemblage-level thinning hypothesis. We also found that responses of individual species to the soil fertility gradient showed virtually no support for interspecific competitive exclusion. The overwhelming influence of density found in this study suggests that plant species richness along many productivity gradients may be strongly influenced by total stem density, and that differences in competitive ability among species, although generally important, are not necessary to create dramatic changes in species richness along fertility gradients. Stevens, M.H.H., and W.P. Carson (1999) The significance of assemblage level thinning for species richness. J. Ecol. 87:490-502 1. A unimodal relationship between species richness and primary productivity is commonly reported. To explain this pattern, the mechanisms proposed in the many hypotheses are generally complex and almost all are without a strong empirical foundation. Here we evaluate the role of self-thinning in plant assemblages: assemblage-level thinning. 2. We developed a simple two-parameter model of species richness that predicts that plant species richness will be determined by a unimodal relationship between total plant density and above-ground biomass. This model provides a very narrowly defined set of testable quantitative predictions, and thus is the first falsifiable model of assemblage-level thinning. We fit this model to the species richness-above-ground biomass data from 14 empirical studies that are often cited as evidence of a general diversity-productivity relationship. In addition, we compared our model to two other models, one more flexible and one more constrained than our own. 3. We found that our model of species richness explained a substantial and statistically significant portion of the species richness observed in 11 of the 14 empirical studies of species richness-biomass patterns. Therefore, given the conservative nature of our model, and the number of previously published data sets explained by this model, we argue that assemblage-level thinning not only provides a viable and exceedingly parsimonious explanation, but may also be a widespread phenomenon.Schnitzer, S., and W.P. Carson (1999) Tropical Environments. Pp 605-609 in Encyclopedia of Environmental Science, Alexander, D.E., and R.W. Fairbridge, Ed. Chapman and Hall, London We hypothesized that severe drought affects the structure of tropical forests by favouring seedlings of some species or groups at the expense of others. To test this hypothesis, we irrigated naturally occurring woody seedlings during an El Nino-related drought in seasonal moist tropical forest on Barro Colorado Island, Panama. We predicted that irrigated plots would retain greater species richness than control plots, and that the benefits of irrigation would increase with the abundance of: trees relative to lianas, wet-forest seedlings relative to dry-forest seedlings, and rare species relative to common species. We also hypothesized that the strength of this filter would increase with increased moisture limitation, predicting that the benefits of irrigation would increase with seedling density and light availability, and decrease with mean seedling age. Irrigation did reduce species losses, but not by limiting the loss of drought-sensitive species as predicted. Instead, mortality in irrigated plots was density dependent, whereas species losses in control plots were well predicted by random thinning, suggesting that density dependence weakened as abiotic stress increased. Irrigation increased seedling growth, but did not affect seedling mortality. Contrary to our predictions, irrigation increased growth in plots dominated by dry-forest species relative to those dominated by wet-forest species, suggesting that dry-forest seedlings either occur in moisture-limited microsites or are more able to utilize dry-season precipitation. The strength of the filter did increase with potential moisture limitation, as irrigation increased seedling growth more in higher light environments. Annual precipitation has declined over much of the humid tropics during the 20th century. Our results suggest that this trend may reduce tropical forest diversity by weakening density-dependent mechanisms that maintain diversity. In addition, plots dominated by dry-forest species experienced higher growth in response to irrigation and also far lower dry-season mortality relative to plots dominated by wet-forest species. While we cannot disentangle the effects of microsite from species composition, these results suggest that dry-forest species may benefit from any increase in dry season length or severity. Research conducted during 'normal' conditions may overlook the impact of severe events and thus fail to identify critical mechanisms structuring ecological communities. Long, Z.T., W.P. Carson, and C.J. Peterson (1998) Can disturbance create refugia from herbivores: an example with hemlock regeneration on treefall mounds. J. Torrey Bot. Soc. 125:165-168 We tested the hypothesis that tip-up mounds of exhumed roots and soil created by treefalls provide a better site for the survival and growth of hemlock (Tsuga canadensis L.) than the areas immediately surrounding the mounds. The xeric and unstable conditions of tip-up mounds may impede the establishment and growth of hemlock relative to surrounding areas. The size and steep walls of tip-up mounds, however, may deter deer from accessing the tops of mounds, thereby allowing hemlocks to escape browsing. Nine years after a catastrophic blowdown in the Allegheny National Forest in northwestern Pennsylvania, we found that hemlocks on the mounds were larger, more abundant, and browsed less often than hemlocks found off of the mounds. The increased growth and survival of hemlocks on tip-up mounds was likely caused by decreased browsing pressure. Peterson, C.J., and W.P. Carson (1996) Generalizing forest regeneration models: the dependence of propagule availability on disturbance history and stand size. Can. J. For. Res. 26:45-52 We summarize three existing conceptual frameworks for forest regeneration in northeastern North America and suggest that none consider both a range of disturbance characteristics and a range of forest conditions at the time of disturbance. We offer a more general conceptual model, within which the existing models can be seen as special cases. We propose that the abundance of characteristic seed-bank, pioneer species, such as L.f. and Rubus spp. (often Rubus allegheniensis T.C. Porter, Rubus hispidus L., or Rubus odoratus L.), is dependent on propagule availability, which in turn is determined by forest age and size. Specifically, following disturbance, large tracts of forest and older forests (ca. >125 years) are predicted to have very low densities of the above pioneers. As a result, population, community, and ecosystem parameters may be substantially different in the regenerating forest than in the familiar cases of regeneration in secondary forests. Indeed, the presettlement forest of much of northeastern North America may have experienced a notable scarcity of pioneers after disturbances, in areas far enough inland for hurricanes to be unimportant. Our hypothesis makes predictions of seed-bank abundance that are well supported in a variety of forest types; we also provide support for our hypothesis with data on regeneration following catastrophic windthrow in Allegheny National Forest, Pennsylvania, and elsewhere. Finally, our hypothesis also predicts that the potential regeneration in much of the secondary forest of northeastern North America should profoundly shift as stands age from roughly 100 to 130 years. Belsky, A.J., W.P. Carson, C.L. Jensen, and G. Fox (1993) Overcompensation in plants: hebivore optimization of red herring? Evol. Ecol. 7:109-121 The increased growth rates, higher total biomass, and increased seed production occasionally found in grazed or clipped plants are more accurately interpreted as the results of growth at one end of a spectrum of normal plant regrowth patterns, rather than as overcompensation, herbivore-stimulated growth, plant-herbivore mutualisms, or herbivore enhanced fitness. Plants experience injury from a wide variety of sources besides herbivory, including fire, wind, freezing, heat, and trampling; rapid regrowth may have been selected for by any one of the many types of physical disturbance or extreme conditions that damage plant tissues, or by a combination of all of them. Rapid plant regrowth is more likely to have evolved as a strategy to reduce the negative impacts of all types of damage than as a strategy to increase fitness following herbivory above ungrazed levels. There is no evolutionary justification and little evidence to support the idea that plant-herbivore mutualisms are likely to evolve. Neither life history theory nor recent theoretical models provide plausible explanations for the benefits of herbivory. Several assumptions underlie all discussions of the benefits of herbivory: that plant species are able to evolve a strategy of depending on herbivores to increase their productivity and fitness; that herbivores do not preferentially regraze the overcompensating plants; that resources will be sufficient for regrowth; and that being larger is always 'better' than being smaller. None of these assumptions is necessarily correct. Facelli, J., and W.P. Carson (1991) Heterogeneity of plant litter accumulation in succcessional communities. Bull. Torrey Bot. Club 118:62-66 Vankat, J.L., and W.P. Carson (1991) Floristics of a chronosequence corresponding to old field deciduous forest succession in southwestern Ohio. 3. Post-disturbance vegetation. Bull. Torrey Bot. Club 118:385-391 Revegetation of 1.5 x 1.5 m plots disturbed by severing roots and rhizomes and turning the topsoil was examined during the first growing season for a chronosequence of five stands. The objective was to compare the disturbance responses of communities of different successional stages. Species richness of post-disturbance vegetation was highest in the 50-year, savanna-like old field, but otherwise was generally lower in later successional stages. Species diversity was largely constant, but was low when one or two species dominated by reproducing vegetatively. Cover was high in the disturbed plots in old fields and low in forests. In general, both annuals and biennials (including short-lived perennials) were lower and woody plants higher as percentages of the flora and in relative cover in later successional stages. Perennial herbs were most important in the two mid-seral old fields. Floristic comparisons with the undisturbed vegetation and seed banks indicate that the seed bank was important in revegetation following disturbance in early-successional stages, as it determines the number and abundance of annuals and biennials. In contrast, remnants of the undisturbed vegetation persisting in the disturbed plots were more important in mid- and late-successional stages through vegetative reproduction of perenniai herbs and woody plants. Cain, M.L., W.P. Carson, and R.B. Root (1991) Long-term suppression of insect herbivores decrease rhizome production and number in Solidago altissima. Oecologia 88:251-257
Peterson, C.J., W.P. Carson, B.C. McCarthy, and S.T.A. Pickett (1990) Microsite variation and soil dynamics within newly-created treefall pits and mounds. Oikos 58:39-46 Carson, W.P., and D.K. Abbiw (1990) The vegetation of a fire protection site on the Accra Plains, Ghana. Afr. J. Ecol. 28:143-146 Carson, W.P., and C.J. Peterson (1990) The role of litter in an old-field plant community: impact of litter quantity in different seasons on plant species richness and abundance. Oecologia 85:8-13
Carson, W.P., and S.T.A. Pickett (1990) Role of resources and disturbance in the organization of an old-field plant community. Ecology 71:226-238 Carson, W.P., and G.W. Barrett (1988) Succession of old-field plant communities: effects of contrasting types of nutrient enrichment. Ecology 69:984-994 |
Download the |