The power of accurate observation is commonly called cynicism by those who have not got it.  - Bernard Shaw

Aaron Stoler

Research Interests

Leaf Litter and Aquatic Forest Nutrient Cycling

 

             When you go into a forest during fall, what do you see on the ground?  Leaves!  When you walk back to the forest in the springtime, what do you see on the ground?  No leaves!  Congratulations, you just passed kindergarten.  Now for the more difficult questions.  What happens to the leaves, how does it happen, and why does it happen?

 

 

             Obviously, the leaves disappear.  Actually, they decompose.  In autumn, the tree absorbs much of the nutrients from the leaves (you notice this when the leaves start turning different colors), and then they fall.  Why would a tree do this?  Well, first of all, not all trees shed their leaves.  Coniferous trees usually keep most of their leaves.  For trees with broad leaves, keeping leaves can be dangerous and highly inefficient.  The cold weather makes photosynthesis more difficult and slower, and also the broad nature of the leaves allow accumulation of snow on the leaves, making the branches heavier and more likely to break.  On the other hand, leaves are costly to regenerate in the spring, so if a tree can somehow mitigate the risks of holding onto leaves during spring, then it probably will.  Pine needless are perfect in this regard, so pine trees typically hold on to their needles during the winter!  Surface area of the needles is low, so snow doesn’t accumulate, and leaves are protected from the environment by a thick, waxy outer layer

 

 

Next question.  How do the leaves disappear?  Well, if the leaves fall onto land, the leaves will gradually turn to mush as rain and snow fall on them.  Water is actually a very corrosive and sticky substance (just because you drink it all the time does not make it unreactive—water has oxygen, and oxygen is one of the most reactive elements common to our world!).  Earthworms and spiders move the leaves around and fungi use the chemicals.  When the leaf falls into water, the same thing happens, but faster.  Water is constantly hitting the leaf, so it breaks down even faster.  Also, you get detritivores (mostly insects) which eat the leaves!

 

And the final question.  Why do the leaves disappear?  If you are looking for a philosophical answer, consult your local deity.  But physically, organisms in the forest rely on either the nutrients in the soil or in the water to form the base food supply which they rely upon.  You can’t grow without food—neither can bacteria,

Stream in Patapsco State Park, Maryland

bugs, frogs, spiders, fish, and everything else in the forest.  The only way that nutrients can get into the soil or water is from stuff dying or from input outside forests.  Conveniently, every autumn, leaves die.  Ecologically speaking, leaves form the base of the nutrient cycle in forests.

 

In addition to putting a whole bunch of good nutrients in the water, leaves and needles also put a bunch of bad chemicals in the water, such as tannins and alkaloids (tannins are the same stuff found in tea and coffee which most people find bitter).  These chemicals are difficult for organisms to digest because they bind to proteins (which is also why people put milk in their coffee—the tannins bind to the milk and make the drink more digestible and less bitter).  Tannins are known anti-microbial agents, they can kill tadpoles, and they can even mummify a live human (No kidding...Wikipedia that one…)!

 

To add to the story, we must recognize that there are MANY types of trees in forests.  Each type of tree has different associated chemical compositions.  Every forest has a unique composition of plant-life; therefore every forest has a unique input of chemical components.  These chemical components generate unique environments.  For example, the input of red maple litter into an aquatic systems pumps a lot of nutritional nitrogen and dissolved carbon into the environment.  The addition of chestnut litter puts a bunch of toxic tannins into the environment.  Something like oak litter decays so slowly that it puts hardly anything at all into the environment.  Where would you rather live?

 

This research question is becoming increasingly important as our forests change.  Fire suppression, clear-cutting, over-grazing by mammals, invasive introductions, disease, and other disturbances (both anthropogenic and natural) are dramatically changing the landscape of eastern American forests.  For example, red maple (Acer rubrum) is taking over most oak forests and eastern hemlock (Tsuga canandensis) is disappearing because deer are eating all the saplings.  How will the loss and gain of such species affect the nutrient and chemical stock available to forested food webs, and how will the food webs change in response?

 

 

Coniferous tree—Lodge pole pine

http://www.scsc.k12.ar.us

Putting it in context—Communities!

Ok.  Let’s sum all that up and just say that leaves are important.

 

Now, think of a pond in a forest.  Pretty nasty place...lots of dirt, lots of leaves, lots of muck, and LOTS of life.  For such dirty places, they are a growth medium for an incredible diversity of life!  Tadpoles, amphipods, crayfish, snails, midges, zooplankton, phytoplankton, periphyton, water bugs, dragonflies, and salamanders (among a bunch of other organisms) need these ponds.   They can’t live in streams because they can’t deal with constantly flowing water, so they live in ponds.

American tree frog

www.wayfaring.info

 

(No, I don’t work with them and they are only in the tropics.  So what?  They’re cool)

The chemistry of the litter changes the conditions of the pond.  The conditions of the pond must meet the tolerance level of the organisms living in that pond.  You wouldn’t live in a place that you can’t tolerate, would you?  Neither would an insect, frog, or anything else that lives in a pond.  That’s the basics of what I’m studying. 

General Interests

 

I am generally interested in ecology and evolution.  At least, that’s what I have to tell people in order for them to keep paying me.

 

I am broadly interested in the feedbacks between ecosystem processes and diversity.  Environmental conditions determine what species are where.  You can’t put a tropical plant in the middle of the arctic and you can’t put a penguin in the middle of the Sahara desert.  Well, you could try.  Good luck.  I’ll alert PETA.

 

This same concept works on very local scales.  Mosquitoes may breed in one part of a forest simply because it is moist and breeding holes are more readily available, but you won’t find them in a stretch of forest one kilometer away because it is too dry.  Indeed, different species have preferences for some habitats over others, and subtle variations in environmental conditions across a heterogeneous landscape can translate into substantial patterns in species segregation.  My interests lie in understanding those patterns and the conditions that drive them, particularly in forests.  Why forests?  It may have something to do with the incredibly high level of productivity and diversity often associated with them.  But tell me honestly...if you could get paid for hiking through a forest, wouldn’t you?

 

Anyway, environmental conditions are not independent of diversity.  The interaction among species (e.g. competition) can determine what species are allowed into an environment, and can also determine the environmental conditions of that environment.  A lot of organisms alter the environment.  Take beavers, for example.  They only come into an environment if there is a stream there, and then they change the environment so other species can exist there.  Or let’s go with a plant example for those of you more botanically inclined (plant people, as I like to refer to you).  Nitrogen fixing legumes come into an environment and provide more nitrogen for surrounding plants, allowing total primary production to increase.  Without them, the soil can provide nutrients for a 5 plants but with them, 10 species can exist (totally random numbers).  The effect of species on other species can work in the opposite direction as well.  Competition between two species for a limiting nutrient can often result in the elimination of weaker competitors.

 

The environment permits species to exist, and the species change the environment to influence the species there.  Ok, so things are complicated.  Deal with it.