Changing leaves activate critical processes

Bruce Hungate

Bruce HungateBy Bruce Hungate
Director of NAU’s Center for Ecosystem Science and Society

Northern Arizona is home to many charismatic trees. Their leaves changing color in the fall inspire photographers, artists and poets. But a few weeks later, the leaves are dead, having lost not just their color but also their charisma – few poems record the aesthetics of the dull brown and soggy dead leaves decomposing on the forest floor. Yet during this time important processes are underway, including exchanges of vital elements, a critical part of the ecosystem.

In death, leaves teem with life, microbial life, such that each leaf is both home and food to thousands of creatures. We are still learning about this complex micro-community, beginning with their basic energy economy. For example, do the microbes get all the nutrition they need from the leaves? Probably not. Like a nation rich in some resources but lacking others, microbes growing on dead leaves need to import nutrients from the surrounding environment, especially nutrients that are rare in leaves, like nitrogen.

But at what rates do these exchanges occur? And is the carbon in leaves really enough to sustain the microbes? And how does it vary from one leaf type to another? These are difficult questions, because, while thousands of studies have measured how rapidly leaves decompose, it’s much more difficult to track the exchanges of materials between decomposing leaves and the surrounding environment.

Scientists from the Center for Ecosystem Science and Society, the Spanish Council for Scientific Research and the University of Barcelona collaborated on a recent study using stable isotopes to mark the carbon and nitrogen in the leaves so that it was distinct from the carbon and nitrogen in the surrounding environment. They discovered that the microorganisms rely strongly on a major import economy for nitrogen, not so surprising, but also for carbon; this was completely unexpected, because dead leaves contain lots of the element.

Drawing of the narrowleaf (top) and fremont (bottom) cottonwood leaves, by Vicent Pastor Urios, father of the first author of the paper.
Drawing of the narrowleaf (top) and fremont (bottom) cottonwood leaves, by Vicent Pastor Urios, father of the first author of the paper.

Furthermore, it turns out that the type of leaf makes a difference: microbes living on the narrowleaf cottonwood leaves, prevalent in the western United States, import quite a bit more carbon, and the microbial communities growing on leaves of Fremont cottonwoods import substantially more nitrogen. This contrast was surprising, because Fremonts typically contain more nitrogen than narrowleafs, so why the microbial community should import nitrogen at a faster rate when growing on Fremonts is puzzling, but points to more questions the research team will pursue in future studies.

Even with these new questions, the findings revise our understanding of the flow of material and energy in stream ecosystems, and highlight the importance of the invisible players, the bacteria and fungi inhabiting decomposing leaves. Few recognize the charisma of these creatures. After all, they excrete compounds that make dead leaves slimy, that give pond scum its name. But when you drill down there is a fascinating community and element economy that is essential to the livelihoods of other more classically charismatic creatures inhabiting rivers. In nature, the negligibly tiny becomes fascinating when you take the time, and in this case, the isotopes, to pay close attention.

These ideas are included in an article published this month in the journal, Oecologia.