As an ecosystem ecologist, I study how the cycle of life and death influences forest structure and changes over time. A walk in the woods might illuminate the forest’s dynamism as you observe squirrels tending to their buried acorn caches and listen to woodpeckers feasting on grubs. In contrast, the plant community may appear more static to the average aspiring Thoreau. However, as you traverse the forest floor, the cycle of life and death is working on overdrive right beneath your feet.
As soon as a leaf drops to the forest floor in autumn, soil microbes immediately begin dining on that leaf. The carbon and nutrients within the dead leaf – elements which only a month earlier fueled photosynthesis and plant growth – are now used on the forest floor to fuel microbial growth. Eventually, as the microbes process these nutrients, eating what they can and spitting out what they can’t, the nutrients that originated from the dead leaf will be taken up by plant roots to build new leaves, and the cycle of life will continue.
The rate at which dead leaves decompose on the forest floor is a key factor influencing nutrient availability in the soil and therefore overall forest structure. We can think about this decomposition rate as the bottleneck in an hourglass timer – the rate of leaf decay influences how quickly nutrients move from dead leaf material to microbes before being made available for plants to take up again.
For decades, scientists have studied how different factors such as climate and the nutrient content of the leaves and soil affect leaf decay rates. We’ve learned two main things from hundreds of such studies: in general, leaves decay faster in warm and wet climates (that’s why we refrigerate our food!), yet characteristics of the leaves themselves (e.g. nutrient content, texture and shape) most strongly influence decay rates, regardless of climate. Most importantly, decay rates vary tremendously across studies – while some leaves almost completely decompose in a matter of weeks, other species of leaves can take years to complete this turn in the cycle of life.
Impressive ecological variation is what fascinates us about the natural world, but this also presents a challenge to ecologists trying to make sense out of such “messiness.”
How can we predict how fast the cycle of life will turn in a forest hosting dozens of tree species or across different forests with distinct dominant tree species? Such predictions are becoming more urgent as climate change induces shifts in the species composition of many forests. We seek to understand how such changes will affect overall forest health.
In the Phillips lab, where I am a PhD student, one way we simplify species-rich forests is by grouping trees based on the type of fungi a tree species’ roots befriend belowground. In most temperate forests, tree species form physical associations with one of two types of fungi: arbuscular or ectomycorrhizal. In a recent study, I synthesized published data from over 200 tree species spanning tropical and temperate forests.
I showed that across temperate forests, leaf litter from trees associated with arbuscular mycorrhizal fungi decomposes faster, on average, than leaf litter from ectomycorrhizal-associated trees. However, in warmer and wetter tropical forests, this distinction disappears. Leaf litters of both arbuscular and ectomycorrhizal species decompose quickly with no statistically significant difference between the two groups.
We are excited about these results because they can help us predict decomposition rates at broad scales across our changing temperate forests. They also inspire more research towards understanding carbon and nutrient cycling differences in tropical compared to temperate forests. While this work focused solely on leaves, we know plant roots also provide a huge input of carbon and nutrients to the soil. Given that other labs have shown root tissue decomposition patterns vary from those of leaves, my next field research project aims to elucidate how root decomposition of both arbuscular and ectomycorrhizal tree species influences soil nitrogen cycling. Perhaps by the end of my PhD, we’ll understand the cycle of life at least a little better, if not the meaning of it as well!