PNAS Science Sessions podcast:





Podcast Interview: Daniel Wieczynski and Van Savage


PNAS: Welcome to Science Sessions. I’m Paul Gabrielsen. In forests, diversity is about more than just a variety of tree species. It’s also about the variety of traits the trees possess. Functional traits such as leaf area, elemental composition, and drought tolerance become important when the forest faces environmental stresses like changes in climate or precipitation. Daniel Wieczynski and Van Savage, of the University of California, Los Angeles, investigated how climate controls trait diversity in the world’s forests. In a recent PNAS paper, they and their colleagues found the factors most connected to forest trait diversity, a finding important to understanding how the climate shapes forests now and how those forests may change in the future. Wieczynski and Savage recently spoke to PNAS about their work. Here’s Wieczynski, explaining functional traits.


Wieczynski: Functional traits are basically just the characteristics of trees that relate directly to growth. In trees, this could be the elemental composition of the leaves, the nitrogen and phosphorus that you'd find in the leaves, or it could be the shapes of the leaves or the density of the wood. One of the most important functional traits in trees is specific leaf area, which is the amount of area in leaves relative to the amount of mass in leaves. A tree with high specific leaf area will have big, broad thin leaves that capture a lot of light and do a lot of photosynthesis. This is as opposed to a tree with low specific leaf area which will have leaves that are thicker and denser and have lower photosynthetic rates.


PNAS: Considering the diversity of functional traits as opposed to considering the diversity of species alone allows researchers to assess the fitness of a forest. Here’s Savage.


Savage: In evolution and ecology, ecology is mainly concerned with what's present and what's not. That has everything to do with what's growing, what's dying, and being replaced. Evolution has everything to do with fitness, in terms of what survives and what doesn't. So it's trying to get measures that are more directly related to things like fitness or growth. And at an individual level, not just at a species level, but an individual level. So, like a more finer-grained resolution.


PNAS: Wieczynski explains why functional trait diversity matters in forests.


Wieczynski: The importance of diversity of functional traits in a forest is not as well understood but there's some indication that forests with higher diversity of functional traits will have a greater ability to respond to shifts in climate in the future. There's also this emerging idea of multifunctionality in forests and so the more diverse collection of traits that exist in the forest, it might be positively related with more functions that that forest, or the more types of services and things that the forest can produce.


PNAS: The researchers took distributions of traits within forests and combined that data with global climate data. Wieczynski: This allows us to create more accurate representation of functional diversity and relate that to variation in the environment, specifically in variety of climatic variables. So, what ends up happening is that in a particular type of environment, some trees will do really well and some trees will do really poorly depending on the traits that they have.


Savage: There are also things, you know, where there are boundaries to what traits you can have. So if temperature is too extreme, there's a limit to how hot almost any plant can tolerate it or how cold it can tolerate it. So also there's extreme ranges and extreme bounds, they can make it so it's a filtering process in the trees you see.


PNAS: They found that the strongest climate drivers of functional trait diversity were vapor pressure and temperature variability. Savage: But I'm a little surprised temperature variability is as important or more important than mean annual temperature. One of the things climate change is going to do is increase the variability a lot.


Wieczynski: But what was really surprising is that if we look at the shapes of trait distributions within individual communities, we see that the shapes of these trait distributions are indicative of the current response to rapid climate warming. But it suggests that currently forests are shifting in functional trait space to deal with shifting climatic conditions, specifically rapid warming. Our finding that temperature variability is actually a very important predictor of functional diversity means that as climate change continues and these extreme events become more common, this will have a pretty profound impact on the functional diversity of forests.


Savage: The different aspects of diversity in forests and functional diversity are affected by different climatic variables. We sort of have the data and methods now that we can tease that apart. And teasing that apart should tell us a lot about which climate change effects to be most worried about in which forests, in which locations and that may help them figure out better management strategies. That's a long-term goal and implication that might be more possible as you can tease apart how each climate variable affects each trait.


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PNAS: Science Sessions, Wieczynski & Savage
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