15 December 2009

Light and the Canopy

Posted by Michael McFadden

Image from NASA/Robert SimonA tree falls in a forest: the question isn’t whether it makes a sound or not, but what other plants take advantage of the light that streams in through the newly-opened hole in the canopy, according to a talk at B21C: Adaptation of Vegetation to Changes in Environmental Forcing II.

Tropical forests are complicated and are subject to all kinds of compounding variables, but in boreal forests and environments—particularly those above 35°N—incoming light is the single most defining variable defining vascular plant species distributions, says Peter Eagleson of MIT, in his talk entitled “Range and Richness of Vascular Land-Plants: The role of variable light.” Incoming solar radiation basically draws envelopes around the areas different species live, he explains.

The idea of a “bioclimatic gradient” isn’t new, and it’s pretty intuitive—clearly, conditions in further-northern latitudes are different from those more southerly, and different conditions will force (or allow) the development of different coping strategies. Generating a predictive model of the “bioclimatic gradient” by simplifying the gradient down to one of incident light, allows us to tie the model directly to latitude, Eagleson suggests. So if we know there’s a species that exists at a certain latitude, we can extrapolate out to the potential climate conditions, or vice versa. Thus, if we know the amount of incident light coming in to a site, we can estimated the range of the species that live there.

Why, exactly, ranges increase with increasing latitude is a huge outstanding question in ecology. Why species richness seems to decrease with latitude is another major outstanding question, which Eagleson thinks is related to the intra-annual variability in “climate-critical” events—e.g. if there are lots of changes in sunlight availability over a year or season, only plants that can deal with variability will persist over time, and overall species richness will go down.

As we build models for how plant species distributions will respond to climate or other changes in future, it’s valuable to know what are the most useful variables for defining ranges and richness, so models can weight those variables appropriately. In this context, Eagleson’s validation of incident light delineating species range, and the recurrence of climatic events delineating species richness, is particularly useful to the next generation of modelers looking to figure out what, exactly, our terrestrial environment will look like in the future.

–Ale Borunda, Columbia University Earth and Environmental Sciences Journalism Graduate Student