Climate regulates the distribution and abundance of symbiotic nitrogen-fixing trees in North America
Symbiotic nitrogen-fixing plants (hereafter “N fixers”) show a distinct spatial distribution. They are relatively rare in higher latitudes compared to low latitudes, but also show strong regional signals. For example, in certain regions such as Texas and Arizona, USA, they are often the dominant player in forests, whereas at similar latitudes to the east they are nearly absent. Why are N fixers more abundant in certain areas? Climate can have a profound influence on plant communities, and might influence N fixers in a different way than non-fixers. How does climate relate to the distribution of N-fixing versus non-fixing trees? Given the latitudinal distribution, we hypothesize that N fixer relative abundance will increase with temperature. However, it is unclear what we should expect for precipitation, given anecdotal evidence that N fixers inhabit wet and dry areas. It is also unclear how wind might influence N fixer relative abundance, but it might be important given its role in seed dispersal. We addressed this question using forest inventory data from the USA and Mexico (>125,000 plots) along with climate data from the Climate Research Unit. We used zero- and one-inflated beta regression to link N fixer relative basal area with climate variables.
As we hypothesized, the relative basal area of N fixers increases with increasing mean annual temperature. The mean diurnal temperature range, in contrast, has no significant influence on N fixer relative basal area. Mean annual precipitation has a significant but non-monotonic effect on N fixer relative basal area, such that N fixer abundance is high under both very dry and wet conditions compared with mesic conditions. Interestingly, mean wind speed has strong negative effect on N fixer relative basal area. The temperature effect might stem from the biphasic temperature response of the nitrogenase enzyme, but could also result from other factors. The precipitation effect might be driven by strong diffusion limitation of soil nitrogen at the dry end and more hydrologic and/or gaseous nitrogen loss at the wet end. The wind effect might result from differences in seed dispersal mechanisms among N fixers and non-fixers, with wind-dispersed non-fixers benefiting more from high wind speed. Our results suggest that climate has a major effect on the abundance distribution of N fixers, and might be useful for forecasting how this important functional group will respond to climate change.