Climate tolerance, range position, and soil fertility affects herbaceous species distribution across topographic microclimates

Background/Question/Methods

Topography alters microclimates in montane environments by affecting evapotranspiration, snow accumulation, runoff, and wind exposure. Cool topographic microclimates have the potential to create microrefugia for plant species vulnerable to warming climates. The capacity for plant species to respond to topographic microclimate may depend on species range position and sensitivity to climate change or non-climate limitations like lack of soil nutrients. We hypothesized species sensitivity to microclimate would be correlated with climate niche. We also hypothesized that north and south-range limit species might occur in cooler and warmer microclimates. We expected that the relationship between climate and microclimate would be weaker on nutrient-poor soils. These hypotheses were tested with presence/absence data from 4000 vegetation plots (ECOSHARE federal plot data and plant community data from a previous study) for common herbaceous species in the Klamath-Siskiyou Mountains. We developed high resolution species distribution models using MaxEnt with solar insolation, elevation, and topographic position for topographic microclimate and temperature and precipitation annual averages and extremes for climate. We tested for the effects of species range limits and soil infertility on model results using an index of range position and plot records on and off low-nutrient serpentine soils.

Results/Conclusions

We found a significant relationship between plant species distribution based on microclimate and climate (R² = 0.71, p < 0.001). While microclimate models had significantly lower predictive power than climate models, the inclusion of microclimate variables into climate models significantly improved model fit. Species microclimate niche was significantly affected by range position and soil fertility. Species at southern range limits occurred in significantly cooler microclimates than the rest of the plant community, particularly on low-nutrient serpentine soils. Northern range limit species occurred in significantly warmer microclimates, but this effect only weakly interacted with soil fertility. Species not at their range limits did not have different microclimate niches on serpentine soils. Overall, our results suggest that topographic microclimate may be important for herbaceous species with distributions sensitive to climate variables, particularly for species at their southern range limits and for plant communities on infertile soils.