COS 90-1 - CANCELLED - Topographic gradients in a semi-arid ecotonal forest drive patterns of structure and diversity

Background/Question/Methods

The effects of climate change on forested ecosystems are determined by the aggregate responses of individual trees to climate variability. However, regional-scale climate predictions may not correspond to the conditions that individuals actually experience due to local variation in landscape features, particularly topography. We hypothesized that in semi-arid forests where trees often face dehydration stress, even small changes in topography may affect tree performance, ultimately causing variation in forest structure and diversity. In a semi-arid forest in the U.S. Great Plains, our goals were to (1) quantify the relationship between topographic and microclimatic gradients, and (2) identify topographic variables associated with variation in forest diversity and structure. In a 20.2-ha forest inventory plot, we sampled microclimatic variables (understory light, air temperature and relative humidity, soil moisture and temperature) along topographic gradients and integrated these with data on the abundances and distributions of woody seedlings, saplings, and adults and topographic variables estimated from a digital elevation model. We ran a principal component analysis on the microclimate variables and extracted the first two components to correlate these to the topographic variables and ran models using both derived categorical habitat types and continuous topographic variables to predict metrics of forest structure and diversity.

Results/Conclusions

Variation in all microclimatic variables corresponded strongly with topography. There was an elevation-driven soil moisture gradient from the river floodplain to the adjacent canyons up to the forest-grassland ecotone. While soil moisture fluctuated throughout the growing season at lower elevations, the mean moisture content was 8.5% higher than along the higher-elevation ecotone. Light availability and soil and air temperatures were higher along the ecotone and upper canyon slopes, and lowest in the mesic canyon bottoms, with air temperature differences equivalent to the mean annual temperature change across three degrees of latitude. Forest structure and diversity correlated with topographic and microclimatic gradients. We observed higher stem density, basal area, biomass, and species richness of adults and saplings in less exposed habitats with higher soil moisture, whereas seedling stem density and richness were higher in high-light, moist habitats. Species exhibited well-defined topographic niches, with high species turnover with elevation. Overall, topography and microclimatic variation strongly influenced forest structure and composition, with more complex and diverse stands on steeper slopes in areas with higher water availability. Our results suggest that as the climate becomes warmer and drier, topographically mediated microclimatic refuges supporting diverse and structurally complex forested ecosystems may shrink in semi-arid regions.