Climate Modifies the Influence of Fine-Scale Topography and Landscape Position on Forest Productivity

Abstract:

Because topography influences moisture and nutrient availability, both landscape position (here, relative elevation and aspect) and fine scale topography (FST, ranging in scale from meters to tens of meters) can influence forest primary productivity. Soil and water both move downslope over time, changing the lateral and vertical distribution of water and nutrients along a hillslope. Climate change can alter aspect-dependent ecosystem processes that govern productivity such as soil water availability, photosynthesis, and the length of the growing season. We investigate how landscape position and FST affect forest productivity as environmental conditions change in a temperate forest in central North America. We use Enhanced Vegetation Index (EVI) as a proxy for productivity, obtained from Landsat images from 1984 to 2017. Elevation and aspect were assigned to each pixel based on a LiDAR Digital Elevation Model. We quantified each pixel’s fine-scale topographical position relative to its surroundings as the difference between its elevation and the average elevation of surrounding pixels. We used the Segmented R analytics package to investigate relationships between EVI and relative elevation, aspect, and FST across time as environmental conditions changed. At elevations above approximately 75% of the maximum elevational gain, the forest exhibited a decline in productivity as elevation increased, highlighting the expected, negative effect of soil moisture shortages on plant productivity in upslope regions. In hot and dry years, the decline in productivity with elevation was steeper and the elevation above which the decline was evident was lower. We further observed that trees in concave positions exhibited greater productivity than those in convex positions, and that this effect was exaggerated in hot and dry years. Finally, we observed that trees on north-facing slopes exhibited greater productivity in hot and dry years. This research underlines the importance of landscape position and FST on forest productivity and the degree to which these topographical features affect ecosystem response to climate change. Spatiotemporal distribution of water and energy is highly related to landscape position and FST which play an important role in the response of such ecosystems to climate change.