Landscape Heterogeneity Modulates Forest Sensitivity to Climate

Abstract:

Elevation dependent snowmelt magnitude and timing strongly influences net ecosystem productivity in forested mountain watersheds. However, previous work has provided little insight into how internal watershed topography may modulate plant available water, microclimate and therefore forest growth. We analyzed 331 tree cores from three coniferous tree species across a range of elevation, landscape positions and aspects in the Lubrecht Experimental Forest, Montana, USA. We compared the annual basal area increment (BAI) to measures of the annual climatic water deficit, hydro-meteorological data in sideslope and hollow positions, and topographic indices derived from a LiDAR digital elevation models. Results indicate strong relationships between measures of BAI and the topographic wetness index, with differing slopes dependent on tree species. Generally, trees located in wetter landscape positions (hollows) exhibited greater annual basal growth relative to trees located in drier landscape positions (sideslopes). At the watershed scale, we evaluated the relationships between convergence and divergence, LiDAR derived estimates of basal area and seasonal patterns of the Landsat derived Normalized Difference Vegetation Index (NDVI; 1984-2012).These indicated differential growth response due to elevation gradients, irradiance and local convergence and divergence. Wetter landscape positions have higher values of greenness and basal area than drier positions. These observations suggest that the topography of semi-arid watersheds is a necessary consideration for quantifying conifer productivity and resiliency, due to its potential to mediate local microclimate and subsurface water redistribution.