Forests, shrubs, and terrain: top-down and bottom-up controls on forest structure

Overstory forest structure responds to terrain-related abiotic factors and to biotic interactions among overstory and understory plants. Unlike species abundance, tree height, biomass, and leaf area in many regions have been poorly quantified in relation to terrain-driven environmental gradients. In addition, the magnitude of understory influences on overstory structure has been poorly characterized for many forest systems. Our primary goal was to identify relationships between terrain (elevation, convexity, exposure), evergreen understory, and overstory structure (height, aboveground biomass, leaf area) in mature deciduous forests of the southern Appalachian Mountains. We used a combination of field point and plot measurements, LiDAR, and satellite image data to sample little-disturbed deciduous forest stands. Height, biomass, and gap frequency were significantly related to changes in elevation, exposure (aspect), and convexity (cove to ridge). Higher evergreen understory density was strongly correlated with decreases in forest height and biomass, with an impact observed across moisture, elevation, and exposure gradients. Canopies on ridges averaged half as tall at the highest evergreen understory densities when compared to those without evergreen shrubs (10 vs. 19 m), and overstory canopy height averaged 6 m shorter on sideslopes with high evergreen understory density compared to those with low evergreen understory density. Canopy height declined from low to high elevations, with larger relative decreases on ridges, but biomass increased from low to high elevations, due primarily to high biomass in coves at mid- to upper elevations. Biomass and canopy height declined from cove to ridge and north- to south-facing slopes. Responses in canopy height and aboveground biomass associated with changes in understory evergreen density were similar to impacts due to terrain. Gaps were more frequent on south-facing slopes. Previous studies at this site and others identify soil moisture and soil N competition as the most plausible mechanisms by which understory shrubs might influence overstory canopy structure, with low light limiting seedling recruitment as an additional mechanism. Our work suggests evergreen understory density, particularly on sideslope and ridge locations, substantially affects overstory canopy height and biomass.