Responses of herbs and shrubs to reduced root competition under canopies and in gaps: a trenching experiment in old-growth Douglas-fir forests

We tested the effect of root trenching on vegetation in closed-canopy and gap locations in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests. Based on theory, we expected belowground competition to be intense in a region with low summer rainfall, and trench responses were expected to he greater in the high light environment of the gaps. We installed 1 m deep trenches around study plots and lined the trenches to prevent reinvasion by tree roots. Soil moisture was measured monthly during the growing season for the first 3 years after trench installation. Vegetation in these trenched plots was compared with control plots 10 years after installation of the plots. Trenched plots with no vegetation manipulation averaged 92% total understory cover, while untrenched plots averaged 47% cover. Contrary to our expectation, both vegetation and soil moisture responses to trenching were greater in areas of high tree canopy cover than in gaps. Trenched plots under closed canopies were moister than control plots throughout the growing season. while the trenching effect became apparent in the overall wetter gaps only at the end of the growing season. We conclude that understory plants at these sites were limited at least as much by belowground competition as by above-ground competition.