Emergent properties of downed woody debris in canopy gaps: A response of the soil ecosystem to manipulation of forest structure

Abstract

Natural forest disturbance events can influence soil biogeochemical processes in two ways - by creating downed woody debris (DWD; fallen tree boles or branches) and by creating canopy gaps that alter forest microclimate. DWD represents a substrate for microbial growth and a persistent store of carbon and nutrients, but microbial activity is also sensitive to temperature and moisture. We studied the potential interaction of DWD and canopy gaps on soil microbial processes, and wondered if microclimatic conditions resulting from the manipulation of forest structure would be enough to inhibit production, thereby altering a critical ecosystem process. Gaps and DWD (>10 cm diameter) were added to a maturing, even-aged, second-growth northern hardwood forest (the Flambeau Experiment; N Wisconsin, USA) to enhance structural complexity and promote key ecosystem processes typically associated with late-successional forests. We investigated the influence of DWD and gaps on soil microbial community composition, extracellular enzyme activity and soil characteristics. Soils were sampled near intermediately and highly decayed DWD and 2 m away from DWD (control) in gaps and closed canopy a decade after manipulation.DWD decomposition influenced the surrounding soil differentially depending on decay class and canopy condition. Mean C- and P-potential extracellular enzyme activities (BG, BX and AP) were enhanced near highly decayed DWD in gaps. The relative abundance of bacteria (actinomycete, anaerobic, gram-negative and gram-positive) remained constant in gaps but decreased from May to August in closed canopy. In gaps, soil total exchangeable cations increased by 34.6%, available phosphorus by 152% and fungal to bacterial ratios by 23.3% but temperatures decreased by 3.42% suggesting that canopy condition continues to affect soil properties and microbial processes a decade after gap creation. These results highlight the contribution of DWD to the forest floor and the influence of decaying wood characteristics on belowground ecosystems critical to future forest productivity. Retaining or adding heterogeneously distributed DWD of varying decay status may be essential to maintain ecosystem functions associated with nutrient cycling and microbial community dynamics in managed forests.