Below- and above-ground effects of deadwood and termites in plantation forests

Deadwood is an important legacy structure in managed forests, providing continuity in shelter and resource availability for many organisms and acting as a vehicle by which nutrients can be passed from one stand to the next following a harvest. Despite existing at the interface between below- and above-ground systems, however, much remains unknown about the role woody debris plays in linking these zones. Moreover, it remains untested whether the accelerative effects of wood-feeding insects on wood decomposition influence tree growth or nutritional status in forests. In this study, we added different quantities of pine logs to the bases of saplings in two-year-old loblolly pine (Pinus taeda L.) plantations in Mississippi, USA. We included a treatment in which subterranean termites (Blattodea: Rhinotermitidae: Reticulitermes) were excluded from logs to determine how these insects affect the release of nutrients from wood and, in turn, tree growth. After 51 months of decomposition, we quantified below-ground effects by measuring microbial biomass, plant-available forms of N, and ectomycorrhizal diversity associated with fine tree roots. Meanwhile, above-ground measurements focused on the elemental concentrations in decomposing wood either protected or unprotected from termites and tree metrics related to growth and nutrient status. We found additions of wood to significantly increase nitrate and potential net nitrification relative to reference treatments but detected no significant effects on tree growth, needle nitrogen concentrations, or ectomycorrhizal diversity. Soil nitrate and potential net nitrification were higher under protected vs. unprotected logs, and plant-available forms of N were mostly more abundant short distances away from both protected and unprotected logs than directly under them. The wood of logs protected from termites had significantly lower concentrations of most elements compared to that of unprotected logs, largely due to the large amounts of soil imported into unprotected logs by termites. Termite exclusion had no measurable effect on tree growth, nutritional status, or ectomycorrhizal diversity, however. Our findings indicate that deadwood and termites both contribute to the spatial heterogeneity of soil properties but may have limited short-term local effects on tree growth. Longer-term studies and studies on less fertile sites are needed.