Experimental Evidence that Forest Structure Controls Detrital Decomposition

Much remains unknown regarding the linkages between forest structure and microclimate as they regulate detrital decomposition. In this study, we use a factorial field experiment that included canopy gap creation and downed woody material (DW) additions conducted in a mature northern hardwood forest. Our objectives were to (1) test the individual and combined effects of canopy gaps and DW additions on detrital mass loss; (2) determine whether the factors regulating mass loss are similar among leaf litter, experimental wood stakes, and coarse DW; and (3) assess the microclimatic variables that most strongly influence mass loss of these detrital types. After three years, leaf litter mass loss within gaps, without or with DW additions, was significantly greater than that of any non-gap treatments. Mass loss of stakes was significantly greater in gaps, intermediate in gaps with DW additions, and lowest in non-gap treatments. Mass loss of wood stakes after 8 years varied by species, with aspen (Populus tremuloides) losing up to 93% and sugar maple (Acer saccharum) up to 82% of its original mass. Fourteen years following treatment, the experimental logs lost 55-70% of their original mass, with ash (Fraxinus spp.) decaying faster than maple. Gap creation and DW additions individually, but not in combination, increased mass loss of coarse DW. For most substrates tested, gaps were consistently and positively related to mass loss, with approximately 10% greater mass loss in gaps compared to non-gaps. The presence of deadwood strongly moderated litter decomposition, had minimal effect on small woody substrates in the short-term after gap creation, but was influential on longer-term decay patterns of larger DW. Predictive models for each substrate varied, though shared some similar drivers. Litter mass loss was positively correlated to increasing gap size, canopy openness, and soil moisture. Stake mass loss was positively correlated to increasing gap size and canopy openness for maple, but soil temperature for aspen. Mass loss for logs was driven by increasing DW volume and gap size for ash, but soil temperature for maple. Smaller-sized materials may be more sensitive to environmental conditions as opposed to logs for which microclimatic influence may lag or remain a minor driver for at least the initial decade of decomposition. Regardless of substrate type, the findings of this work highlight the potential for greater rates of detrital mass loss from forest systems under predicted increases in canopy disturbance rates with climate change and invasive insects and diseases.

Automated summary

This study conducted a field experiment in a mature northern hardwood forest to investigate the effects of canopy gaps and downed wood on detrital mass loss. The results showed that both canopy gaps and downed wood increased detrital mass loss, but their combined effect was not significant. It was also found that leaf litter had significantly greater mass loss within canopy gaps compared to non-gap treatments.