Local controls modify the effects of timber harvesting on surface soil carbon and nitrogen in a temperate hardwood forest

Abstract

Managing for structural complexity to enhance forest health and resiliency is increasingly incorporated in silvicultural treatments. High spatial variability in stands managed for structural complexity could obscure forest management effects on surface soils. Yet few studies have assessed how within-stand variation in forest structure and other local controls influence the effects of timber harvesting on surface soil organic matter dynamics over time. We used a stratified random sampling design to capture variation in stand age, legacy structure, soil type, and topography in a second-growth, oak-hardwood forest in the northeastern U.S. We compared surface soil carbon and nitrogen content and availability in 15 harvested stands managed to promote tree regeneration (n = 144 plots) and five unharvested controls (n = 48 plots). We also examined changes over time since harvest in just the harvested stands using a 25-year chronosequence. Timber harvesting strongly influenced surface soil carbon and nitrogen dynamics. The harvested stands had lower soil carbon and nitrogen, microbial biomass, and carbon mineralization but higher nitrogen mineralization. These differences were more pronounced in the drier soil type with higher organic matter content than in the more moist soil type. Across the 25-year chronosequence, elevation, soil type, and downed woody material density dictated the direction of changes in surface soil carbon and nitrogen over time. Soil carbon and nitrogen accrued over time at drier, higher elevation (∼300 m) sites and was positively associated with higher densities of fine woody material but declined at lower elevations (∼180 m). Proximity to legacy trees was associated with higher soil carbon and nitrogen concentrations and availability. Our findings underscore the importance of silvicultural practices that retain structural legacies in shaping surface soil carbon and nitrogen dynamics over time. Our results also highlight how accounting for spatial variation in local controls on soil carbon and nitrogen, such as topography, can improve detection of changes from forest management practices that increase spatial heterogeneity within stands, such as irregular shelterwood and seed tree regeneration methods.