Evaluating forest subcanopy response to moderate severity disturbance and contribution to ecosystem-level productivity and resilience

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Abstract

North American temperate forests have functioned as a terrestrial carbon (C) sink for more than a century, but the future of this sink is highly uncertain as disturbance frequency increases and regrown forests approach maturity. The subcanopy is integral to the functional recovery of forests, supporting short-term resilience of primary production and longer-term shifts in tree species composition and diversity. However, the factors that contribute to variation in forest subcanopy response to disturbance are not well understood. In this study, we investigated subcanopy shifts in aboveground wood net primary productivity (ANPPw) and composition following experimental moderate severity disturbance emulating natural canopy mortality from age-related senescence. We assessed the importance of variation in disturbance severity, site fertility, and community composition on subcanopy disturbance response and contribution to total (canopy and subcanopy) ANPPw response. We also assessed the effect of the moderate severity disturbance on species composition and diversity, and competitive patterns within the subcanopy layer. Subcanopy aboveground biomass and ANPPw increased substantially relative to pre-disturbance levels by a factor of 1.4 and 22.7, respectively. The subcanopy (stems <8 cm DBH) made up a large component of overall (canopy plus subcanopy) post disturbance ANPPw (16.2%) and disturbance response (post-disturbance ANPPw/pre-disturbance ANPPw; 54.1%). Subcanopy ANPPw, subcanopy post-disturbance ANPPw response, and subcanopy contribution to total post-disturbance ANPPw response were all most strongly predicted by subcanopy community composition in combination with canopy composition and site fertility. Variation in disturbance severity was not a strong predictor of subcanopy ANPPw response to disturbance. Subcanopy compositional trends and growth patterns both indicate likely increased heterogeneity in canopy composition (greater β diversity) and a potential shift toward greater dominance by mid-tolerant Quercus rubra (northern red oak). Our results illustrate the importance of the subcanopy in the response of forest productivity to moderate severity disturbance and illustrate that composition of the subcanopy layer exerts a strong influence on the growth response both of the subcanopy and the forest as a whole. Our findings highlight the unique role of moderate severity disturbance, relative to more severe disturbances, in promoting biological and structural heterogeneity in forest ecosystems and favoring underrepresented mid-tolerant species.

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Fahey, R. T., Stuart-Haëntjens, E. J., Gough, C. M., De La Cruz, A., Stockton, E., Vogel, C. S., & Curtis, P. S. (2016). Evaluating forest subcanopy response to moderate severity disturbance and contribution to ecosystem-level productivity and resilience. Forest Ecology and Management, 376, 135–147. https://doi.org/10.1016/j.foreco.2016.06.001

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