Forest response and recovery following disturbance in upland forests of the Atlantic Coastal Plain

Abstract

Carbon and water cycling of forests contribute significantly to the Earth's overall biogeochemical cycling and may be affected by disturbance and climate change. As a larger body of research becomes available about leaf-level, ecosystem and regional scale effects of disturbances on forest ecosystems, a more mechanistic understanding is developing which can improve modeling efforts. Here, we summarize some of the major effects of physical and biogenic disturbances, such as drought, prescribed fire, and insect defoliation, on leaf and ecosystem-scale physiological responses as well as impacts on carbon and water cycling in an Atlantic Coastal Plain upland oak/pine and upland pine forest. During drought, stomatal conductance and canopy stomatal conductance were reduced, however, defoliation increased conductance on both leaf-level and canopy scale. Furthermore, after prescribed fire, leaf-level stomatal conductance was unchanged for pines but decreased for oaks, while canopy stomatal conductance decreased temporarily, but then rebounded the following growing season, thus exhibiting transient responses. This study suggests that forest response to disturbance varies from the leaf to ecosystem level as well as species level and thus, these differential responses interplay to determine the fate of forest structure and functioning post disturbance.

Decadal-Scale Reduction in Forest Net Ecosystem Production Following Insect Defoliation Contrasts with Short-Term Impacts of Prescribed Fires

“…Resprouting and recovery of ericaceous shrubs following wildland fires or other disturbance is well documented, and is facilitated by extensive belowground stems and root systems [35,59,60]. Our study suggests that compensatory photosynthesis contributed to the maintenance of GEP and ANPP following disturbance [18,23]. At the oak stand, partial defoliation of the canopy in 2008 allowed greater penetration of light lower into the sub-canopy and understory, and understory shrubs and scrub oaks responded with a doubling of leaf area that was stable through the end of the study [33,37,49].…”

“…As oak mortality progressed and canopy gaps expanded, compensatory growth of undamaged canopy trees-primarily chestnut and scarlet oaks as well as pitch and shortleaf pines-were important in maintaining GEP and ANPP. Stomatal conductance and whole-tree transpiration measured using sap flux gauges increased for many of the remaining canopy oaks, suggesting that light use efficiency and photosynthetic rates also increased [23,61]. As canopy gaps persisted, increased productivity of sub-canopy oaks and pine seedlings and saplings became increasingly important in maintaining GEP and ANPP at the oak stand, similar to results reported for sub-canopy species following non-stand-replacing disturbance in other forests (e.g., [18,[22][23][24]62]).…”

“…Stomatal conductance and whole-tree transpiration measured using sap flux gauges increased for many of the remaining canopy oaks, suggesting that light use efficiency and photosynthetic rates also increased [23,61]. As canopy gaps persisted, increased productivity of sub-canopy oaks and pine seedlings and saplings became increasingly important in maintaining GEP and ANPP at the oak stand, similar to results reported for sub-canopy species following non-stand-replacing disturbance in other forests (e.g., [18,[22][23][24]62]). Although canopy oak species in the PNR have twice the average foliar N concentrations, higher maximum stomatal conductance, and greater water use efficiencies (WUEs) compared to pitch and shortleaf pines, pines and oaks have similar light compensation points, quantum yields, and maximum assimilation rates on a leaf area basis [61].…”

“…Recently, it has been hypothesized that relatively high rates of GEP and NEP in intermediate-age forests can be maintained by frequent low-intensity disturbances which delay the onset of steady-state conditions [18,20,21]. Two primary mechanisms are thought to be involved: increased canopy heterogeneity following disturbance allows greater light penetration into the sub-canopy and results in greater light use efficiency and compensatory photosynthesis by the remaining trees, saplings, and understory vegetation [22][23][24]; and nutrients released during and following disturbance are rapidly reabsorbed by recovering vegetation, stimulating foliage production and enhancing photosynthetic rates [18,25].…”