Impact of insect defoliation on forest carbon balance as assessed with a canopy assimilation model

Abstract

Disturbances such as fire, hurricanes, and herbivory often result in the net release of CO 2 from forests to the atmosphere, but the magnitude of carbon (C) loss is poorly quantified and difficult to predict. Here, we investigate the carbon balance of an oak/pine forest in the New Jersey Pine Barrens using the Canopy Conductance Constrained Carbon Assimilation (4C-A) model. The 4C-A model utilizes whole-tree sap-flux and leaf-level photosynthetic gas exchange measurements at distinct canopy levels to estimate canopy assimilation. After model parameterization, sensitivity analyses, and evaluation against eddy flux measurements made in 2006, the model was used to predict C assimilation for an undisturbed year in 2005, and in 2007 when the stand was completely defoliated for 2-3 weeks during an infestation of gypsy moths (Lymantria dispar L.). Following defoliation, only 50% of the foliage reemerged in a second flush. In 2007, canopy net assimilation (A nC ), as modeled with the 4C-A, was reduced to approximately 75% of A nC in 2006 (940 vs. 1240 g C m À2 a À1 ). Overall, net primary production (NPP) in 2007 was approximately 240 g C m À2 a À1 (vs. 250 g C m À2 a À1 in 2006), with 60% of NPP allocated to foliage production, a short-term carbon pool. Woody biomass accumulation, a long-term carbon pool, was reduced by 20% compared with the previous year (72 vs. 57 g C m À2 a À1 in 2006 and 2007, respectively). The overall impact of the defoliation spanned 21% of upland forests (320 km 2 ) in the New Jersey Pine Barrens, representing a significant amount of overall C not being taken up from the atmosphere by the forest, thus not accumulated in the biosphere.

Implementation of a Marauding Insect Module (MIM, version 1.0) in the Integrated BIosphere Simulator (IBIS, version 2.6b4) dynamic vegetation–land surface model

“…Many previous studies have represented insect damage in DVLSMs or less comprehensive climate-driven terrestrial models (Randerson et al, 1996;Krinner et al, 2005;Seidl et al, 2008;Wolf et al, 2008;Albani et al, 2010;Schäfer et al, 2010;Edburg et al, 2011;Medvigy et al, 2012;Mikkelson et al, 2013b;Chen et al, 2015). To our knowledge, however, our study is the first to assess, over daily to centennial timescales, the impacts from insect damage on vegetation dynamics and the carbon, energy, and water cycles in an integrated way (see Sect.…”

“…Medvigy et al (2012) used the Ecosystem Demography version 2 (ED2) DVLSM to simulate the impacts of defoliation by the gypsy moth (Lymantria dispar Linnaeus) on vegetation coexistence and carbon dynamics in the eastern US. Background herbivory or insect outbreaks have also been simulated in DGVMs and other climate-driven terrestrial models (Randerson et al, 1996;Seidl et al, 2008;Wolf et al, 2008;Albani et al, 2010;Schäfer et al, 2010;Chen et al, 2015) less comprehensive than DVLSMs. However, most previous studies lacked realism by representing insect damage as end-of-year instantaneous events (instead of simulating their unfolding over many weeks during the growing season) and/or by imposing the assumed consequences of insect activity (e.g., reduced total canopy conductance) rather than letting the model estimate these changes as a function of the new, insect-modified state of the vegetation.…”

“…Many tree species can produce a second flush of foliage after an early-season defoliation event (Jones et al, 2004;Schäfer et al, 2010). We therefore allowed for the possibility of reflush in MIM, as this phenomenon can substantially influence simulated land-atmosphere exchanges and vegetation competition.…”

Exploring the "overflow tap" theory: linking forest soil CO<sub>2</sub> fluxes and individual mycorrhizosphere components to photosynthesis

“…Clearly, more in situ research is needed under different environmental conditions (e.g. average years versus disturbance year, Vargas, 2009, or variable NPP due to insect defoliation, Schäfer et al, 2010) and using isotopic research. Furthermore, the high frequency data revealed that the shortterm, temperature-independent R a component linked to GPP may lead to uncertain temperature-based night-time R eco corrections in EC flux calculations (Aubinet et al, 2002;Reichstein et al, 2005) so that correlations of GPP and R eco may need to be reconsidered .…”

Contrasting effects of invasive insects and fire on ecosystem water use efficiency

“…1b). At the oak stand, herbivory by gypsy moth during the early summer of 2007 reduced LAI to < 0.5 (see Schäfer et al, 2010). Following the peak of herbivory in June, a second partial leaf-out resulted in a total LAI of only 2.3 (Fig.…”