Abstract
Stomatal conductance controls carbon and water fluxes in forest ecosystems. Therefore, its accurate characterization in land-surface flux models is necessary. Sap-flux scaled canopy conductance was used to evaluate the effect of drought, disturbance, and mortality of three oak species (Quercus prinus, Q. velutina, and Q. coccinea) in an upland oak/pine stand in the New Jersey Pine Barrens from 2005 to 2008. Canopy conductance (GC) was analyzed by performing boundary line analysis and selecting for the highest value under a given light condition. Regressing GC with the driving force vapor pressure deficit (VPD) resulted in reference canopy conductance at 1 kPa VPD (GCref). Predictably, drought in 2006 caused GCref to decline. Q. prinus GCref was least affected, followed by Q. coccinea, with Q. velutina having the highest reductions in GCref. A defoliation event in 2007 caused GCref to increase due to reduced leaf area and a possible increase in water availability. In Q. prinus, GCref quadrupled, while doubling in Q. velutina, and increasing by 50% in Q. coccinea. Tree mortality in 2008 led to higher GCref in the remaining Q. prinus but not in Q. velutina or Q. coccinea. Comparing light response curves of canopy conductance (GCref) and stomatal conductance (gS) derived from gas-exchange measurements showed marked differences in behavior. Canopy GCref failed to saturate under ambient light conditions whereas leaf-level gS saturated at 1,200 μmol m−2 s−1. The results presented here emphasize the differential responses of leaf and canopy-level conductance to saturating light conditions and the effects of various disturbances (drought, defoliation, and mortality) on the carbon and water balance of an oak-dominated forest.
Highlights
Interactions between climate change, forests, and disturbance events will undoubtedly change ecosystem dynamics and ecosystem function (Dale et al, 2000, 2001)
The results presented here emphasize the differential responses of leaf and canopy-level conductance to saturating light conditions and the effects of various disturbances on the carbon and water balance of an oak-dominated forest
Comparing the light response of GCref in a pre-defoliation year (2005) vs. a post-defoliation year (2008), shows that GCref at maximum light conditions increased in Q. prinus from 100 to 130 mmol m−2 s−1, remained constant in Q. velutina at about 130 mmol m−2 s−1, and decreased in Q. coccinea from 120 to 90 mmol m−2 s−1 (Figure 4)
Summary
Interactions between climate change, forests, and disturbance events will undoubtedly change ecosystem dynamics and ecosystem function (Dale et al, 2000, 2001). Some of the significant ecosystem services of forests are carbon (C) sequestration and water (H2O) flux regulation Protecting these ecosystem services are pivotal for human existence, knowledge of their intricate interactions will help guide modeling as well as conservation efforts. Forest disturbance such as insect outbreaks and fire are predicted to increase under global climate change due to an expansion or range shift of insects, higher survival rates of pest insect populations and increased fire proneness due to increased droughts (Ayres and Lombardero, 2000; Seidl et al, 2008; Jönsson et al, 2009; Régnière et al, 2009; Cudmore et al, 2010; Metsaranta et al, 2010). Forest carbon and water cycling are, intermediaries of these disturbance and climate events and, need to be accurately characterized (Ayres and Lombardero, 2000)
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