Sustained carbon uptake and storage following moderate disturbance in a Great Lakes forest

Abstract

Carbon (C) uptake rates in many forests are sustained, or decline only briefly, following disturbances that partially defoliate the canopy. The mechanisms supporting such functional resistance to moderate forest disturbance are largely unknown. We used a large-scale experiment, in which > 6700 Populus (aspen) and Betula (birch) trees were stem-girdled within a 39-ha area, to identify mechanisms sustaining C uptake through partial canopy defoliation. The Forest Accelerated Succession Experiment in northern Michigan, USA, employs a suite of C-cycling measurements within paired treatment and control meteorological flux tower footprints. We found that enhancement of canopy light-use efficiency and maintenance of light absorption maintained net ecosystem production (NEP) and aboveground wood net primary production (NPP) when leaf-area index (LAI) of the treatment forest temporarily declined by nearly half its maximum value. In the year following peak defoliation, redistribution of nitrogen (N) in the treatment forest from senescent early successional aspen and birch to non-girdled later successional species facilitated the recovery of total LAI to pre-disturbance levels. Sustained canopy physiological competency following disturbance coincided with a downward shift in maximum canopy height, indicating that compensatory photosynthetic C uptake by undisturbed, later successional subdominant and subcanopy vegetation supported C-uptake resistance to disturbance. These findings have implications for ecosystem management and modeling, demonstrating that forests may tolerate considerable leaf-area losses without diminishing rates of C uptake. We conclude that the resistance of C uptake to moderate disturbance depends not only on replacement of lost leaf area, but also on rapid compensatory photosynthetic C uptake during defoliation by emerging later successional species.

Observations of stem water storage in trees of opposing hydraulic strategies

“…We conducted our study in a forest plot within the footprint of the US-UMB Ameriflux-affiliated eddy covariance flux station (http://ameriflux. lbl.gov/; Site ID US-UMB; Gough et al 2013), which provides measurements of plot-level meteorological conditions as well as carbon and water vapor fluxes. The study area is dominated (;30% of leaf area and 59.6% of stem area) by early successional bigtooth aspen (Populus grandidentata) and paper birch (Betula papyrifera).…”

“…The study area is dominated (;30% of leaf area and 59.6% of stem area) by early successional bigtooth aspen (Populus grandidentata) and paper birch (Betula papyrifera). Later successional species such as red oak (Quercus rubra), red maple (Acer rubrum), white pine (Pinus strobus), American beech (Fagus grandifolia), and sugar maple (Acer saccharum) are increasing in their relative portion of the stand basal area (Gough et al 2013). Mean canopy height is approximately 25 m, average tree age of canopy dominant trees is 90 years, mean peak growing season LAI is ;3.9 m 2/ m 2 , and average stem density is ;750 trees/ha (for trees with diameter at breast height !8 cm).…”

Species‐specific transpiration responses to intermediate disturbance in a northern hardwood forest

“…We conducted our study in two adjacent forest plots surrounding two Ameriflux-affiliated eddy covariance flux stations (http://ameriflux.lbl.gov/). The experiment and site layout are described in detail in Gough et al [2013]. While neither of our two forest plots is physically bounded, we considered the control plot area to be the 180 ha area composing 99% of the flux footprint of the primary eddy covariance tower.…”

“…We used a threshold frictional velocity value (u * filter, based on the approach of Reichstein et al [2005]) to filter data when turbulent mixing was low. A detailed description of our eddy flux processing is reported in Gough et al [2013]. We used a bilinear periodic approach [Morin et al, 2014] to gap-fill latent and sensible heat flux.…”

“…Nonstand replacing disturbances, often called intermediate disturbances, occur naturally via species-specific pest infestation [e.g., Herms and McCullough, 2014;Simard et al, 2012], strong storms and wind throws [e.g., Uriarte et al, 2012], drought [e.g., Huang and Anderegg, 2012;McDowell et al, 2008;Royer et al, 2010], and ecological succession [e.g., Gough et al, 2013;Hardiman et al, 2013] and anthropogenically via fire management and biomass removals [e.g., Royer et al, 2010;Stephens et al, 2009] and selective logging [e.g., Asase et al, 2014;Asner et al, 2004]. These disturbances impact canopy structure at a diffuse spatial scale ranging from one to several tree crowns per hectare, and may be distributed over MATHENY ET AL.…”