Changes in net ecosystem productivity with forest age following clearcutting of a coastal Douglas-fir forest: testing a mathematical model with eddy covariance measurements along a forest chronosequence

Thinning Can Reduce Losses in Carbon Use Efficiency and Carbon Stocks in Managed Forests Under Warmer Climate

Forest carbon use efficiency (CUE, the ratio of net to gross primary productivity) represents the fraction of photosynthesis that is not used for plant respiration. Although important, it is often neglected in climate change impact analyses. Here we assess the potential impact of thinning on projected carbon cycle dynamics and implications for forest CUE and its components (i.e., gross and net primary productivity and plant respiration), as well as on forest biomass production. Using a detailed process-based forest ecosystem model forced by climate outputs of five Earth System Models under four representative climate scenarios, we investigate the sensitivity of the projected future changes in the autotrophic carbon budget of three representative European forests. We focus on changes in CUE and carbon stocks as a result of warming, rising atmospheric CO2 concentration, and forest thinning. Results show that autotrophic carbon sequestration decreases with forest development, and the decrease is faster with warming and in unthinned forests. This suggests that the combined impacts of climate change and changing CO2 concentrations lead the forests to grow faster, mature earlier, and also die younger. In addition, we show that under future climate conditions, forest thinning could mitigate the decrease in CUE, increase carbon allocation into more recalcitrant woody pools, and reduce physiological-climate-induced mortality risks. Altogether, our results show that thinning can improve the efficacy of forest-based mitigation strategies and should be carefully considered within a portfolio of mitigation options.

Spatial Patterns of Soil Respiration Links Above and Belowground Processes along a Boreal Aspen Fire Chronosequence

Fire in boreal ecosystems is known to affect CO₂ efflux from forest soils, which is commonly termed soil respiration (Rs). However, there is limited information on how fire and recovery from this disturbance affects spatial variation in Rs. The main objective of this study was to quantify the spatial variability of Rs over the growing season in a boreal aspen (Populus tremuloides Michx.) fire chronosequence. The chronosequence included three stands in northern Alberta; a post fire stand (1 year old, PF), a stand at canopy closure (9 years old, CC), and a mature stand (72 years old, MA). Soil respiration, temperature and moisture were measured monthly from May to August using an intensive spatial sampling protocol (n = 42, minimum lag = 2 m). Key aboveground and belowground properties were measured one time at each sampling point. No spatial structure was detected in Rs of the PF stand during the peak growing season (June and July), whereas Rs was auto-correlated at a scale of < 6 m in the CC and MA stands. The PF stand had the lowest mean Rs (4.60 μmol C m^-2 s^-1) followed by the CC (5.41 μmol C m^-2 s^-1), and the MA (7.32 μmol C m^-2 s^-1) stand. Forest floor depth was the only aboveground factor that influenced the spatial pattern of Rs in all three stands and was strongest in the PF stand. Enzyme activity and fine root biomass, on the other hand, were the significant belowground factors driving the spatial pattern of Rs in the CC and MA stands. Persistent joint aboveground and belowground control on Rs in the CC and MA stands indicates a tight spatial coupling, which was not observed in the PF stand. Overall, the current study suggests that fire in the boreal aspen ecosystem alters the spatial structure of Rs and that fine scale heterogeneity develops quickly as stands reach the canopy closure phase (<10 years).

Changes in net ecosystem productivity of boreal black spruce stands in response to changes in temperature at diurnal and seasonal time scales

Net ecosystem productivity (NEP) of boreal coniferous forests is believed to rise with climate warming, thereby offsetting some of the rise in atmospheric CO(2) concentration (C(a)) by which warming is caused. However, the response of conifer NEP to warming may vary seasonally, with rises in spring and declines in summer. To gain more insight into this response, we compared changes in CO(2) exchange measured by eddy covariance and simulated by the ecosystem process model ecosys under rising mean annual air temperatures (T(a)) during 2004-2006 at black spruce stands in Saskatchewan, Manitoba and Quebec. Hourly net CO(2) uptake was found to rise with warming at T(a) < 15 degrees C and to decline with warming at T(a) > 20 degrees C. As mean annual T(a) rose from 2004 to 2006, increases in net CO(2) uptake with warming at lower T(a) were greater than declines with warming at higher T(a) so that annual gross primary productivity and hence NEP increased. Increases in net CO(2) uptake measured at lower T(a) were explained in the model by earlier recovery of photosynthetic capacity in spring, and by increases in carboxylation activity, using parameters for the Arrhenius temperature functions of key carboxylation processes derived from independent experiments. Declines in net CO(2) uptake measured at higher T(a) were explained in the model by sharp declines in mid-afternoon canopy stomatal conductance (g(c)) under higher vapor pressure deficits (D). These declines were modeled from a hydraulic constraint to water uptake imposed by low axial conductivity of conifer roots and boles that forced declines in canopy water potential (psi(c)), and hence in g(c) under higher D when equilibrating water uptake with transpiration. In a model sensitivity study, the contrasting responses of net CO(2) uptake to specified rises in T(a) caused annual NEP of black spruce in the model to rise with increases in T(a) of up to 6 degrees C, but to decline with further increases at mid-continental sites with lower precipitation. However, these contrasting responses to warming also indicate that rises in NEP with climate warming would depend on the seasonality (spring versus summer) as well as the magnitude of rises in T(a).

Elevated CO(2) and temperature alter net ecosystem C exchange in a young Douglas fir mesocosm experiment

We investigated the effects of elevated CO(2) (EC) [ambient CO(2) (AC) + 190 ppm] and elevated temperature (ET) [ambient temperature (AT) + 3.6 degrees C] on net ecosystem exchange (NEE) of seedling Douglas fir (Pseudotsuga menziesii) mesocosms. As the study utilized seedlings in reconstructed soil-litter-plant systems, we anticipated greater C losses through ecosystem respiration (R(e)) than gains through gross photosynthesis (GPP), i.e. negative NEE. We hypothesized that: (1) EC would increase GPP more than R(e), resulting in NEE being less negative; and (2) ET would increase R(e) more than GPP, resulting in NEE being more negative. We also evaluated effects of CO(2) and temperature on light inhibition of dark respiration. Consistent with our hypothesis, NEE was a smaller C source in EC, not because EC increased photosynthesis but rather because of decreased respiration resulting in less C loss. Consistent with our hypothesis, NEE was more negative in ET because R(e) increased more than GPP. The light level that inhibited respiration varied seasonally with little difference among CO(2) and temperature treatments. In contrast, the degree of light inhibition of respiration was greater in AC than EC. In our system, respiration was the primary control on NEE, as EC and ET caused greater changes in respiration than photosynthesis.