Impact of the Mountain Pine Beetle on the net Ecosystem Productivity of Lodgepole Pine Stands and the Role of Secondary Structure
British Columbia, Canada is experiencing a severe mountain pine beetle (MPB) epidemic extending over an area of 135,000 km2. The widespread mortality of lodgepole pine caused by the beetle is having severe implications for Canada's carbon (C) budget. This study used the eddy-covariance technique to examine how the beetle is affecting the net ecosystem productivity (NEP) of two attacked lodgepole pine stands in the central interior of BC over 2 years. MPB-KS is an 83-year-old stand that was first attacked in 2006. By the start of 2007, roughly 60% of the canopy had been beetle attacked and by August 2008 only 21% of the canopy remained healthy. MPB-CR, a 110-year-old stand, first attacked by the beetle in 2003, had >95% pine mortality in 2007, and also differed from MPB-KS in that it had a developed secondary structure (seedlings, saplings, sub- canopy and canopy trees that survive a beetle attack) (SS) and deciduous ground layer. In 2007, MPB-KS had an annual and growing season (GS) NEP of -55 and 36 g C m-2, and in 2008, an annual and GS NEP of -42 and 34 g C m-2, respectively. MPB-CR had an annual and GS NEP of -22 and 23 g C m-2 in 2007 and 2 and 75 g C m-2 in 2008. In both years, MPB-KS was a GS C sink due to the productivity of the healthy portion of the canopy, the many seedlings and the mossy surface layer. Between 2007 and 2008, MPB-CR experienced a reduction in LAI (from 0.91 to 0.78), due to needle-fall, which led to an opening up of the canopy and resulted in a high SS C uptake in 2008. These results were confirmed by foliar CO2 exchange measurements which showed a high productivity for the SS and deciduous vegetation.
Impacts of Nitrogen Fertilization on Carbon and Water Balances in Different-aged Westcoast Douglas-fir Stands
Using the eddy-covariance technique, we have been measuring, since 1998, CO2, water vapour and energy exchange between the atmosphere and the ecosystem in three different-aged Westcoast Douglas-fir (Pseudotsuga menziesii) stands (9, 21 and 60 year-old in 2009, hereafter known as HDF00, HDF88 and DF49, respectively). These stands were fertilized with urea in January 2007. While HDF88 and DF49 were fertilized aerially with 200 kg N ha-1, HDF00, due to its young age and competing brush, was fertilized manually at 60 kg N ha-1. This paper reports the environmental controls of pre-fertilization interannual variability in ecosystem respiration (R), gross photosynthesis (P) and evapotranspiration (E), and discusses the effects of N- fertilization on these fluxes and net ecosystem productivity (NEP = P - R, i.e., carbon sequestration) and water use efficiency (WUE = P/E). For DF49, both pre-fertilization annual P and R responded positively to increases in mean annual air temperature and soil water content, but R was more responsive to both these environmental variables. Using these relationships, we calculated P and R for 2007 and 2008 assuming the stand was not fertilized. A comparison with measured 2007 and 2008 fluxes indicated that fertilization increased P and had almost no effect on R and soil respiration with the consequence that NEP was increased by 170 and 90 g C m- 2 during 2007 and 2008, respectively. Pre-fertilization annual E was modelled as a function of mean annual temperature, annual total photosynthetically active radiation, and precipitation for the previous year. Using this model to calculate 2007 and 2008 values of E for the unfertilized stand and comparing these with measured values in the fertilized stand indicated that fertilization had no effect on E, neither in 2007 nor in 2008. As a result, annual water use efficiency increased from a mean of 4.9 g C (kg water)-1 for 1998-2006 to 5.2 g C (kg water)-1 in 2007 and 2008. A similar analysis was performed in case of the other two young stands to separate the influences of climate, stand age and fertilization.
Carbon Cycling in Balsam Fir Ecosystems Following Forestry Practices
We are using the eddy covariance method to monitor carbon cycling following forestry practices in Abies balsamea (L.) Mill forests. The impact of forestry practices depends on the extent and duration of lost photosynthetic uptake and on the quantity of additional ecosystem respiration due to the decomposition of logging residues. A 33 year-old forest was thinned in 2005, reducing density from 2300 to 1500 trees per hectare, leaf area index from 6.5 to 4.7, and biomass from 46.6 to 36.4 Mg C ha-1. Gross primary production (GPP) in the year after thinning decreased to 11.3 Mg C ha-1 y-1 in comparison to 15.9 Mg C ha-1 y-1 in the year before thinning. GPP was higher in subsequent years because LAI was recovering from disturbance and environmental conditions were more favourable for photosynthesis than in 2004. Interannual variation in the effects of environmental conditions on photosynthesis was apparent when comparing monthly distribution of GPP among years. For example, high GPP in 2007 could be ascribed to greater precipitation in July and August than in other years. Thinning reduced ecosystem respiration (ER) from 11.0 Mg C ha-1 y-1 in 2004 to 8.3 Mg C ha-1 y-1 in 2006. We estimate that most of the readily decomposable logging residues were oxidized in the three years following thinning. Clearcutting released a dense layer of A. balsamea advanced regeneration and stimulated abundant regeneration of Prunus pensylvanica (L.f.) on disturbed ground. GPP increased rapidly as a result, reaching 7.1 Mg C ha- 1 y-1 in the fourth year. ER at the clearcut site was 7 Mg C ha-1 y-1 in the first year, two thirds of that in the undisturbed, 33 year-old forest. ER increased to 8.3 Mg C ha-1 y-1 in the next three years as the more readily decomposable of the 95 Mg C ha-1 of logging residues was oxidized. The clearcut became a smaller source of C in each year, decreasing from 5.8 Mg C ha-1 y-1 in the year after harvest to 1.2 Mg C ha-1 y-1 in 2008. We predict that the clearcut site will become a net sink for carbon approximately six years after harvesting. Our preliminary conclusion is that balsam fir ecosystems recover photosynthetic uptake relatively quickly after disturbance by a variety of means and that this response offsets additional ecosystem respiration due to decomposition of logging residues. Results such as these are essential for increasing our understanding of ecosystem processes so that models can be developed to predict the impact of altered climate regimes on the carbon dynamics of forests under management.
Biometric and Eddy-Covariance Based Estimates of Ecosystem Carbon Exchange in an Age-Sequence of Temperate Pine Forests
We determined and compared annual carbon (C) exchanges from biometric and eddy-covariance (EC) measurements in an age-sequence (6-, 19-, 34-, 69-years old) of managed pine (Pinus strobus L.) forests in southern Ontario from 2005-2007. The biometric approach determined annual above- and belowground tree biomass production from site-specific allometric biomass equations depending on either tree diameter at breast height (DBH) only (method B1) or on DBH with tree height as additional variable (method B2). In addition, detritus production and heterotrophic soil respiration were determined. Data from continuous closed- path measurements at the oldest site and from a roving open-path system among the three younger sites provided EC-based estimates of C exchanges (method EC). The contribution of individual net primary productivity (NPP) components varied considerably with stand age, suggesting different dominant fluxes and uncertainty levels occurring at various forest development stages. All methods produced similar patterns for inter-annual variations with highest (lowest) C fluxes in 2006 (2005). While on an annual basis, differences between methods ranged from ± 4-67% for estimates of annual net ecosystem productivity (NEP), the differences were within ± 15% when averaged over three years, except for the 34-year old stand. Mean annual NEP was estimated by the biometric method B1 (B2) as 1 (N.A.), 394 (634), 134 (265), and 124 (272) g C m-2 y-1 compared to 47, 724, 408, and 119 g C m-2 y-1 by the EC method for the 6-, 19-, 34-, 69-years old stands, respectively. The biometric method B1 agreed best with the EC estimates in the youngest and the oldest stand, but estimated considerably lower productivity rates than the EC method in the two middle-age stands in which method B2 showed a better agreement with method EC by accounting for the vigorous height growth in these stands. Thus, our comparison study shows that the use of inadequate allometric equations may considerably hamper the agreement between biometric and EC estimates. We also observed a strong correlation between annual NEP and tree stem production which suggests that large-scale estimates of annual NEP could be efficiently derived from tree diameter measurements in existing permanent sample plots. Cross-validation of different methodologies is an important tool to define uncertainties around individual method outputs and to improve estimates of C exchange processes in managed forest ecosystems.
RELATIVE CONTRIBUTIONS OF SOIL, FOLIAR AND WOODY-TISSUE RESPIRATION TO TOTAL ECOSYSTEM RESPIRATION IN DIFFERENT-AGE, PLANTED FORESTS
Nitrogen Feedbacks on Ecosystem Production: CLASS-CTEMN+ Model Application to two North American Forest Sites
Nitrogen controls and feedbacks were simulated using a carbon and nitrogen (C&N) coupled process-based model--Canadian Land Surface Scheme and Canadian Terrestrial Ecosystem Model (CLASS-CTEMN+)-- at two North American needle forest sites (Turkey Point mature white pine site-TP39, ON, Canada and Howland main site-Ho1, ME, USA). CLASS-CTEMN+ was improved by incorporating a routine for N2O emissions to atmosphere. Simulated gross ecosystem productivity (GEP), ecosystem respiration (Re) and net ecosystem productivity (NEP) values were compared with eddy covariance flux measurements over 5-years (2003-2007) at TP39 and 9-years (1996-2004) at Ho1 site. Simulated soil nitrogen pools and N2O emissions were also compared with field observations at TP39 in 2006 and 2007. The C&N coupled simulations compare well with the carbon flux observations at both sites (RMSE of 1.5, 0.64, 1.46 and MAE of 1.14, 0.5, 1.02, n=1825, at TP39; and RMSE of 1.71, 1.14, 1.55 and MAE of 1.18, 0.83, 1.06, n=3285, at Ho1, for GEP, Re and NEP respectively); while the non-nitrogen controlled model results showed an overestimation (RMSE of 3.08, 1.31, 1.94; MAE of 2.68, 1.13, 1.6, n=1825, at TP39 and RMSE of 2.2, 1.46, 1.55; MAE of 1.8, 1.25, 1.11, n=3285, at Ho1, for GEP, Re and NEP respectively). CLASS-CTEMN+ simulated mean annual NEP was 261 g C m-2 yr-1 as compared to mean annual observed NEP value of 228 g C m-2 yr-1 over 2003-2007 at TP39. Corresponding mean annual simulated and observed NEP values were 201 and 256 g C m-2 yr-1 over 1996-2004 at Ho1. The model-simulated soil nitrogen pools were similar to observed nitrogen pools. Modeled N2O fluxes also followed the general trend of measurements made using closed chambers at TP39. However, there were uncertainties in chamber-based observed N2O emissions as well. This study will help to understand nitrogen cycle feedbacks on carbon exchanges in forest ecosystems under climate change.