Boreal Forest Transect Case Study Research Articles

Long term aboveground litterfall production in boreal jack pine (Pinus banksiana) and black spruce (Picea mariana) stands along the Boreal Forest Transect Case Study in western central Canada

Boreal Forest Transect Case Study with a natural climate gradient resulting from latitudinal variation provides a spatial proxy for expected climate change, and can help to understand its impact on different ecosystem processes and how the forest species are adapting to these changes. We investigated year to year variation in aboveground litter production for 2001–2010 in 13 different mature jack pine (JP) and black spruce (BS) stands in relationship to climatic variables. Simple linear and step-wise regression analysis showed that tree species interacted differently with climate and stand variables affecting litterfall. Annual total and woody litterfall varied from 57 to 164 g·m−2.y−1 in JP and 18 to 63 g·m−2.y−1in BS. Needle litterfall accounted for about 70% of the total litter production in JP and BS and about 80% in mixed wood stands, and explained significant variation in total litter production. On regional scale, needle litter production was strongly influenced by growing degree days (R2 = 0.96). Woody litterfall decreased with winter precipitation (R2 = 0.95) in JP, and increased with productivity (R2 = 0.88) in BS. Regression models developed here are useful tools in carbon budget models.

Chemical quality of aboveground litter inputs for jack pine and black spruce stands along the Canadian Boreal Forest Transect Case Study

In the Canadian boreal forest, jack pine stands generally have a thin forest floor and occupy sites with coarsetextured soils and good drainage. Black spruce occurs more often on poorly drained sites and develops a thick mossdominated forest floor, but the common attribution of this development to poor quality of black spruce foliar litter has not been tested. We determined needle, twig, cone, and bark litter inputs during 10 y for black spruce and jack pine along the Boreal Forest Transect Case Study in northern Saskatchewan and Manitoba. Analysis of C, N, total phenolics, condensed tannins, and solid-state 13C NMR spectra from years 1–3 showed only small differences between species, notably higher tannins in black spruce cones. There was similarly little difference between area-based inputs, including classes of C structures determined by NMR. Condensed tannin input for black spruce was approximately twice that for jack pine, but both were in the very low range of reported values. Similar analyses showed that black spruce forest floor was less decomposed than that of jack pine, and for both species, aromatic litter C inputs appear to be poorly conserved, with a large influence of mosses and lichen. It is unlikely, however, that these large differences are mainly due to the small differences in aboveground litter inputs.

Long-term forest-floor litter dynamics in Canada's boreal forest: Comparison of two model formulations

Litter decomposition is a key component in ecosystem material cycling that determines (i) forest soil carbon (C) and nutrient content, (ii) release of carbon dioxide to the atmosphere, and (iii) generation and mass transfer of dissolved organic carbon from terrestrial to aquatic ecosystems. In this study, we provide simulations of long term forest-floor litter dynamics generated with both (i) an existing forest nutrient cycling and biomass growth model (ForNBM) with a single-pool formulation of forest-floor litter decomposition (Zhu et al., 2003. Ecol. Model. 169, 347–360), and (ii) a revised version of the model produced by substituting the single-pool formulation with a three-pool version of the formulation tested against data from litterbag experiments (FLDM; Zhang et al., 2010. Ecol. Model. 221, 1944–1953). This is done to determine the importance of subdividing the litter mass into categories of rates of decay (i.e., fast, slow, and very slow) on model accuracy. Forest-forest litter dynamics simulated with the two models are subsequently compared against field measurements collected at several northern jack pine ( Pinus banksiana) stands along a southwest-northeast oriented transect (climate gradient) associated with the Boreal Forest Transect Case Study in northwest Canada. Initial comparison shows that the single-pool formulation underpredicts residual litter mass when forests are <65 years old, largely due to the improper treatment of the very slow decomposing litter component. This underprediction is resolved when the three-pool formulation is used. From a ecosystems-response point of view, the revised ForNBM (with the three-pool formulation) demonstrates that (i) forest-floor litter initially increases with forest growth and reaches a plateau once the forest matures; (ii) the forest floor stores more litter and C at the southern and warmer sites than at the northern sites; and (iii) in a similar climate regime, the forest floor stores more litter and C at productive than at nutrient-poor sites.

Soil carbon stabilization in jack pine stands along the Boreal Forest Transect Case Study

Boreal forests, containing > 20% of the total organic carbon (OC) present at the surface of the Earth, are expected to be highly vulnerable to global warming. The objective of this study was to compare soil OC stocks and chemistry in jack pine stands located along a latitudinal climatic transect in central Canada. Total OC stocks (0-1 m) increased with decreasing mean annual temperature (MAT). We used a combination of physical fractionation of soil OC pools, 13 C isotopic determination and cross-polarization, magic-angle spinning 13 C nuclear magnetic resonance (NMR) spectroscopy to further characterize OC composition at all sites. Soil OC was dominated by labile pools. As illustrated by the C/N ratios, δ 13 C data and results from the 13 C NMR analysis, the light fraction showed little alteration within the soil profiles. Instead, this fraction reflected the importance of fresh litter inputs and showed an increase in root contribution with depth. As opposed to the light fraction, the clay- and silt-stabilized OC exhibited an increase in δ 13 C and a decrease in C/N with depth, indicating an increase in its degree of decomposition. These changes with depth were more marked at the southern than the northern sites. Results hence suggest that if the MAT were to increase in the northern boreal forest the overall jack pine soil OC stocks would decrease but the remaining OC would become more decomposed, and likely more stabilized than what is currently present within the soils.

Application of the forest ecosystem model EFIMOD 2 to jack pine along the Boreal Forest Transect Case Study

Sustainability of forest ecosystems and climate change are two critical issues for boreal forest ecosystems in Canada that require an understanding of the links and balance between productivity, soil processes and their interaction with natural and anth ropogenic disturbances. Forest ecosystem models can be used to understand and predict boreal forest ecosystem dynamics. EFIMOD 2 is an individual tree model of the forest-soil ecosystem capable of modelling nitrogen feedback to productivity in response to changes in soil moisture and temperature. It has been successfully applied in Europe, but has not been calibrated for any forest ecosystem in Canada. The objective of this study was to parameterize and validate EFIMOD 2 for jack pine in Canada. Simulated and measured results agreed for changes in tree biomass carbon and soil carbon and nitrogen with increasing stand age and across a climatic gradient from the southern to northern limits of the boreal forest. Preliminary results from scenario testing indicate that EFIMOD 2 can be successfully applied to predict the impacts of forest management practices and climate change in the absence of natural disturbances on jack pine in the boreal forest of Canada. Model development is underway to represent the effects of natural disturbances. Key words: EFIMOD 2, forest soil, carbon, nitrogen, model, jack pine

Stocks, Chemistry, and Sensitivity to Climate Change of Dead Organic Matter Along the Canadian Boreal Forest Transect Case Study

Improving our ability to predict the impact of climate change on the carbon (C) balance of boreal forests requires increased understanding of site-specific factors controlling detrital and soil C accumulation. Jack pine (Pinus banksiana) and black spruce (Picea mariana) stands along the Boreal Forest Transect Case Study (BFTCS) in northern Canada have similar C stocks in aboveground vegetation and large woody detritus, but thick forest floors of poorly-drained black spruce stands have much higher C stocks, comparable to living biomass. Their properties indicate hindered decomposition and N cycling, with high C/N ratios, strongly stratified and depleted δ13C and δ15N values, high concentrations of tannins and phenolics, and 13C nuclear magnetic resonance (NMR) spectra typical of poorly decomposed plant material, especially roots and mosses. The thinner jack pine forest floor appears to be dominated by lichen, with char in some samples. Differences in quantity and quality of aboveground foliar and woody litter inputs are small and unlikely to account for the contrasts in forest floor accumulation and properties. These are more likely associated with site conditions, especially soil texture and drainage, exacerbated by increases in sphagnum coverage, forest floor depth, and tannins. Small changes in environmental conditions, especially reduced moisture, could trigger large C losses through rapid decomposition of forest floor in poorly drained black spruce stands in this region.

Estimates of soil organic carbon stocks in central Canada using three different approaches

This study compared three estimates of carbon (C) contained both in the surface layer (0–30 cm) and the total soil pools at polygon and regional scales and the spatial distribution in the three prairie provinces of western Canada (Alberta, Saskatchewan, and Manitoba). The soil C estimates were based on data from (i) analysis of pedon data from both the Boreal Forest Transect Case Study (BFTCS) area and from a national-scale soil profile database; (ii) the Canadian Soil Organic Carbon Database (CSOCD), which uses expert estimation based on soil characteristics; and (iii) model simulations with the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS2). At the polygon scale, good agreement was found between the CSOCD and pedon (the first method) total soil carbon values. Slightly higher total soil carbon values obtained from BFTCS averaged pedon data (the first method), as indicated by the slope of the regression line, may be related to micro- and meso-scale geomorphic and microclimate influences that are not accounted for in the CSOCD. Regional estimates of organic C from these three approaches for upland forest soils ranged from 1.4 to 7.7 kg C·m–2 for the surface layer and 6.2 to 27.4 kg C·m–2 for the total soil. In general, the CBM-CFS2 simulated higher soil C content compared with the field observed and CSOCD soil C estimates, but showed similar patterns in the total soil C content for the different regions. The higher soil C content simulated with CBM-CFS2 arises in part because the modelled results include forest floor detritus pool components (such as coarse woody debris, which account for 4–12% of the total soil pool in the region) that are not included in the other estimates. The comparison between the simulated values (the third method) and the values obtained from the two empirical approaches (the first two methods) provided an independent test of CBM-CFS2 soil simulations for upland forests soils. The CSOCD yielded significantly higher C content for peatland soils than for upland soils, ranging from 14.6 to 28 kg C·m–2 for the surface layer and 60 to 181 kg C·m–2 for the total peat soil depth. All three approaches indicated higher soil carbon content in the boreal zone than in other regions (subarctic, grassland).

Influence of nutrients, disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada

The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes—from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by ageclass structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.

Modelling the response of net primary productivity (NPP) of boreal forest ecosystems to changes in climate and fire disturbance regimes

This study reports on the use of the process-based ecosystem model CENTURY 4.0 to investigate the patterns of net primary productivity (NPP) along a transect across the boreal forests of central Canada and the influence of climate change, CO 2 fertilization and changing fire disturbance regimes on changes in NPP over time. Simulated NPP was tested against observed NPP data from northern sites near Thompson (Manitoba) and southern sites near Prince Albert (Saskatcheuan) and shown to be consistent with the data. The temporal dynamics of NPP were very different for the southern, central and northern sites, consistent with the hypothesis that different climate-driven processes regulate forest growth in the various regions of the boreal forest transect case study (BFTCS). The simulations suggest that climate change would result in increased NPP for most sites across the transect. According to the model results, increases in atmospheric CO 2 also show increased NPP. The combined influence of climate change and elevated CO 2 appear to interact in a positive, but non-linear manner. Statistical analysis of the simulation results indicate that changes in NPP are also positively correlated with changes in net N mineralization ( R 2=0.89). This supports the conclusion that feedback via N cycling — a coupling of aboveground production with changes in belowground decomposition — is very important for understanding the NPP dynamics of the boreal forest under a changing climate. It was also found that NPP increases with greater fire frequency under current climate conditions, at least over the range of fire return intervals considered (50–200 years). The influence of other changes in disturbance regimes (e.g. altered fire severity and concurrent changes in climate or CO 2 fertilization), however, were not considered.

Simulating carbon dynamics along the Boreal Forest Transect Case Study (BFTCS) in central Canada: 1. Model testing

CENTURY 4.0, a simulation model of carbon and nitrogen dynamics of terrestrial ecosystems based on the relationships between climate, soil texture, plant productivity, decomposition and human management, was tested against observed data along the boreal forest transect case study (BFTCS) in central Canada. The results show that the simulated average aboveground biomass and net N mineralization were consistent with observed data. The modeled estimates for soil carbon were consistent with those from regional‐scale empirical regression models. High correlation (R2 = 0.92) with data was obtained for the simulation of soil carbon dynamics of the boreal forest, but the model overestimated soil carbon (O–20 cm) by 2–8% for fine‐textured soil and underestimated soil carbon by 5–18% for sandy soil. The effects of climatic variation on temporal changes in biomass and soil carbon storage over the past century were found to be very different for southern and northern sites but relatively insensitive to site‐specific soil texture. The main discrepancies between observed data and CENTURY 4.0 results are associated with the effects of soil texture and an inadequate representation of fire disturbance on C dynamics of boreal forests. Further improvements, particularly in the representation of disturbance regimes and in the simulation of slow pool C dynamics, are suggested to enhance its predictive capability.

Simulating carbon dynamics along the Boreal Forest Transect Case Study (BFTCS) in central Canada: 2. Sensitivity to climate change

The effects of climate change and doubling atmospheric CO2 on carbon dynamics of the boreal forest in the area of the Boreal Forest Transect Case Study in central Canada were investigated using the process‐based plant‐soil model CENTURY 4.0. The results presented here suggest that (1) across the transect climate change would result in increased total carbon in vegetation biomass but decreased overall carbon in soil; (2) increased atmospheric CO2 concentration under current climatic patterns would result in increased total carbon in vegetation and in soil organic matter; and (3) combined climate change and elevated CO2 would increase both net primary productivity and decomposition rates relative to the current climate condition, but their combined action would be a reduction of soil carbon losses relative to those due to climate change alone. The interactive effects of climate change and elevated CO2, however, are not a simple additive combination of the individual responses. The responses to climate change and elevated CO2 vary across the climate gradient from southern to northern sites on the transect. The present simulations indicate that the northern sites are more sensitive to climate change than the southern sites are, but these simulations do not consider likely changes in the disturbance regime or changes in forest species distribution.

The Boreal Forest Transect Case Study: Global change effects on ecosystem processes and carbon dynamics in boreal Canada

The Boreal Forest Transect Case Study: Global change effects on ecosystem processes and carbon dynamics in boreal Canada