Considerable evidence exists that current global temperatures are higher than at any time during the past millennium. However, the long-term impacts of rising temperatures and associated shifts in the hydrological cycle on the productivity of ecosystems remain poorly understood for mid to high northern latitudes. Here, we quantify species-specific spatiotemporal variability in terrestrial aboveground biomass stem growth across Canada's boreal forests from 1950 to the present. We use 873 newly developed tree-ring chronologies from Canada's National Forest Inventory, representing an unprecedented degree of sampling standardization for a largescale dendrochronological study. We find significant regional-and species-related trends in growth, but the positive and negative trends compensate each other to yield no strong overall trend in forest growth when averaged across the Canadian boreal forest. The spatial patterns of growth trends identified in our analysis were to some extent coherent with trends estimated by remote sensing, but there are wide areas where remote-sensing information did not match the forest growth trends. Quantifications of tree growth variability as a function of climate factors and atmospheric CO 2 concentration reveal strong negative temperature and positive moisture controls on spatial patterns of tree growth rates, emphasizing the ecological sensitivity to regime shifts in the hydrological cycle. An enhanced dependence of forest growth on soil moisture during the late-20th century coincides with a rapid rise in summer temperatures and occurs despite potential compensating effects from increased atmospheric CO 2 concentration. drought impacts | climate change | dendrochronology | normalized difference vegetation index | ecology C ircumpolar boreal forests are estimated to store ∼53.9 Pg of carbon or ∼14% of terrestrial vegetation biomass (1). These regions are currently experiencing accelerated changes, including warmer and longer growing seasons, tree line expansion, species migration, increased frequency and severity of drought, and increases in the frequency and severity of disturbances (2-10). These changes create uncertainty about the boreal forests' future role in the global carbon cycle (11). Adding to this uncertainty is the discrepancy over recent changes in the productivity of boreal and other northern latitude forests. Some empirical evidence suggests increases in the forest productivity (12)(13)(14), whereas other studies suggest decreasing productivity over the last decades (7,8,(15)(16)(17). Furthermore, inversion and process-based ecosystem models indicate large carbon sinks (7,8), whereas field-based bottom-up approaches suggest smaller carbon sinks or small carbon sources (3, 18), or large sinks (19). Quantifying

Peatlands store the majority of soil carbon in many northern regions, yet their vulnerability to fire remains poorly understood. We used large‐scale mapping of fire and peatland distributions to explore patterns of burning at two spatial scales. On a landscape scale in central Alberta, we used spatially explicit distributions of peatlands and 50 years of fire perimeter maps to determine whether uplands burn more preferentially than peatlands. Burn area and ignition localities in central Alberta did not occur preferentially in uplands relative to bogs and fens. Extrapolating this result at a regional scale, we used the Peatlands of Canada database and 20 years of historical fire records to estimate annual burn areas for Alberta, British Columbia, Northwest Territories, and Saskatchewan peatlands. Peatland burn areas varied tremendously over time, with high fire activity in the early 1980s and mid‐1990s. On average, fires impacted 1850 km2 of peatland annually across this region of western Canada. Positive relationships between the area of peatland burned and weather variables calculated for each fire event using the Canadian Fire Weather Index, including maximum air temperatures and the duff moisture code, suggest that drier and/or warmer conditions likely would increase the burning of peatlands in western Canada.

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Boreal peatland ecosystems occupy about 3.5 million km 2 of the earth's land surface and store between 250 and 455 Pg of carbon (C) as peat. While northern hemisphere boreal peatlands have functioned as net sinks for atmospheric C since the most recent deglaciation, natural and anthropogenic disturbances, and most importantly wildfire, may compromise peatland C sinks. To examine the effects of fire on local and regional C sink strength, we focused on a 12 000 km 2 region near Wabasca, AB, Canada, where ombrotrophic Sphagnum-dominated bogs cover 2280 km 2 that burn with a fire return interval of 123 AE 26 years. We characterized annual C accumulation along a chronosequence of 10 bog sites, spanning 1-102 years-since-fire (in 2002). Immediately after fire, bogs represent a net C source of 8.9 AE 8.4 mol m À2 yr À1 . At about 13 years after fire, bogs switch from net C sources to net C sinks, mainly because of recovery of the moss and shrub layers. Subsequently, black spruce biomass accumulation contributes to the net C sink, with fine root biomass accumulation peaking at 34 years after fire and aboveground biomass and coarse root accumulation peaking at 74 years after fire. The overall C sink strength peaks at 18.4 mol C m À2 yr À1 at 75 years after fire. As the tree biomass accumulation rate declines, the net C sink decreases to about 10 mol C m À2 yr À1 at 100 years-sincefire. We estimate that across the Wabasca study region, bogs currently represent a C sink of 14.7 AE 5.1 Gmol yr À1 . A decrease in the fire return interval to 61 years with no change in air temperature would convert the region's bogs to a net C source. An increase in nonwinter air temperature of 2 1C would decrease the regional C sink to 6.8 AE 2.3 Gmol yr À1 . Under scenarios of predicted climate change, the current C sink status of Alberta bogs is likely to diminish to the point where these peatlands become net sources of atmospheric CO 2 -C.

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Phosphate (PO4) availability limits the productivity of pine plantations growing on Spodosols of the southeastern USA. Oxalate has been shown to interact with both the sorption and desorption PO4 onto soil mineral surfaces. In addition, organic matter, a crucial component of many soil surfaces, affects the adsorption of PO4. We studied the effects of oxalate and organic matter on PO4 sorption and desorption onto the whole soil and clay‐sized fraction of a spodic horizon from a poorly drained Spodosol of the flatwoods region of the lower Coastal Plain of the southeastern USA. Common batch studies and mass balance of OH‐ production and consumption were used to interpret the processes. Maximum reduction in PO4 sorption was observed in samples where organic matter and oxalate were present. The molar ratio of OH‐ ions released to PO4 sorbed supports the idea of a ligand‐exchange mechanism dominating the PO4 sorption process. Some of the sorption sites appear to be common sites for PO4, oxalate, and organic matter. Phosphorus desorption from the spodic horizon by the action of oxalate was through ligand exchange of oxalate for PO4. The presence of soil organic matter increased the amount of PO4 desorbed by oxalate. Oxalate appeared to form stable soluble complexes with Al in solution, thus inhibiting its reprecipitation.

The responses of high latitude ecosystems to global change involve complex interactions among environmental variables, vegetation distribution, carbon dynamics, and water and energy exchange. These responses may have important consequences for the earth system. In this study, we evaluated how vegetation distribution, carbon stocks and turnover, and water and energy exchange are related to environmental variation spanned by the network of the IGBP high latitude transects. While the most notable feature of the high latitude transects is that they generally span temperature gradients from southern to northern latitudes, there are substantial differences in temperature among the transects. Also, along each transect temperature co-varies with precipitation and photosynthetically active radiation, which are also variable among the transects. Both climate and disturbance interact to influence latitudinal patterns of vegetation and soil carbon storage among the transects, and vegetation distribution appears to interact with climate to determine exchanges of heat and moisture in high latitudes. Despite limitations imposed by the data we assembled, the analyses in this study have taken an important step toward clarifying the complexity of interactions among environmental variables, vegetation distribution, carbon stocks and turnover, and water and energy exchange in high latitude regions. This study reveals the need to conduct coordinated global change studies in high latitudes to further elucidate how interactions among climate, disturbance, and vegetation distribution influence carbon dynamics and water and energy exchange in high latitudes.

A large area of boreal jack pine (Pinus banksiana Lamb.) forest in Canada is recovering from clear-cut harvesting, and the carbon (C) balance of these regenerating forests remains uncertain. Net ecosystem CO 2 exchange was measured using the eddycovariance technique at four jack pine sites representing different stages of stand development: three postharvest sites (HJP02, HJP94, and HJP75) and one preharvest site (OJP). The four sites, located in the southern Canadian boreal forest, Saskatchewan, Canada, are typical of low productivity jack pine stands and were 2, 10, 29, and 90 years old in 2004, respectively. Mean annual net ecosystem production (NEP) for 2004 and 2005 was À137 AE 11, 19 AE 16, 73 AE 28, and 22 AE 30 g C m À2 yr À1 at HJP02, HJP94, HJP75 and OJP, respectively, showing the postharvest jack pine stands to be moderate C sources immediately after harvesting, weak sinks at 10 years, moderate C sinks at 30 years, then weak C sinks at 90 years. Mean annual gross ecosystem photosynthesis (GEP) for the 2 years was 96 AE 10, 347 AE 20, 576 AE 34, and 583 AE 35 g C m À2 yr À1 at HJP02, HJP94, HJP75, and OJP, respectively. The ratio of annual ecosystem respiration (R) to annual GEP was 2.51 AE 0.15, 0.95 AE 0.04, 0.87 AE 0.03, and 0.96 AE 0.03. Seasonally, NEP peaked in May or June at all four sites but GEP and R were highest in July. R at a reference soil temperature of 10 1C, ecosystem quantum yield and photosynthetic capacity were lowest for the 2-year-old stand. R was most sensitive to soil temperature for the 90-year-old stand. The primary source of variability in NEP over the course of succession of the jack pine ecosystem following harvesting was stand age due to the changes in leaf area index. Intersite variability in GEP and R was an order of magnitude greater than interannual variability at OJP. For both young and old stands, GEP had greater interannual variability than R and played a more important role than R in interannual variation in NEP. Based on year-round flux measurements from 2000 to 2005, the 10-year stand had larger interannual variability in GEP and R than the 90-year stand. Interannual variability in NEP was driven primarily by early-growing-season temperature and growing-season length. Photosynthesis played a dominant role in the rapid rise in NEP early in stand development. Late in stand development, however, the subtle decrease in NEP resulted primarily from increasing respiration.