Summary The large magnitude of predicted warming at high latitudes and the potential feedback of ecosystems to atmospheric CO2 concentrations make it important to quantify both warming and its effects on high‐latitude carbon balance. We analysed long‐term, daily surface meteorological records for 13 sites in Alaska and north‐western Canada and an 82‐y record of river ice breakup date for the Tanana River in interior Alaska. We found increases in winter and spring temperature extrema for all sites, with the greatest increases in spring minimum temperature, average 0.47 °C per 10 y, and a 0.7‐day per 10 y advance in ice breakup on the Tanana River. We used the climate records to drive an ecosystem process model, BIOME_BGC, to simulate the effects of climate change on the carbon and water balances of boreal forest ecosystems. The growing season has lengthened by an average of 2.6 days per 10 y with an advance in average leaf onset date of 1.10 days per 10 y. This advance in the start of the active growing season correlates positively with progressively earlier ice breakup on the Tanana River in interior Alaska. The advance in the start of the growing season resulted in a 20% increase in net primary production for both aspen (Populus tremuloides) and white spruce (Picea glauca) stands. Aspen had a greater mean increase in maintenance respiration than spruce, whereas spruce had a greater mean increase in evapotranspiration. Average decomposition rates also increased for both species. Both net primary production and decomposition are enhanced in our simulations, suggesting that productive forest types may not experience a significant shift in net carbon flux as a result of climate warming.

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A long history of fire suppression in the western United States has significantly changed forest structure and ecological function, leading to increasingly uncharacteristic fires in terms of size and severity. Prior analyses of fire severity in California forests showed that time since last fire and fire weather conditions predicted fire severity very well, while a larger regional analysis showed that topography and climate were important predictors of high severity fire. There has not yet been a large-scale study that incorporates topography, vegetation and fire-year climate to determine regional scale high severity fire occurrence. We developed models to predict the probability of high severity fire occurrence for the western US. We predict high severity fire occurrence with some accuracy, and identify the relative importance of predictor classes in determining the probability of high severity fire. The inclusion of both vegetation and fire-year climate predictors was critical for model skill in identifying fires with high fractional fire severity. The inclusion of fire-year climate variables allows this model to forecast inter-annual variability in areas at future risk of high severity fire, beyond what slower-changing fuel conditions alone can accomplish. This allows for more targeted land management, including resource allocation for fuels reduction treatments to decrease the risk of high severity fire.

An ecological process model (BIOME-BGC) was used to assess boreal forest regional net primary production (NPP) and response to short-term, year-to-year weather fluctuations based on spatially explicit, land cover and biomass maps derived by radar remote sensing, as well as soil, terrain and daily weather information. Simulations were conducted at a 30-m spatial resolution, over a 1205 km(2) portion of the BOREAS Southern Study Area of central Saskatchewan, Canada, over a 3-year period (1994-1996). Simulations of NPP for the study region were spatially and temporally complex, averaging 2.2 (+/- 0.6), 1.8 (+/- 0.5) and 1.7 (+/- 0.5) Mg C ha(-1) year(-1) for 1994, 1995 and 1996, respectively. Spatial variability of NPP was strongly controlled by the amount of aboveground biomass, particularly photosynthetic leaf area, whereas biophysical differences between broadleaf deciduous and evergreen coniferous vegetation were of secondary importance. Simulations of NPP were strongly sensitive to year-to-year variations in seasonal weather patterns, which influenced the timing of spring thaw and deciduous bud-burst. Reductions in annual NPP of approximately 17 and 22% for 1995 and 1996, respectively, were attributed to 3- and 5-week delays in spring thaw relative to 1994. Boreal forest stands with greater proportions of deciduous vegetation were more sensitive to the timing of spring thaw than evergreen coniferous stands. Similar relationships were found by comparing simulated snow depth records with 10-year records of aboveground NPP measurements obtained from biomass harvest plots within the BOREAS region. These results highlight the importance of sub-grid scale land cover complexity in controlling boreal forest regional productivity, the dynamic response of the biome to short-term interannual climate variations, and the potential implications of climate change and other large-scale disturbances.

Forests provide a broad set of ecosystem services, including climate regulation. Other ecosystem services can be ecosystem dependent and are in part regulated by local‐scale decision‐making. In the southwestern United States, ongoing climate change is exacerbating a legacy of fire‐exclusion that has altered forest structure and increased high‐severity wildfire risk. Management can mitigate this risk by reducing forest density and restoring frequent surface fires, but at the cost of reduced carbon stocks. We sought to quantify the role of management in building adaptive capacity to projected climate and wildfires and the carbon consequences in a forested watershed. We simulated carbon dynamics under projected climate and wildfires and two management scenarios: prioritized and optimized. The prioritized scenario involved thinning and prescribed burning in areas selected by stakeholders to mitigate high‐severity wildfire risk. The optimized scenario used the probability of high‐severity wildfires to locate thinning treatments and increased prescribed fire area burned relative to the prioritized scenario. Both scenarios reduced wildfire severity and significantly increased net photosynthesis relative to no‐management. However, the optimized scenario decreased management‐related losses by 2.4 Mg • C • ha−1 and wildfire emissions by 2.9 Mg • C • ha−1 relative to the prioritized scenario. By decreasing the area thinned and increasing the area burned relative to the prioritized scenario, the optimized scenario halved the time to realize a net carbon benefit relative to no‐management. Given the increasing climatic and disturbance pressures impacting southwestern forests, management will play a critical role in building adaptive capacity and ensuring the continued provision of ecosystem services.