Increased temperatures and concomitant changes in vegetation patterns are expected to dramatically alter the functioning of northern ecosystems over the next few decades. Predicting the ecosystem response to such a shift in climate and vegetation is complicated by the lack of knowledge about the links between aboveground biota and belowground process rates. Current models suggest that increasing temperatures and rising concentrations of atmospheric CO 2 will be partly mitigated by elevated C sequestration in plant biomass and soil. However, empirical evidence does not always support this assumption, as elevated temperature and CO 2 concentrations also accelerate the belowground C flux, in many cases extending to increased decomposition of soil organic matter (SOM) and ultimately resulting in decreased soil C stocks. The mechanism behind the increase has remained largely unknown, but it has been suggested that priming might be the causative agent. Here, we provide quantitative evidence of a strong coupling between root exudation, SOM decomposition, and release of plant available N caused by rhizosphere priming effects. As plants tend to increase belowground C allocation with increased temperatures and CO 2 concentrations, priming effects need to be considered in our long‐term analysis of soil C budgets in a changing environment. The extent of priming seems to be intimately linked to resource availability, as shifts in the stoichiometric nutrient demands of plants and microorganisms will lead to either cooperation (resulting in priming) or competition (no priming will occur). The findings lead us on the way to resolve the varying response of primary production, SOM decomposition, and release of plant available N to elevated temperatures, CO 2 concentrations, and N availability.

Summary1. Clearcut logging results in major changes in ectomycorrhizal fungal communities, but whether this results in the loss of key functional traits, such as those associated with nutrient acquisition from soil organic matter, is unknown. Furthermore, little is known about the importance of resource partitioning in structuring ectomycorrhizal fungal communities following disturbance because most research on these communities has focussed on life history strategies. By studying functional traits, such as activities of enzymes involved in the catabolism of organic macromolecules in soil, we can determine whether a physiological potential for resource partitioning exists in pioneer ectomycorrhizal communities and whether severe disturbance affects these important ecosystem services. 2. We used activities of key hydrolytic enzymes in the ectomycorrhizospheres of Douglas-fir seedlings regenerating at clearcut sites as a functional trait to test whether these differed from those at recent wildfire sites or control forests. We sampled the most abundant types of ectomycorrhizas from 16-month-old seedlings from sites exposed to (i) low or (ii) high severity wildfire, (iii) sites that had been clearcut logged in the same year as the fire and (iv) sites that contained control stands of mature Douglas-fir. We expected differences in activities among ectomycorrhizas sampled from different disturbance treatments and among those formed by different fungal species. 3. In spite of large differences in soil chemistry, activities of acid phosphomonoesterase, N-acetylglucosaminidase and b-glucosidase, when averaged among the ectomycorrhizas sampled per site, were not affected by disturbance agent. However, activities varied up to sixfold among mycorrhizospheres of different fungal species on the same seedling. Multivariate analysis also indicated some consistent differences in enzyme profiles among ectomycorrhizas formed by specific fungal species, independent of treatment. 4. The finding that ectomycorrhizal fungal communities exposed to different disturbance agents are functionally similar with respect to the activities of three mycorrhizosphere enzymes supports the conclusion that complementarity exists among ectomycorrhizal fungi. The substantial physiological diversity among ectomycorrhizal fungi at the scale of an individual seedling's root tips, especially at control mature forests, indicates the potential for resource partitioning within the ectomycorrhizal community and access to a wider range of nutrient sources by each seedling. 5. Functional similarity among ectomycorrhizal fungal communities across a disturbance severity gradient suggests that dry interior Douglas-fir forests are resilient to severe disturbances such as high severity wildfire and clearcutting with forest floor removal. Moreover, our results suggest that current harvesting practices emulate natural disturbances with respect to site-level mycorrhizosphere enzyme activity. The large variation in activity among fungal species, however, 2010, 24, 1139-...

Wildfire severity in forests is projected to increase with warming and drying conditions associated with climate change. Our objective was to determine the impact of wildfire and clearcutting severity on the ectomycorrhizal fungal (EMF) community of Douglas-fir seedlings in the dry forests of interior British Columbia, Canada. We located our study within and surrounding the area of the McLure fire (August 2003). We hypothesized that disturbance would affect EMF community assembly due to reductions in fungal inoculum. Five treatments representing a range of disturbance severities were compared: high severity burn, low severity burn, screefed clearcut (manual removal of forest floor), clearcut, and undisturbed forest. EMF communities in the undisturbed forest were more complex than those in all disturbance treatments. However, aspects of community assembly varied with disturbance type, where the burn treatments had the simplest communities. After 4 months, regenerating seedlings in the burn treatments had the lowest colonization, but seedlings in all treatments were fully colonized within 1 year. EMF communities were similar among the four disturbance types, largely due to dominance of Wilcoxina throughout the study period. However, forest floor retention influenced community assembly as the EMF in the clearcut treatment, where forest floor was retained, had levels of diversity and richness comparable to the undisturbed forest. Overall, the results suggest that increasing forest floor disturbance can alter EMF community assembly in the first year of regeneration. A correlation between poorly colonized seedlings and seedling productivity also suggests a role for productivity in influencing community assembly.

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Forest land managers rely on predictions of tree mortality generated from fire behavior models to identify stands for post-fire salvage and to design fuel reduction treatments that reduce mortality. A key challenge in improving the accuracy of these predictions is selecting appropriate wind and fuel moisture inputs. Our objective was to evaluate postfire mortality predictions using the Forest Vegetation Simulator Fire and Fuels Extension (FVS-FFE) to determine if using representative fire-weather data would improve prediction accuracy over two default weather scenarios. We used pre- and post-fire measurements from 342 stands on forest inventory plots, representing a wide range of vegetation types affected by wildfire in California, Oregon, and Washington. Our representative weather scenarios were created by using data from local weather stations for the time each stand was believed to have burned. The accuracy of predicted mortality (percent basal area) with different weather scenarios was evaluated for all stands, by forest type group, and by major tree species using mean error, mean absolute error (MAE), and root mean square error (RMSE). One of the representative weather scenarios, Mean Wind, had the lowest mean error (4%) in predicted mortality, but performed poorly in some forest types, which contributed to a relatively high RMSE of 48% across all stands. Driven in large part by over-prediction of modelled flame length on steeper slopes, the greatest over-prediction mortality errors arose in the scenarios with higher winds and lower fuel moisture. Our results also indicated that fuel moisture was a stronger influence on post-fire mortality than wind speed. Our results suggest that using representative weather can improve accuracy of mortality predictions when attempting to model over a wide range of forest types. Focusing simulations exclusively on extreme conditions, especially with regard to wind speed, may lead to over-prediction of tree mortality from fire.