Mycorrhizae play a key role in ecosystem dynamics, and it is important to understand how environmental stress and climate change affect these symbionts. Several climate models predict that the intercontinental western United States will experience an increase in extreme precipitation events and warming temperatures. In 1996, northern Arizona, USA, experienced a 100-year drought that caused high local mortality of pinyon pine (Pinus edulis), a dominant tree of the southwest. We compared trunk growth, water potentials, and ectomycorrhizal dynamics for surviving trees at three high-mortality sites and adjacent low-mortality sites. Four major patterns emerged. First, surviving trees at sites that suffered high mortality exhibited reduced long-term growth and increased water stress relative to adjacent sites where little or no mortality occurred. Second, surviving trees from high-mortality sites had 50% lower ectomycorrhizal colonization and showed a pronounced shift in fungal community composition relative to low-mortality sites. Third, in support of an intermediate-host plant stress hypothesis, trees that experienced intermediate levels of stress supported two-fold greater ectomycorrhizal colonization than trees at the high or low end of a stress gradient. Fourth, we observed a strong correlation between trunk growth and ectomycorrhizal colonization and validated the resulting regression model with independent data. This relationship suggests that tree rings can be used to reconstruct past and predict future ectomycorrhizal colonization. Overall, our findings suggest that predicted climate changes might be accompanied by both qualitative and quantitative changes in ectomycorrhizal dynamics that could affect ecosystems by altering nutrient cycling, carbon dynamics, and host-plant performance.
We examined the roles that seasonal shifts in precipitation and temperature played in the ectomycorrhizal (ECM) colonization of pinyon pine (Pinus edulis Engelm.) at two contrasting sites in northern Arizona. Pinyons growing in ash and cinder soils experienced much greater water and nutrient stress than pinyons growing nearby in sandyloam soils. Over a one year period, we obtained monthly measurements of ECM colonization, root zone soil moisture and temperature, and air temperature and precipitation. Four major patterns emerged. Firstly, although climate as measured by ambient temperature and precipitation did not vary between the two sites, soil temperature was significantly higher and soil moisture significantly lower at the cinder site than at the sandy-loam site. Secondly, ECM colonization was significantly higher at the cinder site for 5 of 12 months. Thirdly, although nearly 70 % of the variation in ECM colonization of pinyons growing in cinder soil was predicted by a combination of soil moisture and soil temperature, these same variables had little predictive power for pinyons growing in sandy-loam soils. Air temperature and precipitation were also significantly correlated with ECM colonization at the cinder site but not the sandy-loam site. Fourthly, a watering experiment showed that ECM colonization significantly increased with supplemental water at the cinder site, but not at the sandy-loam site. Thus, in two sites that did not differ in plant community or climate, ectomycorrhizas in cinder soils were far more sensitive to changes in moisture and temperature than ectomycorrhizas in sandy-loam soils.
The forests in northern Michigan will be affected directly and indirectly by changing climate during the 21st century. This assessment evaluates the vulnerability of forest ecosystems in the eastern Upper Peninsula and northern Lower Peninsula of Michigan under a range of future climates. We synthesized and summarized information on the contemporary landscape, provided information on past climate trends, and described a range of projected future climates. This information was used to parameterize and run multiple vegetation impact models, which provided a range of potential vegetative responses to climate. Finally, we brought these results before a multidisciplinary panel of scientists and land managers familiar with Michigan forests to assess ecosystem vulnerability through a formal consensus-based expert elicitation process. The summary of the contemporary landscape identifies major forest trends and stressors currently threatening forests in the region. Observed trends in climate over the past century reveal that precipitation increased in the area, particularly in summer and fall, and that daily maximum temperatures increased, particularly in winter. Projected climate trends for the next 100 years using downscaled global climate model data indicate a potential increase in mean annual temperature of 2.2 to 8.1 °F for the assessment area. Projections for precipitation indicate an increase in winter and spring precipitation, and summer and fall precipitation projections vary by scenario. We identified potential impacts on forests by incorporating these climate projections into three forest impact models (Tree Atlas, LANDIS-II, and PnET-CN). Model projections suggest that northern boreal species such as black spruce and paper birch may fare worse under future conditions, but other species such as American elm and white oak may benefit from projected changes in climate. Published literature on climate impacts related to wildfire, invasive species, and diseases also contributed to the overall determination of climate change vulnerability. We assessed vulnerability for nine forest communities in the assessment area, which were a combination of U.S. Department of Agriculture, Forest Service Forest Inventory and Analysis program forest types and Michigan Natural Features Inventory natural communities. The basic assessment was conducted through a formal elicitation process of 27 science and management experts from across the state, who considered vulnerability both in terms of potential impacts on a system and in terms of the system's adaptive capacity. Upland spruce-fir, jack pine, lowland conifers, and red pine-white pine forest communities were determined to be the most vulnerable. Barrens and lowland/riparian hardwood communities were perceived as less vulnerable to projected changes in climate. These projected changes in climate and the associated impacts and vulnerabilities will have important implications for economically valuable timber species, forest-dependent wildlife and plants, recreation, and long-range p...
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