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Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.

Bioclimate models incorporating topographic predictors as surrogates for microclimate effects are developed for Populus tremuloides and Picea engelmannii to provide the fine-grained specificity to local terrain required for adapting management of three Colorado (USA) national forests (1.28 million ha) and their periphery to climate change. Models were built with the Random Forests classification tree using presence-absence observations obtained by overlaying species distribution maps on data points gridded at 225 m within the forests and from ground plot observations from adjacent areas. Topographic effects derived from 90-m elevation grids were expressed by weighting aspect by slope angle. Climate estimates were obtained from spline surfaces. Out-of-bag errors were 17 per cent, and classification errors for an independent sample from within the forest were 13 per cent. Topographic variables were second in importance to climate variables for predicting species distributions; their inclusion captured well-known topographic effects on vegetation in mountainous terrain. Predictions made for future climates described by three General Circulation Models and three emissions scenarios were used to map on 90-m grids the habitat expected to be lost, threatened, persistent or emergent. The habitat categories are used to identify those areas where treatments should have highest likelihood of success.

Stable isotope ratios of precipitation are useful tracers of climatic and hydrological processes. To better understand the isotope hydro-climatology of a high-elevation Rocky Mountain valley we collected meteoric water samples from Gunnison, Colorado, USA and determined stable isotope values for 239 individual precipitation events over a nine year period. Annual precipitation in Gunnison is moderately bi-modal with significant winter snowfall and convective summer thunderstorms associated with the North American Monsoon. Stable isotope values of precipitation span a large range, with summer rains as high as δ 2 H = +19‰ and δ 18 O = +4.8‰ (relative to V-SMOW) and winter snowfall as low as δ 2 H = -286‰ and δ 18 O = -36.7‰. These data define a local meteoric water line for Gunnison of δ 2 H = 7.2 δ 18 O – 4.2. Monthly meteoric water lines have slopes similar to the Global Meteoric Water Line (∼8) for winter months and more evaporated slopes (∼6) during the summer. Monthly mean temperature most strongly controls the monthly isotopic composition of precipitation (m = 0.61–0.64 ‰/°C); the slope of the isotope/temperature relationship is steeper in summer than winter.

Reports of forest damage have increased with the frequency of climatic extremes, but longer term impacts of such events on population dynamics of forest trees are generally unknown. Incited by the turn-of-the-century drought, sudden aspen decline (SAD) damaged 535 000 ha of Populus tremuloides Michx. in the Southern Rockies ecoregion of western North America. Although spread of the disease stopped in about 2009, most of the affected stands continued to deteriorate. Remeasurement of plots in southwestern Colorado showed that, since the peak of the epidemic, live basal area in sick plots decreased by an additional 28% to only 38% of that in healthy plots. Sick plots had much more recent damage than healthy plots, with almost three times as much recently dead basal area, over twice the density of recently dead trees, and almost four times as much recent crown loss. The important contributing agents in SAD were still active in sick stands in 2013. Density of small regeneration showed opposite trends, increasing in healthy plots and decreasing in sick plots. Timely regeneration treatments may be needed in some such stands to facilitate recovery. In addition to acute damage from climatic extremes, long-term decline diseases like SAD will likely be a common signature of forest damage from climate change.

Increasing prevalence of conifer needle pathogens globally have prompted further studies on pathogen identification and a better understanding of phylogenetic relationships among needle pathogens. Several Lophodermella species can be aggressive pathogens causing needle cast in natural pine forests in the USA and Europe. However, their relationships with other Rhytismataceae species have historically been based on similarities of only limited phenotypic characters. Currently, no molecular studies have been completed to elucidate their relationships with other Lophodermella needle pathogens. This study collected and sequenced three gene loci, namely: internal transcribed spacer, large ribosomal subunit, and translation elongation factor 1-alpha, from five Lophodermella needle pathogens from North America (L. arcuata, L. concolor, L. montivaga) and Europe (L. conjuncta and L. sulcigena) to distinguish phylogeny within Rhytismatacaeae, including Lophophacidium dooksii. Phylogenetic analyses of the three loci revealed that all but L. conjuncta that were sampled in this study consistently clustered in a well-supported clade within Rhytismataceae. The multi-gene phylogeny also confirmed consistent nesting of L. dooksii, a needle pathogen of Pinus strobus, within the clade. Potential synapomorphic characters such as ascomata position and ascospore shape for the distinct clade were also explored. Further, a rhytismataceous species on P. flexilis that was morphologically identified as L. arcuata was found to be unique based on the sequences at the three loci. This study suggests a potential wider range of host species within the genus and the need for genetic characterization of other Lophodermella and Lophophacidium species to provide a higher phylogenetic resolution.

Mapping geographic mosaics of genetic variation and their consequences via genotype x environment interactions at large extents and high resolution has been limited by the scalability of DNA sequencing. Here, we address this challenge for cytotype (chromosome copy number) variation in quaking aspen, a drought-impacted foundation tree species. We integrate airborne imaging spectroscopy data with groundbased DNA sequencing data and canopy damage data in 391 km 2 of southwestern Colorado. We show that (1) aspen cover and cytotype can be remotely sensed at 1 m spatial resolution, (2) the geographic mosaic of cytotypes is heterogeneous and interdigitated, (3) triploids have higher leaf nitrogen, canopy water content, and carbon isotope shifts (δ 13 C) than diploids, and (4) canopy damage varies among cytotypes and depends on interactions with topography, canopy height, and trait variables. Triploids are at higher risk in hotter and drier conditions.