No abstract

Fire is a keystone process in many ecosystems of western North America. Severe fires kill and consume large amounts of above‐ and belowground biomass and affect soils, resulting in long‐lasting consequences for vegetation, aquatic ecosystem productivity and diversity, and other ecosystem properties. We analyzed the occurrence of, and trends in, satellite‐derived burn severity across six ecoregions in the Southwest and Northwest regions of the United States from 1984 to 2006 using data from the Monitoring Trends in Burn Severity project. Using 1,024 fires from the Northwest (4,311,871 ha) and 497 fires from the Southwest (1,434,670 ha), we examined the relative influence of fine‐scale topography and coarse‐scale weather and climate on burn severity (the degree of change from before the fire to one year after) using the Random Forest machine learning algorithm. Together, topography, climate, and weather explained severe fire occurrence with classification accuracies ranging from 68% to 84%. Topographic variables were relatively more important predictors of severe fire occurrence than either climate or weather variables. Predictability of severe fire was consistently lower during years with widespread fires, suggesting that local control exerted by topography may be overwhelmed by regional climatic controls when fires burn in dry conditions. Annually, area burned severely was strongly correlated with area burned in all ecoregions (Pearson's correlation 0.86–0.97; p < 0.001), while the proportion of area burned severely was significantly correlated with area burned only in two ecoregions (p ≤ 0.037). During our short time series, only ecoregions in the Southwest showed evidence of a significant increase (p ≤ 0.036) in annual area burned and area burned severely, and annual proportion burned severely increased in just one of the three Southwest ecoregions. We suggest that predictive mapping of the potential for severe fire is possible, and will be improved with climate data at the scale of the topographic and Landsat‐derived burn severity data. Although severity is a value‐laden term implying negative ecosystem effects, we stress that severity can be objectively measured and recognize that high severity fire is an important ecological process within the historical range of variability in some ecosystems.

Shifts in species' phenology in response to climate change have wide-ranging consequences for ecological systems. However, significant variability in species' responses, together with limited data, frustrates efforts to forecast the consequences of ongoing phenological changes. Herein, we use a case study of three North American plant communities to explore the implications of variability across levels of organisation (within and among species, and among communities) for forecasting responses to climate change. We show how despite significant variation among species in sensitivities to climate, comparable patterns emerge at the community level once regional climate drivers are accounted for. However, communities differ with respect to projected patterns of divergence and overlap among their species' phenological distributions in response to climate change. These analyses and a review of hypotheses suggest how explicit consideration of spatial scale and levels of biological organisation may help to understand and forecast phenological responses to climate change.

Summary1. Many studies have documented advancement in spring plant phenology; however, studies in dry climates, where water, rather than temperature, is the limiting factor, are rare. To better understand how plants of a water-limited environment may respond to predicted changes in climate, we used a species-rich 20-year data set collected in a semi-arid ecosystem to determine species' relationships with precipitation and temperature for seasons coincident with and previous to flowering. Our data were collected across a 1200-m elevation gradient, allowing us to explore the consistency in relationships with climatic variables from desert scrub to pine forest. A second objective was to document evidence of changes in the onset of spring flowering over this 20-year period. 2. Onset of spring flowering for species at the lowest elevations was most commonly driven by temperature and precipitation conditions of the previous autumn. In contrast, onset of spring flowering for species in high-elevation communities was more often associated with spring temperatures, a pattern consistent with communities of higher latitudes. Despite these coarse patterns, species' relationships to climate variables were highly variable and individualistic.3. Approximately 10% of species showed a significant trend in changes in first flowering date over the period [1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][2003]; most trends were in the direction of later onset. The decrease in autumn precipitation observed over the study period appears to explain the delay in onset observed for many of the species across the elevation gradient. Other species' delays in spring flowering appear to be related to the slight decrease in spring temperature observed over the study period. 4. Synthesis. The south-western USA is expected to become warmer and drier. Climate relationships documented in this study suggest divergent, individualistic changes in the onset of spring flowering. Low-elevation plants may exhibit delayed spring flowering due to changes in the timing or amount of precipitation or insufficient chilling. High-elevation species may show advancement in spring flowering due to warming temperatures. The highly individualistic responses to climate change may result in significant changes in the diversity, composition and abundance of plants in flower. Variable changes in phenology such as these have major implications for species population dynamics and ecosystem functioning.

We related measurements of annual burned area in the southwest United States during 1984–2013 to records of climate variability. Within forests, annual burned area correlated at least as strongly with spring–summer vapour pressure deficit (VPD) as with 14 other drought-related metrics, including more complex metrics that explicitly represent fuel moisture. Particularly strong correlations with VPD arise partly because this term dictates the atmospheric moisture demand. Additionally, VPD responds to moisture supply, which is difficult to measure and model regionally due to complex micrometeorology, land cover and terrain. Thus, VPD appears to be a simple and holistic indicator of regional water balance. Coupled with the well-known positive influence of prior-year cold season precipitation on fuel availability and connectivity, VPD may be utilised for burned area forecasts and also to infer future trends, though these are subject to other complicating factors such as land cover change and management. Assuming an aggressive greenhouse gas emissions scenario, climate models predict mean spring–summer VPD will exceed the highest recorded values in the southwest in nearly 40% of years by the middle of this century. These results forewarn of continued increases in burned forest area in the southwest United States, and likely elsewhere, when fuels are not limiting.