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Aim (1) To calculate annual potential evapotranspiration (PET), actual evapotranspiration (AET) and climatic water deficit (Deficit) with high spatial resolution;(2) to describe distributions for 17 tree species over a 2300-m elevation gradient in a 3000-km 2 landscape relative to AET and Deficit; (3) to examine changes in AET and Deficit between past (c. 1700), present (1971-2000 and future (2020-49) climatological means derived from proxies, observations and projections; and (4) to infer how the magnitude of changing Deficit may contribute to changes in forest structure and composition.Location Yosemite National Park, California, USA.Methods We calculated the water balance within Yosemite National Park using a modified Thornthwaite-type method and correlated AET and Deficit with tree species distribution. We used input data sets with different spatial resolutions parameterized for variation in latitude, precipitation, temperature, soil waterholding capacity, slope and aspect. We used climate proxies and climate projections to model AET and Deficit for past and future climate. We compared the modelled future water balance in Yosemite with current species water-balance ranges in North America. ResultsWe calculated species climatic envelopes over broad ranges of environmental gradients -a range of 310 mm for soil water-holding capacity, 48.3°C for mean monthly temperature (January minima to July maxima), and 918 mm yr )1 for annual precipitation. Tree species means were differentiated by AET and Deficit, and at higher levels of Deficit, species means were increasingly differentiated. Modelled Deficit for all species increased by a mean of 5% between past (c. 1700) and present . Projected increases in Deficit between present and future (2020-49) were 23% across all plots.Main conclusions Modelled changes in Deficit between past, present and future climate scenarios suggest that recent past changes in forest structure and composition may accelerate in the future, with species responding individualistically to further declines in water availability. Declining water availability may disproportionately affect Pinus monticola and Tsuga mertensiana. Fine-scale heterogeneity in soil waterholding capacity, aspect and slope implies that plant water balance may vary considerably within the grid cells of kilometre-scale climate models. Sub-grid-cell soil and topographical data can partially compensate for the lack of spatial heterogeneity in gridded climate data, potentially improving vegetation-change projections in mountainous landscapes with heterogeneous topography.

Mega‐fires are often defined according to their size and intensity but are more accurately described by their socioeconomic impacts. Three factors – climate change, fire exclusion, and antecedent disturbance, collectively referred to as the “mega‐fire triangle” – likely contribute to today's mega‐fires. Some characteristics of mega‐fires may emulate historical fire regimes and can therefore sustain healthy fire‐prone ecosystems, but other attributes decrease ecosystem resiliency. A good example of a program that seeks to mitigate mega‐fires is located in Western Australia, where prescribed burning reduces wildfire intensity while conserving ecosystems. Crown‐fire‐adapted ecosystems are likely at higher risk of frequent mega‐fires as a result of climate change, as compared with other ecosystems once subject to frequent less severe fires. Fire and forest managers should recognize that mega‐fires will be a part of future wildland fire regimes and should develop strategies to reduce their undesired impacts.

Shaded fuelbreaks and larger landscape fuel treatments, such as prescribed ®re, are receiving renewed interest as forest protection strategies in the western United States. The effectiveness of fuelbreaks remains a subject of debate because of differing fuelbreak objectives, prescriptions for creation and maintenance, and their placement in landscapes with differing ®re regimes. A well-designed fuelbreak will alter the behavior of wildland ®re entering the fuel-altered zone. Both surface and crown ®re behavior may be reduced. Shaded fuelbreaks must be created in the context of the landscape within which they are placed. No absolute standards for fuelbreak width or fuel reduction are possible, although recent proposals for forested fuelbreaks suggest 400 m wide bands where surface fuels are reduced and crown fuels are thinned. Landscape-level treatments such as prescribed ®re can use shaded fuelbreaks as anchor points, and extend the zone of altered ®re behavior to larger proportions of the landscape. Coupling fuelbreaks with area-wide fuel treatments can reduce the size, intensity, and effects of wildland ®res. #