We document changes in forest structure between historical (1930s) and contemporary (2000s) surveys of California vegetation through comparisons of tree abundance and size across the state and within several ecoregions. Across California, tree density in forested regions increased by 30% between the two time periods, whereas forest biomass in the same regions declined, as indicated by a 19% reduction in basal area. These changes reflect a demographic shift in forest structure: larger trees (>61 cm diameter at breast height) have declined, whereas smaller trees (<30 cm) have increased. Large tree declines were found in all surveyed regions of California, whereas small tree increases were found in every region except the south and central coast. Large tree declines were more severe in areas experiencing greater increases in climatic water deficit since the 1930s, based on a hydrologic model of water balance for historical climates through the 20th century. Forest composition in California in the last century has also shifted toward increased dominance by oaks relative to pines, a pattern consistent with warming and increased water stress, and also with paleohistoric shifts in vegetation in California over the last 150,000 y.global change | forest | historical ecology | climatic water deficit U nderstanding the patterns and causes of recent changes in vegetation structure is essential to predicting ongoing and future vegetation responses to global climate change. Recent changes in forest structure and tree mortality attributed to increases in temperature and drought have been documented for large areas across the globe (1-5). Declines in the abundance of large trees have attracted particular attention, as large trees contribute disproportionately to forest structure and function, carbon stocks, and the cultural values of forests (6-10). Although land-use change and harvesting of large trees contribute to their decline, studies have found that large trees can suffer disproportionate mortality in response to drought in both temperate and tropical systems, and that declines have occurred in protected areas not subject to logging (5,8,11,12). The mechanisms responsible for large tree vulnerability to water deficit are poorly understood but rest largely on interacting effects of increased vulnerability to cavitation, carbon starvation during drought, and vulnerability to natural enemies (13,14). Here we present, to our knowledge, the first analysis of changes in large tree density across California during the 20th century in relation to changes in water deficit during the same period, and examine changes in species composition that can be linked to historical changes over longer time periods.In the forests of California, comparisons of historical and contemporary forests in selected areas of the Sierra Nevada and Transverse mountains suggest that forests have in-filled with small trees and exhibited declines in larger trees since the early 1900s, patterns attributed to fire suppression, forest exploitation, and changes ...

Aim  Many climate‐linked vegetation models predict major contraction of subalpine forests within the next 100 years, which would require a relatively rapid replacement of high‐elevation species by lower‐elevation species over large portions of subalpine forest. We tested this prediction by comparing empirical data from a historic data set with data collected from re‐sampled sites from 2007–09. Location  Central Sierra Nevada, CA, USA, 2300–3400 m elevation. Methods  We re‐sampled 139 undisturbed historical vegetation plots across 5500 km2 originally sampled from 1929–34 in the subalpine zone of the Sierra Nevada, and compared historical with current forest structure and composition. We compared historic and modern climatic conditions using two high‐elevation climate stations nearby. Results  Subalpine forests experienced a net increase in tree stem density of 30.4%, including a 63.3% increase in small trees. Six of eight tree species showed statistically significant increases in small tree density, including species with distributions at both the upper and lower boundaries of subalpine. Increases in small tree density were partly offset by a 20% decrease in large trees. These shifts were significant throughout the landscape of our study area. Modern stand composition was indistinguishable from historical composition. Daily minimum temperature (+ 1.2°C) and precipitation (+ 15–48%) both increased during the same period. Main conclusions  Warming temperatures plus steady to increasing precipitation have led to less stressful conditions for recruitment and survival of small trees, and are probably contributing to increased mortality of large trees. Tree abundance and composition in the subalpine has not changed in the direction predicted by vegetation models linked to future climate scenarios. Our results underline the fundamental role that moisture balance plays in structuring mediterranean‐zone montane forests. Future shifts in vegetation composition and structure from these regions are likely to depend on interactions between water balance and disturbance factors like fire, insects and disease.

Questions Has tree density changed consistently across vegetation types? Do changes in component species correspond with changes across vegetation types? Do patterns of changes suggest potential drivers of change? Location Northern two‐thirds of the Sierra Nevada, CA, USA, ca. 45 000 km2. Methods Using two data sets that cover the span of elevations and land jurisdictions in the study area, we classified 4321 historical plots and 1000 modern plots into nine broad groups of vegetation types that are widely used by land managers and researchers in the region. We compared tree density and composition between historical and modern plots across and within these nine types. Results In the modern data set, tree density was significantly higher in eight of nine vegetation types. Total density was significantly higher in modern plots for all west slope types, especially for montane hardwood, where modern forests are 128% denser, and mixed conifer forests, which are 69% denser. Relative density of component species was also very different between data sets in these forests, and suggests a shift in dominance toward shade‐tolerant conifers and evergreen oaks. Fire suppression is likely a driving factor in these types but density was also significantly higher in high‐elevation types such as sub‐alpine forest (+20%), where neither fire suppression nor logging have had major impacts on structure. East slope forest types (eastside P. jeffreyi forest and piñon‐juniper woodland) were very similar in both modern and historical data sets, with no significant differences in density or composition. Conclusion West slope forest types, especially montane hardwood and mixed conifer forest, appear the most altered types of the mountain range. These types are more productive but have also been subject to greater disturbance than high‐elevation and east slope forest types. Climate change may also be driving changes across the study area. Species such as Quercus chrysolepis and Calocedrus decurrens, which have each increased markedly in abundance, appear well positioned to dominate in the near future, especially under continued fire suppression and a warmer climate.

van Mantgem. 2015. Increasing elevation of fire in the Sierra Nevada and implications for forest change. Ecosphere 6(7):121. http://dx.doi.org/10.1890/ES15-00003.1Abstract. Fire in high-elevation forest ecosystems can have severe impacts on forest structure, function and biodiversity. Using a 105-year data set, we found increasing elevation extent of fires in the Sierra Nevada, and pose five hypotheses to explain this pattern. Beyond the recognized pattern of increasing fire frequency in the Sierra Nevada since the late 20th century, we find that the upper elevation extent of those fires has also been increasing. Factors such as fire season climate and fuel build up are recognized potential drivers of changes in fire regimes. Patterns of warming climate and increasing stand density are consistent with both the direction and magnitude of increasing elevation of wildfire. Reduction in high elevation wildfire suppression and increasing ignition frequencies may also contribute to the observed pattern. Historical biases in fire reporting are recognized, but not likely to explain the observed patterns. The four plausible mechanistic hypotheses (changes in fire management, climate, fuels, ignitions) are not mutually exclusive, and likely have synergistic interactions that may explain the observed changes. Irrespective of mechanism, the observed pattern of increasing occurrence of fire in these subalpine forests may have significant impacts on their resilience to changing climatic conditions.