California's Historic Legacy For Landscape Change, the Wieslander Vegetation Type Maps

Abstract

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 ...

The relative influence of change in habitat and climate on elevation range limits in small mammals in Yosemite National Park, California, U.S.A.

“…Surveyors color-coded topographic maps to reflect the vegetation communities they observed, and annotated the maps with the spatially dominant plant species in each polygon. These maps were georeferenced and hand-digitized on-screen to produce a spatial digital layer of land cover in the 1930s [37,38]. Given the uncertainty of these assessments from vantage points, this dataset was analyzed at a spatial resolution of 300 m.…”

Predicting habitat suitability for Townsend's big‐eared bats across California in relation to climate change

“…The average temperature in California has increased by approximately 1.11°C since the early 20th century, with warming projected to continue (Frankson et al, 2022 ). However, warming across the state has not been uniform, suggesting that some ecoregions are experiencing more accelerated effects of climate change—for example, monthly minimums in the Sierra Nevada have increased by about 3°C in the past 100 years (Thorne et al, 2006 ). Flooding, drought, and wildfires are ecosystem disturbances, influenced by climate conditions, and are also predicted to increase in the next century (Frankson et al, 2022 ).…”