title: Twentieth-century shifts in forest structure in California: Denser forests, smaller trees, and increased dominance of oaks.
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.
authors.affiliation: Berkeley Initiative for Global Change Biology and Department of Integrative Biology, University of California, Berkeley, CA 94720; Patrick.McIntyre@wildlife.ca.gov.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: McIntyre
authors.affiliation: Information Center for the Environment, Department of Environmental Science and Policy, University of California, Davis, CA 95616;
authors.forename: James H
authors.initials: JH
authors.lastname: Thorne
authors.affiliation: Information Center for the Environment, Department of Environmental Science and Policy, University of California, Davis, CA 95616; Department of Forest Management, University of Montana, Missoula, MT 59812;
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Dolanc
authors.affiliation: United States Geological Survey California Water Science Center, Sacramento, CA 95819;
authors.forename: Alan L
authors.initials: AL
authors.lastname: Flint
authors.affiliation: United States Geological Survey California Water Science Center, Sacramento, CA 95819;
authors.forename: Lorraine E
authors.initials: LE
authors.lastname: Flint
authors.affiliation: Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720; and.
authors.forename: Maggi
authors.initials: M
authors.lastname: Kelly
authors.affiliation: Berkeley Initiative for Global Change Biology and Department of Integrative Biology, University of California, Berkeley, CA 94720; Department of Integrative Biology and Jepson Herbarium, University of California, Berkeley, CA 94720.
authors.forename: David D
authors.initials: DD
authors.lastname: Ackerly
mesh_terms: D044822:Biodiversity; D018533:Biomass; D002140:California; D065928:Forests; D049673:History, 20th Century; D049674:History, 21st Century
keywords: climatic water deficit; forest; global change; historical ecology