The Ancient Blue Oak Woodlands of California: Longevity and Hydroclimatic History

Abstract

Ancient blue oak trees are still widespread across the foothills of the Coast Ranges, Cascades, and Sierra Nevada in California. The most extensive tracts of intact old-growth blue oak woodland appear to survive on rugged and remote terrain in the southern Coast Ranges and on the foothills west and southwest of Mt. Lassen. In the authors' sampling of old-growth stands, most blue oak appear to have recruited to the canopy in the middle to late nineteenth century. The oldest living blue oak tree sampled was over 459 years old, and several dead blue oak logs had over 500 annual rings. Precipitation sensitive tree-ring chronologies up to 700 years long have been developed from old blue oak trees and logs. Annual ring-width chronologies of blue oak are strongly correlated with cool season precipitation totals, streamflow in the major rivers of California, and the estuarine water quality of San Francisco Bay. A new network of 36 blue oak chronologies records spatial anomalies in growth that arise from latitudinal changes in the mean storm track and location of landfalling atmospheric rivers. These long, climate-sensitive blue oak chronologies have been used to reconstruct hydroclimatic history in California and will help to better understand and manage water resources. The environmental history embedded in blue oak growth chronologies may help justify efforts to conserve these authentic old-growth native woodlands.

How unusual is the 2012–2014 California drought?

“…Our October to June precipitation reconstruction is based on existing blue oak tree ring chronologies [ Stahle et al , ] augmented with new specimens collected after the completion of the 2014 growth season and therefore spans 1293 to 2014 CE (Figure ). The bivariate proxy‐observation relationship is linear, stable through time, coherent across the frequency domain, and exceptionally robust by dendroclimate standards ().…”

The value of crossdating to retain high‐frequency variability, climate signals, and extreme events in environmental proxies

“…Splitnose rockfish (Black et al ., , ) was correlated with mean January through March upwelling index averaged across 36°N and 39°N as well as mean January through March 1° gridded Hadley ISST sea surface temperature. Blue oak (Stahle et al ., ) was correlated with prior December through current February NOAA NCDC CA Divisions 5 and 7 precipitation as well as 1° gridded Hadley prior December through current February precipitation. Correlation analysis with gridded Hadley data was performed in the KNMI Climate Explorer (Trouet & Van Oldenborgh, ).…”

“…Five datasets are included in the analysis, three of which have been previously published. Two are terrestrial: a blue oak ( Quercus douglasii ) stand from southern California (Stahle et al ., ) and a Douglas‐fir ( Pseudotsuga menziesii ) stand from the western Cascade Mountains of Oregon (Table ). The remaining datasets are marine, including the bivalve species Arctica islandica from the central coast of Maine, USA, the bivalve species Pacific geoduck ( Panopea generosa ) from the northern British Columbia coast, Canada (Black et al ., ), and splitnose rockfish ( Sebastes diploproa ) from the north‐central California Current, USA (Black et al ., , ).…”

Hydrologic refugia, plants, and climate change

“…With multicentury lifespans and relatively drought-sensitive young life stages (Mahall et al, 2009;Stahle et al, 2013), current tree stand structure creates a living recordlocal adult distributions record historical recruitment limitation and survival, and young life stage distributions provide comparative insights into current limitations on recruitment. Their distributions cross wide regional climatic and local microenvironmental gradients, and experience strong seasonality and interannual variability in rainfall.…”

“…Their distributions cross wide regional climatic and local microenvironmental gradients, and experience strong seasonality and interannual variability in rainfall. With multicentury lifespans and relatively drought-sensitive young life stages (Mahall et al, 2009;Stahle et al, 2013), current tree stand structure creates a living recordlocal adult distributions record historical recruitment limitation and survival, and young life stage distributions provide comparative insights into current limitations on recruitment. This creates ideal conditions to observe the interactions between oak performance, climate and mesic microenvironments, and anticipate future hydrologic refugia.…”