Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes

Abstract

Great Basin bristlecone pine (Pinus longaeva) at 3 sites in western North America near the upper elevation limit of tree growth showed ring growth in the second half of the 20th century that was greater than during any other 50-year period in the last 3,700 years. The accelerated growth is suggestive of an environmental change unprecedented in millennia. The high growth is not overestimated because of standardization techniques, and it is unlikely that it is a result of a change in tree growth form or that it is predominantly caused by CO2 fertilization. The growth surge has occurred only in a limited elevational band within Ϸ150 m of upper treeline, regardless of treeline elevation. Both an independent proxy record of temperature and high-elevation meteorological temperature data are positively and significantly correlated with upper-treeline ring width both before and during the high-growth interval. Increasing temperature at high elevations is likely a prominent factor in the modern unprecedented level of growth for Pinus longaeva at these sites.climate change ͉ dendrochronology ͉ Great Basin ͉ tree rings ͉ treeline B ackground. Bristlecone pine (Pinus longaeva) is notable for its individual trees that attain great age, for its use in the calibration of the radiocarbon timescale, and for its role in providing an element in millennial-scale multiproxy reconstructions of temperature. The ring-width chronologies from long-lived bristlecone pine are annually resolved and can reach back thousands of years, making these high-resolution multimillennial proxy records of climate a rare and valuable resource in paleoclimatology. Uppertreeline bristlecone pine site locations are cold for much of the year and can be extremely dry during the summer growing season. As a result, these high-elevation tree-ring series contain some information on moisture availability, but they also bear an important imprint of temperature variability, so that both types of signal may be present in records from the upper treeline (1-7). There are interannual responses to precipitation variations at all elevations, including some degree of high-frequency variability related to extreme drought conditions at the upper treeline (8), although the variability related to precipitation is more pronounced at lower elevations (1, 9). Conversely, the main decadal to multidecadal ring-width variability at treeline locations may be related more closely to temperature than to precipitation (10). Despite the challenges in using these natural archives of climate successfully, we argue that it is worthwhile to make considerable effort to achieve the best possible use of this concentration of long annual records.

Mountain runoff vulnerability to increased evapotranspiration with vegetation expansion

“…Widespread increases in subalpine tree growth, tree-line altitude, and species distribution with elevation have been reported with recent climate trends in California and elsewhere, implying that rapid vegetation shifts are possible (14)(15)(16). Time series of Sierra Nevada forest greenness indicate a transition from water limitation at low elevation to cold limitation at high altitude, implying that upper elevation ET is sensitive to warming (17).…”

Recent reduction in the frequency of frost accounts for most of the increased growth of a high elevation spruce forest in northwestern China

“…For example, Salzer et al (2009) report that high elevation Great Basin bristlecone pine (Pinus longaeva) at three sites in western North America showed ring growth in the second half of the 20th century that was greater than during any other 50-year period in the last 3700 years. A similar unprecedented increase in growth over a 425-year chronology was reported in Juniperus przeqalskii from the Xiqing Mountains in northeast Tibet in association with an increase in minimum winter temperature of 2.5°C (Gou et al 2007).…”

Complex climate constraints of upper treeline formation in the Pyrenees

“…Radial growth enhancement has been observed in many treelines being mostly attributed to warmer temperatures (Rolland et al 1998;Camarero and Gutiérrez 2004;Salzer et al 2009). …”