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Nonlinear growth responses of Douglas-fir in the Pacific Northwest to summer temperatures in the past decade
Citation:
LEE, E., P. A. BEEDLOW, D. T. TINGEY, AND R. S. WASCHMANN. Nonlinear growth responses of Douglas-fir in the Pacific Northwest to summer temperatures in the past decade. Presented at World Dendro 2010, Rovaniemi, FINLAND, June 13 - 18, 2010.
Impact/Purpose:
Altered seasonal climate patterns resulting from global climate change could affect the productivity of coniferous forests in the Pacific Northwest region of North America.
Description:
Altered seasonal climate patterns resulting from global climate change could affect the productivity of coniferous forests in the Pacific Northwest region of North America. This study examined seasonal patterns of temperature, precipitation, relative humidity and plant available soil moisture in relation to Douglas-fir growth. Basal area increment (BAI) was measured from 1998 through 2008 in five naturally regenerated forest stands along an elevation gradient from the Pacific coast to 1100 m on the west slope of the Cascade Mountains. Circumference from manually read band dendrometers was used to calculate basal area increment (BAI) at approximately four-week intervals. Regression analysis and time series analysis were performed to relate BAI with plant available soil moisture (ASW), air temperature, precipitation and relative humidity. Douglas-fir BAI exhibited a curvilinear (quadratic) response to air temperature with an optimum of approximately 20° C at all sites. BAI increased rapidly in spring with increasing air temperature, rising to a peak in either May or June, followed by a decline in July when air temperatures exceeded the temperature optimum and soil moisture was declining. At the coastal and wet sites, the decline in BAI in July was likely due to limitations in air temperatures above the optimum because soil moisture was adequate for normal growth. BAI continued to decline in late summer at all sites with high temperatures and low soil moisture availability. Summer temperatures in the past decade were the warmest in the western U.S. since measurements have been recorded. During the past decade growth rates of Douglas-fir were significantly reduced in years with aberrantly high summer temperatures in comparison to cooler years. Growth response of Douglas-fir in the Pacific Northwest can be modeled as an upside-down quadratic growth response to temperature, which was also observed for white fir (D’Arrigo et al, 2004). Our findings suggest that the divergence problem in dendroclimatology observed in 20th century reconstructions is due to a nonlinear growth response, as postulated by Loehle (2009). Because summer temperatures in recent decades have reached and exceeded the temperature optimum for tree growth, tree ring chronologies that showed a positive response to temperature in the early part of the century have a less positive or even negative response to warming in the latter part of the century. Tree ring-based temperature reconstructions based on a linear approximation are likely to worsen in the future as temperatures increase.