Thresholds for warming‐induced growth decline at elevational tree line in the Yukon Territory, Canada

Abstract

A few tree ring studies indicate recent growth declines at northern latitudes. The precise causes are not well understood. Here we identify a temperature threshold for decline in a tree ring record from a well‐established temperature‐sensitive site at elevational tree line in northwestern Canada. The positive ring width/temperature relationship has weakened such that a pre‐1965 linear model systematically overpredicts tree ring widths from 1965 to 1999. A nonlinear model shows an inverted U‐shaped relationship between this chronology and summer temperatures, with an optimal July–August average temperature of 11.3°C based on a nearby station. This optimal value has been consistently exceeded since the 1960s, and the concurrent decline demonstrates that even at tree line, trees can be negatively affected when temperatures warm beyond a physiological threshold. If warming continues without significant gains in effective precipitation, the large‐scale greening of recent decades could be replaced by large‐scale browning. Such browning could slow or reverse carbon uptake by northern forests.

Evidence for a weakening relationship between interannual temperature variability and northern vegetation activity

“…The Palmer Drought Severity Index (PDSI), which is defined from a water-balance model forced with climate data, shows a significant increase in drought over the NH in the last several decades, and particularly in southern Europe and eastern Asia and Siberia 9 . Increased drought stress can modify the response of plant growth to temperature change 7 and was hypothesized to be a potential cause for the declined temperature sensitivity of tree growth in tree rings time series during the last part of the twentieth century 10 . The effect of climate drought on the temperature sensitivity of vegetation growth, however, may be partly alleviated by CO 2 -induced increase in vegetation water use efficiency 11,12 .…”

Non-stationary Responses of Tree-Ring Chronologies and Glacier Mass Balance to Climate in the European Alps

“…These findings emphasize how the same climatic conditions drive nonproportional variations in glaciers' mass variations and tree-ring growth, especially in the years when extreme meteorological conditions induce strong variations. The different responses to climate could be related to the fact that these two climatic proxies respond to common but also different climate variables and to the fact that high-altitude trees have threshold temperatures above which they may not respond linearly (D'Arrigo et al, 2004). In particular, we found that tree rings mainly responded to July temperature and glacier mass balance to JJA temperature (Figs.…”

Growth responses of Norway spruce (Picea abies (L.) Karst.) to the climate in the south-eastern part of the Českomoravská Upland (Czech Republic)

“…4). Warm temperatures during the growing season may induce moisture stress and decrease tree growth (Barber et al, 2000;D'Arrigo et al, 2004;Miyamoto et al, 2010;Aakala and Kuuluvainen, 2011). High temperatures increase evapotranspiration from soils and plant tissues, inducing stomatal closure and a decreased net photosynthesis to minimize water losses in response to an increasing moisture stress (Kozlowski, 2002).…”