Recent climatic drying leads to age-independent growth reductions of white spruce stands in western Canada

5 years ago

Recent climatic drying leads to age-independent growth reductions of white spruce stands in western Canada

Michael Michaelian, Edward H. Hogg, Michael E. Undershultz, Trisha I. Hook

Since 2001, climatic conditions have been notably drier than normal across large areas of the western Canadian interior, leading to widespread impacts on the forests of this region. This poses a major concern for the future, given climate change projections for continued warming and drying. We conducted tree-ring analysis in 75 pure stands of white spruce (Picea glauca) across Alberta and west-central Saskatchewan to examine the effects of recent climatic drying on the growth of this important boreal tree species. Allometric equations were used to calculate annual growth in aboveground tree biomass (GBM) from ring width measurements. Results showed an increasing trend in GBM from the 1960s to the 1990s, followed by a sharp decline during the severe drought of 2001–2002. Of the 75 stands, only 18 recovered sufficiently to cause an increase in mean GBM from the predrought decade of 1991–2000 to the subsequent decade of 2001–2010. The remaining 57 stands exhibited a decline in mean GBM between these decades. Climatic drying was a major cause of the growth decline, as shown by the significant stand-level relationship between percentage change in decadal mean GBM and the change in decadal mean values of a climate moisture index from 1991–2000 to 2001–2010. Subsequent analyses of boreal stands sampled across Alberta during 2015 revealed that white spruce growth had declined even further as drought conditions intensified during 2014–2015. Overall, there was a 38% decrease in mean GBM between 1997 and 2015, but surprisingly, the percentage decrease was not significantly different for young, productive stands compared with older, less productive stands. Thus, stand ageing cannot explain the observed decline in white spruce growth during the past quarter century, suggesting that these forests are at risk if the trend towards more frequent, severe drought continues in the region. Annual change in (a) average increment of aboveground tree biomass and (b) the climate moisture index during 1960–2015 for 40 white spruce stands sampled in 2015 across northern and west-central Alberta. Average values are also shown for the youngest and oldest stands within each of 10 study areas. Vertical lines denote drought years having negative average values of the climate moisture index for these 10 study areas.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/gcb.13795