Short- and long-term growth characteristics associated with tree mortality in southwestern mixed-conifer forests

Abstract

Continued increases in global temperatures and incidences of drought have been implicated in elevated tree mortality in many regions, prompting interest in better understanding tree mortality processes. A recent extreme drought in the southwestern U.S. (1996–2003) contributed to elevated tree mortality throughout the region. We used this event to investigate the relationship of short- and long-term tree growth characteristics to recent (1996–2008) tree mortality in the mixed-conifer forests in northern Arizona. We compared radial growth characteristics over a 50-year period between paired live and recently dead white fir (Abies concolor (Gordon & Glend.) Lindl. ex Hildebr.), limber pine (Pinus flexilis E. James), trembling aspen (Populus tremuloides Michx.), and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). We found that (i) dead trees of all species typically had lower mean growth rates than live trees, (ii) dead trees of most species had a greater coefficient of variation in growth over long time periods (>20 years) than live trees, (iii) dead aspen and dead Douglas-fir trees had larger negative growth trends than live trees for some time periods, (iv) dead trees of most species had larger numbers of abrupt growth declines than live trees, and (v) a combination of short- and long-term growth characteristics distinguished live and dead trees, with greater importance of short-term growth for aspen, long-term growth for limber pine, and a mix of short- and long-term growth for white fir and Douglas-fir. These results strongly suggest that recent tree mortality in southwestern mixed-conifer forests is caused by a mixture of short- and long-term processes.