Abstract
Climate change in semi‐arid, midlatitude mountain environments is expected to shift the spatial patterns of temperature, water availability, and vegetation upslope. Vegetation growing near its low‐elevation range limit may prove especially vulnerable to mortality and decline. We investigated the altitudinal pattern of conifer mortality that occurred from 2002 to 2004 in Southern California's San Jacinto Mountains. We found that conifer mortality was focused in the lower portion of the midmontane conifer range, which drove the midmontane conifer distribution upslope. We investigated past reports of conifer mortality in Southern California by searching historical newspaper accounts. We found evidence of previous episodes of conifer mortality that coincided with past droughts, and which may have caused vegetation redistribution in the past. We interpret the early 2000s mortality and associated vegetation redistribution as a response to natural decadal to centennial climate variability. Moreover, we hypothesize this response mode will dominate the early impact of global climate change on semi‐arid forest, which, in turn, may complicate efforts to distinguish between ecological changes attributable to natural climate variability and those attributable to global climate change.
Highlights
[33] The rates of mortality we observed are consistent with those reported for the early 2000s by other researchers working in Southern California’s mountains
[34] Midmontane conifer mortality was greatest at lower elevations (Figure 3)
This focused decline further skewed the distribution of midmontane conifers upslope (Figure 4), and led to an average 37-m rise in cover-weighted elevation (Table 2)
Summary
[2] The broad distribution of natural vegetation in California is expected to move upslope and poleward by 2100 with global climate change [Hayhoe et al, 2004; Loarie et al, 2008]. The mortality coincided with and was similar to that reported for other areas of the southwestern U.S Kelly and Goulden [2008] investigated the distribution of plants in Southern California’s Santa Rosa Mountains following this mortality, and found the cover-weighted mean elevation of the most widespread species shifted upslope by $65 m from 1977 to 2007. This redistribution was caused by the combined effect of plant mortality in the lower parts of species ranges and plant expansion in the upper parts of ranges, a pattern that resembles what may be expected with global climate change. We focused on three questions: (1) How did mortality vary with elevation? (2) How were vegetation distributions impacted? (3) Was the mortality unique in a historical context?
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