Forest mortality in high-elevation whitebark pine (Pinus albicaulis) forests of eastern California, USA; influence of environmental context, bark beetles, climatic water deficit, and warming

Abstract

Whitebark pine ( Pinus albicaulis Engelm.) in subalpine zones of eastern California experienced significant mortality from 2007 to 2010. Dying stands were dense (mean basal area 47.5 m2/ha), young (mean 176 years), and even-age; mean stand mortality was 70%. Stands were at low elevations (mean 2993 m), on northerly aspects, and experienced warmer, drier climates relative to the regional species distribution. White pine blister rust was not observed; mountain pine beetle infestations were extensive. Ring widths were negatively correlated with climatic water deficit and positively correlated with water-year precipitation. Although trees that survived had greater growth during the 20th century than trees that died, in the 19th century trees that eventually died grew better than trees that survived, suggesting selection for genetic adaptation to current climates as a result of differential tree mortality. Air surveys (2006–2010) in the Sierra Nevada, Mt. Shasta, and Warner Mountains showed similar trends to the intensive studies. Observed mortality from air surveys was highest in the Warner Mountains (38%) and lowest in the Sierra Nevada (5%); northern aspects at lower elevations within each mountain region had the highest probabilities of mortality and dying stands had higher climatic water deficit. Scenarios for the future of whitebark pine in California are discussed.

Pinus albicaulis Engelm. (Whitebark Pine) in Mixed-Species Stands throughout Its US Range: Broad-Scale Indicators of Extent and Recent Decline

“…Previous studies have observed >50% mortality of whitebark pine trees at smaller spatial scales [4,11,12], and our results extend this benchmark to the entire US range of whitebark pine. This level of mortality is dramatic and rare in other tree species with such broad geographic distribution and ecological importance, except among species susceptible to broad-scale fungal or insect infestations, such as chestnut blight, Dutch elm disease, and emerald ash borer, all of which have caused greater than 50% mortality in their host populations in eastern Canada [29].…”

“…This level of mortality is dramatic and rare in other tree species with such broad geographic distribution and ecological importance, except among species susceptible to broad-scale fungal or insect infestations, such as chestnut blight, Dutch elm disease, and emerald ash borer, all of which have caused greater than 50% mortality in their host populations in eastern Canada [29]. However, this shift covers less than 30 years and thus represents the recent tail end of the long-term effects of white pine blister rust, which began spreading throughout western North America in the 1940s [13], combined with more recent mortality caused by synergistic stressors such as the mountain pine beetle and climatic factors [2][3][4][5][6][7][8]. The current prevalence of dead vs. live trees in intermediate to large diameter classes (Figure 3) is consistent with a shift toward smaller size-classes observed from a probabilistic sample at a finer spatial scale in the Greater Yellowstone Ecosystem [3].…”

“…In particular, the prevalence of white pine blister rust is almost certainly under-estimated because it is difficult to identify with certainty unless the fruiting bodies of the fungal pathogen are evident. Further, mortality of whitebark pine throughout western North America has been attributed to a combination of factors-including drought, heat, white pine blister rust, the mountain pine beetle, and altered disturbance regimes [2][3][4][5][6][7][8]-and it may be difficult to reliably assign a single causal agent at plots that are measured only once every 10 years. Nonetheless, probabilistic, broad-scale sampling of whitebark pine, combined with studies of the mechanisms driving population shifts, can inform future management of this species of concern.…”

“…is a foundational tree species that has experienced dramatic declines in recent decades [1]. High mortality has been attributed to several factors, including a combination of the native mountain pine beetle, Dendroctonus ponderosae, which kills relatively large trees; the nonnative pathogen Cronartium ribicola, which causes the fatal disease white pine blister rust; altered fire regimes over the past century; and climatic variation including drought and above-normal temperatures [2][3][4][5][6][7][8]. Whitebark pine is considered a foundational species given its importance for snowpack retention [5,9] and provision of food and habitat for wildlife such as grizzly bears (Ursus arctos horribilis), Clark's nutcrackers (Nucifraga columbiana), and American red squirrels (Tamiasciurus hudsonicus) [10].…”

“…Whitebark pine is considered a foundational species given its importance for snowpack retention [5,9] and provision of food and habitat for wildlife such as grizzly bears (Ursus arctos horribilis), Clark's nutcrackers (Nucifraga columbiana), and American red squirrels (Tamiasciurus hudsonicus) [10]. Mortality rates exceeding 50% of trees have been observed in previous studies of whitebark pine at local to regional scales [4,11,12].…”