Novel forest decline triggered by multiple interactions among climate, an introduced pathogen and bark beetles

Abstract

Novel forest decline is increasing due to global environmental change, yet the causal factors and their interactions remain poorly understood. Using tree ring analyses, we show how climate and multiple biotic factors caused the decline of whitebark pine (Pinus albicaulis) in 16 stands in the southern Canadian Rockies. In our study area, 72% of whitebark pines were dead and 18% had partially dead crowns. Tree mortality peaked in the 1970s; however, the annual basal area increment of disturbed trees began to decline significantly in the late 1940s. Growth decline persisted up to 30 years before trees died from mountain pine beetle (Dendroctonus ponderosae), Ips spp. bark beetles or non-native blister rust pathogen (Cronartium ribicola). Climate-growth relations varied over time and differed among the healthy and disturbed subpopulations of whitebark pine. Prior to the 1940s, cool temperatures limited the growth of all subpopulations. Growth of live, healthy trees became limited by drought during the cool phase (1947 -1976) of the Pacific Decadal Oscillation (PDO) and then reverted to positive correlations with temperature during the subsequent warm PDO phase. In the 1940s, the climate-growth relations of the disturbed subpopulations diverged from the live, healthy trees with trees ultimately killed by mountain pine beetle diverging the most. We propose that multiple factors interacted over several decades to cause unprecedented rates of whitebark pine mortality. Climatic variation during the cool PDO phase caused drought stress that may have predisposed trees to blister rust. Subsequent decline in snowpack and warming temperatures likely incited further climatic stress and with blister rust reduced tree resistance to bark beetles. Ultimately, bark beetles and blister rust contributed to tree death. Our findings suggest the complexity of whitebark pine decline and the importance of considering multiway drought-disease-insect interactions over various timescales when interpreting forest decline.

Monoterpene responses to interacting effects of drought stress and infection by the fungus Heterobasidion parviporum in two clones of Norway spruce (Picea abies)

“…Regional-scale vegetation mortality events appear to be increasing in a variety of biomes throughout Earth, and are frequently associated with increased temperatures, droughts, and often (but not always) with outbreaks of biotic agents, such as insects and pathogens (Allen et al, 2010;Das et al, 2016;Wong and Daniel, 2017). The physiological mechanisms underlying tree mortality involve the water and carbohydrates storage and transport systems (McDowell et al, 2011).…”

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

“…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].…”

“…Given the widespread nature of the mortality agents [5,6,13], it is reasonable to assume that such mortality has occurred throughout whitebark pine's geographic range, but information for range-wide quantitative assessments is limited. Most studies have used purposive sampling at whitebark pine-dominated sites, with notable exceptions that included subalpine fir-dominated sites [14], conducted aerial censuses of entire watersheds [12], or conducted probabilistic sampling across a wide variety of environments [3].…”

Separating Trends in Whitebark Pine Radial Growth Related to Climate and Mountain Pine Beetle Outbreaks in the Northern Rocky Mountains, USA

“…Dry conditions have previously been associated with mountain pine beetle outbreaks [53,54]. Continued periods of drought will likely stress whitebark pines and make them more susceptible for mountain pine beetle attack and white pine blister rust infection [19].…”

“…The overlapping effects of mountain pine beetle outbreaks and blister rust will likely intensify the overall decline of whitebark pine. The effects of climate change are important when assessing the resiliency of whitebark pine ecosystems and their potential to recover from mountain pine beetle outbreaks and other landscape-level disturbances [19]. Consequently, an urgency exists to understand the influence of climate change on the severity of mountain pine beetle outbreaks and white pine blister rust infections.…”