Widespread mortality of trembling aspen (Populus tremuloides) throughout interior Alaskan boreal forests resulting from a novel canker disease

Abstract

Over the past several decades, growth declines and mortality of trembling aspen throughout western Canada and the United States have been linked to drought, often interacting with outbreaks of insects and fungal pathogens, resulting in a “sudden aspen decline” throughout much of aspen’s range. In 2015, we noticed an aggressive fungal canker causing widespread mortality of aspen throughout interior Alaska and initiated a study to quantify potential drivers for the incidence, virulence, and distribution of the disease. Stand-level infection rates among 88 study sites distributed across 6 Alaska ecoregions ranged from <1 to 69%, with the proportion of trees with canker that were dead averaging 70% across all sites. The disease is most prevalent north of the Alaska Range within the Tanana Kuskokwim ecoregion. Modeling canker probability as a function of ecoregion, stand structure, landscape position, and climate revealed that smaller-diameter trees in older stands with greater aspen basal area have the highest canker incidence and mortality, while younger trees in younger stands appear virtually immune to the disease. Sites with higher summer vapor pressure deficits had significantly higher levels of canker infection and mortality. We believe the combined effects of this novel fungal canker pathogen, drought, and the persistent aspen leaf miner outbreak are triggering feedbacks between carbon starvation and hydraulic failure that are ultimately driving widespread mortality. Warmer early-season temperatures and prolonged late summer drought are leading to larger and more severe wildfires throughout interior Alaska that are favoring a shift from black spruce to forests dominated by Alaska paper birch and aspen. Widespread aspen mortality fostered by this rapidly spreading pathogen has significant implications for successional dynamics, ecosystem function, and feedbacks to disturbance regimes, particularly on sites too dry for Alaska paper birch.

Determining the novel pathogen Neodothiora populina as the causal agent of the aspen running canker disease in Alaska

“…SAD was reported to not affect saplings (Worrall et al 2008). The aspen running canker in Alaska is most destructive on saplings in the understory of intermediate-aged and mature stands, but is nearly absent from younger aspen growing in early-successional stands (Ruess et al 2021). Aspen running canker has been observed primarily on live Fig.…”

“…standing trees with turgid bark and recently dead trees with conspicuous sunken cankers showing retention of fine terminal twigs in canopies. These symptoms represent very recent mortality (1-2 years) and rarely display the branch dieback and reduced-canopy typical of decline symptoms (personal observations, Ruess et al 2021). In SAD, mature trees show extensive branch dieback while active cankers are generally not observed (Marchetti et al 2011); insects and diseases were reported to be minor secondary contributors to droughtassociated mortality.…”

“…Large, naturally occurring cankers (10-700 cm) on live trees are widespread (Ruess et al 2021) and can be difficult to detect as the lesions are frequently high in the tree and often display little discernible bark discolouration as the inner bark is killed (Fig. 1b).…”

“…1c) elliptical. Inventory data show that lesions can girdle and kill trees within a single season (Ruess et al 2021); however, the latency period of wild cankers is unknown. The disease is called 'aspen running canker' because cambium and phloem of aspen trees are rapidly killed as the diffuse lesions expand along nearly the entire bole and eventually girdle the tree (Fig.…”

“…The result of the canker is whole-tree mortality with neither noticeable branch dieback nor slowly declining health. A recent study of 16 576 aspen trees found the canker in 82% of 88 plots within six Alaskan ecoregions (Ruess et al 2021). Canker incidence was higher in smaller trees, particularly those within intermediate-aged to mature stands, wherein lesions can girdle and kill trees within a single season.…”