Host–parasite distributions under changing climate:Tsuga heterophyllaandArceuthobium tsugensein Alaska

Abstract

Dwarf mistletoes ( Arceuthobium species) influence many processes within forested ecosystems, but few studies have examined their distribution in relation to climate. An analysis of 1549 forested plots within a 14.5 million ha region of southeast Alaska provided strong indications that climate currently limits hemlock dwarf mistletoe ( Arceuthobium tsugense (Rosendahl) G.N. Jones) to a subset of the range of its primary tree host, western hemlock ( Tsuga heterophylla (Raf.) Sarg.), with infection varying from a high of 20% of trees at sea level to only 5% by 200 m elevation. Three types of modeling approaches (logistic, most similar neighbors, and random forests) were tested for the ability to simultaneously predict abundance and distribution of host and pathogen as a function of climate variables. Current distribution was explained well by logistic models using growing degree-days, indirect and direct solar radiation, rainfall, snowfall, slope, and minimum temperatures, although accuracy for predicting A. tsugense presence at a particular location was only 38%. For future climate scenarios (A1B, A2, and B1), projected increases for A. tsugense habitat over a century ranged from a low of 374% to a high of 757%, with differences between modeling approaches contributing more to uncertainty than differences between climate scenarios.

Consequences of climate change for biotic disturbances in North American forests

“…To address this knowledge gap we reviewed the primary literature to determine the most important climate metrics associated with historical outbreaks of the insects and pathogens presented in Tables 2 and 3. Our synthesis included studies that tested for: (1) spatial or temporal coincidence or correlation of outbreak/disease symptoms with climate event or trend (e.g., Greenbank 1963, Breshears et al 2005, Barrett et al 2012) and/or (2) concordance of population fluctuations with process-based models of physiologically explicit mechanisms linking climate and population dynamics (e.g., Tran et al 2007, Powell andBentz 2009). We identified 79 studies from 1950 to 2012 that satisfied the criteria; three of the 79 studies evaluated climate effects on more than one insect species and most (66% or 52) of the studies were focused on insects rather than pathogens.…”

From canopy to seed: Loss of snow drives directional changes in forest composition

“…(Carr) (Sitka spruce) are not considered sensitive to snow loss (Buma & Barrett, ) and may increase in dominance when C. nootkatensis fails to regenerate (Oakes et al, ). Yet, despite extensive research on the drivers of decline (Barrett, Latta, Hennon, & Eskelson, ; Buma et al, ; Hennon et al, ; Hennon, Hansen, & Shaw, ; Hennon & Shaw, ; Hennon, Shaw, & Hansen, ; Schaberg et al, ), little is known about C. nootkatensis regeneration following canopy mortality, and the fate of C. nootkatensis and long‐term dynamics of affected forests remain unknown.…”

A framework to evaluate climate effects on forest tree diseases

“…interaction of slope, insolation and elevation on water stress and frost exposure (Barrett et al, 2012) or fire extirpating and delayed reestablishment of pathogens with dispersal slower than that of host trees, as in the case of some dwarf mistletoes (Hawksworth & Wiens, 1996).…”

“…Very short-term weather factors such as humidity and leaf wetness can determine the success of a pathogen's reproduction and infection for many foliar diseases. Changes in disease expression for pathogens that cause chronic infections such as root diseases (Klopfenstein et al, 2009), stem decays and dwarf mistletoes (Barrett et al, 2012) may show a relationship with climate on the order of decades or longer. Climate as a primary driver in disease could be considered confirmed when most or all of the factors listed above have been evaluated at spatial and temporal scales and they consistently support a particular climate effect stated in the climate-disease hypothesis.…”