Direct and indirect impacts of environmental conditions on leaf consumers of boreal shrubs in interior Alaska, and implications for climate change

Background/Question/Methods

One of the biggest unknowns in predicting the impact of climate change is the extent to which effects on populations are likely to be direct (e.g., driven by temperature or precipitation) versus indirect (e.g., driven by changes in interactions with other species). Interior Alaska has experienced increases in summer and winter temperatures, increases in snow fall, and decreases in annual precipitation in the past few decades. We evaluated whether environmental conditions or host characteristics better explained  damage by herbivores and pathogens on two understory boreal shrubs, Alnus fruticosa (alder) and Vaccinium vitis-idaea (cranberry or lingonberry). Alders fix >70% of the nitrogen found in boreal soils, and cranberries are a key food source for animals, including humans.  We selected twenty-one sites to maximize the range of summer temperatures (driven primarily by slope and aspect) and winter temperatures (driven by elevation and associated temperature inversions).  We measured foliar damage between 2002 and 2008, which included the hottest summer and the third coldest summer on record. We estimated damage by five insect guilds and foliar pathogens, and measured leaf morphological and chemical variables. 

Results/Conclusions

Year explained more of the variation in damage than site for alder, but the reverse was true for cranberry. Damage was higher in warmer sites for cranberry but there was no trend for alder.  Damage by phloem-feeding insects was negatively correlated with hot and dry summer conditions for alder, and with minimum winter temperatures for alder and cranberry.  Chewing and mining damage were negatively correlated with below-snow winter temperatures for both alder and cranberry, and greater under higher relative humidity for alders. Pathogens damage was highest on drought-stressed plants in hot years for alder, and following cold winters for both alder and cranberry. Results suggest that overwintering survival and summer desiccation drive insect damage, while plant water stress drives pathogen damage and damage by phloem feeding insects. We find evidence for both direct effects of environmental conditions on leaf consumers and indirect effects mediated through plant foliar conditions, with stronger direct impacts for herbivores, and stronger indirect impacts on pathogens. If the climatic trends seen in Interior Alaska recently continue, net changes in damage by leaf consumers may be small due to opposing effects of summer and winter changes for herbivores, and opposing impacts on herbivores and pathogens.