Insect seed and cone predation reduces reproductive potential of treeline conifers across northern Canada

AimAltitudinal and latitudinal treeline ecotones have not consistently responded to climate warming in the direction and/or magnitude predicted by climate alone, suggesting that non-climatic mechanisms (e.g. biotic interactions) also mediate treeline range dynamics. Through a collaborative research approach, we assessed environmental conditions associated with pre-dispersal insect cone granivory and how this biotic interaction may govern the reproductive potential, and therefore range dynamics, of spruce-dominated treelines across northern Canada. LocationIn all, 10 boreal forest treelines, tundra and alpine, from Yukon to Newfoundland and Labrador, Canada. TaxaWhite spruce (Picea glauca [Moench] Voss), Black spruce (Picea mariana [Mill.] B.S.P.), Strobilomyia spp., Megastigmus spp. MethodsTreeline sites were assessed for presence and magnitude of pre-dispersal seed granivory by insects along with viable seed availability. We quantified stand density metrics, organic layer depth and understorey vegetation composition at each location and, subsequently, incorporated those variables into generalized linear mixed models to establish predictors of granivory magnitude and viability of available seed. ResultsInsect granivory was widespread across sites; however, site-specific patterns of granivory were associated with increased moss cover and decreased shrub cover and stand density. While all black spruce-dominated sites exhibited seed viability rates > 50%, the number of seeds produced per cone varied, suggesting that within-site abiotic conditions and biotic interaction pressures limit successful colonization of novel environments in advance of seed dispersal. Main ConclusionsThe modelled relationships between granivory, seed viability and environmental conditions represent an essential step towards generalizing how and when biotic interactions across subarctic treelines influence boreal tree range dynamics before seed dispersal. Connections between granivory magnitude and site-level treeline characteristics (e.g. stand density, understorey vegetation) will provide a more comprehensive understanding of treeline range dynamics under continued climate change.