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

Abstract

Aim Altitudinal 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. Location In all, 10 boreal forest treelines, tundra and alpine, from Yukon to Newfoundland and Labrador, Canada. Taxa White spruce (Picea glauca [Moench] Voss), Black spruce (Picea mariana [Mill.] B.S.P.), Strobilomyia spp., Megastigmus spp. Methods Treeline 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. Results Insect 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 Conclusions The 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.

Arctic sea ice retreat fuels boreal forest advance

“…These contrasting dynamics suggest that ecotones in western Siberia will be the next to advance whereas those in northern Canada will likely remain stagnant the longest ( 10 – 12 , 14 , 15 , 32 ). Near the Barents Sea ( 17 , 18 , 20 ) and Hudson Bay ( 2 , 13 , 20 , 21 , 23 ), autumn ice was mostly absent throughout the 19th and 20th centuries ( 44 ), leaving open water effects complete. Now other factors may limit the ecotones there, particularly water stress, fire ( 2 – 5 ), edaphic conditions, and topography ( 10 – 12 ) near Hudson Bay, in addition to herbivory ( 18 , 21 ) and land use ( 2 , 26 ) near the Barents Sea.…”

“…Near the Barents Sea ( 17 , 18 , 20 ) and Hudson Bay ( 2 , 13 , 20 , 21 , 23 ), autumn ice was mostly absent throughout the 19th and 20th centuries ( 44 ), leaving open water effects complete. Now other factors may limit the ecotones there, particularly water stress, fire ( 2 – 5 ), edaphic conditions, and topography ( 10 – 12 ) near Hudson Bay, in addition to herbivory ( 18 , 21 ) and land use ( 2 , 26 ) near the Barents Sea.…”

“…Although some boreal tree species are expanding their geographic ranges at post-Pleistocene rates in Fennoscandia (9) and northwest Alaska (16), many circumarctic ecotones elsewhere are slowmoving or stagnant (2)(3)(4)(5)(6)(8)(9)(10)(11)(12)(13)(14)(15) even where temperatures exceed thresholds thought to determine tree line locations (30). Ecotone inertia appears particularly acute in eastern Siberia (3,8,(27)(28)(29) and northern Canada (2,3,(10)(11)(12)(13)(14)(15)21).…”

“…Because individual trees must pass through several size-related stages to reach sexual maturity (26), previous field studies have focused on life-history complexity and nonlinear responses to warming that could explain slow climate-driven ecotone advance. These studies show that cones and catkins often contain few viable seeds (19)(20)(21)28) at the tree line, where reproduction is sensitive to warmth and precipitation (20,28,31). Cool air temperatures (2-5, 13, 16-18, 24, 30), desiccation (15,32,33), and limited soil nitrogen (34) also hamper germination and adult growth.…”