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

Aim: Altitudinal and latitudinal treeline ecotones have not consistently responded to climate warming the in direction and/or magnitude as predicted by climate alone, suggesting that non-climatic mechanisms (e.g., biotic interactions) are also mediating treeline range dynamics. Through a collaborative research approach, we assessed environmental conditions associated with 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: Ten 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-seed dispersal granivory by insects, and viability of available seed was determined. We quantified stand density metrics, organic layer depth, and understory 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: Our findings reveal the widespread presence of insect granivory 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 greater than 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: Results from the modelled relationships between cone granivory, seed viability, and environmental conditions represent an essential step toward generalizing how and when biotic interactions across subarctic treelines influence boreal tree range dynamics before seed dispersal. Connections between granivory magnitude and site-level stand density will help establish how treeline form (e.g., discrete or diffuse) may drive patterns of future insect outbreaks under continued climate warming.

研究目的：海拔与纬度梯度的林线交错带对气候变暖的响应方向与幅度，并未完全契合仅基于气候因子的预测结果，这表明非气候驱动机制（例如生物交互作用）同样在调控林线的分布范围动态。本研究采用协作研究范式，评估了与昆虫球果食籽行为（insect cone granivory）相关的环境条件，并揭示了该生物交互作用如何调控加拿大北部以云杉为主的林线种群的繁殖潜力，进而影响其分布范围动态。
研究区域：涵盖加拿大从育空地区到纽芬兰与拉布拉多省的10处寒温带森林林线（包括冻原与高山林线）。
研究类群：白云杉（Picea glauca (Moench) Voss）、黑云杉（Picea mariana (Mill.) B.S.P.），以及Strobilomyia属、Megastigmus属物种。
研究方法：针对各林线样地，本研究首先调查了种子扩散前昆虫取食球果种子行为的发生情况与强度，并测定了现存种子的活力。随后，我们量化了各样地的林分密度指标、枯落物层厚度以及林下植被组成，并将这些变量纳入广义线性混合模型（generalized linear mixed models），以明确影响昆虫取食种子强度与现存种子活力的预测因子。
研究结果：结果显示，昆虫取食球果种子的行为在所有样地中广泛存在，但各样地的取食强度格局与苔藓盖度升高、灌丛盖度及林分密度降低显著相关。尽管所有以黑云杉为主的样地的种子活力均高于50%，但每颗球果产出的种子数量存在差异，这表明样地内的非生物条件与生物交互作用压力，会在种子扩散前限制种群向新生境的成功定植。
主要结论：本研究通过球果食籽行为、种子活力与环境条件间的模型关联所得到的结果，为归纳亚北极林线的生物交互作用如何、在何时于种子扩散前调控寒温带树木的分布范围动态，迈出了关键一步。昆虫取食种子强度与样地林分密度间的关联，将有助于阐明在气候持续变暖的背景下，林线形态（例如离散型或弥散型）如何驱动未来的昆虫暴发格局。