Pest defenses under weak selection exert a limited influence on the evolution of height growth and drought avoidance in marginal pine populations

Whilst droughts, intensified by climate change, have been affecting forests worldwide, pest epidemics are a major source of uncertainty for assessing drought impacts on forest trees. Thus far, little information has documented the adaptability and evolvability of traits related to drought and pests simultaneously. We conducted common-garden experiments to investigate how several phenotypic traits (i.e., height growth, drought avoidance based on water-use efficiency inferred from δ13C, and pest resistance based on defense traits) interact in five mature lodgepole pine populations established in four progeny trials in western Canada. The relevance of interpopulation variation in climate sensitivity highlighted that seed-source warm populations had greater adaptive capability than cold populations. In test sites, warming generated taller trees with higher δ13C and increased the evolutionary potential of height growth and δ13C across populations. We found, however, no pronounced gradient in defenses and their evolutionary potential along populations or test sites. Response to selection was weak in defenses across test sites, but high for height growth, particularly at warm test sites. Response to selection of δ13C varied depending on its selective strength relative to height growth. We conclude that warming could promote the adaptability and evolvability of growth response and drought avoidance with limited evolutionary influence from pest (biotic) pressures.

尽管气候变化加剧的干旱已在全球范围内影响森林，但病虫害流行仍是评估干旱对林木影响时的主要不确定性来源。迄今为止，鲜有研究同时记录与干旱和病虫害相关的性状的适应性与演化潜力。本研究开展同质园实验（common-garden experiments），以探究加拿大西部4个子代测定试验（progeny trials）中定植的5个成熟扭叶松（lodgepole pine）种群的多项表型性状（phenotypic traits）间的互作关系——这些性状包括高生长、基于δ13C推断的水分利用效率的避旱性，以及基于防御性状的抗病虫害能力。种群间气候敏感性变异的相关性分析表明，暖气候种源种群相较于冷气候种源种群具备更强的适应性。在试验样地中，增温处理使林木株高更高、δ13C值更高，并提升了各种群高生长与δ13C的演化潜力。但本研究未发现防御性状及其演化潜力沿种群梯度或试验样地呈现显著分布规律。各试验样地中防御性状的选择响应较弱，而高生长的选择响应较强，尤其在暖气候试验样地中表现显著。δ13C的选择响应则取决于其相较于高生长的选择强度大小。本研究结论表明，尽管病虫害（生物）胁迫带来的演化影响有限，但增温可促进生长响应与避旱性的适应性与演化潜力。