Understanding how functional traits affect plant performance and fitness is a key step in unravelling the role of natural selection in shaping the evolutionary trajectory of populations. We examined early-age selection acting on leaf traits via their effects on growth performance and fitness, measured in trees planted in a common-garden field trial embedded in a reforestation planting in Tasmania, Australia. We focused on two important leaf traits - stomatal length and specific leaf area (SLA) - measured two years after planting, and compared interplanted groups originating from dry and wet home-site climates, with the trial site having intermediate long-term mean annual rainfall. Two-year height growth was used as the performance attribute, and the time-averaged tree survival over the subsequent six years as the fitness component. There was evidence for performance-based selection on the leaf traits, with the strength and form of selection depending on the trait and climate group being considered. In this sense, selection in the dry group operated mainly on stomatal length where a combination of directional (favouring longer stomata) and stabilizing selection was detected, whereas selection in the wet group acted only on SLA and was purely stabilizing. Estimates of performance-based correlational selection were not statistically significant. For both climate groups, estimates of fitness-based selection gradients provided evidence for significant directional (but not quadratic) selection on height performance, favouring individuals with faster growth, but did not indicate statistical support for direct effects of the leaf traits on tree survival, conditional on measured performance. These results validated qualitative inferences of selection from the performance-based analysis, and suggested that selection on the leaf traits appeared to be mediated by their effects on early-age height performance, which in turn directly influenced later-age survival. We discuss the mechanisms by which the focal traits may have affected height performance, and likely factors contributing to the different patterns of phenotypic selection observed in the two groups experiencing the same environment. We also provide expressions of analytical derivatives that were developed for the estimation of selection gradients based on a logistic regression model relating a binary fitness response to linear and nonlinear covariate terms for the target regressor variables.

中文翻译：

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基于性能的气孔长度和比叶面积选择的推断随林木卵形桉树原产地气候的变化而变化

了解功能性状如何影响植物的性能和适应性是揭示自然选择在塑造种群进化轨迹中的作用的关键一步。我们研究了早期选择对叶子性状的影响，通过它们对生长性能和适应性的影响，对澳大利亚塔斯马尼亚重新造林种植中的普通花园田间试验中种植的树木进行了测量。我们重点关注两个重要的叶子性状——气孔长度和比叶面积（SLA）——在种植两年后进行测量，并比较了源自干燥和潮湿家乡气候的套种组，试验地点具有中等长期平均年降雨量。两年的高度增长被用作性能属性，随后六年的时间平均树木存活率被用作适应性成分。有证据表明对叶片性状进行基于性能的选择，选择的强度和形式取决于所考虑的性状和气候组。从这个意义上说，干组中的选择主要作用于气孔长度，其中检测到定向（有利于更长的气孔）和稳定选择的组合，而湿组中的选择仅作用于SLA并且纯粹是稳定的。基于绩效的相关选择的估计没有统计学意义。对于两个气候组，基于适应度的选择梯度的估计提供了对高度表现的显着定向（但不是二次）选择的证据，有利于生长较快的个体，但没有表明叶片性状对树木生存的直接影响的统计支持，以测量的性能为条件。这些结果验证了基于性能分析的选择定性推论，并表明对叶片性状的选择似乎是通过其对早期身高性能的影响来介导的，而这反过来又直接影响后期的生存。我们讨论了焦点性状可能影响身高表现的机制，以及导致在经历相同环境的两组中观察到的不同表型选择模式的可能因素。我们还提供了分析导数的表达式，这些表达式是为基于逻辑回归模型估计选择梯度而开发的，该逻辑回归模型将二元适应度响应与目标回归变量的线性和非线性协变量项相关联。