The trade-off between leaf number and individual leaf size on current-year shoots (twigs) is crucial to light interception and thus net carbon gain. However, a theoretical basis for understanding this trade-off remains elusive. Here, we argue that this trade-off emerges directly from the relationship between annual growth in leaf and stem mass, a hypothesis that predicts that maximum individual leaf size (i.e. leaf mass, M max, or leaf area, A max) will scale negatively and isometrically with leafing intensity (i.e. leaf number per unit stem mass, per unit stem volume or per stem cross-sectional area). We tested this hypothesis by analysing the twigs of 64 species inhabiting three different forest communities along an elevation gradient using standardized major axis (SMA) analyses. Across species, maximum individual leaf size (M max, A max) scaled isometrically with respect to leafing intensity; the scaling constants between maximum leaf size and leafing intensity (based on stem cross-sectional area) differed significantly among the three forests. Therefore, our hypothesis successfully predicts a scaling relationship between maximum individual leaf size and leafing intensity, and provides a general explanation for the leaf size-number trade-off as a consequence of mechanical-hydraulic constraints on stem and leaf growth per year.

中文翻译：

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茎和叶的生长速度决定了叶的大小与数量的权衡。

在当年的枝条上，枝叶数与单个枝叶大小之间的权衡对于拦截光线并因此获得净碳增长至关重要。但是，了解这种折衷的理论基础仍然难以捉摸。在这里，我们认为这种权衡直接来自叶片的年生长量与茎质量之间的关系，该假设预测最大个体叶片大小（即叶片质量，M max或叶片面积，A max）将成负比例。等轴测图与叶子强度（即每单位茎质量，每单位茎体积或每茎横截面积的叶子数）。我们通过使用标准化长轴（SMA）分析沿海拔梯度分析了居住在三个不同森林群落中的64种树种的细枝，从而验证了这一假设。跨物种，最大单叶大小（M最大，最大值）关于叶子强度等距缩放；在三片森林之间，最大叶片尺寸和生叶强度（基于茎截面积）之间的缩放常数存在显着差异。因此，我们的假设成功地预测了最大单叶大小与成叶强度之间的比例关系，并为由于每年叶片和叶片生长受到机械液压约束而导致的叶大小数权衡提供了一般性解释。