Variations in leaf economics spectrum traits for an evergreen coniferous species: Tree size dominates over environment factors

Many leaf traits strongly vary with tree size and environmental factors, but the importance of these factors to intraspecific variations of leaf traits in forest trees have rarely been simultaneously evaluated.
We measured needle longevity and specific leaf area (SLA) and nitrogen (N) content of every needle age (0 to 4 year old) for 65 individuals with 0.3-100 cm diameter at breast height (DBH) for an evergreen coniferous species, Pinus koraiensis Sieb. et Zucc., in Northeast China. We simultaneously evaluated effects of tree size (DBH or tree height) and environment factors (light intensity, soil N content and water availability) on the needle longevity, SLA, foliage N content as well as the slopes of regressions of SLA and foliage N content against needle age.
All of the studied leaf traits and slopes of regressions of SLA and foliage N content against needle age were significantly related to tree size. Tree height had a greater impact on SLA and area-based leaf N content (Narea), whereas DBH was more important for needle longevity and mass-based leaf N content (Nmass). The environment variables, light intensity, soil N content and water availability, were rather minor factors for trait variations compared with tree size. Significant influences of light intensity were found only on needle longevity, and soil N and water availability had no effects on the leaf traits.
Our study clearly showed that tree size is an important driver of intraspecific variations in the key leaf traits of Pinus koraiensis in a natural forest. We also emphasize the importance of DBH or tree height varies depending on leaf traits, suggesting various mechanisms of size effects on the intraspecific variations in leaf traits. We suggest that ecological significance of leaf trait variations needs reconsideration incorporating tree size effect.

Methods
The original data on functional traits, environment factors (light, soil nutrient and soil water) were collected at individual scale for an evergreen coniferous species within a wide size range (0.3-100 cm DBH) in northeast China.

诸多叶片功能性状均随树木个体大小与环境因子发生显著变化，但目前鲜有研究同时评估这两类因子对林木叶片性状种内变异的相对重要性。
本研究针对中国东北部的常绿针叶树种红松（Pinus koraiensis Sieb. et Zucc.）的65株个体（胸径（diameter at breast height, DBH）0.3~100 cm），测定了不同针叶龄级（0至4年生）的针叶寿命、比叶面积（specific leaf area, SLA）以及氮（N）含量；同时评估了树木大小因子（DBH或树高）与环境因子（光照强度、土壤氮含量及水分可利用性）对针叶寿命、SLA、叶片氮含量，以及SLA和叶片氮含量随针叶龄级变化的回归斜率的影响。
所有测定的叶片性状，以及SLA、叶片氮含量随针叶龄级变化的回归斜率，均与树木大小显著相关。树高对SLA和单位面积叶氮含量（area-based leaf N content, Narea）的影响更为显著，而胸径则对针叶寿命及单位质量叶氮含量（mass-based leaf N content, Nmass）的影响更为突出。相较于树木大小，光照强度、土壤氮含量与水分可利用性等环境因子对性状变异的贡献相对较小：仅光照强度对针叶寿命存在显著影响，土壤氮含量与水分可利用性未对叶片性状产生显著作用。
本研究明确证实，树木大小是天然林中红松关键叶片性状种内变异的重要驱动因子。同时，本研究强调胸径或树高的相对重要性因叶片性状而异，这表明树木大小对叶片性状种内变异的影响存在多种作用机制。本研究提出，在重新考量叶片性状变异的生态学意义时，需纳入树木大小的影响效应。

研究方法
本研究以中国东北部胸径范围为0.3~100 cm的常绿针叶树种为研究对象，在个体尺度上收集了其功能性状、环境因子（光照、土壤养分与土壤水分）的原始数据。