Are leaf, stem and hydraulic traits good predictors of individual tree growth? (FUN2FUN project)

A major foundation of trait-based ecology is that traits have an impact on
individual performance. However, trait-growth relationships have not been
extensively tested in trees, especially outside tropical ecosystems. In
addition, measuring traits directly related to physiological processes
(‘hard traits’) remains difficult and the differences between inter- and
intraspecific relationships are seldom explored. Here, we use
individual-level data on a set of hydraulic, leaf and stem traits to
explore which traits are the best predictors of basal area increment (BAI)
and growth efficiency (BAI per unit of tree leaf area, GE) among and
within species for six dominant tree species along a water availability
gradient under Mediterranean climate (Catalonia, NE Spain). Measured
traits include: leaf mass per area (LMA), leaf nitrogen concentration (N),
leaf C isotopic composition (d13C), stem wood density (WD), branch-level
estimates of the Huber value (Hv), the sapwood-specific hydraulic
conductivity (KS), the leaf-specific hydraulic conductivity (KL) and
resistance to xylem embolism (P50), and the leaf water potential at turgor
loss (Ptlp). Traits were better predictors of GE than BAI and significant
relationships were largely driven by differences among species means.
Contrary to our initial hypotheses, high values of both growth metrics
were associated with ‘conservative’ leaf and hydraulic traits. In
particular, BAI was negatively associated with wood density and hydraulic
efficiency per unit leaf area (KL), while GE increased with LMA,
allocation to sapwood relative to leaves (Hv) and resistance to xylem
embolism (P50). Climate effects on BAI and GE were indirectly mediated by
changes in traits, stand structure and tree size. Overall, these results
suggest that maintaining functionality over extended periods of time may
be more important that maximum gas exchange or hydraulic capacity to
achieve high radial growth under Mediterranean climates. Our study reveals
that the relationships between ‘functional’ traits and tree performance
along environmental gradients are complex and do not necessarily conform
to simple hypotheses based on our understanding of organ-level processes.
Trait integration along common axes of variation together with a
revaluation of the variables that better reflect whole-tree performance
can greatly improve our understanding of trait-growth relationships.

基于功能性状的生态学（trait-based ecology）的核心理论基础之一认为，功能性状会对个体表现产生影响。然而，性状与生长之间的关联尚未在树木类群中得到广泛验证，尤其是在热带生态系统以外的生境中。此外，直接测定与生理过程紧密相关的硬性状（hard traits）仍存在较大难度，种间与种内性状-生长关联的差异也鲜有研究。本研究利用一套涵盖水力、叶片与茎干功能性状的个体水平数据，针对西班牙东北部加泰罗尼亚地中海气候区沿水分有效性梯度分布的6个优势树种，探究种间与种内水平上，哪些性状可最佳预测断面积生长量（BAI）与生长效率（GE，即单位树木叶面积的BAI）。本次测定的功能性状包括：比叶重（LMA）、叶片氮浓度（N）、叶片碳同位素组成（δ¹³C）、树干木材密度（WD）、分枝水平估算的胡贝尔值（Hv）、边材比导水率（KS）、叶比导水率（KL）、木质部栓塞抗性（P50）以及膨压丧失时的叶水势（Ptlp）。相较于断面积生长量，功能性状对生长效率的预测效果更佳，且显著的性状-生长关联主要由物种均值间的差异驱动。与本研究初始假说相悖的是，两项生长指标的高值均与保守型叶片及水力功能性状呈正相关。具体而言，断面积生长量与木材密度、单位叶面积水力效率（KL）呈负相关；而生长效率则随着比叶重、边材相对于叶片的分配比例（Hv）以及木质部栓塞抗性（P50）的升高而提升。气候对断面积生长量与生长效率的影响，经由功能性状、林分结构以及树木个体大小的变化产生间接介导作用。总体而言，本研究结果表明，在地中海气候条件下，若要实现较高的径向生长，维持长期稳定的生理功能或许比追求最大气体交换效率或水力导水能力更为关键。本研究揭示，沿环境梯度分布的功能性状与树木表现之间的关联极为复杂，未必符合基于器官水平生理过程认知所提出的简单假说。通过整合沿共同变异轴的功能性状关联，并重新审视更能反映整树性能的指标，可大幅提升我们对性状-生长关联的认知水平。