Premature losses of leaf area in response to drought and insect herbivory through a leaf lifespan gradient

Implications of the differences in leaf life span are still subject to debate in the field of ecophysiology.Since leaf traits associated with these differences may be decisive for determining the distribution of tree species,this topic is particularly relevant in the context of climate change.This study analyzes the effects of the differences in leaf life span on premature losses of leaf area owing to insect herbivory and to abiotic stress.Loss of leaf area may be an important determinant of total leaf carbon assimilation.Seven Mediterranean tree species,distributed on four sites with different climates were studied.The species exhibited strong differences in leaf life span and in leaf traits,especially leaf mass per unit area.Premature leaf area losses were estimated in response to insect herbivory and summer drought over two years.The results revealed that,despite having older leaf cohorts with more damage,species with longer leaf duration had lower area lost to herbivores and less damage due to accelerated senescence during the summer drought.With respect to the predicted increase in water stress,deciduous species are at a disadvantage due to their high premature loss of leaf area and thus loss of photosynthetic capacity.

Ontogenetic transition in leaf traits: a new cost associated with the increase in leaf longevity

Aims Recent work has identified a worldwide‘economics’spectrum of correlated leaf traits that mainly reflects the compromises between maximizing leaf longevity and short-term productivity.However,during the early stages of tree growth different species tend to exhibit a common strategy,because competition for soil water and nutrients forces the maximization of short-term productivity owing to the need for rapid growth during the most vulnerable part of the tree’s life cycle.Accordingly,our aim here was to compare the variations that occur during ontogeny in the different leaf traits(morphology and leaf chemical composition)of several coexisting Mediterranean woody species differing in their leaf life spans and to test our hypothesis that tree species with a long leaf life span should exhibit larger shifts in leaf characteristics along ontogeny.Methods Six Mediterranean tree species differing in leaf life span,selected from three plots located in central-western Spain,were studied during three growth stages:seedlings,juveniles and mature trees.Leaf life span,leaf morphology(leaf area,dry weight,thickness and mass per unit area)and chemical composition(N and fibre con-centrations)were measured in all six species.The magnitude of the ontogenetic changes in the different traits was estimated and related to the mean leaf longevity of the different species.Important Findings Along ontogeny,strong changes were observed in all variables analysed.The early growth stages showed lower leaf thickness,leaf thickness and mass per unit area and N,cellulose and hemi-cellulose concentrations than mature trees,but a higher lignin content.However,these changes were especially marked in species with a longer leaf life span at maturity.Interspecific dif-ferences in leaf life span,leaf morphology and chemical com-position were stronger at the mature stage than at the seedling stage.We conclude that greater plasticity and more intense strat-egy shifts along ontogeny are necessarily associated with long leaf life span.Our results thus provide a new aspect that should be incorporated into the analysis of the costs and benefits associ-ated with the different strategies related to leaf persistence dis-played by the different species.Accordingly,the intensity of the alterations in leaf traits among different growth stages should be added to the suite of traits that change along the leaf economics spectrum.

3种地中海橡树在叶脉和气孔性状上的个体发育性变化及其与叶片经济性状的关系

我们比较了3个地中海栎属(Quercus)树种幼苗和成龄树的叶脉和气孔性状。之前的研究表明,幼苗的气体交换率往往比成龄树的要高。我们的研究目标是确认叶脉和气孔的性状在整个植物个体发育过程中是否会随着气体交换率的变化而同步改变。我们提出了以下两个备选假设:幼苗比成龄树有着更高的叶脉和气孔密度(假设1);幼苗在维管组织上的投资更少,从而降低了建设成本(假设2)。本研究是在西班牙中西部进行的,我们对每个树种不同生长阶段的植株随机采集了10个样本。我们测量了气孔和叶脉的平均性状(单位叶面积里的气孔大小和数量、叶脉密度、叶脉体积、叶脉到表皮的距离),比叶质量和叶片厚度。研究结果表明,细脉密度和单位面积叶脉体积随着树龄的增加而增加,这似乎与气体交换率方面的个体发育趋势不一致。这种差异支持了我们的假设2,说明植株在幼苗阶段减少对维管组织的投资以最大限度地提高生长速度是其当务之急。幼苗叶片中较大的叶脉间距可以由叶脉到表皮的较短距离所补偿。因此,薄叶片可能是幼苗的一个必要性状,它可以让水分沿较短路径输送到蒸发点,而无需对高成本的维管组织进行大量投资。