Structural defence is coupled with the leaf economic spectrum across saplings of spiny species

Given that the rate of resource capture constrains plant growth and defence, understanding the linkage between the leaf economic spectrum (LES) and defence and how it contributes to growth is central to predicting species performance. In spite of the prevalence of spiny plants in many plant communities, little is known about how the LES relates to defence and growth rate across these species. We grew 42 spiny species, from diverse environments, under common garden conditions for 15 weeks and measured LES (leaf N, SLA and assimilation rate), defence and growth traits. We assessed general relationships between LES and growth rate and tested whether structural defences (spines, leaf fibre and lignin content) and quantitative chemical defences (condensed tannins) are linked to the LES and growth and if different spine types (i.e. leaf spines, stipular spines, prickles and thorns), with distinct anatomical origins, partition out across the LES. We observed two independent trait axes that together explained ~68% of trait variation across species. The first axis showed that structural defences (spines, leaf fibre and lignin content) trade off with leaf productivity along the LES. Axis 2 revealed that condensed tannins is orthogonal and less integrated with the LES-structural defence axis. Bivariate trait analyses disclosed positive covariations between LES traits and sapling growth rate. All structural defence traits were negatively related to sapling growth. Across spine types, species with leaf spines were associated with the conservative end of the LES, characterized by high structural defences and lower leaf productivity relative to other spine types. Our study shows that the LES and structural defences are coupled in spiny species such that constitutive growth – defence strategies range from fast-growing species with low allocation to defences to slow-growing species that invest heavily in structural defences (dominated by leaf spiny species).资源获取速率是制约植物生长与防御的关键因素，解析叶经济谱（leaf economic spectrum, LES）与防御策略的关联及其对植物生长的贡献，是预测物种表现的核心研究方向。尽管多刺植物在众多植物群落中广泛分布，但目前针对这类物种的叶经济谱与防御、生长速率之间的关联机制仍鲜有研究。我们选取来自不同生境的42种多刺植物，在同质园栽培条件下开展了为期15周的栽培实验，并测定了其叶经济谱相关性状（叶片氮含量、比叶面积（specific leaf area, SLA）与光合速率）、防御性状及生长性状。我们评估了叶经济谱与生长速率之间的总体关联，并检验了两个科学问题：一是结构防御性状（刺、叶片纤维与木质素含量）与定量化学防御性状（缩合单宁（condensed tannins））是否与叶经济谱和生长速率存在关联；二是具有不同解剖起源的不同刺类型（即叶刺、托叶刺、皮刺与茎刺）是否在叶经济谱维度上存在分布分化。研究结果显示存在两个独立的性状轴，二者共同解释了跨物种约68%的性状变异。第一性状轴表明，沿叶经济谱梯度，结构防御性状与叶片生产力存在权衡关系。第二性状轴则显示，缩合单宁与叶经济谱-结构防御性状轴呈正交关系，且二者的整合度较低。双变量性状分析结果显示，叶经济谱相关性状与幼苗生长速率呈显著正相关；而所有结构防御性状均与幼苗生长速率呈显著负相关。在不同刺类型的物种中，具叶刺的物种对应叶经济谱的保守端，相较于其他刺类型物种，其结构防御投入更高、叶片生产力更低。本研究证实，多刺植物的叶经济谱与结构防御性状存在耦合关系，由此形成的组成型生长-防御策略谱系覆盖了从低防御投入的速生物种，到高结构防御投入的慢生物种（该类群以具叶刺的物种为主）。