Constraints on leaf structural traits in wetland plants.

A plant species' ecology is associated with leaf economics, characterized, e.g., by photosynthetic rate, construction costs, and leaf life span. Specific leaf area (SLA, leaf area per leaf dry mass) is often considered to be a key trait in this respect, explaining interspecific variation in leaf economics. To understand factors constraining the specific leaf area, we investigated size-related biomechanical constraints of the traits that determine the SLA-leaf thickness, leaf dry matter content and leaf density-and the constraints these traits exert on each other among 33 herbaceous wetland species of northern Ontario with a wide variety of leaf forms, ranging from wide laminar leaves to long, narrow, and relatively thick columnar leaves. Data from garden experiments were compared with field data. The results agree with biomechanical predictions that lamina thickness and leaf dry matter content are positively and leaf density (fresh mass per volume) negatively associated with leaf length. The traits also constrain each other, but these intertrait relationships are confounded by interspecific variation in leaf length. We conclude that for a full understanding of the adaptive significance of leaf structural design, it is essential to include leaf size in the considerations.