Wind Stress An Experimental Investigation into the Structure-Function Relationship of Leaf Architecture

Abstract

Can a noisy electric leaf blower turn on your science students with a multipurpose experiment investigating functional plant morphology? Will students learn and use leaf terminology and tree species identifications? Our trials with students and secondary school science teachers answer "yes" to all of the above. Plants display an amazing variety of leaf forms. At one extreme are the firm, evergreen, needle-like leaves of some conifers and at the other extreme are the large, soft, twice-compound leaves found in species such as Kentucky coffee tree, honeylocust and devil's walking stick. Evergreens are not necessarily needle-leafed, as evidenced by the familiar holly tree or southern bull bay magnolia. Deciduous leaves can be soft, such as those of cherry trees; or quite firm, as in many oak species. Some trees have very large rounded simple leaves, as found in catalpa; while the large leaf size of other trees is the result of compound structure where each leaf is composed of several leaflets. The leaves of weeping willow and Bradford pear are both simple and of similar length, but contrast markedly in shape, texture and leaf area. In the mid 1980s, biomechanist Dr. Steven Vogel presented a seminar to the Duke University Department of Botany showing the responses of leaves from various species of trees to wind stress. The height, size and leaf area of trees provide obvious competitive advantages in light capture, but these architectural features also subject trees to potential problems: blowdowns exacerbated by drag on leaf surfaces (Vogel 1996; Mayhead 1973); leaf damage caused by wind forces acting on flexible structures (Vogel 1989); and desiccation associated with ustained winds. In a wind tunnel, Vogel subjected freshly collected leaves of various tree species to wind stress. He used stop frame photography to study the role of different leaf architectures on reconfiguration in wind and to evaluate the effect of leaf morphology on drag (Vogel 1989). Vogel's experiments employed various non-leaf controls including rigid metal plates and flexible plastic sheets. Tree species with pinnately compound leaves, such as walnuts, and species with pinnately arranged simple leaves in close linear arrangements, as found in sourwood, experienced less drag than did species with distantly spaced simple leaves. However, the relationship of drag to wind speed was species-specific. In addition, simple leaves of some species such as those of tulip poplars and red maples could roll into cone-like shapes that channel wind down the leaf blade without harm. In contrast, the firm leaves typical of oaks provide a rigid structure that confers resistance to movement in low winds but that is susceptible to mechanical damage at higher wind speeds (Vogel 1993). The experiments Vogel outlined left a lasting impression because they highlighted the structure-function relationship, suggested potential biomechanical limits on leaf design, and presented an experimental approach to interpreting leaf architectures. Moreover, if students were to use such an experimental approach, they would learn different leaf arrangements (alternate versus opposite; simple versus compound; broad leaf versus needle-like) and leaf textures (soft, firm, hairy) as a by-product of conducting a rigorous experiment, rather than as an end point in a descriptive lesson on leaf terminology geared toward tree identification. In the process of formulating a specific question on leaf architecture, students must select and identify tree species from which they will collect samples. Therefore, learning tree species names and using leaf terminology are integrated into the students' experiments on leaf design. Natural selection for different leaf types undoubtedly involves many factors. The behavior of leaves in wind is likely one of those factors. Leaves with a large surface area may be efficient for photosynthesis, but if those leaves are easily damaged by wind, the plant must expend extra energy to produce replacement leaves. The goal of this exercise is to determine experimentally if particular leaf characteristics are especially resistant or vulnerable to wind stress. Careful examination of leaf architecture and learning to identify species by leaf type are two desirable by-products of a dynamic experiment on wind resistance.