Small trees, big problems: Comparative leaf function under extreme edaphic stress

PREMISE OF THE STUDY: The pygmy forest, a plant community of severely stunted conifers and ericaceous angiosperms, occurs on patches of highly acidic, nutrient-poor soils along the coast of Northern California, USA. This system is an excellent opportunity to study the effect of severe nutrient deficiency on leaf physiology in a naturally-occurring ecosystem. In this study, we seek to understand the physiological mechanisms stunting the plants' growth and their implications for whole plant function. METHODS: We measured 14 traits pertaining to leaf photosynthetic function or physical structure on seven species. Samples were taken from the pygmy forest community and from conspecifics growing on higher-nutrient soils, where trees may grow over 30 m tall. KEY RESULTS: Pygmy plants of most species maintained similar area-based photosynthetic and stomatal conductance rates to conspecific controls, but had lower specific leaf area (leaf area divided by dry weight), lower percent nitrogen, and less leaf area relative to xylem growth. Sequoia sempervirens, a species rare in the pygmy forest, had a categorically different response from the more common plants and had remarkably low photosynthetic rates. CONCLUSIONS: Pygmy plants were not stunted by low photosynthetic rates on a leaf-area basis; instead, several species had restricted whole-plant photosynthesis due to low leaf area production. Pygmy plants of all species showed signs of greater carbon investment in their leaves and higher production of nonphotosynthetic leaf tissue, further contributing to slow growth rates.