Plant traits are fundamental to understanding how environmental factors, including soils, radiation, and climate, shape plant species distributions. One trait that has received considerable attention lately is leaf abscission or dehiscence in grasses because it overexpressed within New Zealand’s flora. At the center of the debate over the evolution of dehiscence, with discussion focusing on the role of environmental factors, including fire, temperature, and soil moisture, versus the role of trophic interactions, particularly mammalian browsing. This debate provides an ideal context to examine the development of a suite of traits in response to environmental and trophic selection pressures. To identify the adaptive tradeoffs and ecological consequences to dehiscence in tussock grasses we pursued three goals in this study. First, we tested whether there are dehiscence results in improved C acquisition and photosynthetic light response. Second, we used a physiologically based niche model to explore whether dehiscent and non-dehiscent species occupy distinctive niche spaces that suggest the adaptive advantages of these traits. Finally, we evaluated whether dehiscence promotes leaves of lower tissue quality as an alternative to non-dehiscence to resist herbivory.
Results/Conclusions
There is no consistent evidence supporting the self-shading hypothesis. There was substantial variation in photosynthetic rates and responses to both light and CO2 among our species, but the dehiscence state of the grasses rarely explained a significant amount of the variation in physiological traits. There are three niche axes where average niche occupancy differs between dehiscence state. Dehiscent species had lower threshold minimum temperatures than non-dehiscent species. Dehiscent species also had higher upper maximum temperature thresholds for C-uptake. we found that non-dehiscent species were able to take up sufficient N at lower water availability than dehiscent species. For two key tissue quality metrics there were significant differences between dehiscent and non-dehiscent species, with both indicating that dehiscent species have lower quality live tissue than non-dehiscent species. Phosphorus, the most commonly limiting nutrient in New Zealand ecosystems, was consistently found in lower concentration in dehiscent species than non-dehiscent species, while silica, a key element in herbivore defense and structure, is found in higher concentration in dehiscent species than non-dehiscent species. Our results confirm that there are not fundamental differences in the influence of light on C uptake in dehiscent and non-dehiscent species. However, our niche model suggests that the reversion to the non-dehiscent state may have been in response to a move into lower altitudes.