Drivers of woody plant form and function in relation to water acquisition and use in seasonal forests

Abstract

Understanding plant-water relations is of central interest to ecology, as patterns in water availability drive distribution, composition, structure, and function of plant communities worldwide. Plant-water relations are particularly important in forests that experience water limitation and periods of seasonal drought. However, little is known about the initial post-germination response of multiple species from seasonal forests to water availability, or how species with different growth forms respond over time to distinct water availability patterns. Moreover, few studies have examined woody plant drought resistance across entire communities or key traits that are difficult to measure, such as biomass allocation and rooting depth, but which may mediate responses to seasonal water shortage. In Chapter 1, I determined if legumes have a different regeneration niche than non-legumes. I found that legumes may indeed have a different regeneration niche, in that they germinate rapidly and grow taller than other species immediately after germination, maximizing their performance when light and belowground resources are readily available, and potentially permitting them to take advantage of high light, nutrient, and water availability at the beginning of the wet season. For Chapter 2, I measured drought resistance in all dominant woody species within a dry sclerophyll woodland community to determine community level drought resistance. I found that the leaf and stem xylem of all species was highly drought resistant, with two species being slightly more vulnerable to drought than the others. The overall high resistance to drought and similar levels of resistance between leaves and stems suggest that canopy and understory species in this evergreen forest have evolved similar strategies of drought resistance. However, because two species were more vulnerable to drought than the others, this could lead to shifts in community-level species composition caused by future increases in drought frequency and severity. For Chapter 3, to shed light on rooting depth and biomass allocation, I harvested above- and belowground biomass and measured maximum rooting depth for juvenile and adult lianas, deciduous trees, and evergreen trees. This was the first study to show that lianas had the shallowest roots, followed by deciduous trees, then evergreen trees, which had the deepest roots. In addition, my study was the first to find that contrary to predictions lianas and trees have similar allocation patterns to stems. To test the hypothesis that lianas perform better than trees during seasonal drought, for Chapter 4, I used a common garden experiment with lianas and trees. My results are the first experimentally support the hypothesis that lianas perform better and experience less physiological stress than trees during seasonal drought. Moreover, only trees responded to dry season irrigation with increased growth, whereas lianas did not, suggesting clear differences between growth forms in response to altered rainfall regimes. Ultimately, better dry-season performance may explain why liana abundance peaks in seasonal forests compared to trees. Collectively, these studies show that dry forest tree species differ in their strategies used to cope with seasonal drought, and how they respond to water availability.