Growth rates and functional traits of tropical rainforest and savannah species

Summary: Plant growth rates play a crucial role in vegetation dynamics, for example by influencing competitive ability, or the rate of vegetation recovery post disturbance. Plant traits are assumed to underpin variation in growth rates consistently, but most evidence comes from seedlings or closed forest vegetation. Here I aimed to test whether: (1) Trait-growth relationships are the same in adult plants as is known from seedlings; and (2) Trait-growth relationships are consistent across savannas and closed forests. I used adult growth rate and trait measurements from species in a tropical rainforest in Australia, and three tropical savanna regions (one in Australia, Brazil and South Africa). In all sites I tested clear hypotheses relating to four traits related to carbon gains and losses, namely photosynthetic rate, specific leaf area (SLA), wood density, and the ratio of leaf mass to wood mass on canopy branches. In savannas I also considered bark thickness, which is important for insulation against fire in these fire-prone systems, but assumed to be costly to growth. In tropical forests I further considered whether traits were linked to a species trajectory of growth across its lifetime. I found evidence that traits were related to adult growth rates in a predictable way in tropical forests, and that variation in trait values was linked to three distinct strategies regarding lifetime growth trajectories. Savannas showed some consistency with forests regarding trait-growth relationships, though the relative importance of traits in each site varied. I attributed differences in trait-growth patterns largely to differences in the prevailing disturbance regime in each savanna. My thesis provides strong evidence that traits have a predictable effect on the growth rates of adult plants, but that generalisations are difficult without an understanding of regional differences in ecology, evolutionary history, and disturbance regimes. These regional differences will have consequences for vegetation response to future changes in climate.