The hydraulic limitation of tree height attainment

Tree height is an important determinant of tropical forest structure, biomass and diversity. Maximum tree height globally and in the tropics is linked to water availability. Different ecophysiological mechanisms behind this link have been hypothesised and explored, though with little focus on tropical forest trees. This thesis aims to contribute to understanding how tree height might be limited in tropical forests. We first study (Chapter 3) patterns of tropical forest height at the community and taxon level across neotropical forests. We found that neotropical forests and families are similarly limited by mean annual precipitation (MAP). Tree height increases until a peak at ~2400-2700mm MAP, above ~3000mm tree height decreases. We next study (Chapter 4) the patterns of basal xylem vessel widening with tree height the tropical tree genus Cedrela across a range of water availability. The widening of basal vessels is similar regardless of water availability, therefore in trees of a given height vessel diameter is similar within the study species across its range. This has implications for how trees cope with hydraulically stressful conditions and may suggest mechanisms behind declines in maximum tree height with water availability. We finally study (Chapter 5) the changes in a suite of ecophysiological properties and functional traits with tree height in three species along the shade-tolerance spectrum. We show that with height leaves become smaller and thicker, and the theoretical maximum stomatal conductance per leaf area and intrinsic water use efficiency increase. Additionally, xylem vessels taper at the rate consistent with tapering theory. We show that traits covary for these species with differences in shade-tolerance according to expectations from the literature. Specifically, the slow growing shade-tolerant species has narrower xylem, higher LMA and a relatively isohydric leaf water potential regulation relative to the fast-growing shade-intolerant species. Overall, this thesis shows how neotropical trees are limited in their height attainment and explores what ecophysiological mechanisms may underpin this.