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Strong El Niño events alter tropical climates and may lead to a negative carbon balance in tropical forests and consequently a disruption to the global carbon cycle. The complexity of tropical forests and the lack of data from these regions hamper the assessment of the spatial distribution of El Niño impacts on these ecosystems. Typically, maps of climate anomaly are used to detect areas of greater risk, ignoring baseline climate conditions and forest cover. Here, we integrated climate anomalies from the 1982–1983, 1997–1998, and 2015–2016 El Niño events with baseline climate and forest edge extent, using a risk assessment approach to hypothetically assess the spatial and temporal distributions of El Niño risk over tropical forests under several risk scenarios. The drivers of risk varied temporally and spatially. Overall, the relative risk of El Niño has been increasing driven mainly by intensified forest fragmentation that has led to a greater chance of fire ignition and increased mean annual air temperatures. We identified areas of repeated high risk, where conservation efforts and fire control measures should be focused to avoid future forest degradation and negative impacts on the carbon cycle.

Key messageThe rate of vessel tapering is highly conserved across a precipitation gradient in tropical trees, pointing to a limit on tree height determined by a maximum basal vessel diameter.Maximum tree height in the tropics decreases strongly with decreasing precipitation. The role of hydraulic architecture in controlling this variation in tree height remains unclear. The widening of conducting xylem vessels from the apex to the base of trees, also known as tapering, is important for maintaining the hydraulic conductivity along the tree stem. If in contrast vessel diameter were constant, then resistance would increase with path length constraining flow rates as tree height increases. Whilst previous research has shown that vessel diameter scales with tree height at a similar rate (similar power law exponent) across biomes and taxa, knowledge on these relationships across precipitation gradients within a single species is incomplete, especially for the tropics. Here we report how vessel density and diameter at the tree base differ for two tropical Cedrela species across four sites varying in precipitation from 1014 to 2585 mm year−1. We find that maximum tree height decreases with decreasing precipitation across sites from 42 to 13 m. Despite the strong differences between sites in maximum tree height and water availability, tapering is indeed remarkably conserved and close to published scaling with height based on multi-species analyses. Thus, for a given tree height, basal vessel diameter does not vary between sites, whilst the maximum basal vessel size is two times smaller at the drier site (with the shortest trees) compared to the wettest site (with the tallest trees). This suggests a possible limitation of tree height determined by a maximum basal vessel diameter that can be sustained, given increasing embolism risk with increasing dryness. Our results show no hydraulic adaptation across this wetness gradient and reveal a clear relationship between maximum tree height and maximum basal vessel size.

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.