Aboveground litter production in forests is likely to increase as a consequence of elevated atmospheric carbon dioxide (CO sub(2)) concentrations, rising temperatures, and shifting rainfall patterns. ...As litterfall represents a major flux of carbon from vegetation to soil, changes in litter inputs are likely to have wide-reaching consequences for soil carbon dynamics. Such disturbances to the carbon balance may be particularly important in the tropics because tropical forests store almost 30% of the global soil carbon, making them a critical component of the global carbon cycle; nevertheless, the effects of increasing aboveground litter production on belowground carbon dynamics are poorly understood. We used long-term, large-scale monthly litter removal and addition treatments in a lowland tropical forest to assess the consequences of increased litterfall on belowground CO sub(2) production. Over the second to the fifth year of treatments, litter addition increased soil respiration more than litter removal decreased it; soil respiration was on average 20% lower in the litter removal and 43% higher in the litter addition treatment compared to the controls but litter addition did not change microbial biomass. We predicted a 9% increase in soil respiration in the litter addition plots, based on the 20% decrease in the litter removal plots and an 11% reduction due to lower fine root biomass in the litter addition plots. The 43% measured increase in soil respiration was therefore 34% higher than predicted and it is possible that this 'extra' CO sub(2) was a result of priming effects, i.e. stimulation of the decomposition of older soil organic matter by the addition of fresh organic matter. Our results show that increases in aboveground litter production as a result of global change have the potential to cause considerable losses of soil carbon to the atmosphere in tropical forests.
Each year, an average of 45 tropical cyclones affect coastal areas and potentially impact forests. The proportion of the most intense cyclones has increased over the past four decades and is ...predicted to continue to do so. Yet, it remains uncertain how topographical exposure and tree characteristics can mediate the damage caused by increasing wind speed. Here, we compiled empirical data on the damage caused by 11 cyclones occurring over the past 40 years, from 74 forest plots representing tropical regions worldwide, encompassing field data for 22,176 trees and 815 species. We reconstructed the wind structure of those tropical cyclones to estimate the maximum sustained wind speed (MSW) and wind direction at the studied plots. Then, we used a causal inference framework combined with Bayesian generalised linear mixed models to understand and quantify the causal effects of MSW, topographical exposure to wind (EXP), tree size (DBH) and species wood density (ρ) on the proportion of damaged trees at the community level, and on the probability of snapping or uprooting at the tree level. The probability of snapping or uprooting at the tree level and, hence, the proportion of damaged trees at the community level, increased with increasing MSW, and with increasing EXP accentuating the damaging effects of cyclones, in particular at higher wind speeds. Higher ρ decreased the probability of snapping and to a lesser extent of uprooting. Larger trees tended to have lower probabilities of snapping but increased probabilities of uprooting. Importantly, the effect of ρ decreasing the probabilities of snapping was more marked for smaller than larger trees and was further accentuated at higher MSW. Our work emphasises how local topography, tree size and species wood density together mediate cyclone damage to tropical forests, facilitating better predictions of the impacts of such disturbances in an increasingly windier world.
The global proportion of the most intense cyclones is increasing and it remains uncertain how damage to tropical forests will change with increasing wind speed. We compiled empirical data on the damage caused by cyclones from forests representing tropical regions worldwide to better understand the effects of wind speed, topographical exposure to wind, and tree characteristics on the proportion of damaged trees and on the probability of snapping or uprooting. Our work emphasises how local topography, tree size and species wood density together mediate cyclone damage to tropical forests, facilitating better predictions of the impacts of such disturbances.
We studied the water relations of an upper montane rain forest at an elevation of @?1550 m in the Blue Mountains of Jamaica to investigate whether the growth of the forest is limited by water ...shortage and/or whether the characteristically hard, leathery leaves of tropical montane rain forest trees are an adaptation to water shortage. Daily courses of leaf water potential, leaf relative water content, leaf resistance, and leaf temperature were followed for nine tree species at the driest time of year. The lowest leaf water potential measured was -1.7 MPa. Leaf resistance was usually low and rarely showed increases that might be attributed to water shortage; it was significantly correlated with light levels. Leaf relative water content was always >86% and was not correlated with leaf water potential. Leaf sublethal water deficits (SLD) averaged 27% (12 species). There was no relationship between SLD and leaf structural characteristics. We conclude that water supply rarely, if ever, seriously limits the growth of these montane forests and that their xeromorphic leaves are not an adaptation of water limitation.
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