T h e structure of mangrove vegetation, soil salinity, and topographic relief of the mangrove ecosystem were studied along the south coast of Puerto Rico, Culebra, and Mona Island. All systems, with the exception of the forest at Mona Island, were characterized by having a coastal fringe of live vegetation (usually dominated by the red mangrove), a zone of dead trees, and a hypersaline lagoan or dry salt flat o n the landward side. Mean soil salinities were 44 O/00 for the zone of live trees, 72 0100 for the zone of dead trees, and 87 0/00 for the salt flats. T h e Mona Island forest grew over a sandy hardpan that prevented mixing of sea water and the fresh water. Soil salinities were low in this forest, and trees reached a height of 1 3 m. Tree height was inversely proportional to soil salinity (r = 0.72) between 1 7 and 72 O/OO. W h e n soil salinities exceeded 65 0/00, dead tree basal area was higher than live tree basal area. It is suggested that mangrove growth is limited by soil salinities higher than 9 0 0100. Mangrove succession in offshore overwash islands and arid coastlines is described. It is proposed that cyclic rainfall patterns and hurricanes act as regulators of speed and direction of succession. Rainy periods are asswiated with lower soil salinities and expansion of the red mangrove zone. Droughty periods result in high soil salinities, mangrove mortality, and expansion of salt flats. Hurricanes set back succession and reverse successional trends that reduce mangrove areas. A model of mangrove ecosystem function and implications for management are also discussed.
Twenty-eight worldwide reports of massive mangrove tree mortalities are reviewed. Massive mortality is defined as tree mortalities that occur in response to rapid environmental change and affect all size dasses. Massive mortality occurs in addition to normal tree mortality. Normal tree mortality was described using structural data from 114 mangrove stands. This mortality is density dependent, follows orderly time dependent patterns dictated by stand maturation (related to average tree diameter), and usually occurs in the smaller diameter size classes. Disease and other biotic factors do not appear to be primary causes of massive mangrove mortalities. Instead, these factors appear to attack forests weakened by changes in the physical environment. Mangrove environments are dynamic and cyclical and mangrove associations adapt to such environments by both growing and dying fast. Mangrove species' characteristics such as the capacity to produce large quantities of propagules that take advantage of dispersal agents, sharp species zonations, and even-aged populations contribute to the rapid growth-mortality cycles in mangroves. Humans may tilt the balance towards higher mortality rates by introducing chronic stressors that inhibit regeneration mechanisms.
Aim We developed a set of statistical models to improve spatial estimates of mangrove aboveground biomass (AGB) based on the environmental signature hypothesis (ESH). We hypothesized that higher tidal amplitudes, river discharge, temperature, direct rainfall and decreased potential evapotranspiration explain observed high mangrove AGB. Location Neotropics and a small portion of the Nearctic region. Methods A universal forest model based on site‐level forest structure statistics was validated to spatially interpolate estimates of mangrove biomass at different locations. Linear models were then used to predict mangrove AGB across the Neotropics. Results The universal forest site‐level model was effective in estimating mangrove AGB using pre‐existing mangrove forest structure inventories to validate the model. We confirmed our hypothesis that at continental scales higher tidal amplitudes contributed to high forest biomass associated with high temperature and rainfall, and low potential evapotranspiration. Our model explained 20% of the spatial variability in mangrove AGB, with values ranging from 16.6 to 627.0 t ha−1 (mean, 88.7 t ha−1). Our findings show that mangrove AGB has been overestimated by 25–50% in the Neotropics, underscoring a commensurate bias in current published global estimates using site‐level information. Main conclusions Our analysis show how the ESH significantly explains spatial variability in mangrove AGB at hemispheric scales. This finding is critical to improve and explain site‐level estimates of mangrove AGB that are currently used to determine the relative contribution of mangrove wetlands to global carbon budgets. Due to the lack of a conceptual framework explicitly linking environmental drivers and mangrove AGB values during model validation, previous works have significantly overestimated mangrove AGB; our novel approach improved these assessments. In addition, our framework can potentially be applied to other forest‐dominated ecosystems by allowing the retrieval of extensive databases at local levels to generate more robust statistical predictive models to estimate continental‐scale biomass values.
ABSTRACT.-We studied gas exchange, leaf dimensions, litter production, leaf and litterfall chemistry, nutrient flux to the forest floor, retranslocation rates, and nutrient use efficiency of mangroves in Jobos Bay, Puerto Rico. The fringe forest had a salinity gradient from the ocean (35‰) to a salt flat (100‰) and a basin (about 80‰). Red (Rhizophora mangle), white (Laguncularia racemosa), and black (Avicennia germinans) mangroves were zoned along this gradient. Photosynthetic rates, stomatal conductance, leaf area and weight, leaf specific area, and nutrient use efficiency decreased with increasing salinity, while xylem tension, nutrient retranslocation, and leaf respiration increased with increasing salinity. The concentration of some leaf elements increased with salinity (N, P, Mg, Na) while others decreased (Ca). Leaf specific area was less variable than leaf area or weight.
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