Aim We examined relationships between climate-disturbance gradients and patterns of vegetation zonation and ecotones on a subtropical mountain range.Location The study was conducted on the windward slopes of the Cordillera Central, Dominican Republic, where cloud forest appears to shift in a narrow ecotone to monodominant forest of Pinus occidentalis.Methods Climate, disturbance and vegetation data were collected over the elevation range 1100-3100 m and in 50 paired plots along the ecotone. Aerial photographs were georeferenced to a high-resolution digital elevation model in order to enable the analysis of landscape-scale patterns of the ecotone.Results A Shipley-Keddy test detected discrete compositional ecotones at 2200 and 2500 m; the distributions of tree species at lower elevations were continuous. The elevation of the ecotone determined with aerial photographs was fairly consistent, namely ± 164 m (SD) over its 124-km length, but it exhibited significant landscape variation, occurring at a lower elevation in a partially leeward, western zone. The ecotone also occurred significantly lower on ridges than it did in drainage gullies. Ecotone forest structure and composition differed markedly between paired plots. In pine paired plots, the canopy height was 1.7 times higher and the basal area of non-pine species was 6 times lower than in the cloud forest directly below. Fire evidence was ubiquitous in the pine forest but rare in the abutting cloud forest. Mesoclimate changed discontinuously around the elevation of the ecotone: humidity and cloud formation decreased markedly, and frost frequency increased exponentially.Main conclusions The discreteness of the ecotone was produced primarily by fire. The elevational consistency of the ecotone, however, resulted from the overarching influence of mesoclimate on the elevational patterns of fire occurrence. Declining temperature and precipitation combine with the tradewind inversion to create a narrow zone where high-elevation fires extinguish, enabling fire-sensitive and fire-tolerant taxa to abut. Once established, mesotopography and contrasting vegetation physiognomy probably reinforce this boundary through feedbacks on microenvironment and fire likelihood. The prominence of the pine in this study -and of temperate and fire-tolerant taxa in subtropical montane forests in general -highlights the importance of climatedisturbance-biogeography interactions in ecotone formation, particularly where fire mediates a dynamic between climate and vegetation.

Summary1 Mangrove forests are aected by a variety of natural disturbances that dier in scale, intensity and frequency. Small canopy gaps, although common, have not been well studied. We examined the role of lightning-created canopy gaps in the dynamics of a 47-km 2 intertidal mangrove community in the Dominican Republic, by quantifying the spatial patterns of overstorey tree distributions, spatial and temporal patterns of gap formation, and tree regeneration in gaps and beneath the closed forest. We hypothesized that regeneration in these gaps would maintain and reinforce species' distribution patterns across the intertidal gradient in this mangrove ecosystem. 2 All 52 gaps surveyed in the ®eld were nearly circular canopy openings created by lightning. Expanded gap size indicated an average diameter of 30.4 m. The total area of forest in the gap-phase is currently 1.9%, with an annual formation rate of 0.23% year À 1. Canopy turnover was calculated to range from 421 to 842 years; however, gaps were not randomly distributed across the mangrove forest and canopy turnover in the most gap-prone areas ranged from 194 to 386 years. 3 Seedling density was not dierent under gaps and under the forest canopy, but sapling density and sapling growth rates of three species were signi®cantly greater in canopy gaps. The annual mortality rate of Rhizophora mangle (9%) in canopy gaps was signi®cantly lower than either Laguncularia racemosa (32%) or Avicennia germinans (56%). Gap regeneration was dominated by R. mangle throughout the forest. 4 The results of this study do not support our initial hypothesis but suggest that the distribution and abundance of R. mangle will increase throughout the forest over time, given the present disturbance regime. Field observations indicated that the peat mat collapses in lightning-created gaps following their formation, resulting in increased levels of standing water. We suggest the successful regeneration of R. mangle is favoured in these gaps because of its greater tolerance to¯ooding conditions.

We examined soil porewater concentrations of sulfate, alkalinity, phosphorus, nitrogen, and dissolved organic carbon and solid phase concentrations of pyrite in relation to mangrove species distributions along a 3.1-km-long transect that traversed a 47.1-km mangrove forest in the Dominican Republic. Iron, phosphorus, and sulfur dynamics are closely coupled to the activity of sulfate-reducing bacteria, the primary decomposers in anoxic soils of mangrove ecosystems. Patterns in the chemistry data suggested that sulfate reduction rates and storage of reduced sulfur were greater in the inland basin forest dominated by Laguncularia racemosa than the Rhizophora mangle dominated forest of the lower tidal region. The distribution of Laguncularia was significantly correlated with concentrations of total phosphorus (r= 0.99) and dissolved organic carbon (r= 0.86), alkalinity (r= 0.60), and the extent of sulfate depletion (r= 0.77) in the soil porewater and soil pyrite concentrations (r= 0.72) across the tidal gradient. Leaf tissue chemistry of Laguncularia was characterized by lower C:N and C:P ratios that could fuel the higher rates of decomposition in the Laguncularia-dominated forest. We suggest that a plant-soil-microbial feedback contributes to the spatial patterning of vegetation and soil variables across the intertidal zone of many mangrove forest communities.

No abstract

No abstract

No abstract

Abstract:Tropical montane cloud forests (TMCF) are characterized by short trees, often twisted with multiple stems, with many stems per ground area, a large stem diameter to height ratio, and small, often thick leaves. These forests exhibit high root to shoot ratio, with a moderate leaf area index, low above-ground production, low leaf nutrient concentrations and often with luxuriant epiphytic growth. These traits of TMCF are caused by climatic conditions not geological substrate, and are particularly associated with frequent or persistent fog and low cloud. There are several reasons why fog might result in these features. Firstly, the fog and clouds reduce the amount of light received per unit area of ground and as closed-canopy forests absorb most of the light that reaches them the reduction in the total amount of light reduces growth. Secondly, the rate of photosynthesis per leaf area declines in comparison with that in the lowlands, which leads to less carbon fixation. Nitrogen supply limits growth in several of the few TMCFs where it has been investigated experimentally. High root : shoot biomass and production ratios are common in TMCF, and soils are often wet which may contribute to N limitation. Further study is needed to clarify the causes of several key features of TMCF ecosystems including high tree diameter : height ratio.

Interest in tropical secondary forests has grown as large areas of agriculture have been abandoned in recent decades; yet, there are few long‐term studies of post‐agriculture vegetation recovery in the tropics. In this study, we compared the vegetation structure and floristic composition of old‐growth and 40‐year‐old secondary riparian forests in the Cordillera Central, Dominican Republic. Canopy height and stem density of woody plants were similar between forest types, but basal area of trees was 27 percent lower in secondary forests. Introduced tree species comprised 20 percent of the basal area and dominated the understory of secondary forests. Life‐form diversity was higher in old‐growth forests as arborescent ferns, the palm species, and epiphytic bromeliads, orchids, and bryophytes were much more abundant. The number of species of epiphytic orchids and bromeliads, ground ferns, and herbaceous plants was also significantly higher in old‐growth forests. The species density of woody plants and vines, however, was comparable between forest types, and vine abundance was significantly higher in secondary forests. The high importance of introduced tree species and the delayed recovery of several plant life‐forms have important implications for the conservation of plant diversity in secondary forests in the tropics. The robust regeneration of woody structure despite the long land tenure (ca 60 yr) by farmers is probably due to the nutrient‐rich alluvial soils and low‐intensity agriculture. This study revealed the potential for the rapid recovery of woody plant diversity and structure in fertile secondary forests adjacent to mature forest seed sources and the more delayed recovery of nonwoody plant diversity and abundance. RESUMEN El interés por los bosques tropicales secundarios ha crecido debido a que grandes extensiones de áreas agricolas han sido abandonadas en las últimas décadas. Aun asi, en los trópicos hay pocos estudios a largo plazo sobre la recuperación de la vegetatión de zonas agricolas abandonadas. En nuestro estudio, comparamos la estructura de la vegetación y la composición floristica de bosques ribereños maduros y bosques de 40 años de edad en la Cordillera Central de Repúblics Dominicana. La altura del dosel y la densidad de tallos de plantas leñosas fueron similares en los distintos tipos de bosque, pero el área basal de los árboles fue 27 por ciento más baja en los bosques secundarios. Las especies arb6reas introducidas representaron 20 por ciento del área basal y dominaron el sotobosque de los bosques secundarios. La diversidad de formas de vida fue más alta en los bosques maduros, debido a que los helechos arborescentes, las palmas, bromelias, orquideas, y musgos epifitos fueron mucho más abundantes. El numero de especies de orquideas y bromelias epifitas, helechos de tierra y de plantas herbáceas fue significativamente más alta en los bosques maduros. Sin embargo, el números de especies leñosas y de bejucos fue comparable en los dos tipos de bosque, mientras que la abundancia de bejuc...