Vegetation of the dry bioclimatic areas in the Dominican Republic

We conduct a study of the dry forest in areas of scant rainfall in the Dominican Republic; these are sites with particular endemic habitats, as the samples reveal a high rate of local and insular endemic species, with 84 (24.85%) endemisms. This work covers dry forests rich in coarse spiny plants and includes a statistical (multivariate analysis), phytosociological and catenal study of the vegetation. We analyse the floristic composition, ecology, distribution and richness in endemisms of each association. The bioclimatic analysis reveals the different thermotypes and ombrotypes on the island, and locates the proposed plant associations in the infra- and thermotropical thermotypes, and in the arid, semiarid and dry ombrotypes. As a result of this phytosociological analysis, we propose the new alliance Harrio nashii–Acacion skleroxylae and four plant associations: Harrisio nashii–Prosopidetum juliflorae, Crotono poitaei–Erythroxyletum rotundifolii, Lonchocarpo pycnophylli–Cylindropuntietum caribaeae and Neoabbottio paniculatae–Guaiacetum officinali; these associations connect catenally with the subhumid forest and mangrove swamps.     

Patterns in a species‐rich tropical understory plant community

Understory plants are an important component of the high plant species diversity characteristic of neotropical rain forests. Herbs, shrubs, understory trees, and saplings of canopy trees occupy a broadly uniform environment of abundant rainfall, low light levels, and high humidity. We asked whether this community at the La Selva Biological Station in the Caribbean lowlands of Costa Rica was structured by environmental filters such as soil origin, topographic position, and understory light availability. We used nested quadrats to assess effects of soil origin (recent alluvium, weathered alluvium, residual volcanic soil) and topographic position (ridges, mid‐slopes and flats) on species composition, density, and diversity and measured six edaphic and understory light parameters. Non‐metric multidimensional scaling ordinations were based on frequency of occurrence in 20 quadrats for 272 species in the shrub size class and 136 species in the small‐tree size class for 17 sites. Three axes were correlated with composite environmental variables produced by principal component analysis representing slope, extractable phosphorus, and light. NMS site positions also reflected soil origin, topographic position, and geographic location. The analyses illustrated a complex community structured by species responses to environmental filters at multiple, interdigitated spatial scales. We suggest that light availability affected by canopy dynamics and dispersal limitation provides additional sources of variation in species distributions, which interact with edaphic patterns in complex ways. Abstract in Spanish is available with online material.     

Denitrification studies in a temperate forest catena soil

Summary Formation of ammonium during the reduction of nitrate under moderate and strict anaerobic incubation of two topsoils of a temperate forest catena, an acid mull and an anmoor was studied. In mull, both conditions of incubation caused reduction of nitrate and release of ammonium. The accumulation of ammonium continued even when there was no nitrate left hence indicating the formation of ammonium apparently through desamination of organic matter. Whereas, in anmoor neither any such formation of ammonium nor any significant reduction of nitrate was observed in the case of moderate anaerobic incubation. But under strict anaerobic incubation, progressive disappearance of nitrate was encountered from the beginning up to 30 days and this was accompanied by an increasing accumulation of ammonium in this soil. Yet this accumulation stopped when there was no nitrate left. Thus, the formation of ammonium is caused by the reduction of nitrate in anmoor.     

Spatial relationships of aluminum chemistry in the streams of the Hubbard Brook Experimental Forest,New Hampshire

Aluminum chemistry was evaluated in two headwater streams in the White Mountains of New Hampshire. Observed elevational trends in stream aluminum chemistry may be related to spatial variations of vegetation type and mineral soil depth within the watersheds. At the highest elevations maximum densities of spruce and fir vegetation occur and aluminum appears to be mobilized predominantly by transformations involving dissolved organic matter. At the mid-elevations hardwood vegetation predominates and the mechanism of aluminum mobilization shifts to dissolution by strong acids within the mineral soil. At the lowest elevations, relatively thick mineral soil seems to limit aluminum mobility, resulting in low concentrations in streamwater. Comparison of these results with an earlier study of an adjacent watershed, indicates that subtle differences in watershed characteristics such as tree species distribution and topography may cause significant variations in stream aluminum chemistry. Control of aluminum mobility by imogolite minerals was not indicated by the stream chemistry of these watersheds. To determine the relationship between acidic deposition and aluminum mobility, natural variations which occur in the aluminum cycle must be addressed.     

Effect of submergence on availability of certain plant nutrients in three Ultisol catenas

Soil samples from surface and sub-surface horizons in the well-drained and poorly-drained members of three soil catenas were incubated under submergence or at field capacity to study the effects of these incubation conditions and prior natural drainage on the solubility of four plant micro-nutrients. Iron, Mn, Zn and Cu were extracted by water using a 11 water:soil ratio. The four micronutrient metals were also extracted by DTPA solutions buffered at either pH 5.3 or pH 7.3 to compare the effectiveness of these two extractants under these incubation conditions with acid soils. Generally the extractability of the nutrients was much affected by the horizon (A, E or B) with A horizons having the greatest amounts of all nutrients and undergoing greater changes in water- and DTPA-extractability during incubation. Soil drainage class (wellvs. poorly drained) had few effects. Incubation moisture regime had major effects on water extractable Fe and Mn with lesser effects on Zn and Cu. Submerged soils generally had the greatest levels of water extractable nutrients, though rice uptake did not reflect this. DTPA at pH 5.3 extracted 2 to 3 times as much Fe, Mn, Zn and Cu as did DTPA at pH 7.3 and about 50 to 100 times as much as did water. Correlations between DTPA extractable nutrients and rice uptake were significant only for Fe and Cu and declined during incubation. The changes in all variables during incubation were complex, being related to soil properties such as organic matter content, pH and mineralogy as well as to incubation conditions.     

Spatial and temporal patterns of home range use by olive baboons (Papio anubis) in eastern Laikipia,Kenya

The study investigated the relationship between topography, seasonal food availability and home range use by a troop of olive baboons (Papio anubis) in a dry savannah environment. Valleys were the most utilized parts of the home range in dry seasons while old boma sites were more frequently used in the wet season. Herb‐layer food biomass decreased significantly in dry seasons. The flowers of both Acacia tortilis and Acacia etbaica were available only in dry seasons with valleys having the highest biomass. The biomass of Acacia nilotica seeds was higher in valleys than on other locations. Leaves of the shrub Lycium europaeum were available on all locations in the wet season but valleys had the highest biomass of this food type. Herb‐layer baboon food biomass at ridge tops, slopes, valleys and old boma sites showed similar trends in productivity between seasons. Baboons tracked food along the soil catenas by feeding at ridge tops and old boma sites where foods were available during the wet season and exploiting food sources at the valleys during the dry seasons. Food availability was related to the catena effect and baboons used the catenas in the home range in a predictable manner.     

Proposed mechanism for the origin of sodic patches in Kruger National Park, South Africa

The accumulation of sodium on the footslopes of granitic catenas in semi‐arid southern Africa leads to the formation of sodic patches. Sodic patches are ecologically important for nutrient accumulation, predator evasion and wallowing, but they are often perceived as derelict lands because of vegetation denudation and low aesthetic quality. This negative perception, by both ecologists and tourists, often leads to ill‐advised management and ‘rehabilitation’ measures. In Kruger National Park, sodic patches occur at the riparian‐upland boundary and hence the processes originating in both systems may contribute to their origin. The upland‐based catena and riparian‐based evapotranspiration models were used to explore the hypothesis that these soils originate from both catena and evapotranspiration‐driven hydraulic processes. The models predict vegetation zonation dictated by a salt tolerance gradient and an increase in sodic patch area over time as a result of progressive salt accumulation. Vegetation structure and hillslope morphology across the riparian‐upland boundary and analysis of change in patch configuration over a 50‐year aerial photographic record were used to test the hypotheses. Sodic patches have unique vegetation, occur on the intermediate positions of hillslopes, have increased in area three‐fold over 50 years and are encroaching into the riparian zone. This behaviour is consistent with the assertion that these patches originate from both catena and evapotranspiration processes. Results imply a dynamic aspect of sodic patches, which have been previously viewed as static landscape features in pedogenic time scales. However, we show that they change over relatively smaller time scales meaning that their management should be commensurate with this dynamic nature. Therefore, sodic patches should be managed under a scheme that incorporates their small‐scale expansion and recognizes their ecological importance.