Anthropogenic activities have accelerated the rate of global loss of biodiversity, making it more important than ever to understand the structure of biodiversity hotspots. One current focus is the relationship between species richness and aboveground biomass (AGB) in a variety of ecosystems. Nonetheless, species diversity, evenness, rarity, or dominance represent other critical attributes of biodiversity and may have associations with AGB that are markedly different than that of species richness. Using data from large trees in four environmentally similar sites in the Luquillo Experimental Forest of Puerto Rico, we determined the shape and strength of relationships between each of five measures of biodiversity (i.e., species richness, Simpson’s diversity, Simpson’s evenness, rarity, and dominance) and AGB. We quantified these measures of biodiversity using either proportional biomass or proportional abundance as weighting factors. Three of the four sites had a unimodal relationship between species richness and AGB, with only the most mature site evincing a positive, linear relationship. The differences between the mature site and the other sites, as well as the differences between our richness–AGB relationships and those found at other forest sites, highlight the crucial role that prior land use and severe storms have on this forest community. Although the shape and strength of relationships differed greatly among measures of biodiversity and among sites, the strongest relationships within each site were always those involving richness or evenness.

Evaporation losses from a watertight 6.34 ha rain forest catchment under wet maritime tropical conditions in the Luquillo Experimental Forest, Puerto Rico, were determined using complementary hydrological and micrometeorological techniques during 1996 and 1997. At 6.6 mm d−1 for 1996 and 6.0 mm d−1 for 1997, the average evapotranspiration (ET) of the forest is exceptionally high. Rainfall interception (Ei), as evaluated from weekly throughfall measurements and an average stemflow fraction of 2.3%, accounted for much (62–74%) of the ET at 4.9 mm d−1 in 1996 and 3.7 mm d−1 in 1997. Average transpiration rates (Et) according to a combination of the temperature fluctuation method and the Penman-Monteith equation were modest at 2.2 mm d−1 and 2.4 mm d−1 in 1996 and 1997, respectively. Both estimates compared reasonably well with the water-budget-based estimates (ET − Ei) of 1.7 mm d−1 and 2.2 mm d−1. Inferred rates of wet canopy evaporation were roughly 4 to 5 times those predicted by the Penman-Monteith equation, with nighttime rates very similar to daytime rates, suggesting radiant energy is not the dominant controlling factor. A combination of advected energy from the nearby Atlantic Ocean, low aerodynamic resistance, plus frequent low-intensity rain is thought to be the most likely explanation of the observed discrepancy between measured and estimated Ei.

Tropical montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastal regions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive 2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfalls that alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic landforms that can be solely attributed to their low-latitude locations. While they are relatively understudied, an emerging view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a high frequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in other fluvial systems.

This paper analyzes field survey results and develops a conceptual model of the factors that influence cropping and fallowing practices on small farms in the terra firme landscape near Santarem, Brazil. A multi-fallow cultivation system that used rice, corn and bitter manioc in various relay-intercropping combinations was the most common cultivation practice observed. Five different types of fallow vegetation were identified and used by the farmers: (1) mature forest vegetation greater than 20 years old; (2) secondary forest vegetation 8 to 12 years old; (3) young secondary forest vegetation 3 to 6 years old; (4) brushy vegetation 2 to 4 years old; and (5) weed vegetation less than 2 years old. Distinct relay-intercropping sequences were associated with each of these fallows. We suggest that the selection of fallow length and cropping sequence is subject to the following general constraints: (1) the productivity of the landscape as determined by soil, water and climate; (2) ecological requirements and risks associated with particular crops; (3) land availability and the costs of site preparation, and cultural treatments; (4) the availability of hired labor; (5) the age structure of the families, their subsistence requirements and preferences for particular crops, leisure and non-farm-related production activities; and (6) local economic conditions including land values, access to credit and non-farm-related employment, and the conditions of commodity markets. To maximize agricultural production and general household utility given these constraints, the farmers have several options, including: (1) varying the length of fallows; (2) varying the types and sequences of crops that are planted following a given fallow; (3) modifying the clearing and cultivation practices; (4) improving subsequent yields by managing regeneration within a fallow; (5) developing diversified land use systems that contain combinations of pasture, perennials, semi-permanent annuals, areas of extractive reserves and true shifting cultivation; and (6) increasing production through the use of external inputs like fertilizer, irrigation and farm machinery. Most farmers in the study area have chosen to modify cropping sequences and vary the lengths of natural fallows rather than using expensive external inputs. Our data suggest that a major factor in selecting a fallow length is the cost of land clearance and preparation. Moreover, since clearing costs are dramatically reduced for young secondary vegetation, the reduction in site preparation costs over several short rotations compensates for the lost production caused by using short fallows instead of long fallows.