Short-Term Disappearance of Foliar Litter in Three Species Before and After a Hurricane1

Litter disappearance was examined before (1989) and after (1990) Hurricane Hugo in the Luquillo Experimental Forest, Puerto Rico using mesh litterbags containing abscised Cyrilla racemiflont or Dacryodes excelsa leaves or fresh Prestoea montana leaves. Biomass and nitrogen dynamics were compared among: (i) species; (ii) mid- and high elevation forest types; (iii) riparian and upland sites; and (iv) pre- and post-hurricane disturbed environments. Biomass disappearance was compared using multiple regression and negative exponential models in which the slopes were estimates of the decomposition rates subsequent to apparent leaching losses and the y-intercepts were indices of initial mass losses (leaching). Cyrilla racemiflora leaves with low nitrogen (0.39%) and high lignin (22.1%) content decayed at a low rate and immobilized available nitrogen. Dacryodes excelsa leaves had moderate nitrogen (0.67%) and lignin (16.6%) content, decayed at moderate rates, and maintained the initial nitrogen mass. Prestoea montana foliage had high nitrogen (1.76%) and moderate lignin (16.7%) content and rapidly lost both mass and nitrogen. There were no significant differences in litter disappearance and nitrogen dynamics among forest types and slope positions. Initial mass loss of C. racemiflora leaves was lower in 1990 but the subsequent decomposition rate did not change. Initial mass losses and the overall decomposition rates were lower in 1990 than in 1989 for Dacryodes excelsa. Dacryodes excelsa and C. racemiflora litter immobilized nitrogen in 1990 but released 10-15 percent of their initial nitrogen in 1989, whereas P. montana released nitrogen in both years (25-40%). Observed differences in litter disappearance rates between years may have been due to differences in the timing of precipitation. Foliar litter inputs during post-hurricane recovery of vegetation in Puerto Rico may serve to immobilize and conserve site nitrogen.     

Effects of light,moisture, temperature,and litter on the regeneration of five tree species in the tropical montane wet forest of Puerto Rico

Field experiments were conducted to investigate the effects of light, moisture, temperature, and litter on the regeneration of two early-, one mid-, and two late-successional tropical tree species. High light and litter seem to be universally good cues for regeneration, increasing seed/seedling survival for all species except for Cecropia (an early-successional species) whose small seeds may not be able to penetrate the litter layer. In addition, the high temperature environment in both artificially shaded and nonshaded areas of a natural gap exhibits less seed loss, an increase in the percent and rate of germination, and an increase in seedling survival for Dacryodes (a late-successional species), than the lower temperature environment under an intact canopy. Low soil water is also a good cue for Dacryodes germination as it is for Prestoea and Cecropia. Finally, the lower temperature environment found under the forest canopy (compared to the natural gap) leads to less seed loss and more germination for Guarea (a mid-successional species). Our results suggest that a good patch for regeneration of many species in this forest, early- as well as late-successional species, would have high light and a litter layer that moderates temperature and moisture extremes. The substantial variation in suitability among regeneration filters and species could: (1) contribute to low establishment success, i.e., most dispersed propagules do not become trees, (2) make it difficult to group species into germination strategies, and (3) make it hard to generalize about a net effect of any specific environmental variable on establishment. We suggest that tropical disturbances should be viewed in terms of their impact on a variety of environmental cues, which may signal germination and impact subsequent growth and survival.     

Species-specific Seedling Responses to Hurricane Disturbance in a Puerto Rican Rain Forest1

Seedling dynamics were followed in a Puerto Rican forest for 20 months following a severe hurricane to study the interactive effects of hurricane debris, nutrients, and light on seedling diversity, density, growth, and mortality. Three treatments (debris removal, an unaltered control with hurricane debris, and chemical fertilization added to hurricane debris) altered levels of forest debris and soil nutrients. Canopy openness was measured twice using hemispherical photographs of the canopy. We examined the demographic responses of six common species to treatments over time. Seedling densities increased for all six species but the only significant treatment effects were increased densities of the pioneer tree Cecropia and the shrub Palicourea in the debris removal treatment. Seedling growth declined with declining light levels for four species but not for the pioneer tree Alchornea or the non‐pioneer tree Dacryodes. Only Cecropia and the non‐pioneer tree Chionanthus had treatment effects on growth. Mortality also differed among species and tended to be highest in the fertilized plots for all but Cecropia and Dacryodes. We found only some of the expected differences between pioneer and non‐pioneer plants, as each species had a unique response to the patchy distributions of organic debris, nutrients, and light following the hurricane. High local species diversity was maintained through the individualistic responses of seedlings after a disturbance.     

Leaf Litter Decomposition and Substrate Chemistry of Early Successional Species on Landslides in Puerto Rico1

Decomposition is a critical process for nutrient release and accumulation of soil organic matter in disturbed soils, such as those found on landslides. I conducted a decomposition experiment on five landslides in the Luquillo Mountains of Puerto Rico as part of an investigation of the successional roles of two of the most common plant colonists to landslides, Cecropia schreberiana Miq. (Cecropiaceae) a pioneer tree species, and Cyathea arborea (L.) Sm. (Cyatheaceae) a pioneer tree fern. I compared leaf litter decomposition over one year and the initial and 1‐yr chemistry for both species. Initial litter chemistry differed between the two species, as Cecropia had slightly higher nitrogen (9.2 mg/g) than Cyathea (8.2 mg/g) and higher lignin (28.6%) than Cyathea (26.0%), but water‐soluble carbon and nonpolar extractable carbon (fats and oils, waxes, chlorophylls) were higher in Cyathea than Cecropia. Total carbon, acid‐soluble carbon, total phosphorus, and pH did not differ significantly between leaf litter species. Across all five landslides, Cyathea (k= 0.93 ± 0.06) leaves decomposed significantly faster than Cecropia (k= 0.68 ± 0.06). The differences in these species leaf litter decomposition rates and chemical composition could potentially influence organic matter dynamics and nutrient cycling rates in these early successional systems.     

Barriers to Forest Regeneration in an Abandoned Pasture in Puerto Rico

Sources of forest regeneration (soil seed bank, seed rain) and barriers to seedling establishment were examined in a recently abandoned pasture in eastern Puerto Rico. Few woody species were found in the soil seed bank or in the seed rain. The number of seeds and species in the seed rain and soil seed bank declined with distance from the adjacent secondary forest. Nine species naturally dispersed and colonized plots during the study, with the wind‐dispersed tree Tabebuia heterophylla being the predominant colonizer (91% of all seedlings). Barriers to seedling establishment were determined using a blocked field experiment with eleven woody species representative of a variety of life histories. Each species was planted under the pasture vegetation (control) or in areas where all vegetation was removed (removal). Germination was enhanced for four species in the control treatment, five species were not affected, and two species did not germinate under either treatment. Survival to 6 months was higher in the removal treatment for two species. Seedling biomass was greater in the removal treatment at 12 months for one species. Seed mass was a good predictor of germination success and final shoot biomass, but not survival. This study demonstrates that seeding recently abandoned pastures with a mix of known pioneer species may accelerate the rate of secondary succession, but some species will have to be planted in later successional stages in order to overcome strong barriers to establishment.     

Large seasonal variation of soil respiration in a secondary tropical moist forest in Puerto Rico

Tropical forests are the largest contributors to global emissions of carbon dioxide (CO₂) to the atmosphere via soil respiration (R_s). As such, identifying the main controls on R_s in tropical forests is essential for accurately projecting the consequences of ongoing and future global environmental changes to the global C cycle. We measured hourly R_s in a secondary tropical moist forest in Puerto Rico over a 3‐year period to (a) quantify the magnitude of R_s and (b) identify the role of climatic, substrate, and nutrient controls on the seasonality of R_s. Across 3 years of measurements, mean R_s was 7.16 ± 0.02 μmol CO₂ m^‐2 s^‐1 (or 2,710 g C m^‐2 year^‐1) and showed significant seasonal variation. Despite small month‐to‐month variation in temperature (~4°C), we found significant positive relationships between daily and monthly R_s with both air and soil temperature, highlighting the importance of temperature as a driver of R_s even in warm ecosystems, such as tropical forests. We also found a significant parabolic relationship between mean daily volumetric soil moisture and mean daily R_s, with an optimal moisture value of 0.34 m³ m^‐3. Given the relatively consistent climate at this site, the large range in mean monthly R_s (~7 μmol CO₂ m^‐2 s^‐1) was surprising and suggests that even small changes in climate can have large implications for ecosystem respiration. The strong positive relationship of R_s with temperature at monthly timescales particularly stands out, as moisture is usually considered a stronger control of R_s in tropical forests that already experience warm temperatures year‐round. Moreover, our results revealed the strong seasonality of R_s in tropical moist forests, which given its high magnitude, can represent a significant contribution to the seasonal patterns of atmospheric (CO₂) globally.     

Nutrient Dynamics of Soil Derived from Different Parent Material on Barro Colorado Island,Panama1

I compared the concentrations of N, P, and S in both litter and mineral soil (0–15 cm depth) from three old‐growth, tropical moist forests on Barro Colorado Island (BCI), Panama. Each site was on a different substrate (i.e., parent material), but otherwise had similar climate, vegetation, and topography. There were no site differences in concentrations of N and S for either litter or soil. Concentrations of litter P and soil‐extractable P were greater for the andesite (igneous rock) site than for two sites on different sedimentary rocks; however, concentrations of several other litter and soil P fractions did not differ among sites. Patterns in soil P fractions suggested advanced soil development to the point that parent material has little control of P dynamics. Litter samples from each site, leached in the laboratory, released similar amounts of N, P, and S to the soil, indicating no differences in rates of turnover in the litter and in fluxes from litter into the mineral soil among sites. I expected more site differences in soil nutrient dynamics given vastly different parent materials and soil types (i.e., Oxisol vs. Alfisol) and very shallow soil on BCI that brings the parent material close to the plant root zone. Erosion and soil mixing may explain the uniformity in soil nutrient dynamics across the sites.     

Decay rate and substrate quality of fine roots and foliage of two tropical tree species in the Luquillo Experimental Forest,Puerto Rico

Decomposition rates, initial chemical composition, and the relationship between initial chemistry and mass loss of fine roots and foliage were determined for two woody tropical species, Prestoea montana and Dacryodes excelsa, over a gradient of sites in two watersheds in the Luquillo Experimental Forest, Puerto Rico. At all locations, fine roots decayed significantly more slowly than foliage during the initial 6 months.Substrate quality of the initial tissue showed marked differences between roots and foliage when using cell wall chemistry, secondary chemistry and total elemental analysis as indices. Quantity of acid detergent fiber (ADF) (non-digestible cell wall fiber) and lignin content were higher for roots than leaves: D. excelsa roots had 55.3% ADF and 28.7% lignin while leaves had 36.2% ADF and 11.8% lignin; P. montana roots had 68.0% ADF and 26.8% lignin while leaves had 48.5% ADF and 16.1% lignin. Aluminum concentrations were higher in fine roots (843 mg kg^–1 in D. excelsa, 1500 mg kg^–1 in P. montana) than leaves (244 mg kg^–1 in D. excelsa, 422 mg kg^–1 in P. montana), while calcium concentrations were higher in foliage (5.5 mg g^–1 in D. excelsa, 7.8 mg g^–1 in P. montana) than roots (3.4 mg g^–1 in D. excelsa, 3.1 mg g^–1 in P. montana). Nitrogen did not show any trend with tissue or species type. A linear model between mass remaining after 6 months and initial tissue chemistry could be developed only for calcium (r²=0.64).     

Lack of Ecotypic Differentiation: Plant Response to Elevation,Population Origin,and Wind in the Luquillo Mountains,Puerto Rico1

How important is ecotypic differentiation along elevational gradients in the tropics? Reciprocal transplants of two shrubs, Clibadium erosum (Asteraceae) and Psychotria berteriana (Rubiaceae), and a palm, Preitoea acuminata var. montana (Palmaceae), were used to test for the effect of environment and population origin on growth and physiology in the Luquillo Experimental Forest of Puerto Rico. Two sites were used, one at Pico del Este (1000 m in cloud forest) and one at El Verde (350 m in lower montane rain forest). At the cloud forest site, plastic barriers were erected around a subset of the plants to examine if protection from wind affected survival or biomass accumulation. Survival of C. erosum and P. berteriana was not affected by site, population origin, or the presence of barriers. For P. acuminata var. montana, survival was higher for plants with barriers, but not affected by site and population origin. Plants of C. erosum and P. berteriana at El Verde grew larger than at Pico del Este, but there was no effect of population origin or barrier treatment on biomass accumulation for these species. For P. acuminata var. montana, there was no effect of environment, population origin, or barrier treatment on biomass accumulation. Light‐saturated photosynthetic rate (A_max) of C. erosum, P. berteriana, and P. acuminata var. montana, as well as leaf anatomical characteristics of C. erosum, were unaffected by environment, population origin, and barrier treatment. On balance, there seems to be little evidence of ecotypic differentiation in these species along the gradient.     

Structural response of Caribbean dry forests to hurricane winds: a case study from Guánica Forest, Puerto Rico

Aim Tropical dry forests in the Caribbean have an uniquely short, shrubby structure with a high proportion of multiple‐stemmed trees compared to dry forests elsewhere in the Neotropics. Previous studies have shown that this structure can arise without the loss of main stems from cutting, grazing, or other human intervention. The Caribbean has a high frequency of hurricanes, so wind may also influence forest stature. Furthermore, these forests also tend to grow on soils with low amounts of available phosphorus, which may also influence structure. The objective of this study was to assess the role of high winds in structuring dry forest, and to determine whether soil nutrient pools influence forest response following hurricane disturbance. Location Guánica Forest, Puerto Rico. Methods Over 2000 stems in five plots were sampled for hurricane effects within 1 week after Hurricane Georges impacted field sites in 1998. Sprout initiation, growth, and mortality were analysed for 1407 stems for 2 years after the hurricane. Soil nutrient pools were measured at the base of 456 stems to assess association between nutrients and sprout dynamics. Results Direct effects of the hurricane were minimal, with stem mortality at < 2% and structural damage to stems at 13%, although damage was biased toward stems of larger diameter. Sprouting response was high – over 10 times as many trees had sprouts after the hurricane as before. The number of sprouts on a stem also increased significantly. Sprouting was common on stems that only suffered defoliation or had no visible effects from the hurricane. Sprout survival after 2 years was also high (> 86%). Soil nutrient pools had little effect on forest response as a whole, but phosphorus supply did influence sprout dynamics on four of the more common tree species. Main conclusions Hurricanes are able to influence Caribbean tropical dry forest structure by reducing average stem diameter and basal area and generating significant sprouting responses. New sprouts, with ongoing survival, will maintain the high frequency of multi‐stemmed trees found in this region. Sprouting is not limited to damaged stems, indicating that trees are responding to other aspects of high winds, such as short‐term gravitational displacement or sway. Soil nutrients play a secondary role in sprouting dynamics of a subset of species. The short, shrubby forest structure common to the Caribbean can arise naturally as a response to hurricane winds.     

Disentangling the long‐term effects of disturbance on soil biogeochemistry in a wet tropical forest ecosystem

Climate change is increasing the intensity of severe tropical storms and cyclones (also referred to as hurricanes or typhoons), with major implications for tropical forest structure and function. These changes in disturbance regime are likely to play an important role in regulating ecosystem carbon (C) and nutrient dynamics in tropical and subtropical forests. Canopy opening and debris deposition resulting from severe storms have complex and interacting effects on ecosystem biogeochemistry. Disentangling these complex effects will be critical to better understand the long‐term implications of climate change on ecosystem C and nutrient dynamics. In this study, we used a well‐replicated, long‐term (10 years) canopy and debris manipulation experiment in a wet tropical forest to determine the separate and combined effects of canopy opening and debris deposition on soil C and nutrients throughout the soil profile (1 m). Debris deposition alone resulted in higher soil C and N concentrations, both at the surface (0–10 cm) and at depth (50–80 cm). Concentrations of NaOH‐organic P also increased significantly in the debris deposition only treatment (20–90 cm depth), as did NaOH‐total P (20–50 cm depth). Canopy opening, both with and without debris deposition, significantly increased NaOH‐inorganic P concentrations from 70 to 90 cm depth. Soil iron concentrations were a strong predictor of both C and P patterns throughout the soil profile. Our results demonstrate that both surface‐ and subsoils have the potential to significantly increase C and nutrient storage a decade after the sudden deposition of disturbance‐related organic debris. Our results also show that these effects may be partially offset by rapid decomposition and decreases in litterfall associated with canopy opening. The significant effects of debris deposition on soil C and nutrient concentrations at depth (>50 cm), suggest that deep soils are more dynamic than previously believed, and can serve as sinks of C and nutrients derived from disturbance‐induced pulses of organic matter inputs.     

The Influence of Topography on Tree Growth,Mortality, and Recruitment in a Tropical Montane Forest1

To determine if there were consistent differences in growth, mortality, and recruitment on slopes and ridge crests in tropical montane forests, which could explain the (frequent but not universal) low stature of trees in the ridgetop forests, we analyzed data from long‐term plots in Jamaica (1990–1994; sixteen 200‐m² plots, six on ridge crests and five each on north and south slopes). Mortality was higher on north slopes, while growth and recruitment rates were not significantly different among positions. Soil pH and effects of recent disturbance by Hurricane Gilbert were positively correlated with growth and recruitment, while slope angle and disturbance effects were the best predictors of mortality. The patterns we found in Jamaica, that growth and recruitment were not higher on ridge crests than slopes, are different than those found by Herwitz and Young in Australia where growth and turnover were greater on a ridge crest. Therefore, it is not possible at present to make simple generalizations about dynamics of ridge crest versus slope forests in the montane tropics.     

The Effect of Phosphorus Availability on Decomposition Dynamics in a Seasonal Lowland Amazonian Forest

Once the weathering of parent material ceases to supply significant inputs of phosphorus (P), vegetation depends largely on the decomposition of litter and soil organic matter and the associated mineralization of organic P forms to provide an adequate supply of this essential nutrient. At the same time, the decomposition of litter is often characterized by the immobilization of nutrients, suggesting that nutrient availability is a limiting factor for this process. Immobilization temporally decouples nutrient mineralization from decomposition and may play an important role in nutrient retention in low-nutrient ecosystems. In this study, we used a common substrate to study the effects of native soil P availability as well as artificially elevated P availability on litter decomposition rates in a lowland Amazonian rain forest on highly weathered soils. Although both available and total soil P pools varied almost three fold across treatments, there was no significant difference in decomposition rates among treatments. Decomposition was rapid in all treatments, with approximately 50% of the mass lost over the 11-month study period. Carbon (C) and nitrogen (N) remaining and C:N ratios were the most effective predictors of amount of mass remaining at each time point in all treatments. Fertilized treatments showed significant amounts of P immobilization (P < 0.001). By the final collection point, the remaining litter contained a quantity equivalent to two-thirds of the initial P and N, even though only half of the original mass remained. In these soils, immobilization of nutrients in the microbial biomass, late in the decomposition process, effectively prevents the loss of essential nutrients through leaching or occlusion in the mineral soil.     

Effects of forest clearing and succession on the carbon and nitrogen content of soils in Puerto Rico and US Virgin Islands

Soil samples from mature and secondary forests and agricultural sites in three subtropical life zones of Puerto Rico and the US Virgin Islands were collected to determine the effects of forest conversion to agriculture and succession on soil organic carbon (C) and nitrogen (N) contents. Site characteristics that may affect soil C and N (slope, elevation, aspect, and texture) were as uniform as possible. Carbon contents (to 50 cm depth or bedrock) of cultivated sites, as a percent of corresponding mature forests, were lower in the wet (44%) and moist (31%) than in the dry (86%) life zones whereas N contents were relatively high regardless of life zone (60–130% of the mature forests). Conversion of forests to pasture resulted in less soil C and N loss than conversion to crops. The time for recovery of soil C and N during succession was approximately the same in all three life zones, about 40–50 yr for C about 15–20 yr for N. However, the rate of recovery of soil C was faster in the wet and moist life zone, whereas N appeared to recover faster in the dry life zone. Evidence for loss of soil C during cultivation and gain during succession to soil depths of 50–100 cm is presented.     

Biology of the Mongoose (Herpestes javanicus) in a Rain Forest of Puerto Rico1

Ecological aspects of the mongoose (Herpestes javanicus) were studied in the Luquillo Mountains, a rain forest region in eastern Puerto Rico. Information was obtained by removal trapping of mongoose from grids placed in tree plantations, colorado and tabonuco forests. Trapping efficiency was two mongoose per 100 trap-days, suggesting mongoose abundance is low in wet montane forests of Puerto Rico. Sex ratio was biased (2.6:1) in favor of males. Body masses of male mongoose inhabiting rain forests of the Luquillo Mountains were larger than those in dry forests at Guánica (P < 0.0001). Stomach contents from 18 mongoose were examined, animal matter comprised 75 percent of the total food items encountered. Of these, 33 percent were from vertebrates. The food items most frequently encountered were lizards (Anolis spp.), centipedes (Scolopendra spp.), and cockroaches (Blatellidae).     

The Demography of Miconia prasina (Melastomataceae) During Secondary Succession in Puerto Rico1

Miconia prasina (Melastomataceae) is an important colonizing species during early stages of secondary succession in Puerto Rico but its abundance declines with increasing stand age. We studied its demography for 2 yr (1995–1997) in three populations across a pasture to forest chronosequence (1‐, 12‐, and 25‐yr old) and in one population in a hurricane‐disturbed older forest (>60‐yr old). Vegetative growth was positive in the two young sites, but negative in the two older sites. The highest percentage of plants that flowered occurred in the two younger sites but highest seed production occurred in the 25‐yr‐old site. Although seed germination occurred in the two older sites, no seedlings established. Vegetative reproduction (root sprouts) was found in all sites, with most sprouts occurring in young sites. The 2 yr of census data were used to construct stage‐based transition matrices. In the two young sites, the average population growth rate was positive (λ > 1), while average population growth was negative in the two older sites (λ < 1). Elasticity, a measure of the effects of proportional changes in life‐history transitions to the long‐term population growth rate, varied across both stages and sites. Elasticity shifted from large plants in young sites to small plants in older sites. Across all sites, elasticities related to survival were more important than those of growth or vegetative reproduction. The demographic analysis supported the previous observations of changes in the density of M. prasina based on the chronosequence and also suggests that establishment via seeds must occur prior to abandonment in active pastures.     

Soil Seedbank in a Dipterocarp Rain Forest in Xishuangbanna,Southwest China1

Dipterocarp rain forest reaches its northern latitudinal limit in Xishuangbanna, Southwest China. We studied the soil seedbank of dipterocarp rain forest in Xishuangbanna during the dry and wet seasons. Results showed that there were large seed accumulations in both the dry (mean ± SD; 3925 ± 2533 seeds/m²) and wet seasons (5415 ± 3232 seeds/m²). One hundred and sixteen species of seed plants were identified from germination, 66 percent of which were woody species. Weed or pioneer species dominated the seedbank. The soil seedbank in Xishuangbanna had similar species composition as compared to those in tropical Asia, but higher seed storage reflects the intense disturbance and forest fragmentation in this area.