Edmund V. J. Tanner

EXPERIMENTAL INVESTIGATION OF NUTRIENT LIMITATION OF FOREST GROWTH ON WET TROPICAL MOUNTAINS

This article reviews literature and summarizes experiments to investigate the extent to which productivity of tropical montane rain forests is constrained by low nutrient supply. On any one mountain, with increase in altitude foliar N decreases, and P and K usually decrease, but Ca and Mg show no consistent trend. However for a wide range of sites N, P, K, Mg, and Ca show no trends. Litterfall contents of N and P and often K, Ca, and Mg are lower in montane forests than in lowland forests, mainly because of reduced litterfall mass, but N and P concentrations are also lower in forests above 1500 m. Tropical montane soils usually have more soil organic matter per unit ground area; N mineralization levels are lower at higher altitudes in Costa Rica, and extractable and total soil P are lower in sites with lower litterfall P concentrations. We speculate that many lowland forests are limited by P and many montane forests by N. Fertilization studies on ash-derived montane soils in Hawai‘i showed a trend for a switch from N limitation on young soils to P, or N and P, limitation on soils over older substrates. Jamaican montane trees were limited by N and by P separately. Venezuelan montane trees were limited by N. The sites in Jamaica and Venezuela have soils of indeterminate age. Taken together these results show that nutrient limitation is widespread in montane soils (all sites have responded to at least one nutrient) and that the particular nutrient(s) that limit(s) production may differ for explicable reasons. First results from lowland forests on sandy soils in Kalimantan show N or simultaneous N and P limitation. Many more experiments, especially in lowland forests, are needed to test our speculation that P usually limits productivity in tropical lowland rain forests and that N limits productivity in tropical montane rain forests.

Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 x 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1-10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant (P = 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.

Global soil carbon: understanding and managing the largest terrestrial carbon pool

Carbon stored in soils worldwide exceeds the amount of carbon stored in phytomass and the atmosphere. Despite the large quantity of carbon stored as soil organic carbon (SOC), consensus is lacking on the size of global SOC stocks, their spatial distribution, and the carbon emissions from soils due to changes in land use and land cover. This article summarizes published estimates of global SOC stocks through time and provides an overview of the likely impacts of management options on SOC stocks. We then discuss the implications of existing knowledge of SOC stocks, their geographical distribution and the emissions due to management regimes on policy decisions, and the need for better soil carbon science to mitigate losses and enhance soil carbon stocks.

Relationships among net primary productivity, nutrients and climate in tropical rain forest: a pan-tropical analysis

Tropical rain forests play a dominant role in global biosphere-atmosphere CO(2) exchange. Although climate and nutrient availability regulate net primary production (NPP) and decomposition in all terrestrial ecosystems, the nature and extent of such controls in tropical forests remain poorly resolved. We conducted a meta-analysis of carbon-nutrient-climate relationships in 113 sites across the tropical forest biome. Our analyses showed that mean annual temperature was the strongest predictor of aboveground NPP (ANPP) across all tropical forests, but this relationship was driven by distinct temperature differences between upland and lowland forests. Within lowland forests (< 1000 m), a regression tree analysis revealed that foliar and soil-based measurements of phosphorus (P) were the only variables that explained a significant proportion of the variation in ANPP, although the relationships were weak. However, foliar P, foliar nitrogen (N), litter decomposition rate (k), soil N and soil respiration were all directly related with total surface (0-10 cm) soil P concentrations. Our analysis provides some evidence that P availability regulates NPP and other ecosystem processes in lowland tropical forests, but more importantly, underscores the need for a series of large-scale nutrient manipulations - especially in lowland forests - to elucidate the most important nutrient interactions and controls.

Hurricane effects on forest ecosystems in the Caribbean

Hurricanes are common, potentially catastrophic events for ecosystems in the Caribbean. We synthesize the work reported in this issue, together with the existing literature, to discuss effects of hurricanes on Caribbean ecosystems and to highlight priorities for future work. Comparisons of the impacts of hurricanes on different ecosystems are made difficult by the lack of detailed meteorological data, lack of prehurricane ecological data and differences between studies in types and timing of measurements made. Effects of recent hurricanes on Caribbean forest ecosystems include: defoliation, ranging from complete in lowland wet forest in Nicaragua after Hurricane Joan to negligible in parts of Jamaican montane forest after Hurricane Gilbert; felling of trees by uprooting and snapping (80% in Nicaragua to 14% in Jamaica); and tree mortality, which is rarely recorded and generally low (13% in the Yucatan following Hurricane Gilbert to 3% in Puerto Rican montane forests following Hurricane Hugo). Damage to individual trees varies with topographic location, stand characteristics, tree size (large ones uprooting and small ones snapping in Dominica during Hurricane David, but not in Jamaica), and species characteristics (such as wood density), but it is difficult to generalize about these factors. Effects on animal populations are both direct and through reductions in food supplies. Frugivorous and nectarivorous birds were more severely affected than insectivorous species in the Virgin Islands and Jamaica. There is little information about hurricane effects on insect populations, but populations of two species of walking sticks in Puerto Rico declined sharply after Hurricane Hugo. Numbers of adults of one frog species in Puerto Rico quadrupled after Hurricane Hugo, but numbers of juveniles were severely reduced by the storm. Effects of hurricanes on the physical environment include modified microclimates due to increased light penetration through defoliated canopies and landslides triggered by rainfall. Increased litterfall led to increases in some soil nutrients, and fine root biomass was drastically reduced in a Puerto Rican montane forest. Recovery of forest ecosystems from hurricanes depends on a combination of seedling growth and resprouting of canopy trees. In several studies, seed germination was promoted by higher light and/or higher temperature, but seedling mortality also increased. The relative importance of newly germinated seedlings, advance regeneration, and regrowth of damaged adults has not been studied. The few long-term studies of adult trees show the expected decline in the proportion of pioneer and intolerant species with time after disturbance. Hurricanes may be the most important factor controlling species composition and some aspects of ecosystem dynamics in the Caribbean; there is much still to be learned, and we suggest some priorities for future research.

Damage and responsiveness of Jamaican montane tree species after disturbance by a hurricane

A severe hurricane affected Jamaican montane rain forests in 1988. We made local and widespread measurements of tree condition in three periods: prehurricane 1974- 1984 (preh.); hurricane 1984-1989 (h.); and post-hurricane 1989-1992 (post-h.). In the h. period, 7.22% of stems and 4.72% of the total basal area died; crown loss was the most frequent cause of mortality. Among individual tree species, the hurricane caused a large range in mortality (0-26%) and non-fatal damage. Post-h. mortality was greater than mor- tality in the h. period, and varied among species. Post-h. stem growth rates (all species combined) were more than double the prehurricane rates, but species showed a considerable range from no significant increase to eight times greater. We classified 20 common tree species using damage scores (normalized h. mortality, plus normalized change in mortality from preh. to post-h., plus normalized complete crown loss in h.) and response scores (normalized change, preh. to post-h., in recruitment to the ?3 cm dbh size class, plus normalized change in growth rate from preh. to post-h., plus normalized frequency of sprouts). Species were assigned to one of four groups: resistant (11 species), with low damage and low response; susceptible (5 species), with high damage and low response; resilient (1 species), with high damage and high response; and usurpers (3 species), with low damage and high responsiveness. The grouping of species was broadly related to their regeneration requirements. Most species with seedlings usually found under closed canopy were resistant; three of the four species with seedlings usually found on landslides were also resistant. Species with seedlings most frequently found in gaps included resilient, susceptible, and usurper species, but were not usually resistant. It is likely that the three species classified as usurpers will increase their relative abun- dance in the forest in the next decades and that Cyathea pubescens, which was very susceptible, will decrease in relative abundance of adults. Most of the other species are likely to have small changes in their relative abundances. Thus, at present, hurricanes have few long-term effects on the forests, although a change in the disturbance regime may alter this.

Nitrogen dynamics of tropical agroforestry and annual cropping systems

Abstract The relative importance of the processes of SOM (maintenance of active soil organic matter) and SYNCHRONY (timing of release of organically-bound nutrients to coincide with crop demand) were assessed for their contribution to the maintenance of crop nitrogen availability in alley cropping. Alley cropping is a system of agroforestry where trees and crops are intercropped, the former being periodically pruned to produce mulch. Two maize alley cropping treatments, with Erythrina poeppigiana and with Gliricidia sepium , were compared to sole-cropped maize in an 8 yr old experiment at CATIE in Costa Rica. Maize productivity, maize N uptake, and N release from mulch and crop residue decomposition were measured each month during one cropping cycle. The effects of changes in active soil organic matter (SOM) on available N were assessed by measuring field N mineralization and the size of the microbial N pool through the cropping season. Two sub-treatments were introduced to assess the contribution of a current mulch application to maize N uptake (1) removing the mulch, and (2) applying 15 N labelled mulch. Monthly sampling of 15 N in the mulch, microbial biomass, and maize allowed assessment of the SYNCHRONY of mulch N release and crop uptake. Maize biomass and maize N content, N release from mulch and residue decomposition, and N mineralization were all higher in the alley crop than the sole crop by 2.2-, 2.8-, 5.0- and 2.1-fold respectively. Soil microbial N was not significantly different between treatments, but increased by 80% during the cropping season. Maize grown in the alley crop with the mulch removed contained only 3–15% less N at maturity. Similarly 15 N labelled mulch only contributed about 10% of crop N. The percentage contribution of mulch 15 N to the maize declined from 13–14% 30 days after planting to 8–11% 100 days after planting. Total recovery of mulch N by the maize was only about 10 kg ha −1 and almost all of this was taken up by 60 days after planting. The contribution of mulch N to weed N content declined from 15–24% 7 weeks after mulch application to 2–6% 9 months after application. Mulch N contributed only 3–5% of the microbial N pool at 40 days and this fell to zero by 105 days. The higher rates of N mineralization under the alley crop compared to rates under the sole crop led to faster establishment of the maize in the alley crop and maintained higher rates of N accumulation thereafter. These higher rates of N mineralization resulted from the build up of readily-mineralizable organic N in the soil over the 7 yrs of tree mulch application. The size of the microbial N pool was not to be related to nitrogen availability nor organic residue inputs. Mulch N released during a cropping season accounted for about 15% of the increase in N uptake by maize. Transfer of mulch N to the crop may have been restricted by the low incorporation of mulch N into the microbial biomass. The long-term build-up of the SOM reserve of mineralizable organic N was more important than the SYNCHRONY of mulch N release and crop uptake in determining the substantially higher productivity and N uptake in the alley crop compared to the sole crop.

EFFECTS OF ABOVE- AND BELOWGROUND COMPETITION ON GROWTH AND SURVIVAL OF RAIN FOREST TREE SEEDLINGS

Because reducing aboveground competition for photosynthetically active radiation (PAR) increases growth and survival rates of tropical rain forest seedlings, belowground competition for nutrients is often assumed to be of little or no importance. We tested this assumption. We measured the growth, over one year, of seedlings of Aspidosperma carapanauba (shade tolerator) and Dinizia excelsa (light demander) transplanted into understory (high aboveground competition; 1% incoming photosynthetic photon flux density [PPFD]) and single treefall gap areas (lower aboveground competition; 6% PPFD in centers of gaps) without and with trenches cut around plots (high and low belowground competition). Over the wetter six months, location in gaps significantly increased relative height growth rates by 320% and 570%, and relative leaf production rates by 190% and 280%, while trenching significantly increased height by 330% and 520%, and leaf production by 170% and 260%, for Aspidosperma and Dinizia, respectively, with significant interactions between treatments. Trenching approximately doubled height growth in the understory. Leaf loss rates were unaffected by either treatment but were 3.7 times higher in Dinizia compared with Aspidosperma. Aspidosperma could persist in the understory, as leaf production and loss were balanced at 0.60% PPFD, whereas Dinizia could not, as it required 2.4% PPFD. Reducing either aboveground competition or belowground competition allowed Dinizia to persist as leaf production was increased above unchanging leaf loss rates. Gap and trenching treatments both significantly reduced mortality rates in Dinizia. Location in gaps significantly reduced mortality rates in Aspidosperma. Both species significantly increased allocation to leaves and significantly decreased allocation to roots in response to trenching. The increased growth in the wet season caused by trenching, and increased foliar nitrogen and phosphorus amount, showed that both species were probably competing with adult trees for nutrients as well as PAR. We deduced that phosphorus did not limit growth; however, we could not deduce which nutrient did limit growth. Given that 60–70% of the worlds tropical rain forests occur on low-fertility oxisols, intense adult–juvenile competition both aboveground and belowground may be the norm, not the exception.

Responses of woody plant seedlings to edge formation in a lowland tropical rainforest, Amazonia

Abstract Forest fragments surrounded by pasture are likely to be influenced by edge effects. This paper describes short-term effects of edge creation on woody seedling dynamics in an Amazonian lowland rainforest, using data collected between nine months before and 16 months after creation of an abrupt forest-pasture edge. Starting in December 1989, recruitment, mortality, and growth of plants ⩽250 cm tall were monitored in plots located in the interior and at 5, 10, 25, 50, 100 and 250 m from the edge of a 100-ha forest fragment. In each plot, temperature and relative humidity were measured three times, while hemispherical photographs were taken after edge creation to estimate vegetation cover. Seedling recruitment increased along transects at 5 and 10 m from the edge following edge creation; pioneer species (especially Cecropia spp.) were recruited up to 10 m from the edge where they had previously been absent. Seedling mortality did not increase following edge formation. Seedlings within 10 m of the edge exhibited accelerated vertical growth, with a logarithmic decline in the effect with distance from the edge. Increased vertical growth was most pronounced for plants recruited at about the time of edge formation. Temperature and vapor pressure deficit were elevated following edge formation to ca. 50 m into the forest, and both declined exponentially with distance from the edge. Over the first 15 months following edge creation, recruitment and growth of woody seedlings was increased only up to 10 m from new edges, and mortality was unchanged, suggesting that the edge effect for woody seedlings was only about 10 m wide.

Increased litterfall in tropical forests boosts the transfer of soil CO2 to the atmosphere.

Aboveground litter production in forests is likely to increase as a consequence of elevated atmospheric carbon dioxide (CO2) concentrations, rising temperatures, and shifting rainfall patterns. As litterfall represents a major flux of carbon from vegetation to soil, changes in litter inputs are likely to have wide-reaching consequences for soil carbon dynamics. Such disturbances to the carbon balance may be particularly important in the tropics because tropical forests store almost 30% of the global soil carbon, making them a critical component of the global carbon cycle; nevertheless, the effects of increasing aboveground litter production on belowground carbon dynamics are poorly understood. We used long-term, large-scale monthly litter removal and addition treatments in a lowland tropical forest to assess the consequences of increased litterfall on belowground CO2 production. Over the second to the fifth year of treatments, litter addition increased soil respiration more than litter removal decreased it; soil respiration was on average 20% lower in the litter removal and 43% higher in the litter addition treatment compared to the controls but litter addition did not change microbial biomass. We predicted a 9% increase in soil respiration in the litter addition plots, based on the 20% decrease in the litter removal plots and an 11% reduction due to lower fine root biomass in the litter addition plots. The 43% measured increase in soil respiration was therefore 34% higher than predicted and it is possible that this ‘extra’ CO2 was a result of priming effects, i.e. stimulation of the decomposition of older soil organic matter by the addition of fresh organic matter. Our results show that increases in aboveground litter production as a result of global change have the potential to cause considerable losses of soil carbon to the atmosphere in tropical forests.