Several lines of evidence suggest that nitrogen in most tropical forests is relatively more available than N in most temperate forests, and even that it may function as an excess nutrient in many tropical forests. If this is correct, tropical forests should have more open N cycles than temperate forests, with both inputs and outputs of N large relative to N cycling within systems. Consequent differences in both the magnitude and the pathways of N loss imply that tropical forests should in general be more 15N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition 15 of tree leaves and soils from a variety of tropical and temperate forests. Foliar 8 N values from tropical forests averaged 6.5%o higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in 15N than other tropical forests. The average 315N values for tropical forest soils, either for surface or for depth samples, were almost 8% higher than temperate forest soils. These results provide another line of evidence that N is relatively abundant in many tropical forest ecosystems. Table 1. 315N (%) values of plant species. %N is the nitrogen concentration (%). Species Site Region Country 15N % N Ref

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.

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Several lines of evidence suggest that nitrogen in most tropical forests is relatively more available than N in most temperate forests, and even that it may function as an excess nutrient in many tropical forests. If this is correct, tropical forests should have more open N cycles than temperate forests, with both inputs and outputs of N large relative to N cycling within systems. Consequent differences in both the magnitude and the pathways of N loss imply that tropical forests should in general be more 15N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition 15 of tree leaves and soils from a variety of tropical and temperate forests. Foliar 8 N values from tropical forests averaged 6.5%o higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in 15N than other tropical forests. The average 315N values for tropical forest soils, either for surface or for depth samples, were almost 8% higher than temperate forest soils. These results provide another line of evidence that N is relatively abundant in many tropical forest ecosystems. Table 1. 315N (%) values of plant species. %N is the nitrogen concentration (%). Species Site Region Country 15N % N Ref

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The distribution of tree biomass and the allocation of organic matter production were measured in an ll-yr-old Pinus caribaea plantation and a paired broadleaf secondary forest growing under the same climatic conditions. The pine plantation had significantly more mass aboveground than the secondary forest (94.9 vs 35.6 t ha -1 for biomass and 10.5 vs 5.0 t ha -1 for litter), whereas the secondary forest had significantly more fine roots (~<2 mm diameter) than the pine plantation (10.5 and 1.0 t ha-1, respectively). Standing stock of dead fine roots was higher than aboveground litter in the secondary forest. In contrast, aboveground litter in pine was more than ten times higher than the dead root fraction. Both pine and secondary forests had similar total organic matter productions (19.2 and 19.4tha-lyr -1, respectively) but structural allocation of that production was significantly different between the two forests; 44% of total production was allocated belowground in the secondary forest, whereas 94% was allocated aboveground in pine. The growth strategies represented by fast growth and large structural allocation aboveground, as for pine, and almost half the production allocated belowground, as for the secondary forest, illustrate equally successful, but contrasting growth strategies under the same climate, regardless of soil characteristics. The patterns of accumulation of organic matter in the soil profile indicated contrasting nutrient immobilization and mineralization sites and sources for soil organic matter formation.

We studied the rates and patterns of carbon and nutrient¯uxes in litterfall in ten tropical tree plantation species grown at the USDA Forest Service Arboretum in the Luquillo Experimental Forest, Puerto Rico. The stands were 26-years old and grew under similar climatic and edaphic conditions. Individual plantation species ranked differently in terms of their capacity to return mass and speci®c nutrients to the forest¯oor, and with respect to their ef®ciency of nutrient use. The species that returned the most mass did not return the most P, N, or cations. Moreover, species differed according to the amount of N and P resorption before leaf fall. These differences re¯ect the variation in the ecophysiological response of each species to edaphic and climatic conditions. The difference between average and minimum resorption values of the species studied indicate that other environmental factors, such as heavy winds or the physical effects of heavy rains, can force the shedding of nonsenesced leaves. This higher quality material, although not very much in quantity, can provide a small pulse of available nutrients to the forest¯oor community. The same holds true for other high nutrient/low mass fractions of litterfall such as reproductive parts and miscellaneous materials.In areas with no prevalent or strongly seasonal water limitations, temporal variations of leaf litter on the forest¯oor are the combined result of the rate of fall and decomposition of the falling material, and the diverse responses of species to different environmental cues. Leaf fall was inversely correlated to reduced water availability in three of the species studied. Leaf fall of the other species was correlated either to daylight duration or minimum temperatures. The results highlight the importance of understanding species performance relative to nutrient and mass metabolism before selection for plantation use, or for rehabilitation of degraded lands. #