Litterfall and litter nutrient content in two Brazilian Tropical Forests
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Discussion The values of litterfall reported here are low when compared to values reported in the literature for tropical rain forests (Proctor 1984). Atlantic Forest ecosystems investigated by Varjabedian & Pagano (1988), Oliveira & Lacerda (1993) and Lopes et al. (1994) at latitudes similar to that of Cardoso Island showed 7.9, 8.9, and 8.3 t.ha-1.year-1 of litterfall, respectively (table 6). 
Litterfall at AF was lower than that recorded for semi-deciduous forests of south-east Brazil. Although located in the same latitudinal zone as Cardoso Island, these forests lie on more fertile soils (Pagano 1989a) subjected to a typical wet-dry tropical climate which results in the co-existence of deciduous and semi-deciduous species. These ecosystems present different functional patterns as shown by the amount and by the seasonality of litterfall during the dry season (table 6). Bray & Gorham (1964) found a linear inverse relationship between latitude and litterfall, according to which the expected value for AF would be around 7.5-8.0 t.ha-1.year-1. Although low, the results obtained here are still within the range of those reported by those authors. Meentmeyer et al. (1982) propose that litterfall could be determined by a complex of environmental variables such as potential and actual evapotranspiration and latitude. According to this model, the amount of litter to be produced in south-east Brazil would be around 6.0 t.ha-1.year-1, a value very similar to that obtained here. Lonsdale (1988) presents a multiple regression model with latitude, altitude and precipitation as predictors (log x = - 0.0012L - 0.000072A + 1.04), which estimates AF litterfall to be 5.4 t.ha-1.year-1. However, the model for leaf litterfall (log y = - 0.0090L + 0.80) predicts a full rate of 3.8 t.ha-1.year-1 as compared to 3.9 t.ha-1.year-1 in this study. This could be related to the high precision of the method in quantifying leaves, although its precision is not very good for reproductive parts and twigs. Another model proposed by Vogt et al. (1986) using latitude (y = 9457 - 120x), shown to be more accurate, resulted in an estimate of 6.5 t.ha-1.year-1 for AF litterfall. Local factors, such as nutrient availability (Silver 1994), may explain the results obtained for the Restinga forests. Some of those ecosystems present very low litter production, not fitting well to the values predicted by these models. The present study confirms the observations of Silver (1994) about the availability of nutrients, especially P, as a determinant of litter production. Peak production occurs at the beginning of rainy season when leaf fall is greater as predicted by the production curve. This pattern differs from most investigations conducted in tropical ecosystems, where the highest deposition of litter occurred in the dry season (Klinge & Rodrigues 1968a, Klinge 1977, Silva 1984, Morellato 1987, Dantas & Phillipson 1989, Scott et al. 1992, César 1993a, Ramos & Pellens 1994). In regions where the dry season is not distinct as in the Atlantic Forest, high deposition is observed in the rainy period (Jackson 1978, Domingos et al. 1990, Leitão Filho et al. 1993, Britez 1994). According to Jackson (1978) leaf fall peaks during the rainy season occur in regions where water stress is moderate and is simultaneous with the production of new leaves. This appears to be the strategy at Cardoso Island where there is no water deficit during the winter and the leaves may be renewed in the summer, when the environmental conditions are more favourable. The fact of the peak of RF leaf litter production take place two months after that of AF may be due to greater water holding capacity in AF soil, which could permit leaf renewal early at the first rains (150 mm). The relative contribution of the litter components is similar to data reported in the literature for both ecosystems. The contribution of reproductive parts in AF litter is among the highest reported in the literature (table 6) suggesting a vegetation which is vigorous enough to allocate a great part of its energy for reproduction while at the same time requiring frequent replacement. Mean concentrations of macronutrients in leaf litter at AF are in the range of other rain forests (Proctor 1984). Values at RF were lower, mainly those of N and P, making evident that litter quality is unique to each ecosystem, according to soil properties. High concentrations of N and P in the miscellaneous material may be explained by the presence of residual material from the other fractions and from animal vestiges. Reproductive parts contained high concentrations of K, probably due to the fact that this element is stored primarily in young and metabolically active tissues while deciduous leaves lose this element by translocation and before and after decay by leaching. Ca was concentrated in leaves and twigs, Mg in leaves and S in leaves and reproductive parts. The magnitude of mineral flux unequivocally distinguishes the two forests. RF produces lower amounts of low quality litter leading to a smaller return of mineral elements to soil. This becomes evident by comparing the ratio between litterfall at AF and that at RF (1.6) to the ratio between the return of each element in these ecosystems (N: 3.6; P: 4.0; K: 3.0; Ca: 2.0 and Mg: 1.7). Leaf fall determined the seasonality of mineral return to soil in both areas. A seasonal pattern was therefore derived from curve of litter production and not from concentration values which where quite uniform during the entire period of study.
RF has a reduced stock of available nutrients in the soil and biomass and receives inputs from adjacent systems (Hay & Lacerda 1984). AF, on the other hand, receives proportionally lower additional inputs and its nutrient pools are higher, either in the soil or in biomass. These nutrients are continuously and efficiently recycled, probably due to the better soil condition. The main function of litter at AF is the transference of mineral nutrients from the vegetation to the soil, while at RF litter seems to have also a very important role in improving edaphic conditions, increasing the water and ion retention capacity. Its soils are almost free from clay which makes organic matter from litter the main colloidal source. Micronutrients are not often quantified in litterfall studies, what restricts the possibilities for comparisons. Micronutrient concentrations were constant across the leaf fall cycle, but the values for Na were greater during the winter. This is probably due to greater accumulation of marine aerosols on the forest canopy, as rainfall is lower at that season. The annual return of micronutrients to the soil was higher in the AF site than in RF site, both within ranges reported by Klinge & Rodrigues (1968a, b), Pagano (1989b), Domingos et al. (1990) and César (1993b) for Brazilian ecosystems. The ratio between litterfall mass and litterfall nutrient content gives the nutrient use efficiency (NUE) (Vitousek 1982). The mesophyll semi-deciduous forests at São Paulo are inefficient in the use of nitrogen although great amounts of litter are produced (NUE = 51 - Meguro et al. 1979; NUE = 44 - Pagano 1989a, b; NUE = 45 - César 1993a, b). The Atlantic Forest at Cardoso Island presented a NUE of 62, which is equivalent to that reported by Klinge & Rodrigues (1968a, b) for an Amazonian terra-firme forest. Restinga Forest, on the other hand, with a NUE of 143, is very efficient in the use of N, similar to coniferous forests, showing a high level of adaptation to the low nutrient availability. It is therefore an ecosystem with a highly specialized vegetation, able to develop under conditions that would be adverse to the great majority of species from other plant communities. Acknowledgements - To Kevin Creagan, of Louisiana State University, for the translation, and to Dr. Marcos S. Buckeridge for some suggestions.
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