Abstract
The canopy is host to a large percentage of the flora and fauna in tropical wet forests and is distinct from the forest floor in plant richness, soil type and microclimate. In this study, we examined the influence of tree species and season on soil nutrient cycling processes in canopy soils of four tree species common to Costa Rican wet forests. We also compared the canopy soils to the associated forest floor mineral soils. Both tree species and season had strong effects on canopy soil nutrients and processes. Canopy soils from trees with high litter lignin concentrations had higher net N-mineralization rates and higher dissolved inorganic N concentrations than those with low lignin concentrations. During the dry season, net N-immobilization occurred and dissolved organic and inorganic N and available P concentrations were significantly higher than during the wet season. Overall, canopy soils had higher N levels and higher fungi + bacteria richness than forest floor mineral soils. The differences in canopy soil properties observed among tree species indicates that these species have distinct N cycles that reflect differences in both soil origin and biological controls.
Highlights
The rainforest canopy habitat supports a large percentage of the vascular flora, 25–35% (Gentry and Dodson 1987; Nieder et al 2001) as well as animal biota (i.e. Erwin 1997)
In this study we examined the effect of season on canopy soil nutrients and processes because rates of soil processes may vary with seasons, as microbes respond to changing environmental conditions
While we know there are vast differences between the canopy and forest floor habitats in microclimate (Cardelús and Chazdon 2005), species composition (Gentry and Dodson 1987; Nadkarni and Longino 1990; Watkins et al 2006) and soil composition (Nadkarni et al 2002), we know much less about how tree species influence canopy processes
Summary
The rainforest canopy habitat supports a large percentage of the vascular flora, 25–35% (Gentry and Dodson 1987; Nieder et al 2001) as well as animal biota (i.e. Erwin 1997). The high diversity of epiphytes is supported via nutrients coming principally from canopy soil (Nadkarni 1984), precipitation, throughfall and nitrogen-fixing lichen (Clark et al 1998b, c; Forman 1975; Liu et al 2002; Veneklaas 1990) whereas nutrient sources on the forest floor come principally from forest floor soils. The proportional contributions of each litter source to canopy soil is unknown, in a premontane rainforest in Costa Rica, host tree leaf attrition in the canopy was high, with 70% of intercepted litterfall lost within 2 weeks (Nadkarni and Matelson 1991). The much lower wind speeds in lowland forests (4.5– 11.5 km h−1, our study site La Selva Biological Station, Costa Rica and Brazil, respectively (Kruijt et al 2000; McCay 2003) suggest that residence time is likely higher in the canopy and host tree leaf litter may contribute to canopy soil composition
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