The importance of niche vs. neutral assembly mechanisms in structuring tropical tree communities remains an important unsettled question in community ecology [Bell G (2005) Ecology 86:1757-1770]. There is ample evidence that species distributions are determined by soils and habitat factors at landscape (<10 4 km 2 ) and regional scales. At local scales (<1 km 2 ), however, habitat factors and species distributions show comparable spatial aggregation, making it difficult to disentangle the importance of niche and dispersal processes. In this article, we test soil resource-based niche assembly at a local scale, using species and soil nutrient distributions obtained at high spatial resolution in three diverse neotropical forest plots in Colombia (La Planada), Ecuador (Yasuni), and Panama (Barro Colorado Island). Using spatial distribution maps of >0.5 million individual trees of 1,400 species and 10 essential plant nutrients, we used Monte Carlo simulations of species distributions to test plant-soil associations against null expectations based on dispersal assembly. We found that the spatial distributions of 36 -51% of tree species at these sites show strong associations to soil nutrient distributions. Neutral dispersal assembly cannot account for these plant-soil associations or the observed niche breadths of these species. These results indicate that belowground resource availability plays an important role in the assembly of tropical tree communities at local scales and provide the basis for future investigations on the mechanisms of resource competition among tropical tree species.community assembly ͉ niche differentiation ͉ tropical forest T he high local diversity of tropical tree communities poses a unique challenge for testing niche assembly theories based on resource competition (1). In these species-rich communities, hundreds of tree species can coexist in a single site (2), which renders assessment of the outcome of pairwise competitive interactions intractable. Conversely, the high diversity and relative rarity of most species also means that species seldom encounter each other in ecological neighborhood interactions (3), which suggests that competitive differences among species might not have a predictable effect on community structure. In fact, neutral theories of community assembly assume that there are no competitive differences among species and that ecological communities are assembled by random dispersal. Under neutral community assembly, alpha diversity would be governed by metacommunity diversity and speciation-extinction at macroecological scales (4, 5).Despite the contrasting mechanisms of community assembly proposed by neutral and niche theories, several lines of evidence support each of these perspectives. Tropical tree species differ in their light requirements for regeneration (6) because of a tradeoff between growth rate under high light and survival in the shade (7-9). Seedlings and saplings of different species differ in their resistance to pests, resulting in a frequency-dependent a...

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m -2 year -1 ), phosphorus (P as superphosphate, 5 g P m -2 year -1 ) and potassium (K as KCl, 5 g K m -2 year -1 ) within 38 ha of old-growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images.We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg -1 ), low concentrations of Bray-extractable phosphate (PO4 = 2.2 mg kg -1 ), and modest concentrations of KCl-extractable nitrate (NO3 = 5.0 mg kg -1 ) and KClextractable ammonium (NH4 = 15.5 mg kg -1 ). Added N increased concentrations of KCl-extractable NO3 and acidified the soil by one pH unit. Added P increased concentrations of Bray-extractable PO4 and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m -2 .The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species-rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.

We evaluated spatial heterogeneity for pH and a comprehensive set of nutrient and trace elements in surface (0-0.1 m depth) and subsurface (0.3-0.4 m depth) soils across 26.6 ha of old-growth, lowland, tropical moist forest, established on a highly weathered soil in Panama. Little is known about spatial heterogeneity patterns of soil properties in tropical forest soils. Soil was moderately acidic (pH 5.28) with low concentrations of exchangeable base cations (13.4 cmol c /kg), Bray-extractable PO 4 (2.2 mg/kg), KCl-extractable NO 3 (5.0 mg/kg), and KCl-extractable NH 4 (15.5 mg/kg). The coefficient of variation for soil properties ranged from 24% to >200%, with a median value of 84%. Geostatistical analysis revealed spatial dependence at a scale of 10-100 m for most of the soil properties; however, pH, NH 4 , Al, and B had spatial dependence at a scale up to 350 m. Best-fit models to individual variograms included random, exponential, spherical, Gaussian, linear, and power functions, indicating many different spatial patterns among the set of soil properties. Correlation among individual elements was poor, indicating independent patterns. Our results show complex spatial patterns in soil chemical properties and provide a basis for future investigations on soil-plant relationships and soil nutrient niche differentiation.

Abstract. We used the Hedley sequential fractionation scheme to assess phosphorus (P) chemistry of a strongly weathered soil from a humid lowland forest in Panama. Our analyses were part of a factorial experiment of nitrogen, P, and potassium addition, with nutrients added annually, i.e. a chronic input. The aim was to examine changes in soil P chemistry with 7 years of nutrient addition for soils collected in the wet season and the dry season. The majority of P occurred in fractions extracted by NaOH (24% of the total soil P) and hot concentrated HCl (58% of the total). Organic P (P o ) was~54% of extractable P. Labile P, defined as P o plus inorganic P (P i ) extracted by NaHCO 3 , was largely P o (84% of the NaHCO 3 -extractable P). Chronic P addition increased NaHCO 3 -extractable P o several-fold and NaOH-extractable P i two-fold. Seasonal variation occurred for labile P and NaOH-extractable P, whereas occluded P did not vary throughout the study period. Extractable P was~15% higher in surface than subsurface soil. We added 350 kg P ha -1 during the 7-year period and recovered~55% by sequential extraction. According to biogeochemical theory, added P should show up in fractions with the shortest residence times, e.g. labile P. Our finding that added P accumulated in fractions with presumably long residence times, i.e. extracted by NaOH (bound) and hot concentrated HCl (occluded), suggests that greater attention be paid to the short-term dynamics of bound and occluded P in strongly weathered tropical forest soils.