Joseph B. Yavitt

Soil nutrients influence spatial distributions of tropical tree species

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 (<104 km2) and regional scales. At local scales (<1 km2), 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.

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.

Methane Biogeochemistry and Methanogen Communities in Two Northern Peatland Ecosystems, New York State

Rates of methane (CH4) production vary considerably among northern peat-forming wetlands, and it is not clear whether variability is caused by environmental factors affecting CH4 production or differences in methanogen communities. We investigated CH4 production and emission dynamics concomitantly with 16S rRNA gene sequence-based community analysis of Archaea in two contrasting peat-forming northern wetlands, an ombrotrophic bog and a minerotrophic conifer swamp. Individual measurements of CH4 emissions to the atmosphere followed a lognormal distribution pattern in both sites, and mean rates were 30× greater in the bog site. Rates of CH4 production measured in vitro were initially 3× greater in the bog than in the conifer swamp; although, after 30 days of incubation, production rates were similar suggesting that in situ environmental conditions limited production in the conifer swamp. Amplified ribosomal DNA restriction analysis (ARDRA) and rarefaction techniques indicated that both sites had similar levels of archaeal richness, with 27 unique taxa in the bog and 23 taxa in the conifer swamp. However, the bog had more pronounced dominance of a few taxa, whereas the conifer swamp had more even distribution among taxa. A 16S rRNA gene sequence-based phylogenetic analysis indicated high levels of diversity with similarity to known methanogenic families Methanosarcinaceae, Methanosaetaceae, Methanobacteriaceae, and likely Methanomicrobiaceae as well as two additional lineages previously characterized as groups of yet uncultivated Euryarchaeota commonly occurring in flooded rice soils. Therefore, sites with low and high rates of CH4 production supported very diverse methanogenic communities.

Mountain pine beetle outbreaks in the Rocky Mountains: Regulators of primary productivity?

We consider the hypothesis that mountain pine beetles function as cybernetic regulators of primary productivity in ecosystems of lodgepole pine forest through their selective killing of dominant trees and the subsequent redistribution of resources. Following a recent major beetle outbreak in Yellowstone and Grand Teton national parks, surviving trees did grow significantly faster (P < .1); wood production was redistributed among canopy, subcanopy, and understory trees; and annual wood production per hectare usually returned to pre-attack levels or exceeded them within 10-15 yr. However, reconstructions of annual wood production over the last 70-80 yr indicate that the beetle outbreak did not reduce the variation in productivity; rather, the beetles introduced more variation than would have existed in their absence. Hence, our results do not support the hypothesis that the beetles function as cybernetic regulators (in the strict sense). Nevertheless, the beetle-pine system that we studied shows great resilience, and the effects of beetles on primary productivity do not appear to be as severe as conventional wisdom maintains. Annual wood production per hectare returned quickly to previous levels in the stands we studied, and associated ecological changes can be considered generally benign or even beneficial.

MERCURY CYCLING IN LITTER AND SOIL IN DIFFERENT FOREST TYPES IN THE ADIRONDACK REGION, NEW YORK, USA

The fate of mercury in decomposing leaf litter and soil is key to understanding the biogeochemistry of mercury in forested ecosystems. We quantified mercury dynamics in decomposing leaf litter and measured fluxes and pools of mercury in litterfall, throughfall, and soil in two forest types of the Adirondack region, New York, USA. The mean content of total mercury in leaf litter increased to 134% of its original mass during two years of decomposition. The accumulation pattern was seasonal, with significant increases in mercury mass during the growing season (+4.9% per month). Litterfall dominated mercury fluxes into the soil in the deciduous forest, whereas throughfall dominated fluxes into the coniferous forest. The increase in mercury mass in decomposing deciduous litter during the growing season was greater than could be accounted for by throughfall inputs during the growing season (P < 0.05), suggesting translocation of mercury from the soil to the decomposing deciduous litter. This internal recycling mechanism concentrates mercury in the organic horizons and retards transport through the soil, thereby increasing the residence time of mercury in the forest floor. A mass balance assessment suggests that the ultimate fate of mercury in the landscape depends upon forest type and associated differences in the delivery and incorporation of mercury into the soil. Our results show that incorporation of mercury into decaying leaf litter increases its residence time in the landscape and may further delay the recovery of surface waters, fish, and associated biota following control of mercury emissions to the atmosphere.

Production of methane and carbon dioxide in peatland ecosystems across North America: Effects of temperature, aeration, and organic chemistry of peat

Peat soil from 12 northern peatlands, spanning broad gradients in mean annual temperature (MAT), mean annual precipitation (MAP), and plant species composition, was incubated in vitro at differing temperature (2, 12, 22°C), aeration (anoxic, oxic), and with or without added glucose to evaluate controls on potential production of CH4 and CO2 (and CH4 consumption). Methane production and CH4 consumption (at 12°C) were significantly higher in open (nonforested) than forested peatlands, and varied as a function of MAT at each site, with maximum CH4 production (>450 nmol g‐1 d‐1) and minimum CH4 consumption (‐0.03 h‐1 g‐1) at intermediate MAT (i.e., CH4 production <200 nmol g‐1 d‐1 and CH4 consumption ‐0.06 h‐1 g‐1 at lower and higher MAT). Differences in lignin chemistry of the peat helped explain the variation in CH4 cycling: Added glucose stimulated CH4 production, but only in lignin‐rich peat. Carbon dioxide production (10–60 μmol g‐1 d‐1 at 12°C) showed a strong negative relationship with MAT and with the...

Isolation of a novel acidiphilic methanogen from an acidic peat bog

Acidic peatlands are among the largest natural sources of atmospheric methane and harbour a large diversity of methanogenic Archaea. Despite the ubiquity of methanogens in these peatlands, indigenous methanogens capable of growth at acidic pH values have resisted culture and isolation; these recalcitrant methanogens include members of an uncultured family-level clade in the Methanomicrobiales prevalent in many acidic peat bogs in the Northern Hemisphere. However, we recently succeeded in obtaining a mixed enrichment culture of a member of this clade. Here we describe its isolation and initial characterization. We demonstrate that the optimum pH for methanogenesis by this organism is lower than that of any previously described methanogen.

LITTER DECAY AND LEACHING FROM THE FOREST FLOOR IN PINUS CONTORTA (LODGEPOLE PINE) ECOSYSTEMS

SUMMARY (1) The dynamics of organic matter and nutrient elements in the forest floor of Pinus contorta spp. latifolia (lodgepole pine) forests in Wyoming, U.S.A., were investigated by combining three related measurements: leaching of the forest floor; long-term decay of leaf litter; and steady-state residence times in the forest floor. (2) Elemental fluxes (g m-2 yr-1) resulting from leaching by spring snowmelt were: Ca = 135, Mg = 0.30, K = 0*65, N = 0-46, P = 0.022 and C = 8.3. Over 95% of the N flux was in organic forms. (3) Most of the organic compounds were acidic, and anions of these organic acids were the principal mobile species accompanying cation flux; smaller, but important, contributions were S04 and HCO3. The chemical nature of the dissolved organic compounds changed as snowmelt proceeded, with polyphenolics and carbohydrates (leached from fresh litter) being important in the early stages, and polymeric acid compounds (fulvic and humic acids) dominating in the later stages. (4) About 35% of the original mass of leaf litter remained after 84-96 months. Potassium and magnesium were lost rapidly from leaf litter, whereas there was temporary immobilization of Ca. Nitrogen accumulated in decaying leaves for 72 months, reaching 180% of the original content before net mineralization and release occurred. Relatively small changes in P content of leaf litter were noted during the decay process. The loss of soluble organic constituents (polyphenols, carbohydrates, lipids) was more rapid than for solid residues (holocellulose, lignin). (5) The combination of approaches enabled several features of organic matter and nutrient dynamics in the forest floor, which would not be apparent using any of the methods in isolation, to be deduced: (i) about 30% of annual C release from the forest floor resulted from leaching of organic compounds; (ii) over 20% of the leaching flux for Ca and Mg was attributed to release from 01 litter, compared with less than 10% for K; (iii) high amounts of organic-N leaching from forest-floor layers, combined with rapid accumulation in 0- to 8-year-old litter, indicated rapid translocation of N from subsurface layers to 01 litter via heterotrophs; and (iv) leaching of polyphenols from the forest floor was 50% lower than the apparent loss from 01 litter, suggesting chemical transformation and changes in solubility of these compounds during litter decay. An association of this phenomenon with protein accumulation observed in the 02 horizon is postulated.

Methane production in contrasting wetland sites: response to organic-chemical components of peat and to sulfate reduction.

Abstract We used methane‐production measurements of slurried peat to study controls of methane production in six contrasting Appalachian wetland sites. The sites differed widely in plant‐community composition and in rates of methane production, which varied from 3 μmol/L/day in slurried‐peat samples from a shrub‐dominated bog to 216 μmol/L/day in peat from a spruce‐forested wetland. Three controlling factors of methane production were examined: organic‐chemical components of the peat (e.g., hot‐water soluble, sulfuric acid soluble, sulfuric acid insoluble), concentrations of dissolved organic carbon, and rates of sulfate reduction. Peats from shrub‐dominated sites contained mostly acid‐insoluble organic matter, which was presumably recalcitrant to microbial decomposition. In contrast, peats from moss‐and sedge‐dominated sites contained mostly acid‐soluble organic matter, which was presumably labile. Differences of organic‐chemical components of the peat could explain about 50% of the variation in rates of...

Methanoregula boonei gen. nov., sp. nov., an acidiphilic methanogen isolated from an acidic peat bog

A novel acidiphilic, hydrogenotrophic methanogen, designated strain 6A8(T), was isolated from an acidic (pH 4.0-4.5) and ombrotrophic (rain-fed) bog located near Ithaca, NY, USA. Cultures were dimorphic, containing thin rods (0.2-0.3 μm in diameter and 0.8-3.0 μm long) and irregular cocci (0.2-0.8 μm in diameter). The culture utilized H(2)/CO(2) to produce methane but did not utilize formate, acetate, methanol, ethanol, 2-propanol, butanol or trimethylamine. Optimal growth conditions were near pH 5.1 and 35 °C. The culture grew in basal medium containing as little as 0.43 mM Na(+) and growth was inhibited completely by 50 mM NaCl. To our knowledge, strain 6A8(T) is one of the most acidiphilic (lowest pH optimum) and salt-sensitive methanogens in pure culture. Acetate, coenzyme M, vitamins and yeast extract were required for growth. It is proposed that a new genus and species be established for this organism, Methanoregula boonei gen. nov., sp. nov. The type strain of Methanoregula boonei is 6A8(T) (=DSM 21154(T) =JCM 14090(T)).