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We evaluated the patterns of variation in the activity of four soil enzymes in oak forests soils at spatial scales from l0s of km to <3 m. The four enzymes (beta-glucosidase, chitinase, phenol oxidase, and acid phosphatase) are specific for substrates that vary widely in lability or recalcitrance. Significant variations in enzyme activity were observed at the regional (among forested areas), topographic (along elevation gradients within a watershed) and single-tree (1 m upslope and downslope of an individual tree) scales, but not at the local scale (contiguous watersheds within forested areas). However, the specific patterns of variation in relation to spatial scale were unique to each enzyme system. Detrended correspondence analysis (DCA) ordination of the activity of the four enzymes in each soil sample suggested a strong nutrient availability gradient underlying these spatial scale differences. Exploratory path analysis produced relatively strong predictive models based on soil nutrients, organic matter, and moisture for all the individual enzymes except phenol oxidase. However, path analysis produced an even stronger model for the activity of all four enzymes together, using the DCA axis scores as the dependent variable. The results indicate that the four enzyme systems could help resolve spatial dependencies at a range of scales and could also be used to develop a scale-independent metric to be used for regional analyses in a geographic information system environment.

This study examined foliar nutrient dynamics and nutrient resorption (retranslocation) in three species of Chilean Nothofagus (Fagaceae) that differed in leaf lifespan and elevational distribution. In our central Chile study area the elevations at which these three species are most abundant increase from N. obliqua (deciduous) at low elevations to N. dombeyi at intermediate elevation and N. pumilio (deciduous) at higher elevations up to treeline. We sampled a single stand at 1680 m in which all three species co-occurred. Nothofagus dombeyi leaves were structurally heavier, with specific leaf mass approximately twice that of the two deciduous species. On a concentration basis, foliar N increased in the order N. dombeyi < N. pumilio < N. oliqua and foliar P increase in the order N. dombeyi < N. obliqua < N. pumilio. However, when the differences in specific leaf mass among species were taken into account by calculating N and P content on a leaf area basis, N. dombeyi had the greatest N and P content. N and P remained relatively constant throughout most of the 4-yr N. dombeyi leaf lifespan, then decreased prior to abscission. Nothofagus dombeyi resorbed significantly less N (44-50%) than did the two deciduous species (63-78%), both on proportional and absolute bases. In contrast, N. pumilio and N. dombeyi resorbed similar amounts of P prior to abscission (40-50%), whereas no significant resorption of P from leaves of N. obliqua was noted. We use these results to clarify the relative importance of environmental gradients associated with elevation vs. genetically fixed leaf lifespans in controlling the nutrient dynamics of these congeneric tree species.

The complexity of natural ecosystems makes it difficult to compare the relative importance of abiotic and biotic factors and to assess the effects of their interactions on ecosystem development. To improve our understanding of ecosystem complexity, we initiated an experiment designed to quantify the main effects and interactions of several factors that are thought to affect nutrient export from developing forest ecosystems. Using a replicated 2 × 2 × 4 factorial experiment, we quantified the main effects of these factors and the factor interactions on annual calcium, magnesium, and potassium export from field mesocosms over 4 years for two Vermont locations, two soils, and four different tree seedling communities. We found that the main effects explained 56%-97% of total variation in nutrient export. Abiotic factors (location and soil) accounted for a greater percentage of the total variation in nutrient export (47%-94%) than the biotic factor (plant community) (2%-15%). However, biotic control over nutrient export was significant, even when biomass was minimal. Factor interactions were often significant, but they explained less of the variation in nutrient export (1%-33%) than the main effects. Year-to-year fluctuations influenced the relative importance of the main effects in determining nutrient export and created factor interactions between most of the explanatory variables. Our study suggests that when research is focused on typically used main effects, such as location and soil, and interactions are aggregated into overall error terms, important information about the factors controlling ecosystem processes can be lost.

To contribute to the un-derstanding of the long-term effects of atmospheric deposition on forests of eastern North America, we conducted a set of greenhouse experiments designed to determine the effect of reduced soil Ca:Al ratio on growth and competitive interactions of two common, co-occuring tree species. Second year seedlings of northern red oak (Quercus rubra), an ectomycorrhizal species currently declining in abundance in eastern North America, and yellow-poplar (Liriodendron tulipifera), an arbuscular mycorrhizal species currently increasing in abundance, were grown in intraspecific or interspecific pairs in either field soil or silica sand, and at Ca:Al ratio of 4 or 100. Overall relative growth rates (RGR) of the two species were similar; however, red oak allocated more new biomass to stem wood while yellow-poplar allocated biomass to leaves and roots. Reducing the Ca:Al ratio from 100 to 4 had no major effect on red oak growth but reduced RGR, leaf production, and N uptake significantly in yellow-poplar; leaf [N] was significantly higher in red oaks grown at Ca:Al of 4 than 100. Yellow-poplar grew better in interspecific pairs than in intraspecific pairs. In contrast, competitive mode had no effect on red oak growth. Analysis of soil N pools indicated that red oak was unable to make use of the inorganic N added during biweekly nutrient solution additions. In contrast yellow-poplar was able to deplete soil N pools rapidly and its growth was probably N limited at Ca:Al ratio of 100. The critical Ca:Al threshold for growth decline appeared to be <4.0 for red oak and >4.0 for yellow-poplar. Yellow-poplar outcompetes red oak under N enriched conditions, but its competitive advantage decreases at lowered Ca:Al ratio. We hypothesize that the current shift in tree species composition observed in eastern North America may be the consequence of N enrichment, but that the direction of these shifts may reverse as lowered Ca:Al ratio and other effects of N saturation develop.

The creation of a mathematical simulation model of photosynthetic microbial mats is important to our understanding of key biogeochemical cycles that may have altered the atmospheres and lithospheres of early Earth. A model is presented here as a tool to integrate empirical results from research on hypersaline mats from Baja California Sur (BCS), Mexico into a computational system that can be used to simulate biospheric inputs of trace gases to the atmosphere. The first version of our model, presented here, calculates fluxes and cycling of O 2, sulfide, and dissolved inorganic carbon (DIC) via abiotic components and via four major microbial guilds: cyanobacteria (CYA), sulfate reducing bacteria (SRB), purple sulfur bacteria (PSB) and colorless sulfur bacteria (CSB). We used generalized Monod-type equations that incorporate substrate and energy limits upon maximum rates of metabolic processes such as photosynthesis and sulfate reduction. We ran a simulation using temperature and irradiance inputs from data collected from a microbial mat in Guerrero Negro in BCS (Mexico). Model O 2, sulfide, and DIC concentration profiles and fluxes compared well with data collected in the field mats. There were some model-predicted features of biogeochemical cycling not observed in our actual measurements. For instance, large influxes and effluxes of DIC across the MBGC mat boundary may reveal previously unrecognized, but real, in situ limits on rates of biogeochemical processes. Some of the short-term variation in field-collected mat O 2 was not predicted by MBGC. This suggests a need both for more model sensitivity to small environmental fluctuations for the incorporation of a photorespiration function into the model.