Springer Science and Business Media LLC

Gross primary productivity (GPP) often is estimated at regional and global scales by multiplying the amount of photosynthetically active radiation (PAR) absorbed by the plant canopy (PARa) by light-use efficiency (ε g ; GPP/PARa). Mass flux techniques are being used to calculate ε g . Flux-based estimates of ε g depend partly on how PAR absorption by plants is modeled as a function of leaf area index (LAI). We used CO2 flux measurements from three native grasslands in the Great Plains of USA to determine how varying the value of the radiation extinction coefficient (k) that is used to calculate PARa from LAI affected variability in estimates of ε g for each week. The slope of linear GPP–PARa regression, an index of ε g , differed significantly among the 18 site-years of data, indicating that inter-annual differences in ε g contributed to the overall variability in ε g values. GPP–PARa slopes differed among years and sites regardless of whether k was assigned a fixed value or varied as an exponential function of LAI. Permitting k to change with LAI reduced overall variability in ε g , reduced the slope of a negative linear regression between seasonal means of ε g and potential evapotranspiration (PET), and clarified the contribution of inter-annual differences in precipitation to variation in ε g . Our results imply that greater attention be given to defining dynamics of the k coefficient for ecosystems with low LAI and that PET and precipitation be used to constrain the ε g values employed in light-use efficiency algorithms to calculate GPP for Great Plains grasslands.

Sự chuyển đổi chế độ, hay những thay đổi lớn trong hệ sinh thái, thường liên quan đến các ngưỡng trong các yếu tố như khí hậu, thay đổi đất đai, dòng chảy dinh dưỡng, hoặc các yếu tố khác. Một ví dụ thường được nghiên cứu là hiện tượng phú dưỡng, đây là một vấn đề môi trường nghiêm trọng của các hồ và đập nước liên quan đến sự gia tăng phốt pho (P) vượt ngưỡng. Chúng tôi đã ước lượng các phân phối xác suất của các ngưỡng đối với hiện tượng phú dưỡng của hồ Mendota, Wisconsin, Hoa Kỳ bằng cách sử dụng 30 năm ngân sách P hàng năm. Mặc dù các ngưỡng có khả năng ảnh hưởng tới hiện tượng phú dưỡng của hồ (xác suất 96,6%), nhưng các phân phối xác suất của các ngưỡng lại trải dài trên một khoảng rộng của các tỷ lệ tải P. Các khuyến nghị quản lý nhất quán với các mô hình đơn giản hơn khuyến nghị mục tiêu tải P gần hoặc thấp hơn các tải P thấp nhất được quan sát trong 30 năm qua. Nếu tải trọng tăng, có nguy cơ đáng kể về việc vượt qua một ngưỡng gây ra hiện tượng phú dưỡng bền vững với nồng độ P cao trong hồ và chất lượng nước kém. Ngược lại, nếu tải trọng giảm, sẽ có khả năng vượt qua ngưỡng giảm thiểu, dẫn đến giảm mạnh nồng độ P và cải thiện chất lượng nước. Việc xem xét những rủi ro này sẽ gia tăng ước tính các lợi ích kinh tế ròng của việc tải P thấp hơn. Phân tích của chúng tôi minh họa một quy trình ước lượng các phân phối xác suất cho các ngưỡng của sự chuyển đổi chế độ hệ sinh thái. Mặc dù các phân phối xác suất ngưỡng có thể rộng với đuôi dày, chúng vẫn cung cấp thông tin quan trọng về các hậu quả tiềm tàng của các lựa chọn chính sách thay thế.

Global nitrogen (N) deposition generally reduces ecosystem stability. However, less is known about the responses of ecosystem stability and its driving mechanisms under different N addition gradients. We conducted a four-year N addition experiment in an alpine meadow, using six levels of N addition rates (0, 2, 4, 8, 16, 32 g N m−2 year−1) to examine the effects of N addition on plant community biomass stability and the underlying mechanisms. We found that the stability of ecosystem aboveground net primary productivity (ANPP) decreased linearly with increasing N addition rates, even though it had no effect on plant species richness at low N addition rates and significantly reduced species richness at high N addition rates. The most remarkable finding is that the main mechanism underlying ecosystem stability shifted with N addition rates. The decrease of common species stability contributed most to the reduction of plant community biomass stability under low N addition rates (N0–N4), whereas the decrease of species asynchrony contributed most to the reducing plant community biomass stability under high N addition rates (N8–N32). Our results indicate that species diversity was not a significant predictor of plant community biomass stability in this alpine meadow, which challenges the traditional knowledge. This study highlights the shifts of main mechanism regulating plant community biomass stability under different N addition rates, and suggests that continuous nitrogen deposition in the future may reduce ecosystem stability and potentially impeding the sustainable provision of ecosystem functions and services.

Growing evidence indicates that parasites—when considered—can play influential roles in ecosystem structure and function, highlighting the need to integrate disease ecology and ecosystem science. To strengthen links between these traditionally disparate fields, we identified mechanisms through which parasites can affect ecosystems and used empirical literature searches to explore how commonly such mechanisms have been documented, the ecosystem properties affected, and the types of ecosystems in which they occur. Our results indicate that ecosystem-disease research has remained consistently rare, comprising less than 2% of disease ecology publications. Existing studies from terrestrial, freshwater, and marine systems, however, demonstrate that parasites can strongly affect (1) biogeochemical cycles of water, carbon, nutrients, and trace elements, (2) fluxes of biomass and energy, and (3) temporal ecosystem dynamics including disturbance, succession, and stability. Mechanistically, most studies have demonstrated density-mediated indirect effects, rather than trait-mediated effects, or direct effects of parasites, although whether this is representative remains unclear. Looking forward, we highlight the importance of applying traits-based approaches to predict when parasites are most likely to exert ecosystem-level effects. Future research should include efforts to extend host–parasite studies across levels of ecological organization, large-scale manipulations to experimentally quantify ecosystem roles of parasites, and the integration of parasites and disease into models of ecosystem functioning.

We examined the effects of elevated CO2 and O3 and their interaction on leaf litter chemistry and decomposition in pure stands of aspen (Populus tremuloides) and mixed stands of birch (Betula papyrifera) and aspen at the Aspen Free Air CO2 Enrichment (FACE) experiment. A 935-day in situ incubation study was performed using litterbags filled with naturally senesced leaf litter. We found that elevated CO2 had no overall effects on litter decomposition rates, whereas elevated O3 reduced litter mass loss (−13%) in the first year. The effect of O3 on mass loss disappeared in the second year. For aspen litter but not mixed birch-aspen litter, decomposition rates were negatively correlated with initial concentrations of condensed tannins and phenolics. Most soluble components (94% of soluble sugars, 99% of condensed tannins, and 91% of soluble phenolics) and any treatment effects on their initial concentrations disappeared rapidly. However, the mean residence time (MRT) of birch-aspen litter (3.1 years) was significantly lower than that of aspen litter (4.8 years). Further, because of variation in total litterfall, total litter mass, C, lignin and N remaining in the ecosystem was highest under elevated CO2 and lowest under elevated O3 during the incubation period. Our results indicate that elevated CO2 and O3 can alter short-term litter decomposition dynamics, but longer-term effects will depend more on indirect effects mediated through changes in forest community composition. Treatment effects on soluble components are likely to influence cyclical microbial processes and carbon pulses in the ecosystem only when coupled with increased (CO2) or decreased (O3) litter inputs.

Increasing global temperature and changes in the precipitation regime affect the global carbon cycle by altering the process of organic matter decomposition. Temporary aquatic systems are especially susceptible to climate change. We hypothesized that water availability and temperature affect the early and late stages of decomposition of litter differently and determine the decomposition rates according to litter type. We conducted two decomposition experiments using green (Camellia sinensis L.) and mint (Mentha piperita L.) tea in commercial bags. In the laboratory experiment, we incubated the bags at two contrasting temperatures (4 and 15°C) and in three simulated hydroperiods (M: moist, MS: submerged after 14 days, S: submerged). A field experiment was carried out in winter and spring in nine temporary wetlands (meadows) along a precipitation gradient (from forest to steppe ecosystems) in the Argentinean Patagonia. Water stimulated the leaching of soluble substances in the S treatment and was the conducting factor in early decomposition stages. Temperature stimulated tea decomposition in advanced stages, and both water and temperature exerted a different response depending on the litter type. In the field experiment, mass loss in meadows was determined by the hydroperiod condition, both in winter and spring. Detritus type was the controlling factor in steppe meadows, but on forest meadows water level stimulated both litter types, and temperature increased decomposition. Under the expected increase of temperature and decrease of precipitations in future climate scenarios, organic matter accumulation would increase in steppe meadows and decomposition would be higher in forest meadows.

Permafrost thaw releases nutrients and metals from previously frozen soils and these nutrients may affect important biogeochemical processes including methane (CH4) production and oxidation. Here we assessed how concentrations of nutrients, solutes, and metals varied across four plant communities undergoing permafrost thaw and if these geochemical characteristics affected rates of CH4 production and oxidation. We tested nutrient limitation in CH4 production and oxidation by experimentally adding nitrogen (N), phosphorus (P) and a permafrost leachate to peat across these four plant communities. The upper 20 cm of permafrost contained 715 ± 298 mg m−2 of extractable inorganic N and 20 ± 6 mg m−2 of resin-extractable phosphorus (Presin), for a N:P ratio of 36:1. These low amounts of Presin coincide with high acid-digestible aluminum (Al), iron (Fe), and P concentrations in the permafrost soil and suggest that P may accumulate via sorption and constrain easily available forms of P for plants and microbes. Permafrost leachate additions decreased potential CH4 production rates up to 80% and decreased CH4 oxidation rates by 66%, likely due to inhibitory effects of N in the permafrost. In contrast, organic and inorganic P additions increased CH4 oxidation rates up to 36% in the tall graminoid fen, a community where phosphate availability was low and CH4 production was high. Our results suggest that (1) inorganic N is available immediately from permafrost thaw, while (2) P availability is controlled by sorption properties, and (3) plant community, nutrient stoichiometry, and metal availability modulate how permafrost thaw affects CH4 production and oxidation.

Forests in the northeastern US are experiencing shifts in community composition due to the northward migration of warm-adapted tree species and certain species’ declines (for example, white ash and eastern hemlock) due to invasive insects. Changes in belowground fungal communities and associated functions will inevitably follow. Therefore, we sought to investigate the relative importance of two important tree characteristics—mycorrhizal type [ectomycorrhizal (EcM) or arbuscular mycorrhizal (AM)] and leaf habit (deciduous or evergreen) on soil fungal community composition and organic matter cycling. We sampled soil in the organic and mineral horizons beneath two AM-associated (Fraxinus americana and Thuja occidentalis) and two ECM-associated tree species (Betula alleghaniensis and Tsuga canadensis), with an evergreen and deciduous species in each mycorrhizal group. To characterize fungal communities and organic matter decomposition beneath each tree species, we sequenced the ITS1 region of fungal DNA and measured the potential activity of carbon- and nitrogen-targeting extracellular enzymes. Each tree species harbored distinct fungal communities, supporting the need to consider both mycorrhizal type and leaf habit. However, between tree characteristics, mycorrhizal type better predicted fungal communities. Across fungal guilds, saprotrophic fungi were the most important group in shaping fungal community differences in soils beneath all tree species. The effect of leaf habit on carbon- and nitrogen-targeting hydrolytic enzymes depended on tree mycorrhizal association in the organic horizon, while oxidative enzyme activities were higher beneath EcM-associated trees across both soil horizons and leaf habits.

In the early 1990’s, reserves adjacent to Kruger National Park (KNP) removed their fences to create a continuous landscape within the Kruger to Canyons Biosphere Reserve. Understanding how these interconnected multi-management systems responded to changes in environmental factors and management regimes can help to maintain natural large-scale landscape heterogeneity and ecological resilience. Our objective was to analyze remote sensing-derived vegetation metric changes between the different management types pre- and post-fence removal. The study area included fourteen reserves and the central section of KNP. We calculated the residuals between TIMESAT-derived metrics (from AVHRR NDVI time series) and rainfall to analyze changes in vegetation from 1985 to 2006. We then compared vegetation-rainfall residuals between different management types pre- and post-fence removal using mean–variance plots, nonmetric multidimensional scaling plots, and permutational multivariate analysis of variance to statistically identify and analyze changes. All management types experienced increased greenness. Reserves that removed their fences had greater changes in vegetation post-fence removal compared to reserves that remained fenced and KNP. Our findings suggest managers may need to address landscape changes by implementing management regimes such as reducing artificial surface water to counterbalance increased grazing pressure as a result of increased animal mobility across artificially created resource gradients. Habitat connectivity within and between protected area networks can be achieved by removing fences across adjacent conservation areas thus potentially increasing ecological resilience, which is vital to effective long-term conservation.