Litter quality and soil environmental conditions are well-studied drivers influencing decomposition rates, but the role played by disturbance legacy, such as fire history, in mediating these drivers ...is not well understood. Fire history may impact decomposition directly, through changes in soil conditions that impact microbial function, or indirectly, through shifts in plant community composition and litter chemistry. Here, we compared early-stage decomposition rates across longleaf pine forest blocks managed with varying fire frequencies (annual burns, triennial burns, fire-suppression). Using a reciprocal transplant design, we examined how litter chemistry and soil characteristics independently and jointly influenced litter decomposition. We found that both litter chemistry and soil environmental conditions influenced decomposition rates, but only the former was affected by historical fire frequency. Litter from annually burned sites had higher nitrogen content than litter from triennially burned and fire suppression sites, but this was correlated with only a modest increase in decomposition rates. Soil environmental conditions had a larger impact on decomposition than litter chemistry. Across the landscape, decomposition differed more along soil moisture gradients than across fire management regimes. These findings suggest that fire frequency has a limited effect on litter decomposition in this ecosystem, and encourage extending current decomposition frameworks into disturbed systems. However, litter from different species lost different masses due to fire, suggesting that fire may impact decomposition through the preferential combustion of some litter types. Overall, our findings also emphasize the important role of spatial variability in soil environmental conditions, which may be tied to fire frequency across large spatial scales, in driving decomposition rates in this system.
Control Points in Ecosystems Bernhardt, Emily S.; Blaszczak, Joanna R.; Ficken, Cari D. ...
Ecosystems (New York),
06/2017, Letnik:
20, Številka:
4
Journal Article
Recenzirano
The phrase “hot spots and hot moments” first entered the lexicon in 2003, following the publication of the paper “Biogeochemical hot spots and hot moments at the interface of terrestrial and aquatic ...ecosystems” by McClain and others (Ecosystems 6:301–312, 2003). This paper described the potential for rare places and rare events to exert a disproportionate influence on the movement of elements at the scale of landscapes and ecosystems. Here, we examine how the cleverly named hot spot and hot moment concept (hereafter HSHM) has influenced biogeochemistry and ecosystem science over the last 13 years. We specifically examined the extent to which the HSHM concept has: (1) motivated research aimed at understanding how and why biogeochemical behavior varies across spatiotemporal scales; (2) improved our ability to detect HSHM phenomena; and (3) influenced our approaches to restoration and ecosystem management practices. We found that the HSHM concept has provided a highly fertile framework for a substantial volume of research on the spatial and temporal dynamics of nutrient cycling, and in doing so, has improved our understanding of when and where biogeochemical rates are maximized. Despite the high usage of the term, we found limited examples of rigorous statistical or modeling approaches that would allow ecosystem scientists to not only identify, but scale the aggregate impact of HSHM on ecosystem processes. We propose that the phrase “hot spots and hot moments” includes two implicit assumptions that may actually be limiting progress in applying the concept. First, by differentiating “hot spots” from “hot moments,” the phrase separates the spatial and temporal components of biogeochemical behavior. Instead, we argue that the temporal dynamics of a putative hot spot are a fundamental trait that should be used in their description. Second, the adjective “hot” implicitly suggests that a place or a time must be dichotomously classified as “hot or not.” We suggest instead that each landscape of interest contains a wide range of biogeochemical process rates that respond to critical drivers, and the gradations of this biogeochemical topography are of greater interest than the maximum peaks. For these reasons, we recommend replacing the HSHM terminology with the more nuanced term ecosystem control points. “Ecosystem control” suggests that the rate must be of sufficient magnitude or ubiquity to affect dynamics of the ecosystem, while “points” allows for descriptions that simultaneously incorporate both spatial and temporal dynamics. We further suggest that there are at least four distinct types of ecosystem control points whose influence arises through distinct hydrologic and biogeochemical mechanisms. Our goal is to provide the tools with which researchers can develop testable hypotheses regarding the spatiotemporal dynamics of biogeochemistry that will stimulate advances in more accurately identifying, modeling and scaling biogeochemical heterogeneity to better understand ecosystem processes.
Boreal wetlands are at risk of degradation from anthropogenic activities including oil sands energy extraction. Despite efforts to monitor the impacts of oil sands energy extraction-related ...activities on wetland ecology, few studies examine the impacts of diverse human development types on wetland plant communities. Here, we sought to quantify the effects of human development in the Athabasca, Peace River, and Cold Lake Oil Sands Regions in Alberta, Canada, and to examine its impact on wetland plant community composition. Across the region, we found that total development and development related to energy and mining were both low; ~80% of the study area was undeveloped. Despite the low spatial extent, total anthropogenic development was negatively correlated with site-level conservatism (a metric of plant tolerance to environmental perturbation) in all five wetland classes examined. This suggests that wetlands surrounded by human development are inhabited by generalist species that are tolerant of environmental disturbance. Moreover, distinct floristic groups within each wetland class could be distinguished based on their total developed area, providing additional evidence that human development affects plant composition and diversity, despite its limited extent in the study area. In contrast to total development, energy and mining development had an inconsistent or no detectable impact on wetland plant community composition at the regional level, likely because although oils sands surface mining is intensive, it is spatially restricted to a small area within the oil sands region. Our findings show that wetland plant communities in the oil sands region are impacted by multiple types of human land use concurrently; further research should aim to evaluate both the distinct impacts of different land use types using gradients of development intensity, as well as the cumulative impacts of multiple land use types happening concurrently.
Display omitted
•Wetlands in the oil sands region are surrounded by low human development (HD).•Energy and mining development influences bog and fen plant community composition in particular.•Wetlands with the highest HD were characterized by low-quality generalist plant species.
Changes in fire frequency from historical norms are becoming more frequent due to both changes in management and climate change factors. There is uncertainty about whether increasing fire frequency ...will lead to decreased carbon pools due to shorter inter-fire recovery periods, or increased carbon pools due to lowered fire intensity due to lighter fuel loads. Additionally, data are needed to determine whether plant and soil carbon pools respond similarly and whether ecosystem responses are consistent across environmental gradients that can affect fire intensity, such as soil moisture. We measured soil and vegetation carbon pools and fluxes at sites that had experienced different experimental burn treatments over the previous 8 years and across a range of soil moisture in a longleaf pine (
Pinus palustris
) ecosystem in North Carolina, USA. We found that increasing fire frequency, assessed by either the number of days since a previous fire or the number of fires a plot had experienced over the previous 8 years, significantly reduced carbon stocks in the litter pool and soil carbon pool and reduced the productivity of understory plants. Total carbon stocks also significantly declined, and there was a marginally significant shift away from soil carbon and toward tree carbon as being the dominant carbon pool in the system with increasing fire. None of the results showed any interaction with soil moisture, suggesting that in this landscape, fire effects are consistent across an important environmental gradient. Over the timeframe of this study, management that increases prescribed fire frequency appears to reduce carbon storage.
The carbon economy of drought Ficken, Cari D.; Warren, Jeffrey M.
Plant and soil,
02/2019, Letnik:
435, Številka:
1/2
Journal Article
Recenzirano
Odprti dostop
Aims
The effects of drying and wetting on soil carbon processes are regulated by the responses of plants, plant-associated microbes, and free-living microbes. Whether these groups respond similarly ...to drying and wetting is not clear, however, making it difficult to predict the net effect of drought on soil carbon processes.
Methods
We imposed a drought-rewetting event on mesocosms planted with maple (
Acer saccharum
Marshall
;
arbuscular mycorrhizal fungi host) or oak (
Quercus alba
L.; ectomycorrhizal fungi host) saplings. In each mesocosm, we used mesh to create chambers separating roots, mycorrhizal fungi hyphae, and free-living microbes. We compared respiration and carbon degrading enzyme activity across chambers throughout the dry-rewetting cycle.
Results
Respiration from all chambers declined during drought, and chambers differed in their drought-sensitivity. In maple mesocosms, respiration from the Hyphae+Microbes chamber was the most drought-resistant; in oak mesocosms respiration from the Microbes chamber was the most drought-sensitive. Respiration did not recover after rewatering, indicating a persistent drought legacy. In contrast, enzyme activity returned to control functioning after 2 weeks of well-watered conditions.
Conclusions
Our results suggest that belowground biota differ in their sensitivity to and recovery from drought, which affects the carbon processes differently. An improved ability to partition carbon fluxes into biotic sources can help to constrain predicted carbon fluxes under future climate scenarios.
Questions
Disturbances can cause fluctuations in resource availability that influence plant performance. In systems with such dynamics, inter‐specific differences in resource capture may promote ...co‐existence by partitioning competition between periods of high or low resource availability. Such differences in resource use strategy have been described with the Plant Economics Spectrum, which hypothesizes that functions related to resource use and processing should co‐vary and can be predicted from plant traits. In pyrogenic systems, fires are associated with short‐term increases in soil nitrogen availability (“pulses”), and thus contribute to a fluctuating resource supply. In this study, we sought to understand whether plants differed in their capacity to capture a nitrogen pulse, and to what extent that ability influenced biomass recovery.
Methods
In two consecutive greenhouse experiments, we tested whether two functions — nitrogen assimilation (Experiment 1) and biomass regrowth after disturbance (Experiment 2) — co‐varied, and how each function corresponded to leaf and root functional traits.
Results
In Experiment 1, four co‐occurring shrubs differed in their temporal patterns of nitrogen uptake, and nitrogen uptake was positively correlated with resource‐acquisitive leaf traits (leaf percent nitrogen). In Experiment 2, the biomass regrowth of a resource acquisitive and a resource conservative species was the same regardless of competitive environment (i.e., when grown in pots of mixed‐species or same‐species pairs). Rather than being associated with the capture of new nitrogen, biomass resprouting of both species was associated with the size of below‐ground resource stores and specific root length.
Conclusions
Our work suggests that resource acquisition and processing may be decoupled from each other after disturbance, and also highlights the need for explicit tests of the relationships between root traits and above‐ground plant function.
We compared two metrics of plant performance — nitrogen assimilation and biomass resprouting — of shrubs native to frequently disturbed longleaf pine forest. Biomass resprouting after disturbance was independent of nitrogen assimilation. Increased nitrogen capture was associated with resource‐acquisitive plant traits (leaf nitrogen content); increased resprouting after disturbance was associated with resource‐conservative plant traits (below‐ground biomass and specific root length).
Biological indicators are commonly used to evaluate ecosystem condition. However, their use is often constrained by the availability of information with which to assign species‐specific indicator ...values, which reflect species' responses to the environmental conditions being evaluated by the indicator. As these responses are driven by underlying traits, and trait data for numerous species are available in publicly accessible databases, one possible approach to approximating missing bioindicator values is through traits. We used the Floristic Quality Assessment (FQA) framework and its component indicator of disturbance sensitivity, species‐specific ecological conservatism scores (C‐scores), as a study system to test the potential of this approach. We tested the consistency of relationships between trait values and expert‐assigned C‐scores and the trait‐based predictability of C‐scores across five regions. Furthermore, as a proof‐of‐concept exercise, we used a multi‐trait model to try to reconstruct C‐scores, and compared the model predictions to expert‐assigned scores. Out of 20 traits tested, there was evidence of regional consistency for germination rate, growth rate, propagation type, dispersal unit, and leaf nitrogen. However, the individual traits showed low predictability (R2 = 0.1–0.2) for C‐scores, and a multi‐trait model produced substantial classification errors; in many cases, >50% of species were misclassified. The mismatches may largely be explained by the inability to generalize regionally varying C‐scores from geographically neutral/naive trait data stored in databases, and the synthetic nature of C‐scores. Based on these results, we recommend possible next steps for expanding the availability of species‐based bioindication frameworks such as the FQA. These steps include increasing the availability of geographic and environmental data in trait databases, incorporating data about intraspecific trait variability into these databases, conducting hypothesis‐driven investigations into trait–indicator relationships, and having regional experts review our results to determine if there are patterns in the species that were correctly or incorrectly classified.
Boreal peatlands provide numerous ecosystem services ranging from carbon sequestration to the provisioning of habitat for species integral to Indigenous communities. In the Oil Sands Region of ...Alberta, Canada, human development related to oil and gas extraction occurs in a wetland-dominated landscape. Wetland monitoring programs can determine the extent to which development impacts wetlands, but existing monitoring programs focus on characterizing biodiversity across the region and on compliance and regulatory monitoring that assumes impacts from oil sands development do not extend past lease boundaries. This is unlikely to be true since some impacts, such as particulate deposition, can extend over large areas contingent on local weather and topography. To inform the development of a new regional wetland monitoring program to assess the cumulative effects of oil sands development on wetlands, we synthesized information on the scope of wetland research across the Oil Sands Region, including the anthropogenic stressors that impact wetlands and the wetland characteristics sensitive to different disturbances. We developed a conceptual model linking human development with wetland ecology in the region to make explicit the relationships among oil sands development stressors and different components of wetland ecosystems. By highlighting testable relationships, this conceptual model can be used as a collection of hypotheses to identify knowledge gaps and to guide future research priorities. relationships among We found that the majority of studies are short-term (77% were ≤ 5 years) and are conducted over a limited spatial extent (82% were sub-regional). Studies of reclaimed wetlands were relatively common (18% of all tests); disproportionate to the occurrence of this wetland type. Results from these studies likely cannot be extrapolated to other wetlands in the region. Nevertheless, the impacts of tailings contaminants, wetland reclamation activities, and surface water chemistry are well-represented in the literature. Research on other types of land disturbance is lacking. A coordinated, regional monitoring program is needed to gain a complete understanding of the direct and indirect impacts of human development in the region and to address remaining knowledge gaps.
Aim
Humans impact biodiversity by altering land use and introducing nonnative species. Yet the extent to which coexistence processes, such as competition and niche shifts, mediate these relationships ...is not clear. This study compares how human development influences wetland plant diversity by examining patterns of species richness, niche specialization and nonnative species occurrences along a human development gradient.
Location
Alberta, Canada.
Taxon
Plants.
Methods
We computed species richness and niche specialization (a measure of the range of human development extents over which a species occurs) from species occurrence data across 1582 wetlands. We tested associations between human development extent and species richness, niche specialization and nonnative species using linear mixed models. We used nonmetric multidimensional scaling ordination to examine whether community composition differed among wetlands surrounded by different human development extents.
Results
Species richness and niche specialization show contrasting relationships with human development: richness was highest and niche specialization was lowest at intermediate human development extents, suggesting that competitive ability and environmental filtering may contribute to low richness at low and high development extents, respectively. Wetlands surrounded by the highest and lowest human development extents had similar levels of richness and niche specialization, but differed in community composition. The proportion of nonnative species increased with increasing human development, alternatively suggesting that the substitution of native species by nonnatives in developed areas may contribute to reduced richness and influence community assembly.
Main conclusions
These findings demonstrate that human land development plays a major role in shaping species richness by influencing the number of nonnative species and the niche specialization of species inhabiting a wetland. Furthermore, these findings suggest that the proportion of nonnative species is an overlooked factor potentially influencing plant richness; including this variable may help clarify the inconsistent responses of diversity to human development over large spatiotemporal scales.
Many ecosystems experience drastic changes to soil nutrient availability associated with fire, but the magnitude and duration of these changes are highly variable among vegetation and fire types. In ...pyrogenic pine savannas across the southeastern United States, pulses of soil inorganic nitrogen (N) occur in tandem with ecosystem-scale nutrient losses from prescribed burns. Despite the importance of this management tool for restoring and maintaining fire-dependent plant communities, the contributions of different mechanisms underlying fire-associated changes to soil N availability remain unclear. Pulses of N availability following fire have been hypothesized to occur through (1) changes to microbial cycling rates and (2) direct ash deposition. Here, we document fire-associated changes to N availability across the growing season in a longleaf pine savanna in North Carolina. To differentiate between possible mechanisms driving soil N pulses, we measured net microbial cycling rates and changes to soil δ15N before and after a burn. Our findings refute both proposed mechanisms: we found no evidence for changes in microbial activity, and limited evidence that ash deposition could account for the increase in ammonium availability to more than 5–25 times background levels. Consequently, we propose a third mechanism to explain post-fire patterns of soil N availability, namely that (3) changes to plant sink strength may contribute to ephemeral increases in soil N availability, and encourage future studies to explicitly test this mechanism.
PDF
