Browsing by Author "REYNOLDS, JF"

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    Biological feedbacks in global desertification
    (Science, 1990) Shlesinger; WB; REYNOLDS, JF; Cunningham, GL; Huennecke, LF; Jarrell, WM; Virginia, RA; Whitford, WG
    Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.
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    Contingency in ecosystem but not plant community response to multiple global change factors
    (New Phytologist, 2012) Bradford, MA; Wood, SA; Maestre, FT; REYNOLDS, JF; Warren, RJ
    Community and ecosystem responses to global environmental change are contingent on the magnitude of change and interacting global change factors. To reveal whether responses are also contingent on the magnitude of each interacting factor, multifactor, multilevel experiments are required, but are rarely conducted. We exposed model grassland ecosystems to six levels of atmospheric CO2 and six levels of nitrogen enrichment, applying the latter both chronically (simulating deposition) and acutely (simulating fertilization). The 66 treatments were maintained for 6 months under controlled growing conditions, with biomass harvested every 28 d and sorted to species. Aboveground plant productivity responses to CO2 were contingent on nitrogen amount, and the responses to nitrogen amount were dependent on whether applications were chronic or acute. Specifically, productivity responses to increasing CO2 concentrations were accentuated with higher nitrogen enrichments, and productivity was greater when higher nitrogen enrichments were applied acutely. Plant community composition was influenced only by nitrogen enrichment, where the co-dominant grass species with the greatest leaf trait plasticity increasingly dominated with higher nitrogen amounts. Community processes are considered to be unpredictable, but our data suggest that the prediction of the impacts of simultaneous global changes is more complex for ecosystem processes, given that their responses are contingent on the levels of interacting factors.
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    Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis
    (Ecology Letters, 2011) Eldridge; DJ; Bowker, MA; Maestre, FT; Roger, E; REYNOLDS, JF; Whitford, WG
    Encroachment of woody plants into grasslands has generated considerable interest among ecologists. Syntheses of encroachment effects on ecosystem processes have been limited in extent and confined largely to pastoral land uses or particular geographical regions. We used univariate analyses, meta-analysis and structural equation modelling to test the propositions that (1) shrub encroachment does not necessarily lead to declines in ecosystem functions and (2) shrub traits influence the functional outcome of encroachment. Analyses of 43 ecosystem attributes from 244 case studies worldwide showed that some attributes consistently increased with encroachment (e.g. soil C, N), and others declined (e.g. grass cover, pH), but most exhibited variable responses. Traits of shrubs were associated with significant, though weak, structural and functional outcomes of encroachment. Our review revealed that encroachment had mixed effects on ecosystem structure and functioning at global scales, and that shrub traits influence the functional outcome of encroachment. Thus, a simple designation of encroachment as a process leading to functionally, structurally or contextually degraded ecosystems is not supported by a critical analysis of existing literature. Our results highlight that the commonly established link between shrub encroachment and degradation is not universal.
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    Plant responses to precipitation in desert ecosystems: integrating functional types, pulses, thresholds, and delays
    (Oecologia, 2004) Ogle, K; REYNOLDS, JF
    The 'two-layer' and 'pulse-reserve' hypotheses were developed 30 years ago and continue to serve as the standard for many experiments and modeling studies that examine relationships between primary productivity and rainfall variability in aridlands. The two-layer hypothesis considers two important plant functional types (FTs) and predicts that woody and herbaceous plants are able to co-exist in savannas because they utilize water from different soil layers (or depths). The pulse-reserve model addresses the response of individual plants to precipitation and predicts that there are 'biologically important' rain events that stimulate plant growth and reproduction. These pulses of precipitation may play a key role in long-term plant function and survival (as compared to seasonal or annual rainfall totals as per the two-layer model). In this paper, we re-evaluate these paradigms in terms of their generality, strengths, and limitations. We suggest that while seasonality and resource partitioning (key to the two-layer model) and biologically important precipitation events (key to the pulse-reserve model) are critical to understanding plant responses to precipitation in aridlands, both paradigms have significant limitations. Neither account for plasticity in rooting habits of woody plants, potential delayed responses of plants to rainfall, explicit precipitation thresholds, or vagaries in plant phenology. To address these limitations, we integrate the ideas of precipitation thresholds and plant delays, resource partitioning, and plant FT strategies into a simple 'threshold-delay' model. The model contains six basic parameters that capture the nonlinear nature of plant responses to pulse precipitation. We review the literature within the context of our threshold-delay model to: (i) develop testable hypotheses about how different plant FTs respond to pulses; (ii) identify weaknesses in the current state-of-knowledge; and (iii) suggest future research directions that will provide insight into how the timing, frequency, and magnitude of rainfall in deserts affect plants, plant communities, and ecosystems.
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    Towards a global drylands observing system: Observational requirements and institutional solutions
    (Land Degradation & Development, 2011) Verstraete; MM; Hutchinson, CF; Grainger, A; Smith, M Stafford; Scholes, RJ; REYNOLDS, JF; Barbosa, P; Léon, A; Mbow, C
    Quantitative data on dryland changes and their effects on the people living there are required to support policymaking and environmental management at all scales. Data are regularly acquired by international, national or local entities, but presently exhibit specific gaps. Promoting sustainable development in drylands necessitates a much stronger integration, coordination and synthesis of available information. Space-based remote sensing systems continue to play an important role but do not fulfill all needs. Dedicated networks and observing systems, operating over a wide range of scales and resolutions, are needed to address the key issues that concern decision-makers at the scale of local communities, countries and the international community. This requires a mixture of 'bottom-up' and 'top-down' design principles, and multiple ownership of the resultant system. This paper reviews the limitations of current observing systems and suggests establishing a Global Drylands Observing System, which would capitalize on the achievements of systems already established to support the other Rio Conventions. This Global Drylands Observing System would provide an integrated, coherent entry point and user interface to a range of underlying information systems, identify and help generate missing information, propose a set of standards for the acquisition, archiving and distribution of data where these are lacking, evaluate the quality and reliability of these data and promote scientific research in these fields by improving access to data. The paper outlines the principles and main objectives of a Global Drylands Observing System and calls for renewed efforts to invigorate cooperation mechanisms between the many global environmental conventions. Copyright © 2010 John Wiley & Sons, Ltd.