ALBERT

Description:    After rainfall and soils, fire and herbivory are two of the main determinants of savanna ecosystems. Although the interactive effects of fire and herbivores on soil and vegetation are widely acknowledged few studies have addressed these two factors in concert, and none of the studies has focused on the Kalahari sand system. We experimentally studied how annual late dry season fires and grazing affect herbaceous plant species composition, above- and belowground biomass, and soil and grass nutrient concentrations in the nutrient-poor semi-arid Kalahari system in northern Botswana. Four treatments (fire, grazing, fire + grazing, and no-fire–no-grazing) were applied for two consecutive years in the late dry season. Plant species composition was affected by treatment and year. The no-fire–no-grazing treatment was distinctly different from all the other treatments in terms of species composition. Beta diversity was lower on the fire treatment and grazing treatment, but not where fire and grazing were combined. Fire and grazing alone or in combination did not have a substantial effect on biomass, soil and plant nutrients or plant species alpha diversity. Plant nitrogen was the only element that differed between treatments, with high concentrations on all the grazed treatments in the first year and low levels on the fire-alone treatment during the second year. The results show that fire and grazing mainly affect species composition and large-scale biodiversity patterns as indicated by the no-fire–no-grazing treatment being distinctly different from other treatments, suggesting the evolutionary adaptation of this dystrophic Kalahari sand system to herbivory and fire. Content Type Journal Article Pages 1-17 DOI 10.1007/s10021-012-9611-6 Authors Gaseitsiwe S. Masunga, Okavango Research Institute, University of Botswana, Private Bag 285, Maun, Botswana Stein R. Moe, Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, 1432 Aas, Norway Baipidi Pelekekae, Department of Wildlife and National Parks, P.O. Box 131, Gaborone, Botswana Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    Inorganic nitrogen (N) availability hot spots have been documented in many ecosystems, but major uncertainties remain about their prevalence, timing, and causes. Using a novel mathematical definition of hot spots, spatially explicit measurements of KCl-extractable inorganic N, 2-week soil incubations in the field, ion-exchange resins deployed for 1 year, and a set of associated biotic and abiotic variables, we investigated inorganic N availability hot spots within a 0.89 km 2 alpine-subalpine ecosystem in the Colorado Front Range. Measurements of KCl-extractable NH 4 + and NO 3 − taken on multiple dates showed that hot spots of N availability were present in some but not all parts of the study site and that hot spot location varied over the course of the season. Ion-exchange resins showed that over a 1-year period hot spots were important contributors to resin-available N at the landscape level, with 14% of resin locations accounting for 58% of total resin-extractable inorganic N. The KCl-extractable and resin-available inorganic N measurements showed that although spatial variation in the timing of hot spots (that is, hot moments) spreads the influence of short-term hot spots across the landscape to some extent, spatial variation in inorganic N availability is still important when integrated over 1 year. Resin-available N was poorly correlated with the biotic and abiotic variables that we measured, though we did observe that hot spots of resin-available N were twice as common below tree and shrub canopies than in herbaceous areas. Beyond this relationship with canopy structure, neither KCl-extractable nor resin-available inorganic N hot spots were closely related to plant species identity. Instead, the most effective predictor of KCl-extractable NH 4 + was the size of the soil organic matter (SOM) N pool, with nearly all hot spots appearing in soils that had greater than 1.4% SOM N. Content Type Journal Article Pages 1-16 DOI 10.1007/s10021-011-9450-x Authors Anthony Darrouzet-Nardi, Department of Ecology and Evolutionary Biology, University of Colorado at Boulder, N122 Ramaley, 334 UCB, Boulder, Colorado 80309-0334, USA William. D. Bowman, Department of Ecology and Evolutionary Biology, University of Colorado at Boulder, N122 Ramaley, 334 UCB, Boulder, Colorado 80309-0334, USA Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    Woody vegetation has expanded in coverage over the past century in many places globally, exemplified by pinyon-juniper changes in the Southwestern United States. Extreme drought is one of the few non-management drivers besides fire that might reverse such cover changes, but this has not been well documented. Here, we assess 68 years of tree cover dynamics across an elevation gradient of a pinyon-juniper woodland using aerial photographs (1936 and 1959) and QuickBird imagery (2004). Canopy cover increased 32% from 1936 to the onset of a major drought (2002). The largest relative increase in canopy cover occurred from 1936 to 1959 at the higher elevations, but these gains were eliminated by fires occurring from 1959 to 2002, during which time lower elevations with low canopy cover exhibited the greatest relative increases. The 2002–2004 drought reduced canopy cover by 55%, which eliminated gains in cover that occurred since 1936. Relative tree cover loss was highest at low elevations with low tree cover, but absolute tree cover loss was greater in areas of high tree cover, which increased with elevation. The loss of more than half of the canopy cover during a 2-year drought period was much greater than losses due to fire or possible increases due to historic land use (for example, grazing). These results suggest that regional-scale climatic influences may be more important than land use legacies in controlling tree cover of these and perhaps other semiarid woodlands over longer time scales—notable given that similar episodes of tree mortality are projected in coming decades with climate change. Content Type Journal Article Pages 1-14 DOI 10.1007/s10021-011-9458-2 Authors Michael J. Clifford, Earth and Environmental Science Department, Lehigh University, 1 West Packer Avenue, Bethlehem, Pennsylvania 18015, USA Neil S. Cobb, Merriam-Powell Center for Environmental Research, Department of Biological Sciences, Northern Arizona University, Peterson Hall, Box 6077, Flagstaff, Arizona 86011, USA Michaela Buenemann, Department of Geography, New Mexico State University, MSC MAP, P.O. Box 30001, Las Cruces, New Mexico 88003, USA Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    In water-limited ecosystems, where potential evapotranspiration exceeds precipitation, it is often assumed that plant invasions will not increase total ecosystem water use, because all available water is evaporated or transpired regardless of vegetation type. However, invasion by exotic species, with high water use rates, may potentially alter ecosystem water balance by reducing water available to native species, which may in turn impact carbon assimilation and productivity of co-occurring species. Here, we document the impact of invasion by an understory exotic woody species ( Acacia longifolia ) in a semi-arid Mediterranean dune pine forest. To quantify the effects of this understory leguminous tree on the water use and carbon fixation rates of Pinus pinaster we compare an invaded and a non-invaded stand. A.   longifolia significantly altered forest structure by increasing plant density and leaf area index in the mid-stratum of the invaded forest. A.   longifolia contributed significantly to transpiration in the invaded forest (up to 42%) resulting in a slight increase in stand transpiration in the invaded relative to non-invaded forest. More importantly, both water use and carbon assimilation rates of P.   pinaster were significantly reduced in the invaded relative to non-invaded stand. Therefore, this study shows that exotic plant invasions can have significant impacts on hydrological and carbon cycling even in water-limited semi-arid ecosystems through a repartitioning of water resources between the native and the invasive species. Content Type Journal Article Pages 1-16 DOI 10.1007/s10021-011-9453-7 Authors Katherine G. Rascher, Experimental and Systems Ecology, University of Bielefeld, Universitätstr. 25, 33615 Bielefeld, Germany André Große-Stoltenberg, Institute of Landscape Ecology, University of Münster, Münster, Germany Cristina Máguas, Centre for Environmental Biology (CBA), University of Lisbon, Lisbon, Portugal Christiane Werner, Experimental and Systems Ecology, University of Bielefeld, Universitätstr. 25, 33615 Bielefeld, Germany Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    Various studies over the last 15 years have attempted to describe the processes of N retention, saturation and NO 3 − leaching in semi-natural ecosystems based on stable isotope studies. Forest ecologists and terrestrial biogeochemists have used 15 N labelled NO 3 − and NH 4 + tracers to determine the fate of atmospheric deposition inputs of N to terrestrial ecosystems, with NO 3 − leaching to surface waters being a key output flux. Separate studies by aquatic ecologists have used similar isotope tracer methods to determine the fate and impacts of inorganic N species, leached from terrestrial ecosystems, on aquatic ecosystems, usually without reference to comparable terrestrial studies. A third group of isotopic studies has employed natural abundances of 15 N and 18 O in precipitation and surface water NO 3 − to determine the relative contributions of atmospheric and microbial sources. These three sets of results often appear to conflict with one another. Here we attempt to synthesize and reconcile the results of these differing approaches to identifying both the source and the fate of inorganic N in natural or semi-natural ecosystems, and identify future research priorities. We conclude that the results of different studies conform to a consistent conceptual model comprising: (1) rapid microbial turnover of atmospherically deposited NO 3 − at multiple biologically active locations within both terrestrial and aquatic ecosystems; (2) maximum retention and accumulation of N in carbon-rich ecosystems and (3) maximum leaching of NO 3 − , most of which has been microbially cycled, from carbon-poor ecosystems exposed to elevated atmospheric N inputs. Content Type Journal Article Pages 1-17 DOI 10.1007/s10021-011-9461-7 Authors Chris J. Curtis, Environmental Change Research Centre, University College London, Pearson Building, Gower Street, London, WC1E 6BT UK Chris D. Evans, CEH Bangor, Environment Centre Wales, Deiniol Road, Bangor, Gywnedd LL57 2UP, UK Christine L. Goodale, Department of Ecology & Evolutionary Biology, Cornell University, E215 Corson Hall, Ithaca, New York 14853, USA Tim H.E. Heaton, NERC Isotope Geosciences Laboratory, BGS Keyworth, Nottingham, NG12 5GG UK Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    Climate change is altering long-term climatic conditions and increasing the magnitude of weather fluctuations. Assessing the consequences of these changes for terrestrial ecosystems requires understanding how different vegetation types respond to climate and weather. This study examined 20 years of regional-scale remotely sensed net primary productivity (NPP) in forests of the northern Lake States to identify how the relationship between NPP and climate or weather differ among forest types, and if NPP patterns are influenced by landscape-scale evenness of forest-type abundance. These results underscore the positive relationship between temperature and NPP. Importantly, these results indicate significant differences among broadly defined forest types in response to both climate and weather. Essentially all weather variables that were strongly related to annual NPP displayed significant differences among forest types, suggesting complementarity in response to environmental fluctuations. In addition, this study found that forest-type evenness (within 8 × 8 km 2 areas) is positively related to long-term NPP mean and negatively related to NPP variability, suggesting that NPP in pixels with greater forest-type evenness is both higher and more stable through time. This is landscape- to subcontinental-scale evidence of a relationship between primary productivity and one measure of biological diversity. These results imply that anthropogenic or natural processes that influence the proportional abundance of forest types within landscapes may influence long-term productivity patterns. Content Type Journal Article Pages 1-12 DOI 10.1007/s10021-011-9460-8 Authors John B. Bradford, USDA, Forest Service, Northern Research Station, Grand Rapids, Minnesota 55744, USA Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    Increases in the elevation of the soil surfaces of mangroves and salt marshes are key to the maintenance of these habitats with accelerating sea level rise. Understanding the processes that give rise to increases in soil surface elevation provides science for management of landscapes for sustainable coastal wetlands. Here, we tested whether the soil surface elevation of mangroves and salt marshes in Moreton Bay is keeping up with local rates of sea level rise (2.358 mm y −1 ) and whether accretion on the soil surface was the most important process for keeping up with sea level rise. We found variability in surface elevation gains, with sandy areas in the eastern bay having the highest surface elevation gains in both mangrove and salt marsh (5.9 and 1.9 mm y −1 ) whereas in the muddier western bay rates of surface elevation gain were lower (1.4 and −0.3 mm y −1 in mangrove and salt marsh, respectively). Both sides of the bay had similar rates of surface accretion (~7–9 mm y −1 in the mangrove and 1–3 mm y −1 in the salt marsh), but mangrove soils in the western bay were subsiding at a rate of approximately 8 mm y −1 , possibly due to compaction of organic sediments. Over the study surface elevation increments were sensitive to position in the intertidal zone (higher when lower in the intertidal) and also to variation in mean sea level (higher at high sea level). Although surface accretion was the most important process for keeping up with sea level rise in the eastern bay, subsidence largely negated gains made through surface accretion in the western bay indicating a high vulnerability to sea level rise in these forests. Content Type Journal Article Pages 1-13 DOI 10.1007/s10021-011-9443-9 Authors Catherine E. Lovelock, School of Biological Sciences, University of Queensland, St Lucia, Queensland 4072, Australia Vicki Bennion, School of Biological Sciences, University of Queensland, St Lucia, Queensland 4072, Australia Alistair Grinham, School of Environmental Engineering, University of Queensland, St Lucia, Queensland 4072, Australia Donald R. Cahoon, United States Geological Survey, Patuxent Wildlife Research Center, 10300 Baltimore Avenue, BARC-EAST Building #308, Beltsville, Maryland 20705, USA Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    Carbon (C) inputs and nutrient availability are known to affect soil organic carbon (SOC) stocks. However, general rules regarding the operation of these factors across a range of soil nutrient availabilities and substrate qualities are unidentified. “Priming” (stimulated decomposition by labile C inputs) and ‘preferential substrate utilization’ (retarded decomposition due to shifts in community composition towards microbes that do not mineralize SOC) are two hypotheses to explain effects of labile C additions on SOC dynamics. For effects of nutrient additions (nitrogen and phosphorus) on SOC dynamics, the stoichiometric (faster decomposition of materials of low carbon-to-nutrient ratios) and ‘microbial mining’ (that is, reduced breakdown of recalcitrant C forms for nutrients under fertile conditions) hypotheses have been proposed. Using the natural gradient of soil nutrient availability and substrate quality of a chronosequence, combined with labile C and nutrient amendments, we explored the support for these contrasting hypotheses. Additions of labile C, nitrogen (N), phosphorus (P), and combinations of C and N and C and P were applied to three sites: 2-year fallow grassland, mature grassland and forest, and the effects of site and nutrient additions on litter decomposition and soil C dynamics were assessed. The response to C addition supported the preferential substrate hypothesis for easily degradable litter C and the priming hypothesis for SOC, but only in nitrogen-enriched soils of the forest site. Responses to N addition supported the microbial mining hypothesis irrespective of C substrate (litter or SOC), but only in the forest site. Further, P addition effects on SOC support the stoichiometric hypothesis; P availability appeared key to soil C release (priming) in the forest site if labile C and N is available. These results clearly link previously contrasting hypotheses of the factors controlling SOC with the natural gradient in litter quality and nutrient availability that exists in ecosystems at different successional stages. A holistic theory that incorporates this variability of responses, due to different mechanisms, depending on nutrient availability and substrate quality is essential for devising management strategies to safeguard soil C stocks. Content Type Journal Article Pages 1-10 DOI 10.1007/s10021-011-9440-z Authors Alexandru Milcu, Division of Biology, NERC Centre for Population Biology, Imperial College London, Silwood Park, Ascot, SL5 7PY UK Angela Heim, Department of Earth and Environmental Sciences, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany Richard J. Ellis, Veterinary Laboratories Agency, VLA Weybridge, New Haw, KT15 3NB UK Stefan Scheu, J.F. Blumenbach Institute of Zoology and Anthropology, Georg-August-University Göttingen, Berliner Str. 28, 37073 Göttingen, Germany Pete Manning, Division of Biology, NERC Centre for Population Biology, Imperial College London, Silwood Park, Ascot, SL5 7PY UK Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    Fire influences carbon dynamics from local to global scales, but many uncertainties remain regarding the remote detection and simulation of heterogeneous fire effects. This study integrates Landsat-based remote sensing and Biome-BGC process modeling to simulate the effects of high-, moderate-, and low-severity fire on pyrogenic emissions, tree mortality, and net ecosystem production. The simulation area (244,600 ha) encompasses four fires that burned approximately 50,000 ha in 2002–2003 across the Metolius Watershed, Oregon, USA, as well as in situ measurements of postfire carbon pools and fluxes that we use for model evaluation. Simulated total pyrogenic emissions were 0.732 Tg C (2.4% of equivalent statewide anthropogenic carbon emissions over the same 2-year period). The simulated total carbon transfer due to tree mortality was fourfold higher than pyrogenic carbon emissions, but dead wood decomposition will occur over decades. Immediately postfire, burned areas were a simulated carbon source (net C exchange: −0.076 Tg C y −1 ; mean ± SD: −142 ± 121 g C m −2 y −1 ). As expected, high-severity, stand-replacement fire had disproportionate carbon impacts. The per-unit area effects of moderate-severity fire were substantial, however, and the extent of low-severity fire merits its inclusion in landscape-scale analyses. These results demonstrate the potential to reduce uncertainties in landscape to regional carbon budgets by leveraging Landsat-based fire products that account for both stand-replacement and partial disturbance. Content Type Journal Article Pages 1-18 DOI 10.1007/s10021-011-9444-8 Authors Garrett W. Meigs, Department of Forest Ecosystems and Society, Oregon State University, Corvallis, Oregon 97331, USA David P. Turner, Department of Forest Ecosystems and Society, Oregon State University, Corvallis, Oregon 97331, USA William D. Ritts, Department of Forest Ecosystems and Society, Oregon State University, Corvallis, Oregon 97331, USA Zhiqiang Yang, Department of Forest Ecosystems and Society, Oregon State University, Corvallis, Oregon 97331, USA Beverly E. Law, Department of Forest Ecosystems and Society, Oregon State University, Corvallis, Oregon 97331, USA Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840

Description:    In disturbance-prone ecosystems, organisms often persist in spatial refugia during stressful periods. A clear example is the colonization of abandoned river channels by pioneer riparian trees. Here, we examine the prominence of this establishment pathway for a foundation tree species (Fremont cottonwood, Populus fremontii ) within the riparian corridor of a large river, the Sacramento River in central California. We quantified the total proportion of forest that initiated as a result of channel abandonment for a 160-km reach, analyzed concurrent patterns of tree establishment with floodplain accretion and sedimentation history, and developed a conceptual model of biogeomorphic evolution of abandoned channels. Historical air photo analysis indicated that stands associated with abandoned channels comprised more than 50% of the total extant cottonwood forest area. Tree-ring evidence showed that cottonwood stands commonly developed immediately following abandonment, and the recruitment window ranged from 4 to 40 years, but was less than 10 years at most sites. Rates of floodplain rise and fine sediment accumulation were high in young sites and decreased logarithmically over time. Together, these results suggest that abandoned channels are an important refuge for cottonwood recruitment, that the greatest opportunity for colonization occurs within a short period after the cutoff event, and that sedimentation processes influence the duration of the colonization window. On rivers where tree recruitment along the active channel is severely limited by hydrologic regulation and/or land management, abandoned channel refugia may play an even more important role in sustaining an ecologically functional riparian corridor. Preserving bank erosion, active meander corridors and forest regeneration zones created by cutoff events are therefore key conservation measures on shifting rivers. Content Type Journal Article Pages 1-15 DOI 10.1007/s10021-011-9446-6 Authors John C. Stella, Department of Forest and Natural Resources Management, State University of New York College of Environmental Science and Forestry, One Forestry Drive, Syracuse, New York 13210, USA Maya K. Hayden, Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720, USA John J. Battles, Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720, USA Hervé Piégay, CNRS UMR 5600 EVS/Site ENS Lyon, University of Lyon, 15 Parvis René Descartes, BP 7000, 69342 Lyon Cedex 07, France Simon Dufour, COSTEL-CNRS LETG UMR 6554, Département de Géographie, Université Rennes 2, Place du Recteur Henri Le Moal, 35043 Rennes Cedex, France Alexander K. Fremier, Department of Fish and Wildlife Resources, University of Idaho, Box 441136, Moscow, Idaho 83844-1136, USA Journal Ecosystems Online ISSN 1435-0629 Print ISSN 1432-9840