Resilience of Sandhills Grassland to Wildfire During Drought

In the Nebraska Sandhills, one of the largest contiguous grassland ecoregions remaining in North America, sandy textured soils are stabilized by fine root biomass from predominantly warm-season grasses. Concerns over destabilization have led to management that aims to avoid an undesirable state change toward mobile sand dunes. In 2012, the Sandhills experienced extreme drought conditions that coincided with the worst wildfire year on state record. According to state-and-transition models and ecosystem managers, the combination of wildfire and drought conditions should cause a state transition due to a lack of recovery of grassland vegetation and a loss of sand dune stability. To test this hypothesis, we implemented a time-since-fire study to track biomass recovery of Sandhills grassland vegetation following a wildfire on The Nature Conservancy's Niobrara Valley Preserve in burned and unburned areas. Two yr following the wildfire, aboveground herbaceous biomass in burned areas had recovered to levels that did not differ from unburned areas, maintaining the stability of the sand dunes. This provides evidence that counters current land management frameworks that portray Sandhills grassland as highly vulnerable to destabilization when wildfires occur during severe drought conditions.The Rangeland Ecology & Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact [email protected] for further information

Geoecological drivers of cerrado heterogeneity and 13C natural abundance in oxisols after land-use change Fatores geoecológicos das diferentes formações de cerrados - uma hipótese baseada em abundância natural de 13C e fotografia aérea

The 13C natural abundance technique was applied to study C dynamics after land-use change from native savanna to Brachiaria, Pinus, and Eucalyptus in differently textured Cerrado Oxisols. But due to differences in the d13C signatures of subsoils under native savanna and under introduced species, C substitution could only be calculated based on results of cultivated soils nearby. It was estimated that after 20 years, Pinus C had replaced only 5 % of the native C in the 0-1.2 m layer, in which substitution was restricted to the top 0.4 m. Conversely, after 12 years, Brachiaria had replaced 21 % of Cerrado C to a depth of 1.2 m, where substitution decreased only slightly throughout the entire profile. The high d13C values in the subsoils of the cultivated sites led to the hypothesis that the natural vegetation there had been grassland rather than Cerrado sensu stricto, in spite of the comparable soil and site characteristics and the proximity of the studied sites. The hypothesis was tested using aerial photographs of 1964, which showed that the cultivated sites were located on a desiccated runoff head. The vegetation shift to a grass-dominated savanna formation might therefore have occurred in response to waterlogging and reduced soil aeration. A simple model was developed thereof, which ascribes the different Cerrado formations mainly to the plant-available water content and soil aeration. Soil fertility is considered of minor significance only, since at the studied native savanna sites tree density was independent of soil texture or nutrient status.<br>A abundância natural de 13C foi utilizada para estudar a dinâmica de carbono em latossolos de textura média e argilosa após plantação de pastagens (Brachiaria decumbens) e reflorestamentos (Pinus caribaea e Eucalyptus citriodora). Considerando as diferenças de d13C nos subsolos das savanas nativas e das espécies introduzidas, pôde-se calcular a substituição de carbono somente para os solos argilosos, baseando-se em resultados publicados de d13C em solos adjacentes. Estimou-se que somente 5 % do carbono nativo foram substituidos após 20 anos de plantação com pinus na camada de 0-1,2 m, ficando a substituição restrita aos primeiros 0,4 m do solo. Na pastagem de 12 anos, 21 % do carbono novo foram introduzidos na camada de de 0-1,2 m, sendo a substituição diminuída gradativamente pelo perfil inteiro. De acordo com os altos valores de d13C nos subsolos das áreas cultivadas, levantou-se a hipótese de que a vegetação natural teria sido de campo limpo em vez de cerrado stricto sensu apesar da semelhança dos solos e da proximidade dos sítios estudados. A hipótese foi testada com fotos aéreas de 1964, mostrando que as áreas cultivadas encontravam-se numa cabeceira de escoamento ressecada. Portanto, a vegetação dominada por gramíneas pode ter sido induzida pela hidromorfia e pela baixa aeração do solo. Com base nessas observações, desenvolveu-se um modelo simplificado, atribuindo às diferentes formações de cerrados o acesso de água para as plantas e a aeração do solo. A fertilidade do solo foi considerada de menor importância porque a densidade arbórea das áreas de cerrados encontrava-se independente da textura ou do nível de nutrientes dos solos

The cascading effects of birch on heather moorland: a test for the top-down control of an ecosystem engineer

1. Single species can have a disproportionate effect on ecosystem function and diversity, yet our understanding of the importance of single species in driving terrestrial ecosystems during succession remains poor. 2. Utilizing a long-term experiment, where birch was planted on heather moorland 20 years ago, the cascading effects of a single tree species (Betula pubescens) on ecosystem characteristics (plant species richness, soil chemistry, soil fauna and decomposition rates) were tested. 3. Under the birch, plant species richness decreased and the vegetation composition changed, with lower cover of grasses and Vaccinium myrtillus. The depth of the soil organic horizon, its moisture content and percentage carbon were all smaller under the birch than under the heather. Concentrations of available phosphorus and mineralizable-N were significantly greater in the soil under birch than under the heather plots. Decomposition was faster in the birch than in the heather plots. The abundance and species richness of collembola and oribatid, mesostigmatid and prostigmatid mites were all significantly greater under the birch than under the heather. 4. The durability of the engineering effects of the birch was studied in a second experiment. Plots were established in first generation birch woodland that had developed on Calluna-dominanted moorland. The plots were cleared of birch and planted with heather. After 20 years soil chemical properties, microarthropod communities and decomposition rates were not significantly different between plots with and without the birch. However, the mass of the soil O-horizon was significantly greater in the felled birch plots than in the control birch plots, providing the first indication of a change towards soil properties more typical of a Calluna moorland. Thus for most of the birch engineering effects measured here their durability in the absence of the engineering species is at least 20 years. 5. This work has provided experimental evidence that birch acts as a top-down engineer, driving cascading effects on both above- and below-ground communities, soil chemical and physical properties and ecosystem processes. The work also shows that the role of birch in driving changes in the ecosystem is durable 20 years after the removal of the birch