Search

[1] Forest canopies increase atmospheric inputs of nutrients to forest ecosystems by trapping dust and particulates. In tropical dry forests, this mechanism may contribute significant amounts of phosphorus, often a limiting nutrient in these ecosystems. Shifting cultivation may reduce the atmospheric inputs of phosphorus through the removal and restructuring of the forest canopy. We studied the impacts of land clearing by shifting cultivation in the southern Yucatan peninsula by measuring P in atmospheric bulk deposition and throughfall in two secondary forest stands and one mature forest stand to determine whether mature forests have greater throughfall P inputs than regenerating areas. From May to November 2007, we sampled rainfall in an open field and throughfall in three adjacent forest stands of different ages: 6 year old, 20 year old and mature (>60 year). We analyzed subsamples for inorganic and organic P. During the 7 month wet season, cumulative P input for the open field was 0.28 kg/ha, the 6 year stand accumulated 0.44 kg/ha, the 20 year stand accumulated 0.55 kg/ha and the mature stand accumulated 0.81 kg/ha. Organic P inputs were ∼50% of total P inputs in the open field, and 30–38% of total P inputs for the forest stands. The mature forest had significantly higher P concentrations and inputs than the open field or secondary forest stands, and forest stands had significantly greater P inputs than the open field, but there were no significant differences between the older and younger secondary forests. The repeated clearing of forests may thus reduce important P inputs to the ecosystem in the long-term.

[1] Changes in vegetation cover are known for their ability to modify the surface energy balance and near-surface microclimate conditions. A major change in vegetation composition that has been occurring in many dryland regions around the world is associated with the replacement of arid grasslands by desert shrublands. The impact of shrub encroachment on regional climate conditions remains poorly investigated, and, to date, it is unclear how this shift in plant community composition may affect the microclimate. Here we used concurrent meteorological observations at two adjacent sites dominated by Larrea tridentata shrubs and native grass species, respectively, in the northern Chihuahuan desert to investigate differences in nighttime air temperatures between the shrubland and grassland vegetation covers. The nighttime air temperature was found to be substantially higher (>2°C) in the shrubland than in the grassland, especially during calm winter nights. These differences in surface air temperature were accompanied by differences in longwave radiation and sensible and ground heat fluxes. We developed a one-dimensional model to show how longwave radiation emitted by the ground at night can explain the higher nighttime air temperature over the shrubland. Because of the larger fraction of bare soil typically existing in the shrub cover, the ground surface remains less insulated and more energy flows into the ground at the shrubland site than in the grassland during daytime. This energy is then released at night mainly as longwave radiation, which causes the differences in the nighttime air temperatures between the two land covers.

[1] Shallow coastal lagoons are environments where a dynamic equilibrium exists between water quality and seagrass cover. Dense seagrass canopies limit the resuspension of bed sediments thereby creating a clearer water column and a positive feedback for seagrass growth. Positive feedbacks are often associated with the existence of bistable dynamics in ecosystems. For example, a bare and a seagrass covered sediment bed could both be stable states of the system. This study describes a one-dimensional hydrodynamic model of vegetation-sediment-water flow interactions and uses it to investigate the strengths of positive feedbacks between seagrass cover, stabilization of bed sediments, turbidity of the water column, and the existence of a favorable light environment for seagrasses. The model is applied to Hog Island Bay, a shallow coastal lagoon on the eastern shore of Virginia. The effects of temperature, eutrophication, and bed grain size on bistability of seagrass ecosystems in the lagoon are explored. The results indicate that under typical conditions, seagrass is stable in water depths < 2.2 m (51% of the bay bottom deep enough for seagrass growth) and bistable conditions exist for depths of 2.2–3.6 m (23% of bay) where the preferred state depends on initial seagrass cover. The remaining 26% of the bay is too deep to sustain seagrass. Decreases in sediment size and increases in water temperature and degree of eutrophication shift the bistable range to shallower depths, with more of the bay bottom unable to sustain seagrass.

[1] Well-defined vegetation patterns are a widespread feature of dryland landscapes, where spotted, banded, or other organized systems of vegetation patches are bordered by bare soil. A number of different mechanisms have been invoked to explain the emergence of these patterns and to associate their geometrical features to the driving (biotic and abiotic) processes. A commonly accepted theory of pattern formation is based on the interplay between mechanisms of short-range facilitation and long-range competition with surrounding vegetation. Recently associated with the emergence of vegetation patterns, these short- and long-range interactions have been seldom interpreted as a means for plants to enhance their chances of survival in arid environments. Here we develop a simplistic facilitation-competition model to show how these spatial interactions increase the productivity of dryland ecosystems. Stable (sparsely) vegetated states are shown to occur even in systems in which unvegetated conditions are the only stable state of the local dynamics (i.e., in the absence of spatial interaction). Pattern emergence is also interpreted as an easily detectable sign of the occurrence of highly effective resource allocation and use due to spatial interactions. Moreover, the resilience of vegetated states is qualitatively related to pattern geometry.

[1] This paper examines the effect of hyetograph shape on the potential for landsliding in soil-mantled landscapes. An existing pore pressure response model (Iverson, 2000) is used to study the effects of unsteady rainfall infiltration in hillslopes, and the effect of slope and convergent topography is expressed using a steady state model of slope-parallel subsurface flow. Slope stability is assessed using an infinite slope analysis. This theoretical framework is coupled with simple hyetograph models and to intensity-duration-frequency functions to determine the return period of landslide-triggering rainfall. Results also show that hyetographs with a peak at the end of a rainfall event have a stronger destabilizing effect than hyetographs with a constant rainfall or with a peak at the beginning of a storm. Thus the variability of hyetograph shapes adds uncertainty to the assessment of landsliding triggered by rainfall.

[1] We develop a modeling framework to investigate the influence of the North Atlantic Oscillation (NAO) on phenological variability in Europe through its influence on the distribution of wintertime synoptic-scale surface temperature variability. The approach employs an eigendecomposition of NCEP daily winter surface temperature estimates from the latter twentieth century to represent the spatial structure in the surface temperature field. The subset of statistically significant principal components are modeled as first-order autoregressive AR(1) processes, while the residual variance is modeled as spatially uncorrelated AR(1) noise. For those principal component time series that exhibit a statistically significant seasonal relationship with the NAO index, the parameters of the AR(1) model are conditioned on the phase (“high,” “neutral,” or “low”) of the NAO. This allows for realistic simulations of synoptic scale surface temperature variability over Europe as it is influenced by the NAO index. The model is applied to the simulation of trends in growing degree days (GDD) over Europe where simulated GDD variations are shown to agree well with growing degrees days from the data and evidence from available phenological records. Preliminary application of this model to a climate change scenario involving an increasing NAO 50 years into the future suggests the potential for a continued advancement of the start of the growing season.