Todd M. Scanlon

Positive feedbacks promote power-law clustering of Kalahari vegetation.

The concept of local-scale interactions driving large-scale pattern formation has been supported by numerical simulations, which have demonstrated that simple rules of interaction are capable of reproducing patterns observed in nature. These models of self-organization suggest that characteristic patterns should exist across a broad range of environmental conditions provided that local interactions do indeed dominate the development of community structure. Readily available observations that could be used to support these theoretical expectations, however, have lacked sufficient spatial extent or the necessary diversity of environmental conditions to confirm the model predictions. We use high-resolution satellite imagery to document the prevalence of self-organized vegetation patterns across a regional rainfall gradient in southern Africa, where percent tree cover ranges from 65% to 4%. Through the application of a cellular automata model, we find that the observed power-law distributions of tree canopy cluster sizes can arise from the interacting effects of global-scale resource constraints (that is, water availability) and local-scale facilitation. Positive local feedbacks result in power-law distributions without entailing threshold behaviour commonly associated with criticality. Our observations provide a framework for integrating a diverse suite of previous studies that have addressed either mean wet season rainfall or landscape-scale soil moisture variability as controls on the structural dynamics of arid and semi-arid ecosystems.

On soil moisture–vegetation feedbacks and their possible effects on the dynamics of dryland ecosystems

[1] Soil moisture is the environmental variable synthesizing the effect of climate, soil, and vegetation on the dynamics of water-limited ecosystems. Unlike abiotic factors (e.g., soil texture and rainfall regime), the control exerted by vegetation composition and structure on soil moisture variability remains poorly understood. A number of field studies in dryland landscapes have found higher soil water contents in vegetated soil patches than in adjacent bare soil, providing a convincing explanation for the observed preferential establishment of grasses and seedlings beneath tree canopies. Thus, because water is the limiting factor for vegetation in arid and semiarid ecosystems, a positive feedback could exist between soil moisture and woody vegetation dynamics. It is still unclear how the strength of such a feedback would change under different long-term rainfall regimes. To this end, we report some field observations from savanna ecosystems located along the south-north rainfall gradient in the Kalahari, where the presence of relatively uniform sandy soils limits the effects of covarying factors. The data available from our field study suggest that the contrast between the soil moisture in the canopy and intercanopy space increases (with wetter soils under the canopy) with increasing levels of aridity. We hypothesize that this contrast may lead to a positive feedback and explore the implications of such a feedback in a minimalistic model. We found that when the feedback is relatively strong, the system may exhibit two stable states corresponding to conditions with and without tree canopy cover. In this case, even small changes in environmental variables may lead to rapid and largely irreversible shifts to a state with no tree canopy cover. Our data suggest that the tendency of the system to exhibit two (alternative) stable states becomes stronger in the more arid regions. Thus, at the desert margins, vegetation is more likely to be prone to discontinuous and abrupt state changes.

Determining land surface fractional cover from NDVI and rainfall time series for a savanna ecosystem

Savanna ecosystems are water limited and responsive to rainfall on short time scales, characteristics that can be exploited to estimate fractional cover of trees, grass, and bare soil over large-scale areas from synthesis of remote sensing and rainfall measurements. A method is presented to estimate fractional cover based upon the differing ways in which grasses and trees respond to rainfall, and implementation of this method is demonstrated along the Kalahari Transect (KT), an aridity gradient in southern Africa. Seasonally averaged normalized difference vegetation index (NDVI) and the sensitivity of the NDVI to interannual variations in wet season rainfall are used as state-space variables in a linear unmixing model. End members for this analysis were determined on the basis of best fit to the observed data. The realized end members were consistent with the qualitative characteristics of trees (high NDVI, low sensitivity of NDVI to interannual variations in rainfall), bare soil (low NDVI, low sensitivity), and the transient grass/ bare soil area (moderate NDVI, high sensitivity). Observed sensitivity of NDVI to rainfall was measured as the relationship between wet season NDVI and normalized rainfall over a 16-year period (1983–1998). The unmixing model yields a north-to-south decrease in tree fractional cover that corresponds to the decrease in mean wet season precipitation from 1600 to 300 mm along the KT (R 2 =.87). The fractional tree cover results compare favorably with available ground-based observations. The potential extent of grass cover is limited by the dominance of trees on the northern end of the transect, peaks at the location with approximately 450 mm of mean wet season rainfall, and is limited by rainfall in the arid southern portion of the transect. With mean NDVI for grass inferred from the data, yearly estimates of tree, grass, and bare soil fractional cover can be derived. These annual estimates, which are difficult to obtain from traditional unmixing procedures, are important parameters in fuel load and land–atmosphere exchange models. No calibration or training sets were required for this analysis, and this method has the additional capability to predict fractional-cover components for future rainfall scenarios. D 2002 Published by Elsevier Science Inc.

Modeling transport of dissolved silica in a forested headwater catchment: Implications for defining the hydrochemical response of observed flow pathways

Groundwater, subsurface stormflow, and overland flow components of discharge, derived from a hydrological model that was applied to a forested headwater catchment in north central Virginia, were used with measured stream water and lysimeter concentrations of dissolved silica to investigate the hydrochemical behavior of the catchment. Concentrations in base flow, taken to be a reflection of groundwater, vary with discharge, an observation in conflict with the typical assumption of constant concentration used in end-member mixing analyses. This observed flow dependence was modeled by considering the concentration in groundwater to be related to the saturation deficit in this zone. A positive correlation between the average groundwater saturation deficit and base flow dissolved silica concentrations is consistent with batch experiments and petrographic analysis of regolith core samples, which both indicate an increase in silica available for dissolution with depth in the groundwater zone. In the absence of subsurface storm flow zone sampling during rainfall events a constant concentration was assumed for this zone. Concentration-discharge (C-Q) paths in the stream were used to evaluate the modeled stream silica concentrations. An inconsistency in the direction of the modeled C-Q rotations suggests that the storm flow zone dissolved silica concentration may also vary with time, because of the “flushing” of high-concentration, preevent soil water on the rising limb of the storm hydrograph. For this catchment in Virginia the assumption of a constant concentration for subsurface storm flow, as well as for base flow, appears to be invalid.

Shallow subsurface storm flow in a forested headwater catchment: Observations and modeling using a modified TOPMODEL

Transient, perched water tables in the shallow subsurface are observed at the South Fork Brokenback Run catchment in Shenandoah National Park, Virginia. Crest piezometers installed along a hillslope transect show that the development of saturated conditions in the upper 1.5 m of the subsurface is controlled by total precipitation and antecedent conditions, not precipitation intensity, although soil heterogeneities strongly influence local response. The macroporous subsurface storm flow zone provides a hydrological pathway for rapid runoff generation apart from the underlying groundwater zone, a conceptualization supported by the two-storage system exhibited by hydrograph recession analysis. A modified version of TOPMODEL is used to simulate the observed catchment dynamics. In this model, generalized topographic index theory is applied to the subsurface storm flow zone to account for logarithmic storm flow recessions, indicative of linearly decreasing transmissivity with depth. Vertical drainage to the groundwater zone is required, and both subsurface reservoirs are considered to contribute to surface saturation.

Ecohydrological optimization of pattern and processes in water-limited ecosystems: A trade-off-based hypothesis

The coupled nature of hydrological and ecological dynamics is perhaps nowhere more evident than in semi-arid ecosystems. Frequently stressed and sensitive to change (Guenther et al., 1996), semi-arid ecosystems are responsive to climate variability over relatively short time scales, and water is the main driving force in shaping the vegetation distribution and composition (Rodriguez-Iturbe et al., 1999; Smit and Rethman, 2000). The dynamical nature of the vegetation response to water availability is a prominent feature of semi-arid ecosystem function, as is evident from satellite observations (Goward and Prince, 1995; Scanlon et al., 2002). Despite the close coupling that exists between water and vegetation structure, the challenge of predicting vegetation response to changing climate in these environments is particularly daunting (Daly et al., 2000). Specifically, a central challenge is defining the ecologically and hydrologically relevant processes that led to the formation of vegetation patterns in water-limited ecosystems. The complex interactions between plants, soils, and climates in semiarid ecosystems make it difficult to define specific ecohydrological optimization mechanisms that underlie observed landscape-scale patterns in vegetation structure. Regional models of semi-arid vegetation structure are often biogeographical in nature, making predictions based exclusively on the role of external factors such as mean annual rainfall, or soil infertility imposed by geologic constraints. These kinds of relationships often yield reasonable predictions of savanna ecosystem structure (Sankaran et al., 2005; Huxman et al., 2005), but provide little additional insight in the specific ecohydrological processes that maintain vegetation-climate co-organization. At the scale of plant canopies, it has also been recognized that the role of vegetation itself

Inferred controls on tree/grass composition in a savanna ecosystem: Combining 16‐year normalized difference vegetation index data with a dynamic soil moisture model

[1] A two-layer soil moisture model was developed to infer the controls on the tree/grass coexistence in a savanna ecosystem. The model was applied along a mean annual precipitation gradient in southern Africa, known as the Kalahari Transect (KT), which follows a north-south decline in mean annual rainfall from � 1600 mm/yr to � 250 mm/yr between the latitudes 12� –26� S. Satellite-derived fractional covers for trees, grass, and bare soil were used as input to the model, with the fractional grass cover responsive to interannual variability in rainfall, as defined by observed statistics. Other inputs to the model included satellite-based radiation budget measurements and interpolated, groundbased rainfall measurements. The soil moisture model structure allows both trees and grass to have access to the upper zone, while trees alone can extract water from the lower zone. We focus our analysis on the wet season months of November–March, in which 87% of the annual rainfall is received along the KT. Simulations were performed on daily time steps for 16 years, representing a range of interannual rainfall variability, at 196 equally spaced positions along the transect. The results indicate that two distinct areas of the KT exist in terms of vegetation-rainfall relationships. North of 18� S, the trees and grass are rarely water stressed during the wet season and a large portion of the water balance is accounted for by leakage through the bottom of the root zone in sandy soils. The vegetation cover in the northern end of the transect does not reach its potential in terms of water exploitation and is most likely nutrient limited. In the southern portion of the KT, tree fractional cover is such that trees become water stressed in drier than average wet seasons and suffer no water stress during wetter than average wet seasons. The extent of the fractional grass cover for individual years is controlled by the water demand stress. We find that observed tree/grass fractional cover in the waterlimited region of the transect is best explained by considering, on an individual basis, an allowable range of stress for trees and grass, while minimizing the amount of water that is unexploited by the vegetation and lost as leakage from the root zone. INDEX TERMS: 1655 Global Change: Water cycles (1836); 1818 Hydrology: Evapotranspiration; 1866 Hydrology: Soil moisture; 1878 Hydrology: Water/energy interactions; KEYWORDS: savanna, soil moisture, satellite, Africa

Turbulent transport of carbon dioxide and water vapor within a vegetation canopy during unstable conditions: Identification of episodes using wavelet analysis

The net exchange of CO 2 between the biosphere and atmosphere is realized as a difference between the fluxes associated with photosynthesis and respiration. This paper contrasts the turbulent transport mechanics of two dominant pathways affecting this exchange. Using high-frequency measurements from an experiment conducted at the Duke Forest in North Carolina, wavelet analysis is applied to time series of carbon dioxide and water vapor concentrations in order to (1) determine the dominant eddy sizes involved in the net exchange of these constituents, (2) resolve the eddy size and timescales involved in the intermittent release of CO 2 from the forest floor to the atmosphere, and (3) relate the boundary layer turbulent characteristics to the transport of air enriched in CO 2 from soil respiration. During the daytime hours, when photosynthesis and soil respiration are active in this pine forest and evapotranspiration is taking place, air enriched in both CO 2 and water vapor is indicative of transport from the forest floor. Thus the coherent turbulent structures associated with these transport events are identified and conditionally analyzed from the time series by wavelet transforms, which retain information in the time domain as well as the frequency domain. The dominant flux-carrying eddies between the canopy and atmosphere were approximately 63 m in diameter, about four times the height of the canopy. Eddies that were most effective in transporting air enriched in CO 2 from below the canopy to the atmosphere were found to be approximately 8 m in diameter, on the order of one half the canopy height. Conditional sampling shows that the prevalence of air enriched in both CO 2 and water vapor is related to the rate of turbulent kinetic energy production measured from 24 approximately half-hour time series corresponding to unstable atmospheric conditions.

δ2H isotopic flux partitioning of evapotranspiration over a grass field following a water pulse and subsequent dry down

The partitioning of surface vapor flux (FET) into evaporation (FE) and transpiration (FT) is theoretically possible because of distinct differences in end-member stable isotope composition. In this study, we combine high-frequency laser spectroscopy with eddy covariance techniques to critically evaluate isotope flux partitioning of FET over a grass field during a 15 day experiment. Following the application of a 30 mm water pulse, green grass coverage at the study site increased from 0 to 10% of ground surface area after 6 days and then began to senesce. Using isotope flux partitioning, transpiration increased as a fraction of total vapor flux from 0% to 40% during the green-up phase, after which this ratio decreased while exhibiting hysteresis with respect to green grass coverage. Daily daytime leaf-level gas exchange measurements compare well with daily isotope flux partitioning averages (RMSE = 0.0018 g m−2 s−1). Overall the average ratio of FT to FET was 29%, where uncertainties in Keeling plot intercepts and transpiration composition resulted in an average of uncertainty of ∼5% in our isotopic partitioning of FET. Flux-variance similarity partitioning was partially consistent with the isotope-based approach, with divergence occurring after rainfall and when the grass was stressed. Over the average diurnal cycle, local meteorological conditions, particularly net radiation and relative humidity, are shown to control partitioning. At longer time scales, green leaf area and available soil water control FT/FET. Finally, we demonstrate the feasibility of combining isotope flux partitioning and flux-variance similarity theory to estimate water use efficiency at the landscape scale.

Decreased atmospheric sulfur deposition across the southeastern U.S.: When will watersheds release stored sulfate?

Emissions of sulfur dioxide (SO2) to the atmosphere lead to atmospheric deposition of sulfate (SO4(2-)), which is the dominant strong acid anion causing acidification of surface waters and soils in the eastern United States. Since passage of the Clean Air Act and its Amendments, atmospheric deposition of SO2 in this region has declined by over 80%, but few corresponding decreases in streamwater SO4(2-) concentrations have been observed in unglaciated watersheds. We calculated SO4(2-) mass balances for 27 forested, unglaciated watersheds from Pennsylvania to Georgia, by using total atmospheric deposition (wet plus dry) as input. Many of these watersheds still retain SO4(2-), unlike their counterparts in the northeastern U.S. and southern Canada. Our analysis showed that many of these watersheds should convert from retaining to releasing SO4(2-) over the next two decades. The specific years when the watersheds crossover from retaining to releasing SO4(2-) correspond to a general geographical pattern of later net watershed release from north to south. The single most important variable that explained the crossover year was the runoff ratio, defined as the ratio of annual mean stream discharge to precipitation. Percent clay content and mean soil depth were secondary factors in predicting crossover year. The conversion of watersheds from net SO4(2-) retention to release anticipates more widespread reductions in streamwater SO4(2-) concentrations in this region.