Kelly K. Caylor

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.

Trends in savanna structure and composition along an aridity gradient in the Kalahari

Abstract The Kalahari sand sheet occupies 2.5 million ha in southern Africa. It is an area with relatively similar deep aeolian soils, and a strong south to north gradient in rainfall, from 200 to 1000 mm mean annual precipitation (MAP) in the region studied. This provides an excellent basis for gradient studies at the subcontinental scale. This paper briefly reviews the literature on the vegetation of the Kalahari and describes the vegetation structure and composition at 11 new sites. There is a clear gradient in woody plant biomass (as indexed by basal area) from south to north. Above the minimum level of 200 mm MAP, the woody basal area increases at a rate of ca. 2.5 m2.ha−1 per 100 mm MAP. Mean maximum tree height also increases along the gradient, reaching 20 m at ca. 800 mm MAP. The number of species to contribute > 95% of the woody basal area increases from one at 200 mm to 16 at 1000 mm MAP. Members of the Mimosaceae (mainly Acacia) dominate the tree layer up to 400 mm MAP. They are replaced by either the Combretaceae (Combretum or Terminalia) or Colophospermum mopane of the Caesalpinaceae between 400 and 600 mm MAP, and by other representatives of the Caesalpinaceae above 600 mm MAP. The vegetation is largely deciduous up to 1000 mm MAP, except for species that apparently have access to groundwater, which may be locally dominant above about 600 mm MAP. Abbreviation: MAP = Mean annual precipitation.

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.

Partitioning evapotranspiration across gradients of woody plant cover: Assessment of a stable isotope technique

in the stable isotopic composition of water vapor (d 2 H). Our technique employs a newly‐developed laser‐based isotope analyzer and the Keeling plot approach for surface flux partitioning. The applicability of the technique was verified by comparison to separate, simultaneous lysimeter and sap flow estimates of ET partitioning. The results showed an expected increase in fractional contribution of transpiration to evapotranspiration as woody cover increased—from T/ET =0 .61 at 25% woody cover toT/ET = 0.83 at 100% cover. Further development of this technique may enable field characterization of evapotranspiration partitioning across diverse woody cover gradients, a central issue in addressing dryland ecohydrological responses to land use and climate change. Citation: Wang, L., K. K. Caylor, J. C. Villegas, G. A. Barron‐ Gafford, D. D. Breshears, and T. E. Huxman (2010), Partitioning evapotranspiration across gradients of woody plant cover: Assessment of a stable isotope technique, Geophys. Res. Lett., 37, L09401, doi:10.1029/2010GL043228.

Climatological determinants of woody cover in Africa

Determining the factors that influence the distribution of woody vegetation cover and resolving the sensitivity of woody vegetation cover to shifts in environmental forcing are critical steps necessary to predict continental-scale responses of dryland ecosystems to climate change. We use a 6-year satellite data record of fractional woody vegetation cover and an 11-year daily precipitation record to investigate the climatological controls on woody vegetation cover across the African continent. We find that—as opposed to a relationship with only mean annual rainfall—the upper limit of fractional woody vegetation cover is strongly influenced by both the quantity and intensity of rainfall events. Using a set of statistics derived from the seasonal distribution of rainfall, we show that areas with similar seasonal rainfall totals have higher fractional woody cover if the local rainfall climatology consists of frequent, less intense precipitation events. Based on these observations, we develop a generalized response surface between rainfall climatology and maximum woody vegetation cover across the African continent. The normalized local gradient of this response surface is used as an estimator of ecosystem vegetation sensitivity to climatological variation. A comparison between predicted climate sensitivity patterns and observed shifts in both rainfall and vegetation during 2009 reveals both the importance of rainfall climatology in governing how ecosystems respond to interannual fluctuations in climate and the utility of our framework as a means to forecast continental-scale patterns of vegetation shifts in response to future climate change.

Global synthesis of vegetation control on evapotranspiration partitioning

Evapotranspiration (ET) is an important component of the global hydrological cycle. However, to what extent transpiration ratios (T/ET) are controlled by vegetation and the mechanisms of global-scale T/ET variations are not clear. We synthesized all the published papers that measured at least two of the three components (E, T, and ET) and leaf area index (LAI) simultaneously. Nonlinear relationships between T/ET and LAI were identified for both the overall data set and agricultural or natural data subsets. Large variations in T/ET occurred across all LAI ranges with wider variability at lower LAI. For a given LAI, higher T/ET was observed during later vegetation growing stage within a season. We developed a function relating T/ET to the growing stage relative to the timing of peak LAI. LAI and growing stage collectively explained 43% of the variations in the global T/ET data set, providing a new way to interpret and model global T/ET variability.

Termite mounds can increase the robustness of dryland ecosystems to climatic change

Termites can stabilize tropical grasslands Spotty vegetation patterns in tropical savannas and grasslands can be a warning sign of imminent desertification. However, Bonachela et al. find that termites can also produce spotty patterns. Their theoretical study, confirmed by field data from Kenya, shows that patterns produced by termite mounds are not harbingers of desertification. Indeed, the presence of termites buffers these ecosystems against climate change. Science, this issue p. 651 Termites shape vegetation patterns in arid landscapes and buffer ecosystems against desertification. Self-organized spatial vegetation patterning is widespread and has been described using models of scale-dependent feedback between plants and water on homogeneous substrates. As rainfall decreases, these models yield a characteristic sequence of patterns with increasingly sparse vegetation, followed by sudden collapse to desert. Thus, the final, spot-like pattern may provide early warning for such catastrophic shifts. In many arid ecosystems, however, termite nests impart substrate heterogeneity by altering soil properties, thereby enhancing plant growth. We show that termite-induced heterogeneity interacts with scale-dependent feedbacks to produce vegetation patterns at different spatial grains. Although the coarse-grained patterning resembles that created by scale-dependent feedback alone, it does not indicate imminent desertification. Rather, mound-field landscapes are more robust to aridity, suggesting that termites may help stabilize ecosystems under global change.

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

On the calibration of continuous, high‐precision δ18O and δ2H measurements using an off‐axis integrated cavity output spectrometer

The (18)O and (2)H of water vapor serve as powerful tracers of hydrological processes. The typical method for determining water vapor delta(18)O and delta(2)H involves cryogenic trapping and isotope ratio mass spectrometry. Even with recent technical advances, these methods cannot resolve vapor composition at high temporal resolutions. In recent years, a few groups have developed continuous laser absorption spectroscopy (LAS) approaches for measuring delta(18)O and delta(2)H which achieve accuracy levels similar to those of lab-based mass spectrometry methods. Unfortunately, most LAS systems need cryogenic cooling and constant calibration to a reference gas, and have substantial power requirements, making them unsuitable for long-term field deployment at remote field sites. A new method called Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) has been developed which requires extremely low-energy consumption and neither reference gas nor cryogenic cooling. In this report, we develop a relatively simple pumping system coupled to a dew point generator to calibrate an ICOS-based instrument (Los Gatos Research Water Vapor Isotope Analyzer (WVIA) DLT-100) under various pressures using liquid water with known isotopic signatures. Results show that the WVIA can be successfully calibrated using this customized system for different pressure settings, which ensure that this instrument can be combined with other gas-sampling systems. The precisions of this instrument and the associated calibration method can reach approximately 0.08 per thousand for delta(18)O and approximately 0.4 per thousand for delta(2)H. Compared with conventional mass spectrometry and other LAS-based methods, the OA-ICOS technique provides a promising alternative tool for continuous water vapor isotopic measurements in field deployments.