An ecohydrological framework to explain shifts in vegetation organization across climatological gradients

Spatial patterns found in vegetated ecosystems exhibit different degrees of organization in stand density that can be interpreted as an indicator of ecosystem health. In semiarid environments, it is possible to observe transitions from over-dispersed individuals (e.g., an ordered lattice) to under-dispersed individuals (e.g., clumped points). These configurations correspond to different strategies of adaptation or optimization, whose understanding may help to predict some of the consequences of environmental changes for both ecosystem services and water resources. For this reason, we have developed a theoretical framework that characterizes the dispersion of individuals through a generalized double Poisson distribution and estimates the landscape-wide statistics using a soil moisture model accounting for tree canopies and root systems overlapping. Considering both the shading effect (light interception) of the canopies and the partitioning of water fluxes due to the presence of multiple individual root systems in one point, the optimum spacing between individuals at a given stand density is determined. This framework allows identifying the climatic boundaries for different landscape patterns in terms of optimal water use and stress. This simple scheme explains well the observed patterns of vegetation in arid and semiarid ecosystems.

How to cite: Manfreda, S., K. K. Caylor, S. Good, An Ecohydrological framework to explain shifts in vegetation organization across climatological gradientsEcohydrology, 10(3), 1-14, (doi: 10.1002/eco.1809), 2017. [pdf]

On the Vulnerability of Water Limited Ecosystems to Climate Change

Society is facing growing environmental problems that require new research efforts to understand the way ecosystems operate and survive, and their mutual relationships with the hydrologic cycle. In this respect, ecohydrology suggests a renewed interdisciplinary approach that aims to provide a better comprehension of the effects of climatic changes on terrestrial ecosystems. With this aim, a coupled hydrological/ecological model is adopted to describe simultaneously vegetation pattern evolution and hydrological water budget at the basin scale using as test site the Upper Rio Salado basin (Sevilleta, NM, USA). The hydrological analyses have been carried out using a recently formulated framework for the water balance at the daily level linked with a spatial model for the description of the spatial organization of vegetation. This enables quantitatively assessing the effects on soil water
availability on future climatic scenarios. Results highlighted that the relationship between climatic forcing (water availability) and vegetation patterns is strongly non-linear. This implies, under some specific conditions which depend on the ecosystem characteristics, small changes in climatic conditions may produce significant transformation of the vegetation patterns.

How to cite: Manfreda, S., K.K. Caylor, On The Vulnerability of Water Limited Ecosystems to Climate ChangeWater, 5(2), 819-833; (doi:10.3390/w5020819), 2013. [pdf]

Modelling Vegetation Patterns in Semiarid Environments

The aim of this work is to deepen our understanding on the mutual relationship between climate, vegetation and soil water budget within an ecohydrological framework. To this end a coupled hydrological/ecological model is adopted to describe simultaneously soil water budget and vegetation pattern evolution in a semiarid river basin in New Mexico (USA). This basin represents an ideal area to study the properties of water-controlled ecosystems. Analyses have been carried out using a recently formulated framework for the water balance at the daily level linked with a vegetation model for the description of the spatial organization of vegetation. Using this approach, we identified the dynamic water stress of vegetation during the growing season, taking into account effects of morphology on the spatial distribution of solar radiation and the initial soil moisture condition at the beginning of the growing season. Several different variants of the vegetation model have been tested with the aim to identify the main drivers for the spatial organization of the vegetation. Results clearly show that the observed vegetation patterns emerge from the minimization of water stress and the maximization of water use.

How to cite: Manfreda, S., T. Pizzolla, K.K. Caylor, Modeling Vegetation Patterns in Semiarid Environment, Procedia Environmental Science, 19, 168-177, (doi: 10.1016/j.proenv.2013.06.019) 2013. [pdf]

Effects of morphology on global solar radiation and potential evapotranspiration

In this work we present an application of two different models for the calculation of extraterrestrial solar radiation and main components of surface radiation balance under clear sky conditions. These models account for the effects of the morphology on solar radiation and potential evapotranspiration exploiting the slope and aspect of the considered surfaces. The solar radiation was evaluated with two algorithms (Allen et al., 2006 and Kumar et al., 1997) and is used in the Penman-Monteith equation to estimate the potential evapotranspiration. By comparing the maps and the profiles obtained with these two models we highlighted the main differences due to the structure of the two different algorithms considered. Results show that the two methods produces almost same results when applied at the yearly scale, while the algorithm by Allen et al. (2006) outperform the one proposed by Kumar et al. (1997) at the daily time scale. Results highlighted the role of morphology (slope and aspect) on the global solar radiation and evapotranspiration at the local scale.

How to cite: Pizzolla, T., A. Acampora, S. Manfreda, Effetti legati alla morfologia nella stima della Radiazione Solare globale e dell’Evapotraspirazione potenzialeL’Acqua, n.2, 45-53, 2012. [pdf]

Ecohydrology: A new Interdisciplinary Approach to Investigate on Climate – Soil – Vegetation Interactions

Society is facing growing environmental problems that require new research efforts to understand of the way that ecosystems operate and survive and their mutual relationships with the hydrologic cycle. This is fundamental to advance predictive models used by researchers, industry, and environmental managers. In this frame, Ecohydrology faces this task with the aim to provide improved forecasting and mitigation of flood and drought risk, better understanding of implications of land use changes on terrestrial ecosystems (such as deforestation or desertification), improved weather and climate predictions, better comprehension of climatic changes effects on terrestrial ecosystems. The scope of the present paper is to address the most challenging questions of ecohydrology providing a review of some of the most recent results in this emerging field.

How to cite: Salvatore Manfreda, Ecohydrology: A new Interdisciplinary Approach to Investigate on Climate – Soil – Vegetation Interactions, Annals of Arid Zone, Volume 48, Number 3 e 4, September and December, Pages 219-228, ISSN 0570-1791, 2009. 

Dynamic response of grass cover to rainfall variability: implications for the function and persistence of savanna ecosystems

Savanna grass cover is dynamic and its annual extent resonates with wet season rainfall, as shown by satellite observations of normalized difference vegetation index (NDVI) time series for the Kalahari Transect (KT) in southern Africa. We explore the hydrological significance of the dynamic grass cover by applying a soil moisture model to the water-limited portion of the KT, which spans a north-south gradient in mean wet season rainfall, r’, from approximately 700 to 300 mm. Satellite-derived tree fractional cover, xt, is shown to be highly correlated with ground meteorological measurements of r’ (R2 =0,94) in this region. By implementing a simple expression for grass growth and decay in the model that factored in only xt and near-surface soil moisture, we were able to effectively reproduce the satellite-derived fractional grass cover, xg , along the transect over a 16-year period (1983–1998). We compared the results from dynamic grass model with those yielded by a static grass cover model in which xg was set to its 16-year average for each simulation. The dynamic quality of the grass was found to be important for reducing tree stress during dry years and for reducing the amount of water that is lost from the overall root zone during the wet years, relative to the static grass case. We find that the dynamic grass cover acts as a buffer against variability in wet season precipitation, and in doing so helps to maximize ecosystem water use. The model results indicate that mixed tree/grass savanna ecosystems are ideally suited to reach a dynamic equilibrium with respect to the use of a fluctuating limiting resource (water) by having functional components that respond to variability in rainfall over long timescales (trees) and short timescales (grasses).

How to cite: Scanlon, T.M., K.K. Caylor, S. Manfreda, S.A. Levin, I. Rodríguez-Iturbe, Dynamic Response of Grass Cover to Rainfall Variability: Implications the Function and Persistence of Savanna EcosystemsAdvances in Water Resources, 28(3), 291-302, (doi: 10.1016/j.advwatres.2004.10.014), 2005.  [pdf]

On the coupled geomorphological and ecohydrological organization of river basins

This paper examines the linkage between the drainage network and the patterns of soil water balance components determined by the organization of vegetation, soils and climate in a semiarid river basin. Research during the last 10 years has conclusively shown an increasing degree of organization and unifying principles behind the structure of the drainage network and the three-dimensional geometry of river basins. This cohesion exists despite the infinite variety of shapes and forms one observes in natural watersheds. What has been relatively unexplored in a quantitative and general manner is the question of whether or not the interaction of vegetation, soils, and climate also display a similar set of unifying characteristics among the very different patterns they presents in river
basins. A recently formulated framework for the water balance at the daily level links the observed patterns of basin organization to the soil moisture dynamics. Using available geospatial data, we assign soil, climate, and vegetation properties across the basin and analyze the probabilistic characteristics of steady-state soil moisture distribution. We investigate the presence of organization through the analysis of the spatial patterns of the steady-state soil moisture distribution, as well as in the distribution of observed vegetation patterns, simulated vegetation dynamic water stress and hydrological fluxes such as transpiration. Here we show that the drainage network acts as a template for the organization of both vegetation and hydrological patterns, which exhibit self-affine characteristics in their distribution across the river basin. Our analyses suggest the existence of a balance between the large-scale determinants of vegetation pattern reflecting optimality in the response to water stress and the random small-scale patterns that arise from local factors and ecological legacies such as those caused by dispersal, disturbance, and founder effects.

How to cite: Caylor, K.K., S. Manfreda, I. Rodríguez-Iturbe, On the Coupled Geomorphological and Ecohydrological Organization of River BasinsAdvances in Water Resources, 28(1), 69-86, (doi: 10.1016/j.advwatres.2004.08.013), 2005.  [pdf]