Paolo D'Odorico

On the spatial and temporal links between vegetation, climate, and soil moisture

The impact of climate fluctuations can be observed in the dynamics of vegetation and most particularly in the sensitive environment of savannas. In this paper we present a model for the local competition for soil moisture among neighboring vegetation. The initial condition for the model is a random field where at each point the soil moisture is the mean water content when there are no spatial interactions between sites. The mean soil moisture values account for stochasticity of climate and losses from evapotranspiration and leakage which depend on the existing water content. A spatial dynamics is then implemented based on the explicit minimization of the global water stress over the region. This approach explains the coexistence of herbaceous and woody plants in savannas as well as the changes in canopy density that have been documented in the southwest of the United States as a function of regional climatic fluctuations.

Global land and water grabbing

Societal pressure on the global land and freshwater resources is increasing as a result of the rising food demand by the growing human population, dietary changes, and the enhancement of biofuel production induced by the rising oil prices and recent changes in United States and European Union bioethanol policies. Many countries and corporations have started to acquire relatively inexpensive and productive agricultural land located in foreign countries, as evidenced by the dramatic increase in the number of transnational land deals between 2005 and 2009. Often known as “land grabbing,” this phenomenon is associated with an appropriation of freshwater resources that has never been assessed before. Here we gather land-grabbing data from multiple sources and use a hydrological model to determine the associated rates of freshwater grabbing. We find that land and water grabbing are occurring at alarming rates in all continents except Antarctica. The per capita volume of grabbed water often exceeds the water requirements for a balanced diet and would be sufficient to improve food security and abate malnourishment in the grabbed countries. It is found that about 0.31 × 1012 m3⋅y−1 of green water (i.e., rainwater) and up to 0.14 × 1012 m3⋅y−1 of blue water (i.e., irrigation water) are appropriated globally for crop and livestock production in 47 × 106 ha of grabbed land worldwide (i.e., in 90% of the reported global grabbed land).

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.

Preferential states of seasonal soil moisture: the impact of climate fluctuations.

The impact of climate fluctuations on the dynamics of soil moisture is studied through a stochastic model of soil water balance. The analysis focuses on the changes of soil water content induced by the interannual variability of rainfall observed at the decade-to-century timescale. Extensive data analyses have been performed to characterize the statistical properties of such a variability. Particular attention is paid to the year-to-year variability of the average value of soil moisture during the growing season because of its relevance to the mechanisms affecting the physiology of plants and the dynamics of ecosystems. It is found that the probability distribution of the average seasonal soil moisture may be either unimodal or bimodal depending on the different combinations of climate, soil, and vegetation parameters. The possible occurrence of a double mode has both hydrologic and ecologic implications that are analyzed here.

A Probabilistic Analysis of Fire-Induced Tree-Grass Coexistence in Savannas

Fires play an important role in determining the composition and structure of vegetation in semiarid ecosystems. The study of the interactions between fire and vegetation requires a stochastic approach because of the random and unpredictable nature of fire occurrences. To this end, this article develops a minimalist probabilistic framework to investigate the impact of intermittent fire occurrences on the temporal dynamics of vegetation. This framework is used to analyze the emergence of statistically stable conditions favorable to tree‐grass coexistence in savannas. It is found that these conditions can be induced and stabilized by the stochastic fire regime. A decrease in fire frequency leads to bush encroachment, while more frequent and intense fires favor savanna‐to‐grassland conversions. The positive feedback between fires and vegetation can convert states of tree‐grass coexistence in semiarid savannas into bistable conditions, with both woodland and grassland as possible, though mutually exclusive, stable states of the system.

Robustness of variance and autocorrelation as indicators of critical slowing down.

Ecosystems close to a critical threshold lose resilience, in the sense that perturbations can more easily push them into an alternative state. Recently, it has been proposed that such loss of resilience may be detected from elevated autocorrelation and variance in the fluctuations of the state of an ecosystem due to critical slowing down; the underlying generic phenomenon that occurs at critical thresholds. Here we explore the robustness of autocorrelation and variance as indicators of imminent critical transitions. We show both analytically and in simulations that variance may sometimes decrease close to a transition. This can happen when environmental factors fluctuate stochastically and the ecosystem becomes less sensitive to these factors near the threshold, or when critical slowing down reduces the ecosystems capacity to follow high-frequency fluctuations in the environment. In addition, when available data is limited, variance can be systematically underestimated due to the prevalence of low frequencies close to a transition. By contrast, autocorrelation always increases toward critical transitions in our analyses. To exemplify this point, we provide cases of rising autocorrelation and increasing or decreasing variance in time series prior to past climate transitions.

Stochastic soil moisture dynamics along a hillslope

The spatial and temporal dynamics of soil water content along a hillslope is the result of a number of complex and mutually interacting processes. This paper deals with the role of subsurface, unsaturated, lateral water flow and its links to climate, soil, and hillslope characteristics. The analysis focuses on the soil moisture dynamics at the daily time scale, averaged over the plant rooting depth, and accounts for the stochastic nature of precipitation as well as for the non-linear dependence of transpiration and hydraulic conductivity on soil moisture. The lateral fluxes of soil moisture are described by means of the one-dimensional Richards equation, and the probabilistic soil moisture dynamics is numerically investigated considering different conditions of climate, pedology, vegetation, and hillslope geometry. From the analysis two different regimes emerge: a humid one, characterized by an unsaturated lateral flow with significant spatial gradients of soil moisture along the hillslope, and a dry one, in which the topography does not affect the spatial distribution of the soil moisture. In the humid regime, the long-term average spatial pattern of soil water content is studied at different points along the hillslope using the mean, rms, and pdfs of soil moisture, as well as the components of the long-term water balance. All these analyses show how the soil moisture dynamics is the result of complex and non-local interactions between climate, soil, vegetation, and hillslope shape.

Noise-Induced Phenomena in the Environmental Sciences

Randomness is ubiquitous in nature. Random drivers are generally considered a source of disorder in environmental systems. However, the interaction between noise and nonlinear dynamics may lead to the emergence of a number of ordered behaviors (in time and space) that would not exist in the absence of noise. This counterintuitive effect of randomness may play a crucial role in environmental processes. For example, seemingly “random” background events in the atmosphere can grow into larger instabilities that have great effects on weather patterns. This book presents the basics of the theory of stochastic calculus and its application to the study of noise-induced phenomena in environmental systems. It will be an invaluable reference text for ecologists, geoscientists, and environmental engineers interested in the study of stochastic environmental dynamics.

Ecological feedbacks following deforestation create the potential for a catastrophic ecosystem shift in tropical dry forest

The long-term ecological response to recurrent deforestation associated with shifting cultivation remains poorly investigated, especially in the dry tropics. We present a study of phosphorus (P) dynamics in the southern Yucatán, highlighting the possibility of abrupt shifts in biogeochemical cycling resulting from positive feedbacks between vegetation and its limiting resources. After three cultivation–fallow cycles, available soil P declines by 44%, and one-time P inputs from biomass burning decline by 76% from mature forest levels. Interception of dust-borne P (“canopy trapping”) declines with lower plant biomass and leaf area, limiting deposition in secondary forest. Potential leaching losses are greater in secondary than in mature forest, but the difference is very small compared with the difference in P inputs. The decline in new P from atmospheric deposition creates a long-term negative ecosystem balance for phosphorus. The reduction in soil P availability will feed back to further limit biomass recovery and may induce a shift to sparser vegetation. Degradation induced by hydrological and biogeochemical feedbacks on P cycling under shifting cultivation will affect farmers in the near future. Without financial support to encourage the use of fertilizer, farmers could increase the fallow period, clear new land, or abandon agriculture for off-farm employment. Their response will determine the regional balance between forest loss and forest regrowth, as well as the frequency of use and rate of recovery at a local scale, further feeding back on ecological processes at multiple scales.

Tree‐grass coexistence in Savannas: The role of spatial dynamics and climate fluctuations

The codominance of trees and grasses in savannas is explained as resulting from the minimization of vegetation stress. It is shown that under different climate, soil and vegetation conditions local interactions dictated by the spatial competition for soil moisture lead to an optimal state of minimum global water stress involving a stable coexistence of trees and grasses. The optimal state matches the observed canopy cover and soil moisture characteristics. Moreover, changes in canopy cover in the savanna environments are also suggested to result from climate fluctuations.