Francesco Laio

Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: II. Probabilistic soil moisture dynamics

A stochastic model for soil moisture dynamics at a point is studied in detail. Rainfall is described as a marked Poisson process, producing a state-dependent infiltration into the soil. Losses due to leakage and evapotranspiration also depend on the existing level of soil moisture through a simplifying but realistic representation of plant physiological characteristics and soil properties. The analytic solution of the steady-state probability distributions is investigated to assess the role of climate, soil, and vegetation in soil moisture dynamics and water balance.

Plants in water-controlled ecosystems : active role in hydrologic processes and response to water stress. III. Vegetation water stress

The reduction of soil moisture content during droughts lowers the plant water potential and decreases transpiration; this in turn causes a reduction of cell turgor and relative water content which brings about a sequence of damages of increasing seriousness. A review of the literature on plant physiology and water stress shows that vegetation water stress can be assumed to start at the soil moisture level corresponding to incipient stomatal closure and reach a maximum intensity at the wilting point. The mean crossing properties of these soil moisture levels crucial for water stress are derived analytically for the stochastic model of soil moisture dynamics described in Part II (F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe. Adv. Water Res. 24 (7) (2001) 707-723). These properties are then used to propose a measure of vegetation water stress which combines the mean intensity, duration, and frequency of periods of soil water deficit. The characteristics of vegetation water stress are then studied under different climatic conditions, showing how the interplay between plant, soil, and environment can lead to optimal conditions for vegetation.

Hydrologic controls on soil carbon and nitrogen cycles. I. Modeling scheme

Abstract The influence of soil moisture dynamics on soil carbon and nitrogen cycles is analyzed by coupling an existing stochastic soil moisture model [Adv. Water. Resour. 24 (7) (2001) 707; Proc. R. Soc. Lond. A 455 (1999) 3789] to a system of eight nonlinear differential equations that describe the temporal evolution of the organic matter and the mineral nitrogen in the soil at the daily to seasonal time scales. Special attention is devoted to the modeling of the soil moisture control on mineralization and immobilization fluxes, leaching losses, and plant nitrogen uptake, as well as to the role played by the soil organic matter carbon-to-nitrogen ratio in determining mineralization and immobilization. The model allows a detailed analysis of the soil nitrogen cycle as driven by fluctuations in soil moisture at the daily time scale resulting from the stochastic rainfall variability. The complex ensuing dynamics are studied in detail in a companion paper [Adv. Water Resour. 26 (1) (2003) 59], which presents an application to the Nylsvley savanna in South Africa. The model accounts for the soil moisture control on different components of the nitrogen cycle on a wide range of time scales: from the high frequency variability of leaching and uptake due to the nitrate flushes after persistent rainfall following a period of drought, to the low frequency temporal dynamics of the soil organic matter pools. All the fluctuations in the various pools are statistically characterized in relation to their dependence on climate, soil, and vegetation characteristics.

Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: IV. Discussion of real cases

Three water-controlled ecosystems are studied here using the stochastic description of soil moisture dynamics and vegetation water stress proposed in Part II (F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 707-723) and Part III (A. Porporato, F. Laio, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 725-744) of this series of papers. In the savanna of Nylsvley (South Africa) the very diverse physiological characteristics of the existing plants give rise to different strategies of soil moisture exploitation. Notwithstanding these differences, the vegetation water stress for all the species turns out to be very similar, suggesting that coexistence might be attained also through differentiation of water use. The case of the savanna of Southern Texas points out how rooting depth and interannual rainfall variability can impact soil moisture dynamics and vegetation water stress. Because of the different responses to water stress of trees and grasses, external climatic forcing could be at the origin of the dynamic equilibrium allowing coexistence in this ecosystem. Finally, the analysis of a short grass steppe in Colorado provides an interesting example of the so-called inverse texture effect, whereby preferential conditions for vegetation are dependent on soil texture and rainfall. Sites which are more favorable during wet conditions may become less suitable to the same vegetation type during drier years. Such an effect is important to explain the predominance of existing species, as well as to investigate their reproductive strategies.

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.

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.

Hydrologic controls on soil carbon and nitrogen cycles. II. A case study

The nitrogen and carbon cycles in the broad-leafed savanna at Nylsvley (S. Africa) are modeled using the stochastic approach presented by Porporato et al. [Adv Water Res (this issue)]. An accurate representation of the hydrological mechanisms that control the nitrogen cycle at the daily time scale is shown to be necessary to capture the impact of the high-frequency variability of the soil moisture on the nitrogen and carbon dynamics. The fluctuations of the random precipitation forcing propagate to soil moisture, carbon, and nitrogen dynamics, giving rise to a gamut of fluctuations at different time scales. Long simulations are carried out to achieve a probabilistic characterization of the dynamics of the state variables under different rainfall regimes.

Recent History and Geography of Virtual Water Trade

The global trade of goods is associated with a virtual transfer of the water required for their production. The way changes in trade affect the virtual redistribution of freshwater resources has been recently documented through the analysis of the virtual water network. It is, however, unclear how these changes are contributed by different types of products and regions of the world. Here we show how the global patterns of virtual water transport are contributed by the trade of different commodity types, including plant, animal, luxury (e.g., coffee, tea, and alcohol), and other products. Major contributors to the virtual water network exhibit different trade patterns with regard to these commodity types. The net importers rely on the supply of virtual water from a small percentage of the global population. However, discrepancies exist among the different commodity networks. While the total virtual water flux through the network has increased between 1986 and 2010, the proportions associated with the four commodity groups have remained relatively stable. However, some of the major players have shown significant changes in the virtual water imports and exports associated with those commodity groups. For instance, China has switched from being a net exporter of virtual water associated with other products (non-edible plant and animal products typically used for manufacturing) to being the largest importer, accounting for 31% of the total water virtually transported with these products. Conversely, in the case of The United states of America, the commodity proportions have remained overall unchanged throughout the study period: the virtual water exports from The United States of America are dominated by plant products, whereas the imports are comprised mainly of animal and luxury products.

Intensive or extensive use of soil moisture: Plant strategies to cope with stochastic water availability

Some plants rely on a dependable winter recharge, as opposed to others that quickly respond to the intermittent and uncertain rainfall during the growing season. Using a stochastic model for the soil moisture process and a quantitative measure of plant water stress, we find climate, soil, and vegetation characteristics leading to the dominance or possible coexistence of these two strategies of water use.

An Analysis of the Soil Moisture Feedback on Convective and Stratiform Precipitation

Abstract Land–atmosphere interactions in midlatitude continental regions are particularly active during the warm season. It is still unclear whether and under what circumstances these interactions may involve positive or negative feedbacks between soil moisture conditions and rainfall occurrence. Assessing such feedbacks is crucially important to a better understanding of the role of land surface conditions on the regional dynamics of the water cycle. This work investigates the relationship between soil moisture and subsequent precipitation at the daily time scale in a midlatitude continental region. Sounding data from 16 locations across the midwestern United States are used to calculate two indices of atmospheric instability—namely, the convective available potential energy (CAPE) and the convective inhibition (CIN). These indices are used to classify rainfall as convective or stratiform. Correlation analyses and uniformity tests are then carried out separately for these two rainfall categories, to asse...