Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: I. Scope and general outline

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Abstract

This series of four papers studies the complex dynamics of water-controlled ecosystems from the hydro-ecological point of view [e.g., I. Rodriguez-Iturbe, Water Resour. Res. 36 (1) (2000) 3–9]. After this general outline, the role of climate, soil, and vegetation is modeled in Part II [F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 707–723] to investigate the probabilistic structure of soil moisture dynamics and the water balance. Particular attention is given to the impact of timing and amount of rainfall, plant physiology, and soil properties. From the statistical characterization of the crossing properties of arbitrary levels of soil moisture, Part III develops an expression for vegetation water stress [A. Porporato, F. Laio, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 725–744]. This measure of stress is then employed to quantify the response of plants to soil moisture deficit as well as to infer plant suitability to given environmental conditions and understand some of the reasons for possible coexistence of different species. Detailed applications of these concepts are developed in Part IV [F. Laio, A. Porporato, C.P. Fernandez-Illescas, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 745–762], where we investigate the dynamics of three different water-controlled ecosystems.

Introduction

Water-controlled ecosystems are complex, evolving structures whose characteristics and dynamic properties depend on many interrelated links between climate, soil, and vegetation (Fig. 1). On the one hand, climate and soil control vegetation dynamics (e.g. [4], [15], [17], [21], [22]); on the other hand, vegetation exerts important control on the entire water balance and is responsible for many feedbacks to the atmosphere (e.g. [18], [38], [46]). Many important issues depend on the quantitative understanding of this dynamics, including environmental preservation and proper management of resources (e.g. [2], [3], [27], [40], [41]). In fact, ecohydrology itself may be defined as the science which seeks to describe the hydrologic mechanisms that underlie ecologic patterns and processes [36].

Soil moisture is the key variable which synthesizes the action of climate, soil, and vegetation on the water balance and the dynamic impact of the water balance on plants (e.g. [21], [23], [25], [27], [44], [45]). Many ecosystems of tropical and subtropical latitudes suffer water stress, which is in turn controlled by the temporal fluctuations of soil moisture (e.g. [26], [42]). Although other sources of stress (fire, grazing, nutrient availability, etc.) are certainly also present, in many of the world ecosystems soil moisture is the most important resource affecting vegetation structure and organization.

Vegetation itself plays a special role in water-controlled ecosystems: plants have an active role in water use that heavily conditions the water balance of the system. At the same time plants are also impacted by the arid conditions and the water stress they produce. The connections between the role of plants in the water balance and their water-stress response is a fascinating and in great part unexplored topic which lies at the heart of ecohydrology. Differences in soil moisture dynamics are among the principal reasons for the existence of particular functional vegetation types (e.g., grasslands, savannas, forests, etc.). Special adaptation to water stress and intra/inter-species interactions are likely connected to the dynamics of the climate–soil–vegetation system and, moreover, the coexistence of different functional vegetation types may be explained through the emergence of specific temporal niches of soil water availability [7], [39]. The dynamics of the climate–soil–vegetation interactions are critically influenced by the scale at which the phenomena are studied as well as by the physiological characteristics of vegetation, the pedology of the soil, and the type of climate. Thus such dynamics are fundamentally different between, say, forests, savannas, and grasslands.

In terms of water availability, ecosystem response is controlled not only by rainfall scarcity, but also by its intermittent and unpredictable nature, by its coupling with the temperature changes throughout the year and by the soil characteristics, which control the infiltration process. The uncertainty of both the timing and amount of rainfall has induced vegetation to develop different strategies to respond to water stress and optimize reproduction and productivity (e.g. [27]). Any effort towards hydro-ecological modeling has to take into account the stochastic character of soil moisture dynamics, with fluctuation at different temporal and spatial scales. We concentrate here on a modeling scheme towards a quantitative description of the temporal dynamics of the climate–soil–vegetation system. Simplifying, yet realistic, assumptions will be made in order to keep analytical tractability and, at the same time, to be able to achieve results with general physical interpretation. This kind of approach stems from the conviction that only models with a framework of general characteristics can provide a valuable interface between the real changing environment and data obtained from specific experiments. In this respect the philosophy of this work is akin to that of Noy-Meir [27], Eagleson [9], [10], Eagleson and Tellers [11], Eagleson and Segarra [12], and Paruelo and Sala [28] among others.

Section snippets

Scale issues and stochastic fluctuations in soil moisture dynamics

There are two characteristics which make especially daunting the quantitative analysis of this problem: (1) the very large number of different processes and phenomena which make up the dynamics, and (2) the extremely large degree of variability in time and space that the phenomena present.

The first of the above characteristics obviously calls for simplifying assumptions in the modeling scheme while still preserving the most important features of the dynamics. The second of the above

Topographic effects and interaction with the water table

In this series of papers the temporal dynamics of soil moisture is modeled at a point without consideration of either the effects due to lateral moisture contribution or those resulting from the structure of the root vertical distribution. The point water balance is thusnZrds(t)dt=R(t)−I(t)−Q[s(t),t]−E[s(t)]−L[s(t)],where n is porosity; Zr depth of active soil or root depth; s(t) relative soil moisture content (0⩽s(t)⩽1); t time; R(t) rainfall rate; I(t) rate of losses due to canopy

Soil moisture and the cycles of nutrients

In water-controlled ecosystems plant formations are developed mainly in response to the water balance of the ecosystem, although the structure is also modeled by the nutrient status of the soil [43]. In many regions of the world water availability is the key factor determining ecological function by controlling the duration of the period for which processes such as primary production and nutrient mineralization can occur. The life history of vegetation depends not only on the amount of rainfall

Scope of the study

This series of papers is organized in four parts. After this general outline, the second part [19] deals with the role of climate, soil, and vegetation on the soil moisture dynamics. There, a review is presented of Rodriguez-Iturbe et al. [33], where a similar stochastic model for soil moisture dynamics was proposed and analytically solved for steady-state conditions. Some components of the model are improved here to provide a more realistic description of the dynamics, and a detailed

Acknowledgments

This paper was partially funded by the National Science Foundation grants EAR-9996180 and EAR-9705861.

References (46)

  • P. D'Odorico et al.

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

    Water Resour. Res.

    (2000)
  • P.S. Eagleson

    Climate, soil, and vegetation. 1. Introduction to water balance dynamics

    Water Resour. Res.

    (1978)
  • P.S. Eagleson

    Ecological optimality in water-limited natural soil–vegetation systems. 1. Theory and hypothesis

    Water Resour. Res.

    (1982)
  • P.S. Eagleson et al.

    Ecological optimality in water-limited natural soil–vegetation systems. 2. Tests and applications

    Water Resour. Res.

    (1982)
  • P.S. Eagleson et al.

    Water-limited equilibrium of savanna vegetation systems

    Water Resour. Res.

    (1985)
  • H. Gitay et al.

    What are functional types and how should we seek them?

  • T.C. Hsiao

    Plant responses to water stress

    Ann. Rev. Plant Physiol.

    (1973)
  • H.G. Jones

    Plants and microclimate: a quantitative approach to environmental plant physiology

    (1992)
  • Karlin S. 11th R.A. Fisher Memorial Lecture, Royal Society, 1983 April...
  • P.J. Kramer et al.

    Water relations of plants and soils

    (1995)
  • J. Kutzbach et al.

    Vegetation and soil feedbacks on the response of the African monsoon to orbital forcing in the early to middle Holocene

    Nature

    (1996)
  • O.L. Lange et al.

    Water and plant life: problems and modern approaches

    (1976)
  • W. Larcher

    Physiological plant ecology

    (1995)
  • Cited by (0)

    View full text