The ecohydrology of ecosystem transitions: a meta‐analysis

Abstract

A vast body of literature demonstrated that anthropogenic disturbances such as overgrazing and fire are key drivers of abrupt transition between vegetation types in ecosystems. In this study, we propose that the hydrological context (described in terms of rainfall, evapotranspiration and water yield) is a first-order, primordial determinant of the propensity of ecosystems to undergo transition. This implies that the anthropogenic disturbance is a second-order determinant that is strongly conditioned by the first one. Through the meta-analysis of existing studies, a collection of 685 geo-referenced study cases was organized to study the hydrological characteristics of three climatic regions and three ecosystems that vary in their relation between woody and grassy plants. Thus, humid, sub-humid and dry climatic regions, respectively, receiving >1000, 500-1000 and <500 mm year À1 , were studied, and possible transition mechanisms among grasslands/savannas, shrublands and forests were analysed. The results showed that the ecohydrological context determines the probabilities of ecosystems transitions in different climatic regions and the prevalence of alternative transition mechanisms. We showed that transition of forests into other ecosystems is highly improbable in high-precipitation regions, more probable and likely subject to a bi-stable and reversible regime in sub-humid regions, and highly probable and irreversible in dry regions. Factors such as runoff, deep-water drainage, fire, flammable/ nonflammable biomass and overgrazing were considered as hypothetical transition mechanisms. As a novel finding, we demonstrate that ecohydrology, as a determinant of transition, is a factor that operates at a hierarchical level higher than that of the human-driven disturbance. A synthetic graphical model was proposed to characterize resilience (the capacity of ecosystems to withstand transition) in the three study climatic regions.

Forests, atmospheric water and an uncertain future: the new biology of the global water cycle

“…Those concerned with carbon recognise that water is the most limiting factor for terrestrial ecosystem carbon uptake, and that uncertainties over water imply uncertainties over biomass and carbon fixation (Polis 1999;Good et al 2013;Bernacchi and VanLoocke 2015;Thorley et al 2015;Viglizzo et al 2016;Taylor et al 2017;Zhu et al 2017). Furthermore, those concerned with environmental conservation, stability and the maintenance of species diversity recognise both the significance of freshwater biodiversity (supporting over 126,000 species of plants and animals, many of them vulnerable, on 0.8% of the world's surface, Garcia-Moreno et al 2014) and the links between terrestrial diversity and moisture (Kreft and Jetz 2007;Sheil et al 2016;Viglizzo et al 2015).…”

Changes in evapotranspiration and phenology as consequences of shrub removal in dry forests of central Argentina

“…The changes we and others have observed in LST and albedo may also have a major impact on local climate by increasing the proportion of long-wave radiation that is emitted by the surface and retained by the atmosphere (Lee, 2010). In a recent analysis on ecohydrology of ecosystem transitions, Viglizzo et al (2014) suggest that not only anthropogenic but also biophysical factors could play a critical role in determining vegetation changes. They proposed that the hydrological context more than any other variable is what defines the propensity of transitions after a human disturbance, being higher, less resilient and more permanent in arid and semi-arid areas.…”

“…In a recent analysis on ecohydrology of ecosystem transitions, Viglizzo et al . () suggest that not only anthropogenic but also biophysical factors could play a critical role in determining vegetation changes. They proposed that the hydrological context more than any other variable is what defines the propensity of transitions after a human disturbance, being higher, less resilient and more permanent in arid and semi‐arid areas.…”

A role for meta‐analysis in hydrology

“…Retrospective syntheses through meta‐analysis provide the ability to consider a wide range of possibilities, states, and outcomes that is larger and more exhaustive than what could be possibly captured by a single primary study. Recent examples of retrospective synthesis meta‐analyses include Zhuo, Dai, and Han (); Viglizzo, Nosetto, and Jobbágy, Ricard, Frank (); and Evaristo and McDonnell (). If we examine these three studies further, it is clear where and how meta‐analysis can go beyond statements and contributions of the primary studies they include—and explore new questions, only possible with large “sample size” meta‐analysis.…”

“…Viglizzo et al () used meta‐analysis to understand the drivers of ecosystem transitions, for example, grasslands to shrublands and forests to shrublands. They performed a meta‐analysis of 685 studies in three climatic regions and three ecosystems.…”