Date Published: April 10, 2017
Publisher: Public Library of Science
Author(s): Elena Roitberg, Maxim Shoshany, Lucas C.R. Silva.
Following a predicted decline in water resources in the Mediterranean Basin, we used reaction-diffusion equations to gain a better understanding of expected changes in properties of vegetation patterns that evolve along the rainfall transition between semi-arid and arid rainfall regions. Two types of scenarios were investigated: the first, a discrete scenario, where the potential consequences of climate change are represented by patterns evolving at discrete rainfall levels along a rainfall gradient. This scenario concerns space-for-time substitutions characteristic of the rainfall gradient hypothesis. The second, a continuous scenario, represents explicitly the effect of rainfall decline on patterns which evolved at different rainfall levels along the rainfall gradient prior to the climate change. The eccentricity of patterns that emerge through these two scenarios was found to decrease with decreasing rainfall, while their solidity increased. Due to their inverse modes of change, their ratio was found to be a highly sensitive indicator for pattern response to rainfall decline. An eccentricity ratio versus rainfall (ER:R) line was generalized from the results of the discrete experiment, where ERs above this line represent developed (recovered) patterns and ERs below this line represent degraded patterns. For the rainfall range of 1.2 to 0.8 mm/day, the continuous rainfall decline experiment with ERs that lie above the ER:R line, yielded patterns less affected by rainfall decline than would be expected according to the discrete representation of ecosystems’ response. Thus, for this range, space-for-time substitution represents an overestimation of the consequences of the expected rainfall decline. For rainfall levels below 0.8 mm/day, eccentricity ratios from the discrete and continuous experiments practically converge to the same trend of pattern change along the ER:R line. Thus, the rainfall gradient hypothesis may be valid for regions characterized by this important rainfall range, which typically include desert fringe ecosystems.
The Mediterranean Basin is recognized in a distinctive number of studies as a hot-spot of climate change [1–3]. Mariotti et al. () predicted a 10–30% decrease in water resources in the Eastern Mediterranean by the end of this century, due to the combined effect of decreasing rainfall and increasing atmospheric temperatures.
Differences between the continuous and discrete rainfall decrease scenarios are most distinctive along the rainfall gradient between 1.2 and 0.8 mm/day. While there was a consistent and significant change in the ERs obtained at the discrete rainfall levels, only minor or negligible decreases in ERs were exhibited in the continuous experiments:
Reaction-diffusion equations (RDE) were used with the aim of gaining a better understanding of the expected changes in the properties of patch patterns that evolve along a rainfall transition between semi-arid and arid rainfall regions following a predicted decline in water resources in the Mediterranean Basin. The characteristic equilibrium patterns that emerge as a result of the three rainfall regimes are: spotty vegetation patterns at rainfall levels lower than 0.8 mm/day, transition between spotty vegetation and spotty soil patterns through striped and labyrinth-like patterns for rainfall levels between 0.8 and 1 mm/day, and high vegetation cover together with spotty and striped soil patterns at rainfall levels higher than 1 mm/day. Pattern characteristics of eccentricity and solidity responded inversely to the decline in rainfall, and the ratio between the two parameters is suggested to be an efficient morphometric indicator. The eccentricity ratio versus rainfall line (ER:R line), together with a line that distinguishes between soil dominance and vegetation dominance at the 1 mm/day rainfall level allows to differentiate between four typologies of developed and degraded patterns. For the range between 1.2 and 0.8 mm/day rainfall, the continuous rainfall decline yielded patterns with higher vegetation connectivity than patterns that evolved at corresponding discrete rainfall levels, as inferred by ERs above the ER:R line. We hypothesize that this result represents “pattern inheritance” of plant-soil relationships that evolved prior to the decline in rainfall levels. Thus, for this range, space-for-time substitution represents an overestimation of the consequences of the expected rainfall decline (e.g., ). This result is also in line with suggestions made by Dunkerley () regarding the robustness of banded vegetation patterns to both grazing pressures and rainfall change. For rainfall levels lower than 0.8 mm/day, the eccentricity ratios from the discrete and continuous experiments converge practically to the same trend of pattern change along the ER:R line. Thus, the rainfall gradient hypothesis may be valid for regions within this important rainfall range, which typically includes desert fringe ecosystems.