Semiarid ecosystems of Chile with Mediterranean climate support high demographic rates and maintain important economies associated with the productivity of the natural environment; however, they strongly depend on the water availability and their future is compromised by climate variations. This study tries to define the role of hydrological variables on the growth of herbaceous biomass under the current climate and under future weather variations. For this, the authors used meteorological stations which let them estimate the water balance at plot scale and spectral reflectance sensors which let them follow the dynamics of the NDVI of herbaceous plants. In this context, the NDVI value was about 0.7, which means approximately 3700 kg DM ha1. However, under the worst-case climate change scenarios (RCP 8.5), annual precipitation showed a reduction of up to 31.6% regarding the present period. In this situation, the NDVI could be reduced up to 80% respect to the current situation. Thus, if climate changes to its worst scenario, it could threaten the maintenance and productivity of these ecosystems.
Top1. Introduction
Arid and semi-arid regions currently cover more than 45% of the earth's surface. They constitute the largest biome in the world, play an important role in the global carbon balance and it is expected that their extension and importance will become more relevant in the future due to the predictions about the increase in aridity (Ahlström et al., 2015, Plaza et al., 2018, Schimel, 2010). Among these zones there are various types of ecosystems with a scattered woodland structure, such as the African tropical savannas, the Californian rangelands, the Iberian dehesas, or the Mediterranean espinal mainly located in central Chile and widely established between the IV and VIII regions of this country (the regions of Chile are mostly numbered from I to XII following a locational gradient from the north to the south).
The Mediterranean ecosystems of espinal are the most extensive agroforestry systems in Chile and they occupy an area of around 2 million ha (Olivares, 2017). They are open savanna-like forests with a vegetal composition mainly dominated by two vegetation covers, trees and herbaceous plants, which appear with different combinations and densities (Ovalle et al., 2006). The name of espinal is due to the main tree species, the Acacia caven or the espino (the common name in Spanish). The espinal constitutes a space that must be conserved, since it supports high levels of biodiversity within a wide variety of environments and represents an important resource for local economies, especially those associated with livestock (Root-Bernstein and Jaksic, 2013).
Much of the livestock activity is conditioned by the natural productivity of the environment, mainly herbaceous plants, so that, when productivity is low, farmers must provide food supplements (Olivares, 2017, Ovalle et al., 2015). Consequently, the resources generated by the espinal, both natural and economic, are mainly dependent on the plant biomass produced by the ecosystem, especially the annual herbaceous plants. Despite this, the main element that controls the mechanisms that regulate growth and spatio-temporal variation of plant biomass, water (Asbjornsen et al., 2011, Krause et al., 2017), constitutes a studied variable of little exhaustive, until now, in this type of environment.
Hydrological variables are commonly used to explain the behavior of vegetation biomes (Cox et al., 2016) because both plant growth and development directly depend on water resources to a suitable development (Lozano-Parra et al., 2018b, Meza et al., 2018). Therefore, understanding the interactions that are established between water resources and vegetation is crucial for efficient management and conservation of these ecosystems, since any variation in hydrometeorological conditions could modify plant phenology, affecting both economic and environmental resources (Goodwell et al., 2018, Popp et al., 2009).
For example, in ecosystems similar to espinal like dehesas, it has been reported that vegetation can modify the water and energy balances by interception of rain and solar radiation (Lozano-Parra et al., 2015, Moreno et al., 2013). Thus, Lozano-Parra et al. (2015) observed that the role of vegetation on water resources reaching the soil was more critical in drier environmental conditions. In this context, the interception capacity by vegetation was greater than in wetter situations. It was due to rainfall events with low precipitation never triggered soil hydrological response. In this study, authors also highlighted that dry or wet episodes can appear in any climatic season, i.e., they were independent of seasonality. This implies that, if in the present century the dry episodes become more frequent because of a climate change, an important surface could become drier and less productive (van der Molen et al., 2011).
The climate change in which we are currently immerse (with variations in precipitation and an increase in temperatures) greatly affects the Mediterranean region (IPCC, 2018). However, although there is a general consensus on global warming, there is no consensus on its materialization. For example, it is not clear whether precipitation will be seasonally concentrated or if rainfall intensity will increase (Rodell et al., 2018, Wu et al., 2013). This lack of agreement entails a lack of a sufficiently detailed scenario that allows us to analyze what the future consequences of climate change will be. For this reason, it is necessary to carry out studies that analyze the effects of climate change on ecosystems.