Responses of mediterranean riparian forests to water availability: insights of present and future conditions. A case study in la tordera catchment

During the last decades, most of the studies based on climate change effects on vegetation physiology have focused on upland forests and species at their border of distribution, since, in both conditions, species are highly affected by water scarcity. Although few studies have focused on water availability effects on riparian vegetation, these species could be extremely affected by water scarcity as they are usually found in wet environments and they may not have enough ecophysiological controls to cope with prolonged drought periods. Moreover, riparian zones are considered hotspots of nitrogen (N) processes. These ecotones can reduce part of the N loads received from adjacent ecosystems before they reach the stream, mainly via denitrification and vegetation uptake. In Mediterranean regions, where shallow organic soil layers are disconnected from groundwater, denitrification process is limited by the weak anoxic conditions on riparian forests soils. There, vegetation uptake becomes the main driver for N removal in Mediterranean riparian zones. Climate change effects on riparian vegetation may cascade down and modify this well-known capacity to remove N from riparian zones. The findings from two Mediterranean riparian forests in La Tordera catchment showed that riparian tree species are already experiencing the effects of drought periods in the Mediterranean region. Quercus robur species, typically found in mid-European floodplains, is already experiencing tree growth decline at its southernmost distribution range edge. On the other hand, riparian tree species at Mediterranean forests showed high dependence on soil water availability during summer, obtaining more than 80% of the water transpired from the vadose zone. Phreatophitic species, Alnus glutinosa and Populus nigra, took up water from the groundwater compartment during spring but soil water was their main water source during summer. A. glutinosa did not present ecophysiological controls to avoid drought situations, while P. nigra increased its iWUE during dry years. Fraxinus excelsior was cohabiting with both species in the riparian forest. This species, located far away from the stream channel, was more depending on soil water availability and did not present any ecophysiological mechanism to cope with summer drought. Conversely, the N-fixing invasive species Robinia pseudoacacia, which was co-occurring all across the riparian forest, showed high plasticity to cope with different water availability conditions. Additionally, the findings obtained from the effects of riparian vegetation on water and N fluxes highlighted the high spatial heterogeneity of Mediterranean riparian forests within relatively small distances (~25 m). The studied Mediterranean riparian forest showed a remarkable spatial heterogeneity on water availability, with groundwater levels increasing from the near-stream zone (~0.6m deep) to the hillslope edge (~2.2m deep). Shallow groundwater tables enhanced the connectivity between vadose zone and groundwater at the near-stream zone, allowing greater transpiration and N uptake rates compared to the hillslope edge. Conversely, denitrification rates were generally low across all the riparian area due to water limitation and its weak anoxic conditions. Nevertheless, these soil conditions promote respiration rates all across the riparian forest soil, and thus, riparian soils emitted large CO2 fluxes. Finally, simulation based on climate change projections suggested a future increase in soil N concentrations as well as a reduction of the effective N-removal area of this riparian zones. The feasible substitution of autochthonous species by the already present R. pseudoacacia may homogenize soil N availability across the riparian area but would not increase the future soil N availability. Overall, our findings highlight the spatial heterogeneity of Mediterranean riparian zones and the need to better evaluate spatio-temporal processes to understand their mechanisms. N retention in Mediterranean riparian soils occur mainly by vegetation uptake. Yet, future climate projections may exacerbate water scarcity problems, inhibiting denitrification rates and reducing vegetation uptake. Therefore, these results challenge the well-accepted capacity to reduce N loads reaching the stream, and suggest that Mediterranean riparian soils can become a potential source of N to adjacent aquatic ecosystems in the future.