The influence of Mediterranean riparian forests on stream nitrogen dynamics: A review from a catchment perspective

Lupon A., Sabater F., Bernal S. (2017) The influence of Mediterranean riparian forests on stream nitrogen dynamics: A review from a catchment perspective. Limnetica. 36: 507-523.
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Doi: 10.23818/limn.36.18

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Riparian zones are considered natural filters of nitrogen (N) within catchments because they can substantially diminish the exports of N from terrestrial to aquatic ecosystems. However, understanding the influence of riparian zones on regulating N exports at the catchment scale still remains a big challenge in ecology, mainly because upscaling plot scale results is difficult, as it is disentangling the effects of riparian, upland, and in-stream processes on stream water chemistry. In this review, we summarize previous studies examining key hydrological and biogeochemical processes by which Mediterranean riparian zones regulate catchment water and N exports.We focus onMediterranean regions because they experience a marked climatic seasonality that facilitates disentangling the close link between climate, riparian hydrology, and stream N exports. We show that Mediterranean riparian soils can be hot spots of N mineralization and nitrification within catchments given their relatively moist conditions and large stocks of N-rich leaf litter. Extremely large nitrification rates can occur during short-Time periods (i.e. hot moments) and lead to increases in stream N loads, suggesting that riparian soils can be a potential source of N to adjacent aquatic systems. Moreover, riparian trees can contribute to decrease riparian groundwater level during the vegetative period, and promote reverse fluxes from the stream to the riparian zone. During periods of high hydrological retention, stream water exports to downstream ecosystem decrease, while stream water chemistry is mostly determined by in-stream processes. Riparian tree phenology can also affect catchment N exports by shaping the temporal pattern of both light and litter inputs into the stream. In spring, light enhances in-stream photoautotrophic N uptake before riparian leaf-out, while riparian leaf litter inputs promote in-stream N mineralization in summer and fall. Finally, we illustrate that the impact of Mediterranean riparian zones on stream hydrology and biogeochemistry increases along the stream continuum, and can ultimately influence catchment N exports to downstream ecosystems. Overall, findings gathered in this review question the well-established idea that riparian zones are efficient N buffers, at least for Mediterranean regions, and stress that an integrated view of upland, riparian, and stream ecosystems is essential for advancing our understanding of catchment hydrology and biogeochemistry. © Asociación Ibérica de Limnología, Madrid. Spain.

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Growth and water use performance of four co-occurring riparian tree species in a Mediterranean riparian forest

Nadal-Sala D., Sabaté S., Sánchez-Costa E., Poblador S., Sabater F., Gracia C. (2017) Growth and water use performance of four co-occurring riparian tree species in a Mediterranean riparian forest. Forest Ecology and Management. 396: 132-142.
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Doi: 10.1016/j.foreco.2017.04.021

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Mediterranean riparian zones act as vegetation shelters for several deciduous tree species at the edge of their bioclimatic distribution, e.g. alder (Alnus glutinosa), black poplar (Populus nigra) or ash (Fraxinus excelsior). Current global warming and human induced disturbances may worsen their growing conditions. Under such circumstances, black locust (Robinia pseudoacacia) is outcompeting autochthonous tree species. Here, we provide evidences of black locust better growth and water use performance than alder and ash. We compare the temporal and spatial patterns of transpiration and the stem basal area increments of alder, black poplar, common ash and black locust, all of them co-occurring in a mixed riparian Mediterranean forest. Black locust presented the lowest transpiration values per basal area unit (4.0 mm·m−2·growing season−1). Although tree transpiration was mainly driven by energy availability instead of water, ash transpiration was constrained by water availability at soil water contents below 0.08 cm3·cm−3. Black locust was the only tree species growing all over the water availability gradient present in the study site, and it did not present any significant difference in sap flow values across this gradient. Furthermore, black locust and black poplar were the species with higher growth-based water use efficiency (5.4 g·cm−1·m−3 and 3.6 g·cm−1·m−3, respectively); ash and alder were the less efficient ones (2.8 g·cm−1·m−3 and 1.9 g·cm−1·m−3respectively). The good performance of black locust is relevant to understand its great successful invasion of Mediterranean riparian forests, particularly after human-induced disturbances, as forest management. © 2017 Elsevier B.V.

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Soil water content drives spatiotemporal patterns of CO2 and N2O emissions from a Mediterranean riparian forest soil

Poblador, S., Lupon, A., Sabaté, S., Sabater, F. (2017) Soil water content drives spatiotemporal patterns of CO2 and N2O emissions from a Mediterranean riparian forest soil. Biogeosciences. 14: 4195-4208.
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Doi: 10.5194/bg-14-4195-2017

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