The influence of the invasive alien nitrogen-fixing Robinia pseudoacacia L. on soil nitrogen availability in a mixed Mediterranean riparian forest

Poblador S., Lupon A., Martí E., Sabater F., Sabaté S., Bernal S. (2019) The influence of the invasive alien nitrogen-fixing Robinia pseudoacacia L. on soil nitrogen availability in a mixed Mediterranean riparian forest. European Journal of Forest Research. 138: 1083-1093.
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Doi: 10.1007/s10342-019-01226-x

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Robinia pseudoacacia L. occupies large areas of Mediterranean riparian zones of the northeast of the Iberian Peninsula. This study investigates the influence of the invasive alien nitrogen-fixing R. pseudoacacia on leaf litter nitrogen (N) inputs and soil N availability in a mixed riparian forest in NE Spain. We measured annual leaf litter N inputs, decomposition rates, soil N processes, and soil N concentrations at three sections (near-stream, intermediate, and hillslope) across a riparian forested zone. Moreover, we explored changes in soil N availability associated with the presence of R. pseudoacacia by means of an empirical forest floor model. Leaf litter N content was higher for R. pseudoacacia than for the native non-fixing species. Although the contribution of R. pseudoacacia to annual leaf litter N inputs increased from the near-stream to the hillslope section, soil N mineralization, nitrification, and N availability were similar among sections. Simulations suggest that soil N availability was higher at the near-stream than at the hillslope section without the presence of R. pseudoacacia. However, this pattern smoothed down as R. pseudoacacia spread across the riparian forest. Overall, our results suggest that the spreading of R. pseudoacacia across the riparian zone contributed to homogenize soil N availability over time, and highlight that an integrated spatiotemporal view of the invasive process is needed to assess its impact on soil N biogeochemistry. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.

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Riparian forest transpiration under the current and projected Mediterranean climate: Effects on soil water and nitrate uptake

Poblador S., Thomas Z., Rousseau-Gueutin P., Sabaté S., Sabater F. (2019) Riparian forest transpiration under the current and projected Mediterranean climate: Effects on soil water and nitrate uptake. Ecohydrology. 12: 0-0.
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Doi: 10.1002/eco.2043

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Vegetation plays a key role in riparian area functioning by controlling water and nitrate (N─NO3 −) transfers to streams. We investigated how spatial heterogeneity modifies the influence of vegetation transpiration on soil water and N─NO3 − balances in the vadose soil of a Mediterranean riparian forest. On the basis of field data, we simulated water flow and N─NO3 − transport in three riparian zones (i.e., near-stream, intermediate, and hillslope) using HYDRUS-1D model. We investigated spatiotemporal patterns across the riparian area over a 3-year period and future years using an IPCC/CMIP5 climate projection for the Mediterranean region. Potential evapotranspiration was partitioned between evaporation and transpiration to estimate transpiration rates at the area. Denitrification in the forest was negligible, thus N─NO3 − removal was only considered through plant uptake. For the three riparian zones, the model successfully predicted field soil moisture (θ). The near-stream zone exchanged larger volumes of water and supported higher θ and transpiration rates (666 ± 75 mm) than the other two riparian zones. Total water fluxes, θ, and transpiration rates decreased near the intermediate (536 ± 46 mm transpired) and hillslope zones (406 ± 26 mm transpired), suggesting that water availability was restricted due to deeper groundwater. Transpiration strongly decreased θ and soil N─NO3 − in the hillslope and intermediate zones. Our climate projections highlight the importance of groundwater availability and indicate that soil N─NO3 − would be expected to increase due to changes in plant-root uptake. Lower water availability in the hillslope zone may reduce the effectiveness of N─NO3 − removal in the riparian area, increasing the risk of excess N─NO3 − leaching into the stream. © 2018 John Wiley & Sons, Ltd.

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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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Does soil moisture overrule temperature dependence of soil respiration in Mediterranean riparian forests?

Chang C.T., Sabaté S., Sperlich D., Poblador S., Sabater F., Gracia C. (2014) Does soil moisture overrule temperature dependence of soil respiration in Mediterranean riparian forests?. Biogeosciences. 11: 6173-6185.
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Doi: 10.5194/bg-11-6173-2014

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Soil respiration (SR) is a major component of ecosystems' carbon cycles and represents the second largest CO2 flux in the terrestrial biosphere. Soil temperature is considered to be the primary abiotic control on SR, whereas soil moisture is the secondary control factor. However, soil moisture can become the dominant control on SR in very wet or dry conditions. Determining the trigger that makes soil moisture as the primary control factor of SR will provide a deeper understanding on how SR changes under the projected future increase in droughts. Specific objectives of this study were (1) to investigate the seasonal variations and the relationship between SR and both soil temperature and moisture in a Mediterranean riparian forest along a groundwater level gradient; (2) to determine soil moisture thresholds at which SR is controlled by soil moisture rather than by temperature; (3) to compare SR responses under different tree species present in a Mediterranean riparian forest (Alnus glutinosa, Populus nigra and Fraxinus excelsior). Results showed that the heterotrophic soil respiration rate, groundwater level and 30 cm integral soil moisture (SM30) decreased significantly from the riverside moving uphill and showed a pronounced seasonality. SR rates showed significant differences between tree species, with higher SR for P. nigra and lower SR for A. glutinosa. The lower threshold of soil moisture was 20 and 17% for heterotrophic and total SR, respectively. Daily mean SR rate was positively correlated with soil temperature when soil moisture exceeded the threshold, with Q10 values ranging from 1.19 to 2.14; nevertheless, SR became decoupled from soil temperature when soil moisture dropped below these thresholds. © 2014 Author(s).

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