Responses of Mediterranean riparian forests to water availability: Insights of present and future conditions. A case study in La Tordera catchment = Respostes dels boscos de ribera mediterranis a la disponibilitat d'aigua: estudi de les condicions presents i futures a la conca de Tordera

[eng] 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.[cat] Durant les darreres dècades, la majoria d'estudis sobre els efectes del canvi climàtic en la vegetació s'han centrat en boscos d'alta muntanya amb un gradient latitudinal i espècies en els seus límits de distribució geogràfica, ja que, en ambdues situacions, els individus poden trobar-se afectades per l'escassetat d'aigua. Tot i que pocs estudis s'han centrat en els efectes de disponibilitat d'aigua en la vegetació de ribera, aquestes espècies podrien veure's extremadament afectades per l'escassetat d'aigua, donat que normalment es troben en ambients humits i poden no disposar de controls ecofisiològics suficients per afrontar períodes sequera prolongada. D'altra banda, les zones de ribera es consideren hotspots dels processos del nitrogen (N). Aquests ecotons poden reduir part de les càrregues N que arriben dels ecosistemes adjacents abans d'arribar al riu, principalment a través de la desnitrificació i l'absorció per part de la vegetació. A les regions mediterrànies, on els horitzons orgànics dels sòls orgànics queden desconnectats de l'aigua del freàtic, el procés de desnitrificació està limitat per les baixes condicions anòxiques dels sòls de ribera. Així doncs, l'absorció de N per part de la vegetació es converteix en el principal procés de retenció de N a les zones de ribera mediterrànies. Els efectes del canvi climàtic en la vegetació de ribera poden també modificar aquesta capacitat d'eliminar N de les zones de ribera. Els resultats obtinguts en dos boscos de ribera mediterranis de la conca de La Tordera mostren que les espècies arbòries de ribera estan experimentant els efectes de la sequera a la regió mediterrània. Quercus robur, que normalment es troba en planes d'inundació d'Europa central, estan experimentant un declivi en el creixement anual dels individus en el seu límit de distribució geogràfica més meridional. D'altra banda, les espècies arbòries dels boscos de ribera mediterranis una elevada dependència a la disponibilitat d'aigua del sòl durant el període d'estiu, obtenint més del 80% de l'aigua transpirada a la zona de no saturada del sòl. Les espècies freatòfiles, Alnus glutinosa i Populus nigra, obtenen l'aigua principalment del freàtic durant la primavera, però el sòl no saturat és la principal font d'obtenció d'aigua durant l'estiu. A. glutinosa no presenta controls ecofisiològics per evitar les situacions de sequera, mentre que P. nigra augmenta la seva iWUE durant els anys secs. Fraxinus excelsior conviu amb ambdues espècies al bosc de ribera. Aquesta espècie, situada a les zones allunyades del llit del riu, presenta una forta dependència a la disponibilitat d'aigua del sòl tot i no tenir mecanismes ecofisiològics per afrontar la sequera estival. Contràriament, l'espècie invasora fixadora de N, Robinia pseudoacacia, es troba distribuida al llarg de tot el bosc de ribera i presenta una gran plasticitat per fer front a les diferents condicions de disponibilitat d'aigua. A més, els resultats obtinguts sobre els efectes de la vegetació de ribera sobre els fluxos d'aigua i N han destacat la gran heterogeneïtat espacial dels boscos de ribera mediterranis en distàncies relativament curtes (~ 25 m). El bosc de ribera mediterrània estudiat mostra una notable heterogeneïtat espacial en la disponibilitat d'aigua, amb nivells freàtics decreixents des de la zona pròxima a la llera del riu (~ 0,6 m de profunditat) fins a les zones més allunyades (~ 2,2 m de profunditat). Els nivells freàtics més superficials faciliten l'intercanvi entre la zona saturada i no saturada del sòl, permetent també una major transpiració de la vegetació i absorció de N per part d'aquesta a les zones més pròximes a la llera del riu. Per contra, les taxes de desnitrificació són baixes a tota la zona de ribera degut a la limitació del sòl de ribera en quant a contingut d'aigua del sòl, i la conseqüent anòxia que afavoreix l'activitat dels bacteris desnitrificadors. No obstant això, aquestes condicions aeròbiques del sòl, amb relativa humitat, promouen altes taxes de respiració a tot el sòl forestal de ribera, i la conseqüent elevada emissió de CO2. Finalment, la simulació basada en les projeccions del canvi climàtic suggereixen un augment futur de les concentracions del N al sòl, així com una reducció de l'àrea de ribera capaç de retenir N de forma efectiva. La possible substitució de les espècies autòctones per la invasora R. pseudoacacia podria homogeneïtzar la disponibilitat de N del sòl al llarg de la zona de ribera, però no augmentaria la futura disponibilitat de N del sòl. Així doncs, els nostres resultats destaquen l'heterogeneïtat espacial de les zones de ribera mediterrànies i la necessitat d'avaluar els processos a escala espaciotemporal per comprendre'ls millor. La retenció de N en sòls de ribera mediterranis es produeix principalment per l'absorció d'aquest per part de la vegetació. No obstant, les projeccions climàtiques poden agreujar els problemes d'escassetat d'aigua, inhibint les taxes de desnitrificació i reduint l'absorció de N per part de la vegetació. Per tant, aquests resultats qüestionen la capacitat de les zones de ribera mediterrànies de reduir les càrregues de N que arriben als rius, així com suggereixen que els sòls de ribera mediterranis poden esdevenir una font potencial de N als ecosistemes aquàtics adjacents en el futur

The influence of riparian evapotranspiration on stream hydrology and nitrogen retention in a subhumid Mediterranean catchment

Contiene 6 figuras, 3 tablasRiparian evapotranspiration (ET) can influence stream hydrology at catchment scale by promoting the net loss of water from the stream towards the riparian zone (i.e., stream hydrological retention). However, the consequences of stream hydrological retention on nitrogen dynamics are not well understood. To fill this gap of knowledge, we investigated changes in riparian ET, stream discharge, and nutrient chemistry in two contiguous reaches (headwater and valley) with contrasted riparian forest size in a small forested Mediterranean catchment. Additionally, riparian groundwater level (hgw) was measured at the valley reach. The temporal pattern of riparian ET was similar between reaches, was positively correlated with hgw (ρ = 0.60), and negatively correlated with net riparian groundwater inputs (ρ < -0.55). During the vegetative period, stream hydrological retention occurred only at the valley reach (59% of the time), and was accompanied by in-stream nitrate release and ammonium uptake. During the dormant period, when the stream gained water from riparian groundwater, results showed small influences of riparian ET on stream hydrology and nitrogen concentrations. Despite being a small component of annual water budgets (4.5%), our results highlight that riparian ET drives stream and groundwater hydrology in this Mediterranean catchment and, furthermore, question the potential of the riparian zone as a natural filter of nitrogen loads.Financial supported was provided by the Spanish Government through the projects MONTES-Consolider (CSD2008-00040-MONTES), MEDFORESTREAM (CGL2011-30590), and MEDSOUL (CGL2014-59977-C3-2). AL was supported by a FPU PhD fellowship from the Spanish Ministry of Education and Science (AP-2009-3711) and the MEDSOUL project. SB work was funded by the Spanish Research Council (JAE-DOC027), the Spanish CICT (Juan de la Cierva contract JCI-2008-177), European Social Funds (FSE), and the MEDFORESTREAM and NICUS (CGL-2014-55234-JIN) projects. SP was supported by a FPI PhD fellowship from the Spanish Ministry of Economy and Competitiveness (BES-2012-054572).Peer reviewe

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

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

Supply, demand, and in-stream retention of dissolved organic carbon and nitrate during storms in Mediterranean forested headwater streams

The capacity of headwater streams to transform and retain organic matter and nutrients during base flow conditions has been largely demonstrated in the literature. Yet, most solute exporting occurs during storms, and thus, it becomes essential to understand the role of in-stream processes in regulating solute concentrations and exports during storm flow conditions. In this study, we explored patterns of solute supply, solute demand, and resulting in-stream solute retention for a number of individual storms from two Mediterranean streams (intermittent and perennial) that together encompassed a wide range of hydrological conditions. Our results indicate that more than 70% of the individual storms were chemodynamic (i.e., solute concentrations either increased or decreased with increasing discharge) at the two sites, for both dissolved organic carbon (DOC) and nitrate (NO−3). At the perennial stream, DOC and NO−3 concentrations did not show any clear pattern of storm response during both dry and wet periods, though deviations from chemostasis were generally larger for those events showing higher concentrations during storm flow. At the intermittent stream, DOC and NO−3 showed positive divergences from chemostasis during the wet period. In this site, DOC showed no clear pattern of storm response during the dry period, while many storms showed low NO−3 concentrations compared to chemostasis, suggesting either limited NO−3 sources or in-stream retention. At the two streams, in-stream biogeochemical demand during individual storms was either similar or higher than during base flow conditions for both DOC and NO−3. In-stream NO−3 demand resulted in substantial whole-reach retention during storms (up to 40%), indicating that in-stream biogeochemical processes substantially reduced downstream flux of terrestrial NO−3 inputs during storm events. Conversely, whole-reach DOC retention was relatively low (<10%), suggesting little ability to regulate DOC export and an energy subsidy to downstream ecosystems during storms. This study indicates that in-stream biogeochemical demand during storms can counterbalance solute supply to some extent and stresses the importance of considering the potential role of in-stream processes in shaping stream solute export during storms

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

Riparian zones play a fundamental role in regulating the amount of carbon (C) and nitrogen (N) that is exported from catchments. However, C and N removal via soil gaseous pathways can influence local budgets of greenhouse gas (GHG) emissions and contribute to climate change. Over a year, we quantified soil effluxes of carbon dioxide (CO2) and nitrous oxide (N2O) from a Mediterranean riparian forest in order to understand the role of these ecosystems on catchment GHG emissions. In addition, we evaluated the main soil microbial processes that produce GHG (mineralization, nitrification, and denitrification) and how changes in soil properties can modify the GHG production over time and space. Riparian soils emitted larger amounts of CO2 (1.2-10 g C m−2 d−1) than N2O (0.001-0.2 mg N m−2 d−1) to the atmosphere attributed to high respiration and low denitrification rates. Both CO2 and N2O emissions showed a marked (but antagonistic) spatial gradient as a result of variations in soil water content across the riparian zone. Deep groundwater tables fueled large soil CO2 effluxes near the hillslope, while N2O emissions were higher in the wet zones adjacent to the stream channel. However, both CO2 and N2O emissions peaked after spring rewetting events, when optimal conditions of soil water content, temperature, and N availability favor microbial respiration, nitrification, and denitrification. Overall, our results highlight the role of water availability on riparian soil biogeochemistry and GHG emissions and suggest that climate change alterations in hydrologic regimes can affect the microbial processes that produce GHG as well as the contribution of these systems to regional and global biogeochemical cycles

Early-Career Coordinated Distributed Experiments: Empowerment Through Collaboration

Este artículo contiene 7 páginas, 1 tabla, 3 figuras.Coordinated distributed experiments (CDEs) enable the study of large-scale ecological patterns in geographically dispersed areas, while simultaneously providing broad academic and personal benefits for the participants. However, the effective involvement of early-career researchers (ECRs) presents major challenges. Here, we analyze the benefits and challenges of the first CDE exclusively led and conducted by ECRs (i.e. ECR-CDE), which sets a baseline for similar CDEs, and we provide recommendations for successful CDE execution. ECR-CDEs achieve most of the outcomes identified in conventional CDEs as well as extensive benefits for the young cohort of researchers, including: (i) receiving scientific credit, (ii) peer-training in new concepts and methods, (iii) developing leadership and communication skills, (iv) promoting a peer network among ECRs, and (v) building on individual engagement and independence. We also discuss the challenges of ECR-CDEs, which are mainly derived from the lack of independence and instability of the participants, and we suggest mechanisms to address them, such as resource re-allocation and communication strategies. We conclude that ECR-CDEs can be a relevant tool to empower ECRs across disciplines by fostering their training, networking and personal well-being.The authors were supported by the following founding: NC the support of the Beatriu de Pinós postdoctoral program of the Government of Catalonia’s Secretariat for Universities and Research of the Ministry of Economy and Knowledge (BP2016- 00215), EE by a predoctoral grant from the Basque Government (2014-2017), AB by a Generalitat de Catalunya—Beatriu de Pinós (BP-00385-2016), AMG-F by a predoctoral research grant (BES-2013-065770) from the Spanish Ministry of Economy and Competitiveness, MAr by a postdoctoral grant from the Basque Government, MIA by a Juan de la Cierva postdoctoral grant (FJCI-2015-26192), PR-L by a Margalida Comas postdoctoral contract (PD/031/2018) funded by the Government of the Balearic Islands and the European Social Fund, AP by a Ramón Areces Foundation Postdoctoral Scholarship, and AL by a Kempe Foundation stipend. DOMIPEX project was founded by the First Call of Collaborative Projects among Young Researchers of the Iberian Association of Limnology (AIL; 2013-2015).Peer reviewe

How trees explain the passing of time

La dendrocronologia és la ciència que estudia la relació entre el creixement dels arbres i les condicions meteorològiques o climàtiques al llarg del temps. Les diferents variables ambientals (p.e. temperatura de l’aire i precipitació) o els diferents esdeveniments que es donen al voltant dels arbres (p.e. incendis forestals, esllavissades, tales forestals, entre altres) influencien en el creixement anual d’aquests individus, quedant registrat en els seus anells de creixement. Així doncs, aquesta és una eina cabdal per a estimar la capacitat d’aquests organismes a sobreviure en una climatologia canviant, com en l’actual canvi climàtic. El 10 de març del 2018 es va realitzar una sortida a la font del Sot de Dosrius on es van mostrar els diferents tipus d’anells de creixement que poden fer els arbres mitjançant l’obtenció de mostres de diferents espècies. L’objectiu d’aquest article és presentar quin és el potencial de la dendrocronologia, així com introduir algunes directrius sobre com analitzar aquest tipus de mostres i obtenir-ne informació científica de rellevància en estudis d’ecologia.Dendrochronology is the science that studies the relationship between tree growth and meteorological or climatic conditions over time. Environmental variables (e.g. air temperature and precipitation) and events that occur around trees (e.g. wildfires, avalanches and tree felling) influence the annual growth of trees, and these disturbances are recorded in their rings. Therefore, these rings are a crucial tool for estimating the capacity of trees to survive in changing environmental conditions such as the period of climate change we are now experiencing. On 10 March 2018, we carried out a field survey at the Font del Sot in Dosrius, where we collected tree-ring cores from several species in order to show the types of rings that different tree species have. The aim of this paper is to explain the importance of dendrochronology and to introduce some guidelines on how to analyse these samples and obtain relevant scientific information for ecology studies

Els boscos de ribera

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

Este artículo contiene 11 páginas, 4 figuras, 3 tablas.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.Financial support was provided by the Spanish Government through the projects MONTES-Consolider (CSD2008-00040- MONTES), MEDFORESTREAM (CGL2011-30590), CANTERA (RTI2018-094521-B-100), and MEDSOUL (CGL2014-59977-C3-2). Sílvia Poblador was supported by a FPI PhD fellowship from the Spanish Ministry of Economy and Competitiveness (BES-2012-054572). Anna Lupon was supported by a Kempe Foundation post-doctoral grant (Sweden) and a Juan de la Cierva contract (FJCI-2016-28416). Susana Bernal was supported by a Ramon y Cajal fellowship (RYC 2017- 22643). We also thank site cooperators, including Vichy Catalan and the Catalan Water Agency (ACA) for permission to sample at the Font del Regàs catchment. Sílvia Poblador, Santiago Sabaté, and Francesc Sabater are members of the research group FORESTREAM (AGAUR, Catalonia 2017SGR976).Peer reviewe