Search

[Display omitted] • Mediterranean riparian forests are negatively impacted by drought. • Growth responses to climate and drought of four tree species were investigated. • Growth was reduced by drought and low spring-summer river flow. • Populus nigra and Fraxinus angustifolia were the most responsive species to drought. • Ash earlywood anatomy, and wood C and O isotopes also responded to drought. Mediterranean riparian forests have been altered by past use and are also negatively impacted by climate and hydrological droughts. However, we lack data on their historical changes in extent combined with multi-proxy, long-term assessments of tree growth and leaf gas exchange responses to climate, drought severity and river flow. These evaluations must also consider their current stand structure and the amount of lying deadwood which are proxies of river dynamics and forest maturity. To fill these research gaps, we studied four riparian tree species (Populus alba , Populus nigra , Fraxinus angustifolia and Ulmus minor) inhabiting a Mediterranean riparian forest located in north-eastern Spain. We quantified and analyzed: stand structure; lying deadwood; radial growth; relationships between growth, climate variables, the SPEI drought index and river flow; stable C (δ13C) and O (δ18O) isotopes in wood of P. alba , P. nigra and F. angustifolia ; and earlywood anatomy in F. angustifolia. Mature sites were dominated by P. nigra and F. angustifolia and showed the highest amount of decayed lying deadwood. Radial growth was reduced by drought and low spring-summer river flow. We found the highest growth responses to 3- (P. nigra , r = 0.62; P. alba , r = 0.46) or 12-month SPEI (F. angustifolia , r = 0.54; U. minor , r = 0.53). The coordinated decrease in δ18O and intrinsic water-use efficiency (WUE) in P. alba and P. nigra could involve an increase in stomatal conductance rate. P. alba and P. nigra were more enriched in δ18O than F. angustifolia , suggesting the former used more enriched shallow groundwater in dry periods. The F. angustifolia WUE and P. nigra δ18O series were positively and negatively correlated with the SPEI, respectively. The F. angustifolia hydraulic diameter decreased in response to drought, whereas its vessel density and WUE were positively associated. Overall, P. nigra and F. angustifolia were the species most responsive to drought. [ABSTRACT FROM AUTHOR]

Tree species inhabiting riparian forests under Mediterranean climate have evolved to face summer water shortage but may fail to cope with future increases in drought severity. Thus, understanding tree growth phenological variations in response to environmental conditions is necessary to assess the impact of seasonal drought in riparian forests. In this study, we investigated the response of stem radial growth to climate in the narrow-leaved ash (Fraxinus angustifolia) over its distribution in southern Europe. We simulated intra- and inter-annual growth patterns using the Vaganov-Shashkin (VS) model considering five sites subjected to summer drought but showing different climate conditions. The growth pattern in this species varied from unimodal in cool-wet sites to facultative bimodal in warm-dry sites. Bimodal patterns were characterized by two growth peaks coinciding with favorable climate conditions in spring and autumn. The spring growth peak occurs earlier (May) in warm-dry sites than in wet-cool sites (June–July). The variation in the season growth length and growth timing suggests different strategies adopted by this species to cope with summer drought. The VS model revealed different growth patterns across which would be relevant in predicting the response of this and other riparian tree species to climate warming and aridification. Differences in the length of the growing season, timings of growth peaks and the shift from unimodal to bimodal growth patterns should be considered when assessing growth adjustments to future climate scenarios. [Display omitted] • Ring-width and daily climatic data were used to adjust the VS model in narrow-leaved ash. • The ash growing season varied in length and pattern, but always showed a spring growth peak. • Ash growth shifted from unimodal in wet-cool sites to facultative bimodal in dry-warm sites. • The use of process-based models allows assessing intra-annual growth responses to climate. [ABSTRACT FROM AUTHOR]

One of the most salient causes of the degradation of freshwater systems is the physical habitat changes attributed to river damming. Environmental flows reduce such degradation but are still generally based on the requirements of aquatic species and disregard other biotic components of the ecosystem, such as riparian vegetation. Nevertheless, when environmental flow methods claim to consider riparian vegetation habitats and propose specific flows, their outcomes are rarely predicted quantitatively prior to their implementation. We used a dynamic floodplain vegetation model to analyze the riparian patch dynamics predicted for different flow regimes in two river stretches and to assess vegetation requirements to ensure long-term ecological maintenance and vitality of riparian structure in rivers with altered flow regimes. Furthermore, we assessed the capability of flushing flows to restore and manage riparian vegetation and the efficiency of environmental flows to satisfy riparian vegetation requirements. We found that vegetation encroachment is mainly prevented by floods with a recurrence interval of at least 2 years but that environmental flow regime planning aimed at complying with riparian vegetation requirements is watershed-specific. Additionally, reservoir flows controlled vegetation encroachment without causing severe geomorphic impacts on downstream river channels and with minor water losses to dam managers. [ABSTRACT FROM AUTHOR]

Riparian ash forests subjected to seasonal drought are among the most endangered ecosystems in Europe. They are threatened by climate warming causing aridification and by land-use changes modifying river flow. To assess the impacts of these two stress factors on riparian forests, we studied radial growth and xylem anatomical traits in five narrow-leaved ash (Fraxinus angustifolia) stands across wide climatic and ecological gradients from northern Italy to southern Portugal. Radial growth rates and earlywood hydraulic diameter (Dh) were directly correlated, whilst earlywood vessel density and growth rates were inversely associated. Ash growth positively responded to precipitation. Higher and lower rates of growth increase in response to precipitation were found in dry (annual precipitation 357–750 mm, annual water balance −39 to −48 mm) and wet (annual precipitation 1030 mm, annual water balance 27 mm) sites, respectively. Wet conditions in autumn and winter of the year prior to tree-ring formation lead to larger Dh values, except in the wet site where warmer conditions from prior autumn to current spring were positively associated to wider vessels. Growth was also enhanced by a higher river flow, reflecting higher soil moisture due to elevated groundwater table levels. Peaks in river flow from late winter to early spring increased Dh in dry-continental sites. Growth and potential hydraulic conductivity in drought-prone riparian ash forests are differently impacted by climate variability and river flow depending on site and hydrological conditions. Nevertheless, covariation between radial growth and the earlywood vessel diameter was found, regardless of site specific differences. Wood production and hydraulic conductivity are coordinated through the production of large earlywood vessels which may allow reaching higher growth rates. • Mediterranean riparian forests are threatened but understudied ecosystems. • We quantified radial growth and earlywood anatomy in Fraxinus angustifolia. • Growth rate and the earlywood hydraulic diameter (Dh) covaried. • Growth increased in response to higher precipitation. • Earlywood Dh increased in response to wet prior autumn-winter conditions. [ABSTRACT FROM AUTHOR]

Abstract: This study was carried out in eight stands of wetland woods located in the north-western part of the Iberian Peninsula. The number of sampling sites per stand depended on the physical heterogeneity and area of each stand. Biological data recorded per site included species composition and a list of structural parameters (relative cover in each forest layer, tree height, tree diameter at breast height, number of live and dead trunks and individuals, and morphological adaptations to flooding). Hydro-geomorphologic, geological and climatic variables were also obtained. The floristic data set was subjected to a non-hierarchical cluster analysis and a principal component analysis, which provided an overall image of vegetation grouping based on community composition that only considered perennial species (trees, shrubs, non-annual herbaceous plants and climbers). Three groups of plant communities were distinguished, with an overstorey that was dominated by Betula alba, Salix atrocinerea and Alnus glutinosa, respectively, which presented different environmental drivers. The structural features of each group that was characterised by a different overstorey species were analysed and differences were tested. The number of trunks per individual, number of dead trunks, canopy height and the presence of adaptations to flooding were the structural parameters that best separated the wetland wood groups. Environmental variables were correlated with the ordination axis to analyse community distribution trends and the hydrological regime was found to be the key environmental factor controlling both floristic and structural patterns. [Copyright &y& Elsevier]