Contribution to the ecology of the Middle Ebro riparian woodlandsimplications for management

Abstract

The riparian woodland ecology of one of the largest regulated Mediterranean rivers, the Ebro River (NE Spain), was examined with the ultimate purpose of guiding through future management plans. To that end, some aspects of the tree structure, functioning and functions were explored in relation to their primary driver, the hydrogeomorphic regime, at different spatial and temporal scales. An 8-km river segment representative of the meandering Middle Ebro reach was selected for all studies. The first task was to study the structure and health of the riparian woodlands at the reach- and patch-scale, within the framework of an 80-year-chronosequence defined according to regulation-induced hydrogeomorphic changes. The changes in the forest structure and species composition were analysed using historical aerial photographs dated in 1927, 1957, 1981 and 2003 and a detailed field survey carried out in 2006 and 2007. The results revealed a dichotomous situation, with pioneer forests limited to narrow bands running parallel to the main channel and in-channel areas, contrasting with generalized senescence in the rest of the floodplain covered by natural vegetation. This situation was caused by a loss of hydrogeomorphic dynamism, and by the deficient replacement of pioneer species (Populus nigra, Salix alba and Tamarix spp.) by late-seral species (Fraxinus angustifolia and Ulmus minor). In a second step, research was made on two key processes for forest structure dynamics: sexual regeneration and response of mature tree populations to the local hydrological gradient. Seed dispersal, germination and greenhouse studies showed that the sexual regeneration strategies of one of the apparently more negatively species affected by the new hydrogeomorphic regime, P. alba, were equivalent to those of other pioneer species worldwide, highlighting the potential for forest restoration based on the renaturalization of the hydrogeomorphic regime. The relationship between the mortality rates of P. alba, P. nigra, S. alba and Tamarix spp. and the local hydrology was examined in detail after monitoring the water table levels in 2007, 2008 and 2009. All the species exhibited significant relationships, which was interpreted as an evidence of their phreatophytic (i.e., groundwater dependent) nature. This information was used to propose some hydrologic thresholds for the conservation of their mature populations. Surprisingly, the shift in composition from Salicaceae to Tamaricaceae spp. with drier conditions, observed in other semi-arid regions of the world, was not detected in the Ebro. Only S. alba significantly lost importance with drier hydroperiods, relative to the other species. The role of leaf nutrient content in riparian trees as indicator of nutrient limitation in forested floodplains was examined to discuss the consequences of possible nutrient use differences on the competition between the two dominant tree genera in semi-arid regions of the world: Populus and Tamarix. To that end, the seasonal and spatial variability in leaf nitrogen (N) and phosphorus (P) content was analyzed in 720 leaves (2 tree genera x 3 seasons x 12 geomorphic sites x 10 tree replicates). The results showed that Tamarix is more variable than Populus in terms of leaf N and P content, which seems to support the hypothesis that Tamarix is better adapted to low disturbance and high stress scenarios (floodplain stabilization and water and nutrient stress), compared to Populus, more effective in high-disturbance and low-stress environments (higher flooding disturbance but water and nutrient availability), which would give a novel explanation to the success of Tamarix in regulated rivers. However, these differences were obscured by a stronger spatial component that affected both genera, especially through phosphorus limitation along the flooding gradient, as shown by linear mixed effects (LME) models. In general, at shorter flood durations, N and P loss by trees with litterfall was lower and N:P ratio increased. The response of litter production to hydrologic gradients was also examined, as a key ecological function of the riparian woodlands for the floodplain ecosystem. In 2007, litterfall was monitored monthly in 12 plots covering a variety of forest patches differing in structure, soil and hydroperiod. LME models and hierarchical partitioning were used to assess the spatial variability in litter production and to explore the relative importance of 17 parameters (representing the forest patch structure, soil, hydroperiod and litterfall chemistry) as potential litter production predictors. The results suggested that, although production in the Ebro riparian woodlands (mean = 563 g¿m-2¿y-1) is medium-to-high compared to global and regional means, P limitation might be reducing it in the outer forest patches due to an insufficient input of nutrient-laden sediments with the lack of overbank floods. Forest structure was responsible for 30% of the explained variability, whereas flooding regime and soil fertility, directly or indirectly by litterfall chemistry parameters, explained the remaining 70%. Based on all these results, which highlighted the disturbance-dependency of the Middle Ebro riparian woodlands, the recovery of some hydrogeomorphic dynamism at different scales (basin, reach and site) is recommended to guarantee the self-sustainability of their structure, functioning and free ecological services that they provide. Hopefully, these recommendations might help to take successful management decisions in other semi-arid and Mediterranean watersheds with hydrogeomorphic constraints similar to those of the Middle Ebro.