Seasonal patterns of litterfall in the floodplain forest of a large Mediterranean river

Abstract

Litterfall is a key process controlling trophic and nutrient dynamics in coupled river-floodplain ecosystems. Litterfall quantity, quality and timing may be partly controlled by the local hydrology and forest structure. In the Middle Ebro (a large Mediterranean river in NE Spain), the importance of those factors has recently been assessed for litterfall quantity and quality but never for litterfall timing. In this study, the litterfall seasonal patterns (i.e., the intra-annual phenology of total vertical litterfall, the contribution of different species as well as the litter fractions and their nutrient concentration) in the floodplain forest of the Middle Ebro in 2007 were described to assess the role of their controlling factors. To that end, litterfall was collected monthly in a series of 12 plots representing the variety (in terms of forest structure and hydroperiod) of forest patches existing in the floodplain. The sampled material was sorted by litter fractions and species, and their C, N and P concentrations were analysed. The results showed that, as in most deciduous temperate forests, the bulk of litterfall occurred in autumn (42 % of total) with leaf fall (57 % of all litter fractions) for the 6 dominant tree species. However, the amount of litterfall collected during the rest of the year (especially in spring = 25 % and summer = 20 %) and from other litter components (woody = 22 % and reproductive = 10 %, with significantly higher N and P concentrations) was not negligible. This litterfall leads to seasonal imbalances in the quantity and quality of litter inputs to the riparian system throughout the year, which consequently might control the detritus-based community structure, food webs and nutrient cycling. Two species of Populus (P nigra and P alba) exhibited a second leaf fall peak in the summer. However, cross correlation analyses showed that the temporal dynamics of litter and leaf fall were similar between plots and dominant tree species. This result suggests that, unlike litterfall quantity and quality, litterfall phenology was not controlled by the differences in forest structure and hydrology at the patch level but rather by some other factors operating at a broader spatial scale, such as the regional climate.