Encroachment of upland Mediterranean plant species in riparian ecosystems of southern Portugal

Abstract

Riparian ecosystems have unique biodiversity, are highly sensitive to disturbance and anthropogenic influence. As world water resources become scarcer, scientists predict greater competition among species for water resources. Indeed, increased encroachment of upland plants into the riparian zone is already occurring, irreversibly changing riparian plant communities. Since semi-arid regions such as Mediterranean-type ecosystems are likely to follow this same trajectory, assessing the contributions of riparian versus upland (sclerophyllous) plants to community composition is important. A survey of seventy 2 km-long riparian transects on the Sado and Guadiana watersheds in southern Portugal assessed (1) the woody riparian plant community composition, (2) how much richness is due to strictly riparian plants versus sclerophyllous upland plants, and (3) which combinations of biotic and abiotic factors allow higher species richness in the strictly riparian, sclerophyllous, and overall plant communities. The survey detected 53 different woody plant species (28 endemic) across all communities. Riparian community richness was on average 16 species, seven of which were strictly riparian and the remainder being sclerophyllous, exotic species or fruit trees. Sclerophyllous plant species occurred consistently across sampling units (90% of transects). On average, 46% of the total woody plant community richness was due to strictly riparian plants and 28% was due to sclerophyllous plants. Community richness was positively affected by the area of shrubs in the riparian zone and by the absence of human activities and goats. Surrounding landscape pattern only affected the strictly riparian plant richness. These results suggest that natural and human-mediated disturbances in riparian ecosystems create gaps and clearings for which riparian and sclerophyllous plants compete. Establishment success seems to be related to the propagule pressure of the neighbouring landscape, its diversity and density, as well as the presence of herbivores. Preserving strictly riparian plants, removing exotic 123Biodivers Conserv (2010) 19:2667-2684 DOI 10.1007 species, preventing grazing, and promoting riparian values (recreation, aesthetics and the provision of ecosystem services) will aid the future conservation of the unique biodiversity of riparian ecosystems.

Riparian vegetation research in Mediterranean-climate regions: common patterns, ecological processes, and considerations for management

“…However, in specific cases, the reduction in channel mobility may increase the overall landscape diversity by facilitating expansion of woodland communities (Liébault & Piégay, 2002;Kondolf et al, 2007;Piégay et al, 2009) and human disturbances can increase the total species diversity and evenness (Kladis et al, 2011). Lastly, human-mediated invasion by nonnative species can result in altered competitive hierarchies both for establishing seedlings and for adult plants Aguiar et al, 2006;Stromberg et al, 2007;Mortenson & Weisberg, 2010;Santos, 2010).…”

“…This requires that we consider various human priorities (e.g., aesthetic, economic, cultural values) along with biophysical functions to address multiple management objectives (Piégay et al, 2005;Santos, 2010;Cottet et al, 2010). Assessment methods will need to consider the role of novel riparian habitats, for example those dominated by non-native species (e.g., Stromberg et al, 2007).…”

“…Particularly in arid and semi-arid regions, riparian areas host high local plant diversity and a greater proportion of the biome's tree species than their counterparts in temperate or tropical regions. For example, in Southern Portugal, Santos (2010) found that 46% of the total woody plant community richness resulted from strictly riparian species, compared to 28% from sclerophyllous plants. Moreover, community richness was positively affected by the area of shrubs in the riparian zone and by the absence of human activities and grazing animals.…”

Riparian vegetation patterns according to hydrogeomorphological factors at different spatial and temporal scales in Mediterranean rivers.

“…There is a particular concern about the invasive cane or giant reed A. donax L. (Sanz Elorza et al, 2004), declared by the International Union for Conservation of Nature as one of the most dangerous and harmful invasive alien plants worldwide (Bañares et al, 2004). A similar invasion phenomenon has been observed in numerous studies where encroachment of terrestrial species as has been observed along other semi-arid rivers (Huxman et al, 2005;Santos, 2010).…”

“…Furthermore, their topographical, sedimentological and hydrological heterogeneity make them one of the richest ecosystems in terms of species diversity (Naiman et al, 1993). This fact is specially relevant in the riparian zones of Mediterranean-climate regions (Santos, 2010), where they support a dense and productive closed-canopy forest ecosystem relative to the surrounding landscape, which is typically a matrix of xeric woodlands, shrub and grassland communities (Stella et al, 2012). In addition to their high natural biodiversity, riparian zones provide numerous ecosystem functions and services related to water quality, microclimate, structural habitat for wildlife and fish, an energy base for the food web, and bank stability, among others (Naiman et al, 2005).…”