Advantages of the point-intercept method for assessing functional diversity in semi-arid areas

Abstract

© iForest -Biogeosciences and Forestry IntroductionSemi-arid ecosystems are characterized by water scarcity and often by low soil productivity. Due to global change, they are highly vulnerable to losses of biodiversity, which underpins many critical ecosystem services (Reynolds et al. 2007). The more dramatic effects on these ecosystems are often preceded by subtle changes in relative species' abundance and/or in the dominance of specific functional traits (Chapin et al. 2000, Scheffer et al. 2001. Even in less abundant functional groups (e.g., nitrogen-fixers), the loss of critical traits that ensure ecosystem functioning and resilience may have important consequences (Grime 1998, Walker et al. 1999. The detection of such changes may be a hint of significant ecosystem transitions.Species richness has traditionally been used to assess an ecosystem's response to environmental factors, and has been related to ecosystem multi-functionality (Maestre et al. 2012a). These relationships are largely modulated by other community attributes, such as species evenness and functional identity and divergence, which often respond more rapidly to environmental constraints than richness, and may have a strong impact on ecosystem processes (Chapin et al. 2000, Mouillot et al. 2011, Maestre et al. 2012b. Functional diversity, defined as the value, range, and relative abundance of the functional traits of biological communities in a given ecosystem, was shown to be a better and more universal predictor of ecosystem vulnerability than species diversity, which does not reflect the uneven role played by species in the maintenance of ecosystem processes (Tilman et al. 1997, Díaz et al. 2007). Functional diversity is usually assessed by the use of several metrics (e.g., communityweighted mean and functional richness, evenness, and divergence). Recent investigations have demonstrated the better predictive ability of indexes that consider species abundances rather than richness alone (Schleuter et al. 2010, Mouillot et al. 2011. However, there is no consensus on the best field method for functional diversity assessment. Biomass (Prieur-Richard et al. 2002), frequency (De Bello et al. 2005), and most commonly cover (Frenette-Dussault et al. 2012, Lavorel et al. 2008 Semi-arid areas are particularly susceptible to the loss of biodiversity as a consequence of global change. Species functional traits are key drivers of functioning and resilience of ecosystems, thus monitoring of functional trait diversity is urgently needed. The assessment of functional diversity requires the quantification of species and/or their traits in the field, though there is no consensus on the best plant-sampling method to be used. The aim of this study was to compare the performance of the point-intercept (PT) method with two area-based approaches, the modified-Whittaker (MW) and Dengler (DE) methods, to assess functional diversity in semi-arid areas. The herbaceous community of a savanna-like Mediterranean woodland was surveyed at the two extremes of a regional...

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