Original articleHow habitat area, local and regional factors shape plant assemblages in isolated closed depressions
Introduction
A closed depression can be considered as an aquatic island in a terrestrial landscapes, a sort of insular system. Plant assemblages in these habitat islands are related to habitat area, local environmental and regional ecological processes. Firstly, the number of plants present in the system is obviously a function of the system area. All else being equal, a larger closed depression will contain a greater number of species because larger habitats have, on one hand, more habitats and, on the other, more individuals. More individuals decrease the probability of species extinction due to genetic drift, to random local extinctions or to unpredictable disturbances (Rosenzweig, 1995). Secondly, plant assemblages are dependent on biotic and abiotic conditions characterizing a closed depression (Kirkman et al., 2000), i.e., the local factors. For example, plant composition of young water-filled closed depressions differs greatly from plant assemblages of old peat-filled closed depressions (Klinger, 1996, Frankl and Schmeidl, 2000, Tiner, 2003, Hérault and Thoen, 2008). Lastly, species accumulation in a given closed depression is the result of successive establishment of species dispersed from other closed depressions, i.e., the regional factor (Leibowitz, 2003). While hydrochory may play a crucial role in the structure of plant assemblages in river (Andersson et al., 2000) or in lowland wetlands (Middleton, 2003), seed dispersal by running water is impossible between upland closed depressions. Moreover, dispersal limitation greatly varies with the nature of the surrounding matrix (Murphy and Lovett-Doust, 2004). And it may be expected that a closed depression surrounded by forests may be less colonized by aquatic plants owing to the lower frequentation of these closed depressions by waterfowls (Green et al., 2002).
Processes of plant assemblages in upland closed depressions from European landscapes have not attracted many ecological researches. In particular, one issue that has received very little attention is how plant assemblages respond to spatial isolation (Leibowitz, 2003). Studies from other fragmented ecological systems (e.g., Médail et al., 1998, Dupré and Ehrlén, 2002, Hérault and Honnay, 2005) have concluded that the species specific response to area, local and regional factors is closely linked to syndromes of species life-history traits, i.e., Emergent Groups. An Emergent Group is defined as a set of species having similar combinations of morphological, physiological and phenological life-history traits (Lavorel et al., 1997, Hérault, 2007) so that convergent ecological and evolutionary strategies can be expected between members of a given Emergent Group (Holt, 2006, Scheffer and van Nes, 2006). Which plant life-history traits are needed to counterbalance random local extinctions in a fragmented habitat? Plants have to disperse to new closed depressions, they have to become established, and once established they have to persist as long as possible (Weiher et al., 1999). Dispersal occurs in two dimensions: a temporal and a spatial one. Dispersal in time depends on seed longevity (HilleRisLambers et al., 2005) while dispersal in space depends on a several seed key-properties (mass and dispersal mode, see Hewitt and Kellman, 2002). Establishment is firstly linked to germination requirements (chilling, drying, scarification, …, see Baskin and Baskin, 2001). But the seed size is also critical for establishment, as larger seeds often result in larger and more competitive seedlings (Moles and Westoby, 2004). Additionally, the plant ability to vegetatively reproduce is crucial when few individuals initially colonize a closed depression (Hooftman and Diemer, 2002). And finally, persistence is correlated with many life-history traits (Weiher et al., 1999). The most successfully used are related to the mode of life (life-form, pollination vector, root anchorage for aquatic plants), to the competitive ability (plant height, seed mass) and to the strategy of resource acquisition (e.g., onset of flowering). The latter could be crucial in forested landscapes where vernal plants have evolved to capture resources before canopy closure in late spring (Lapointe, 2001).
The first aim of this study is to delineate plant Emergent Groups having similar life-history trait combinations implied in dispersal, establishment and persistence in temperate closed depressions. Next, the following hypotheses will be tested: (H1) the probability of species presence in a given closed depression increases with the closed depression area, (H2) plant assemblages in closed depressions from forested landscapes are more linked to habitat connectivity than in open ones, e.g., plant dispersal is easier in open than forested landscapes and (H3) the highlighted patterns in H1 and H2 strongly depend on the plant colonization ability, estimated by a combination of dispersal, establishment and persistence life-history traits.
Section snippets
Materials and methods
The 169 investigated closed depressions were situated throughout the Lorraine region in Central-Western Europe (49°26′–50°10′N, 5°42′–6°32′E). The climate is sub-continental with an Atlantic influence; the mean annual temperature is 9 °C. Annual rainfall ranges between 700 and 1000 mm. Among isolated wetlands, small closed depressions encompass all landscape elements where a sediment depository is encircled by hillslopes (Gillijns et al., 2005). It has been hypothesized that some closed
Results
Nine Emergent Groups were delineated using our classification procedure: 2 aquatic, 4 herbaceous, 2 woody and the Bog Mosses. Details on their characteristic syndromes of life-history traits are available in Table 2 and statistics are given in detail in Appendix 3 Comparison of the life-history trait values between the two aquatic Emergent Groups, Appendix 4 Comparison of the life-history trait values between the 4 herbaceous Emergent Groups, Appendix 5 Comparison of the life-history trait
Habitat area limits plant colonization
The theory of island biogeography, originally written for terrestrial islands in an inhospitable sea, predicts that, all else being equal, the probability of presence of a given species increases with the island size (MacArthur and Wilson, 1967). Our results fall short of these expectations in open landscapes (H1 only validated in open landscapes). Indeed, values of the area limitation index are highly differentiated between Emergent Groups in forested landscapes (Table 3).
In open landscapes,
Conclusion
We have shown that plant assemblages in isolated closed depressions are influenced by the surrounding landscape: plant assemblages are severely colonization-limited in forests while local environmental variables and habitat area are the main sources of variability in open landscapes (Fig. 1). Moreover, the relative importance of local versus regional factors shaping the plant assemblages clearly depends on the Emergent Group of interest. Indeed, plants from different Emergent Groups respond in
Acknowledgements
The authors thank Jeff Nekola for useful comments on the manuscript. The comments of the anonymous reviewers have resulted in significant improvements to the manuscript.
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