Effects of area, age and diversity of forest patches in Belgium on plant species richness, and implications for conservation and reforestation

Abstract

The distribution of 203 forest plant species over 234 isolated forest patches in the western part of Belgium and the most northern part of France was studied. An analysis that considered species richness in the context of the SLOSS (single large or several small reserves) debate gave no evidence of habitat subdivision reducing total plant species richness in the forests. The presence of some functional ecological plant species groups was correlated with habitat features and patch area. Habitat diversity was found to be important in explaining the presence of species groups of high conservation value, but patch age (as an indicator for habitat quality) also played a major role. Habitat diversity was not a surrogate for patch age. For most of the species groups, patch area sensu stricto is a redundant variable in explaining species richness relative to habitat diversity and patch age; area-dependent stochastic extinctions of forest plant species are of minor importance, at least at the present level of forest fragmentation. In contrast we suggest that extinction of forest plants occurred and still occurs mainly in a deterministic way. Finally, we conclude that even small forest fragments can be very important for maintaining plant species diversity, at least if they are of high habitat quality and if the forest management is appropriate.

Introduction

Forest plant diversity is affected by internal and external variables (Honnay et al., 1998a). One of these internal variables is forest patch area. This can be seen as an independent factor influencing species richness through stochastic area-dependent extinction (Shaffer, 1981, Pimm et al., 1988) or as a simple surrogate for habitat diversity (the so-called habitat diversity hypothesis; Lack, 1976). Where the effect of forest habitat diversity on forest plant diversity has been investigated at the forest patch level (Peterken and Game, 1984, Hobbs, 1988, Dzwonko and Loster, 1988, Dzwonko and Loster, 1992) or at least discussed (Simberloff and Gotelli, 1984, Dzwonko and Loster, 1989), the overall conclusions tend to support the habitat diversity hypothesis. These conclusions, however, remain rather hypothetical. In some cases only few forest patches were studied while in others statistical analysis did not allow unequivocal separation of the effects of patch area and habitat diversity.

In determining plant species richness of forests, another important internal variable relates to the length of the forest land use. Historical land use has an important effect on forest plant species composition and richness (Peterken, 1981, Hermy, 1994, Koerner et al., 1997). Recent forest established on former agricultural land usually is of low quality for many forest plant species because of the high nutrient levels in the soil, particularly the high soil phosphate content, which stimulates the growth of highly competitive species, suppressing the more stress-tolerant forest plant species (Pigott, 1971, Grime et al., 1988, Hermy, 1992, Wulf, 1994). In addition, soil structure and drainage may be disturbed in recent forest, hampering the establishment of certain forest species (Froment and Tanghe, 1967). All these aspects relating to these differences between old and recent forests may be summarised by the term `habitat quality'.

External variables affecting forest plant species richness mostly relate to the landscape ecological context of the forest, e.g. the isolation and characteristics of the surrounding landscape matrix (Ranney et al., 1981, van Ruremonde and Kalkhoven, 1991, Laurence and Yensen, 1991, Grashof-Bokdam, 1997). In this study, the influence of patch isolation on species richness is only handled in a preliminary phase.

In addition to plant species richness, plant species `quality' is another component in the evaluation of forest conservation sites. Comparison based on total species richness is a quantitative approach (Peterken, 1974), whereas quality of species can be judged from the occurrence of extinction-prone species (Terborgh, 1974).

This paper examines the effect of forest patch area sensu stricto and of habitat diversity and patch age on forest plant species richness in Flemish forests. Moreover, the historical contribution of stochastic extinction processes of forest plant species in Flanders is compared with that of deterministic processes. Seeking correlations with species richness in order to derive practical conservation guidelines has been criticised by authors who advocate an autecological approach (Zimmerman and Bierregaard, 1986, Soulé and Simberloff, 1986). As the resources are not available to study each individual plant species, we believe that the use of functional ecological plant groups and of well-considered independent variables can yield pragmatic conservation principles (Skov et al., 1995).