Can We Extrapolate the California Model of Grassland-Shrubland Ecotone?

Abstract

In California, a number of studies on the interaction between shrubs (Baccharis pilularis) and herbaceous species in annual grasslands have led to the proposal of a conceptual functional model for the dynamics of ecotones between species patches. In our study Chile we used a similar shrub (Baccharis linearis) and annual grassland species to test the robustness of the hypothesis that ecotones between similar landscape elements under similar climate constraints also have similar dynamics. The herbaceous and shrubby ecotone vegetation was described using cover measurements along the same transects in aerial photographs of 1955, 1962, and 1980, and in the field in 1987. The herbaceous species were then determined and quantified by their mass on both sides of the ecotone. Colonization capacity of B. linearis was estimated by describing its establishment on perturbed sites and estimating its seed dispersion. Field and laboratory experiments evaluated the effect of herbaceous species on shrubs at seedling stages. We found similarities between the Chilean and California dynamics in the strong inhibition posed by the annual herbaceous strata to shrub colonization. In Chile this inhibition results in little or no shrub invasion for at least three decades. We also found, just as in California, a reduction of the herbaceous biomass under the shrub canoies, once these get established. However, in Chile, grassland species under the canopy of Baccharis shrubs have a higher biomass and relatively higher species diversity than in California. This difference is related to the fact that herbivores and especially leporids, which in Chile are introduced, play completely different roles in the two regions. In California, herbivores, including leporids, tend to be restricted to areas under the shrub canopies and consequently remove most of the grasses under the shrubs, whereas in Chile leporids are not restricted to shrub patches and the total herbivore effect on forbs under and around shrub patches is much smaller. Consequently, in Chile there is no facilitation of shrub invasion due to herbaceous plant removal by herbivores, and the ecotone is less mobile than in California. The 30-yr stability of the ecotone between Baccharis and grassland shown by our data seems to support this proposition. Our results also suggest that the most abundant species of annual grass in California (Bromus mollis), highly influential in the inhibition of Baccharis colonization, does not behave in a comparable way to its Chilelean congener Bromus berterianus, mainly because it germinates and emerges later in the season than Baccharis linearis. In the Chilean case, herbs such as Erodium cicutarian and Trifolium species seem to be more influential on shrub seedling survival. We conclude that in spite of the strong climatic, physiognomic, and taxonomic similarities between the two areas, ecotones in analogous patches are only partially similar. Consequently, eventually management extrapolations between these regions should be done with extreme care. More generally, our study suggests that extrapolation of management decisions between similar regions, even as similar as chaparral-matorral ecosystems, should be postponed until shown that analogous elements really play sufficiently similar roles in the different areas.