Susan Walker

Properties of ecotones: Evidence from five ecotones objectively determined from a coastal vegetation gradient

Abstract Several properties have been suggested to be characteristic of ecotones, but their prevalence has rarely been tested. We sampled five ecotones to seek evidence on seven generalizations that are commonly made about ecotones: vegetational sharpness, physiognomic change, occurrence of a spatial community mosaic, many exotic species, ecotonal species, spatial mass effect, and species richness higher or lower than either side of the ecotone. The ecotones were in a sequence from scattered mangroves, through salt marsh, rush-marsh, scrub, woodland, to pasture. We developed a method to objectively define, by rapid vegetational change, the position and depth of an ecotone, identifying five ecotones. Their positions were consistent across three sampling schemes and two spatial grain sizes. One ecotone is a switch ecotone, produced by positive feedback between community and environment. Another is anthropogenic, due to clearing for agriculture. Two others are probably environmental in cause, and one may be largely a relict environmental ecotone. Sharp changes in species composition occurred. Three ecotones were associated with a change in plant physiognomy. In two, the ecotone was located just outside a woodland canopy, in the zone influenced by the canopy. Community mosaicity was evident at only one ecotone. There were few strictly ecotonal species; many species occurred more frequently within ecotones than in adjacent vegetation, but there were never significantly more ecotonal species than expected at random. There was little evidence for the spatial mass effect reducing ecotonal sharpness, or leading to higher species richness within ecotones. Species richness was higher than in the adjacent habitat in only one ecotone. It seems that supposedly characteristic ecotone features depend on the particular ecological situation, and the ecology of the species present, rather than being intrinsic properties of ecotones. Nomenclature: Connor & Edgar (1987) and references therein, and Stace (1997), except where indicated.

Evidence on ecotone concepts from switch, environmental and anthropogenic ecotones

. Four contrasting ecotones were sampled to address three questions: (1) Are there ‘ecotonal’ species, (2) Do ecotones possess higher (or lower) species richness than the adjacent communities? and (3) Are exotic species more likely to occur in ecotones? One ecotone was edaphic, one was apparently caused by a positive-feedback switch, one was environmental/anthropogenic and one was entirely anthropogenic. The exact position of each ecotone was established from the spatial change in ordination scores. Ecotonal species, in the sense of species mainly restricted to the ecotone at the site, were present in all four ecotones. All but one of the ecotonal species were native. The switch ecotone and the purely anthropogenic ecotone also contained native species that were significantly more frequent in the ecotone than in either adjacent community. Species richness was intermediate between that of the two adjacent communities in three of the ecotones. In the environmental/anthropogenic ecotone, species richness was higher than in adjacent communities, but not significantly so. There were appreciable numbers of exotic species in the two ecotones with anthropogenic influence, one of which had a proportion of exotic species intermediate between the two adjacent communities. Contrary to theory, the proportion of exotic species in the second ecotone was significantly lower than in either adjacent community. We conclude that all three features we examined depend on the particular ecological conditions and the ecology of the species present; they are not intrinsic properties of ecotones.

TESTS FOR NONEQUILIBRIUM, INSTABILITY, AND STABILIZING PROCESSES IN SEMIARID PLANT COMMUNITIES

Models of semiarid vegetation dynamics incorporating concepts of equilibrium and stability have been largely replaced by nonequilibrium models in recent years. However, neither equilibrium nor disequilibrium and neither stability nor instability have been formally demonstrated. Most records of semiarid vegetation are inadequate for conclusive tests. We tested for equilibrium at seven study sites in six locations and vegetation types, in grazed semiarid grassland in central Otago, New Zealand, using data collected twice annually over 5–13 yr. Some sites showed overall directional change in composition over time, but others appeared to fluctuate about an average equilibrium state. At one site, an instantaneous perturbation experiment was used to test formally for stability, and to seek evidence for intrinsic stabilizing processes. In this experiment, three herbicide treatments removed or decreased different components of the community, and recovery and convergence with the untreated control was monitored ov...

Alluvial grasslands in south-eastern New Zealand: vegetation patterns, long-term and post-pastoral change.

Grasslands in alluvial systems are studied in 12 valleys and intermontane basins in south‐eastern New Zealand Vegetation patterns and their environmental determinants are described overall, and in five different valley groups (clusters) Data from grazing‐exclosures are used to examine effects of the cessation of grazing on vegetation, and long‐term plots and management‐induced contrasts at fencehnes are used to examine grazing effects Climate (minimum temperature of the coldest month and rainfall) and landscape position are the determinants of the primary vegetation gradient A secondary vegetation gradient represents vegetation modification (i e, the degree of exotic species dominance) in alluvial systems Exclosures indicate post‐pastoral decreases in exotic species dominance, in terms of abundance, in two alluvial sites, and increased exotic dominance in three alluvial sites In terms of species numbers, a decrease in exotic proportion is indicated at one site, and an increase is indicated at four sites Differences between the exclosed and continually grazed vegetation in native species richness, and in the amount and proportion of native btomass, are negatively related to the biomass of exotic species present in the ungrazed vegetation, and are not related to the environmental characteristics of the sites Historic data are insufficient to identify general trends in grazed alluvial grasslands In three sites for which data are available, the dominance of exotic species had increased, and the proportional contributions of all native species, and of native grasses, had decreased with time In terms of species number, however, proportions of native species had increased at two sites, and decreased at one site Widespread and well established exotic grasses appear to have reached their potential environmental range in the alluvial systems studied, but the more recent invader, Hieracium pilosella, may not yet occupy its potential range The practicability of post‐pastoral conservation of native vegetation in alluvial systems might be indicated by the identification of modification thresholds However, threshold identification requires data from a greater number of long‐term plots than presently exists in southern alluvial grasslands

Seasonal changes in semi‐arid vegetation of Central Otago, New Zealand

Abstract Seasonal changes in the semi‐arid grassland vegetation of Central Otago, New Zealand, are examined using data collected in nine pairs of spring and autumn sampling dates at five long‐term plots, and data from consecutive autumn, spring, and summer surveys across an environmental gradient. Seasonal changes are small in relation to those occurring in the vegetation from year to year, and to spatial differences in vegetation across the landscape. Seasonal changes in community characteristics and in the abundance of most species differ between years. Climatic control of seasonal change appears to be complex and site‐specific. Greater seasonal variation, in both species presence and species abundance, is found in shady sites and on deeper soils. Spring vegetation tends to be more variable and less well structured than autumn vegetation. It is suggested that the annual soil moisture cycle of winter recharge and summer depletion leads to a random element in the spring vegetation (i.e., the relatively wi...