This study examined the patterns of plant functional trait variation in relation to geomorphology, disturbance and a suite of other environmental factors in the riparian margin of the Upper Hunter River, New South Wales, Australia. Vegetation was surveyed on three geomorphic surfaces (point bar, bench and bank) along a 5.5-km stretch of the Upper Hunter River. Functional traits relating to plant growth and reproduction were collected for the identified species. anova and principal components analysis were used to compare the trait assemblages of species associated with each geomorphic unit. Pearson's correlation coefficients were used to investigate trait variation with respect to environmental variables. There were clear differences in the plant functional trait assemblages associated with the three geomorphic units. Generally the point bar was associated with species that were herbaceous, with small seed mass, a short stature and a high specific leaf area (SLA). Conversely, the bench was associated with grasses that had unassisted seed dispersal and intermediate seed mass and SLA, while species on the bank had tall stature, large seed mass, a high SLA and a perennial life cycle. Variation along the primary gradient of plant functional trait composition was most strongly related to disturbance frequency and to a lesser extent soil nutrients and the proportion of clay and silt, while variation along the secondary gradient was associated with variation in substrate texture as well as soil nutrients.
Many studies have shown that soil disturbance facilitates establishment of invasive, non-native plant species, and a number of mechanisms have been isolated that contribute to the process. To our knowledge no studies have isolated the role of altered soil compaction, a likely correlate of many types of soil disturbance, in facilitating invasion. To address this, we measured the response of seeded non-native and native plant species to four levels of soil compaction in mesocosms placed in an abandoned agricultural field in the Methow Valley, Washington, USA. Soil compaction levels reflected the range of resistance to penetration (0.1-3.0 kg cm -2 ) measured on disturbed soils throughout the study system prior to the experiment. Percent cover of non-native species, namely Bromus tectorum and Centaurea diffusa, decreased by 34% from the least to the most compacted treatments, whereas percent cover of native species, mostly Pseudoroegneria spicata and Lupinus spp., did not respond to compaction treatments. Experimental results were supported by a survey of soil penetration resistance and percent cover by species in 18 abandoned agricultural fields. Percent cover of B. tectorum was negatively related to soil compaction levels, whereas none of the native species showed any response to soil compaction. These results highlight a potentially important, though overlooked, aspect of soil disturbance that may contribute to subsequent non-native plant establishment.
Riparian and riverine systems around the world are subject to human mediated degradation. It is increasingly recognized that rehabilitation of these systems is important to retain biodiversity and important ecosystem services. Plant functional traits represent a potentially useful but underutilized tool in the restoration or rehabilitation of ecosystems such as riparian systems. This study uses a functional trait approach to provide insight into the changes in ecosystem function that have occurred with the loss of native plant species and their replacement by exotics. We examined the plant functional traits associated with extinct native, extant native and extant exotic species in the riparian margin of the Hunter River, New South Wales, Australia. Comparisons of 11 traits were conducted using univariate (analysis of variance and x 2 ) and multivariate (principal component analysis and analysis of similarity) methods. Both extant native and extant exotic species had traits associated with a short lived, ruderal strategy: high specific leaf area (SLA), soft leaves, herbaceous growth forms and therophyte life form. In contrast, extinct native species had traits such as low SLA, tough leaves, tree and shrub growth form and phanerophyte life form. This shift in plant trait assemblages associated with the transition from a native to predominately exotic flora has important implications for ecosystem processes in this riverine system. We suggest that such a functional trait approach can be integrated with knowledge of other ecosystem components to provide an understanding of ecosystem function that can be used to guide rehabilitation initiatives.
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