Succession of Exotic and Native Species Assemblages within Restored Floodplain Forests: A Test of the Parallel Dynamics Hypothesis

Comparing long-term patterns of spread of native and invasive plants in a successional forest

A fundamental question in invasive plant ecology is whether invasive and native plants have different ecological roles. Differences in functional traits have been explored, but we lack a comparison of the factors affecting the spread of co-occurring natives and invasives. Some have proposed that to succeed, invasives would colonize a wider variety of sites, would disperse farther, or would be better at colonizing sites with more available light and soil nutrients than natives. We examined patterns of spread over 70 years in a regenerating forest in Connecticut, USA, where both native and invasive species acted as colonizers. We compared seven invasive and 19 native species in the characteristics of colonized plots, variation in these characteristics, and the importance of site variables for colonization. We found little support for the hypotheses that invasive plants succeed by dispersing farther than native plants or by having a broader range of site tolerances. Colonization by invasives was also not more dependent on light than colonization by natives. Like native understory species, invasive plants spread into closed-canopy forest and species-rich communities despite earlier predictions that these communities would resist invasion. The biggest differences were that soil nitrate and the initial land cover being open field increased the odds of colonization for most invasives but only for some natives. In large part, though, the spread of native and invasive plants was affected by similar factors.

Threats, biodiversity drivers and restoration in temperate floodplain forests related to spatial scales

Floodplain forests offer a diversity of habitats and resources for a very wide range of plant and animal species. They also offer many benefits to humankind and are considered essential to the mitigation of the effects of climate change. Nevertheless, throughout the world they are suffering the most intense of anthropogenic pressures so are, of all ecosystems, among the most endangered. Here, we bring together and synthesise existing ecological understanding of the mechanisms underlying the high heterogeneity and diversity of temperate floodplain forests and of the pressures threatening their high biological value due to habitat homogenisation. Floodplain forests depend on the periodic disturbances under which they evolved, including fluvial dynamics, traditional management practices and the activities of herbivores. However, they have been heavily degraded by climate change, invasion of exotic species, river-flow regulation, landscape fragmentation, eutrophication and the cessation of traditional management. We can now observe two general trends in temperate floodplain forests: (1) Due to intensive landscape exploitation, they are now more open and thus prone to the spread of competitive species, including of invasive exotics and (2) Due to the cessation of traditional management, along with modified hydrological conditions, they are composed of species in the later successional stages (i.e., more shade-tolerant and mesic) while light-demanding species are quickly vanishing. Restoration practices have brought about contrasting results when restoration of floodplains to their natural states has been problematic. This is likely because of interplay between various natural and artificial processes not previously taken into proper consideration. We would like to draw attention to the fact that restoration projects or the preservation of existing floodplain forest ecosystems should combine the restoration of watercourses with the mitigation of other important threats acting at different scales of the landscape (spread of invasive species, eutrophication of watersheds and inappropriate forest management).

Fine-root traits are linked to species dynamics in a successional plant community

Despite the importance of fine roots for the acquisition of soil resources such as nitrogen and water, the study of linkages between traits and both population and community dynamics remains focused on aboveground traits. We address this gap by investigating associations between belowground traits and metrics of species dynamics. Our analysis included 85 species from a long-term data set on the transition from old field to forest in eastern North America (the Buell-Small Succession Study) and the new Fine-Root Ecology Database. Given the prominent roles of life form (woody vs. non-woody) and species origin (native vs. exotic) in defining functional relationships, we also assessed whether traits or their relationships with species dynamics differed for these groups. Species that reached their peak abundance early in succession had fine-root traits corresponding to resource acquisitive strategies (i.e., they were thinner, less dense, and had higher nitrogen concentrations) while species that peaked progressively later had increasingly conservative strategies. In addition to having more acquisitive root traits than native species, exotics diverged from the above successional trend, having consistently thinner fine roots regardless of the community context. Species with more acquisitive fine-root morphologies typically had faster rates of abundance increase and achieved their maximal rates in fewer years. Decreasing soil nutrient availability and increasing belowground competition may become increasingly strong filters in successional communities, acting on root traits to promote a transition from acquisitive to conservative foraging. However, disturbances that increase light and soil resource availability at local scales may allow acquisitive species, especially invasive exotics, to continue colonizing late into the community transition to forest.

Vascular Plant Biodiversity of Floodplain Forest in Morava and Dyje Rivers Confluence (Forest District Soutok), Czech Republic

This paper presents an evaluation of full-area floristic investigation of floodplain forests in Soutok forest district (Židlochovice Forest State Enterprise) based on an individual forest stand inventory. The study area encompasses 5103 ha of forests, where 1186 segments were inventoried, and 71 223 single records about presence of vascular plant species were done. We found 761 taxa (species, subspecies and hybrids), out of which 655 were herbs, 106 woody plants, 156 were endangered species and 177 adventive species. The average area of a segment was 4.3 ha. The mean number of species per segment was 64.42 in a range of 4–180.

An Assessment of Long-Term Compliance with Performance Standards in Compensatory Mitigation Wetlands

Under the US Clean Water Act, wetland restoration is used to compensate for adverse impacts to wetlands. Following construction, compensation wetlands are monitored for approximately 5 years to determine if they comply with project-specific performance standards. Once a compensation site complies with performance standards, it is assumed that the site will continue to meet standards indefinitely. However, there have been few assessments of long-term compliance. We surveyed, in 2012, 30 compensation sites 8-20 years after restoration to determine whether projects continued to meet performance standards. Additionally, we compared floristic quality of compensation sites to the quality of adjacent natural wetlands to determine whether wetland condition in compensation sites could be predicted based on the condition of nearby wetlands. Compensation sites met, on average, 65% of standards during the final year of monitoring and 53% of standards in 2012, a significant decrease in compliance. Although forested wetlands often failed to meet standards for planted tree survival, the temporal decrease in compliance was driven by increasing dominance by invasive plants in emergent wetlands. The presumption of continued compliance with performance standards after a 5-year monitoring period was not supported. Wetlands restored near better quality natural wetlands achieved and maintained greater floristic quality, suggesting that landscape context was an important determinant of long-term restoration outcomes. Based on our findings, we recommend that compensation wetlands should be monitored for longer time periods, and we suggest that nearby or adjacent natural wetlands provide good examples of reasonably achievable restoration outcomes in a particular landscape.