Native woody plant recruitment in lowland forests invaded by non-native ground cover weeds and mammals

Abstract

Globally, lowland forests have been depleted, fragmented, and degraded by land clearance and conversion by humans. Many remnants are also invaded by non-native plants and mammals, which can exacerbate biodiversity loss and impede ecosystem recovery. We examined the effects of non-native ground cover weeds and mammals on the seedling recruitment of native woody plants in lowland forests in northern New Zealand by following establishment over 2 years at sites experiencing different levels of weed cover, with or without supplemental seed addition, and with or without mammal exclusion. In general, native seedling recruitment was highest where seeds had been added and mammals excluded. Native seedling recruitment was negatively correlated with weed cover at sites invaded by Asparagus scandens or Tradescantia fluminensis, but only where seeds had been added. These results suggest that attempts to facilitate native seedling recruitment by sowing native seeds will be most successful where ground cover weeds and introduced mammals are low in abundance. Seedling recruitment was highest for Piper excelsum, Myrsine australis and Melicytus ramiflorus, so these species could be good options for lowland-forest restoration projects where ground cover weeds are present.

Managing for biodiversity: impact and action thresholds for invasive plants in natural ecosystems

“…Several other studies have also provided evidence for negative impact thresholds for a variety of invasive plants, including the shrub Baccharis halimifolia in Mediterranean saltmarshes (Fried and Panetta 2016), the monocarpic perennial Heracleum mantegazzianum in northern European grasslands (Thiele et al 2010b), the vine Delairea odorata in northern Californian coastal scrub and riparian communities (Alvarez and Cushman 2002), and the scrambling herbs Tradescantia fluminensis, Plectranthus ciliatus and Asparagus scandens in New Zealand temperate forests (McAlpine et al 2015). Fried and Panetta (2016) found that native species' responses to invasion were complex and, in general, non-linear across a gradient of B. halimifolia cover.…”

“…following a threshold relationship. McAlpine et al (2015) reported that patterns of native plant species decline varied amongst the three weed species in the temperate forest, with only T. fluminensis and P. ciliatus exerting negative threshold effects on native species richness at approximately 50 % weed volume. It is clear from these studies that rates of species decline and the position of the threshold zone varies from one invasive species to another, and may depend upon the functional identity of the native vegetation within the recipient community.…”

“…In other cases (e.g. T. fluminensis; McAlpine et al 2015), rates of loss of native species follow a sigmoidal relationship, whereby the decline in species richness beyond the threshold zone is initially rapid but slows with increasing weed abundance, eventally approaching an asymptote as weed abundance approaches 100%. A negative sigmoidal-threshold relationship occurs where a tolerant subset of native species is retained even at very high invader abundance.…”

“…For widespread and dominant invasive plants with demonstrably negative effects on native ecosystems, there is growing evidence that ecosystem responses are non-linear, such that they occur only once a particular level of invasive plant abundance has been exceeded; that is, a negative impact threshold relationship (Alvarez and Cushman 2002, Gooden et al 2009, Thiele et al 2010b, McAlpine et al 2015, Fried and Panetta 2016. In this paper we describe a mechanistic framework for such relationships and explore the role of thresholds in triggering invasive plant control.…”

Can a reduced rate of herbicide benefit native plants and control ground cover weeds?

“…Widespread ground cover weed species such tradescantia (Tradescantia fluminensis) and climbing asparagus (Asparagus scandens) have well-documented negative impacts on native plant communities in New Zealand (Kelly & Skipworth 1984a;Standish et al 2001;McAlpine et al 2017), and are thus frequently targeted in weed control programmes. At sites with few native species in the understory, non-target damage may not be a problem.…”