Abstract
Harsh habitats dominated by invasive species are difficult to restore. Invasive grasses in arid environments slow succession toward more desired composition, yet grass removal exacerbates high light and temperature, making the use of “nurse plants” an appealing strategy. In this study of degraded subtropical woodlands dominated by alien grasses in Hawai'i, we evaluated whether individuals of two native (Dodonaea viscosa, Leptocophylla tameiameia) and one non-native (Morella faya) woody species (1) act as natural nodes of recruitment for native woody species and (2) can be used to enhance survivorship of outplanted native woody species. To address these questions, we quantified the presence and persistence of seedlings naturally recruiting beneath adult nurse shrubs and compared survival and growth of experimentally outplanted seedlings of seven native woody species under the nurse species compared to intact and cleared alien-grass plots. We found that the two native nurse shrubs recruit their own offspring, but do not act as establishment nodes for other species. Morella faya recruited even fewer seedlings than native shrubs. Thus, outplanting will be necessary to increase abundance and diversity of native woody species. Outplant survival was the highest under shrubs compared to away from them with few differences between nurse species. The worst habitat for native seedling survival and growth was within the unmanaged invasive grass matrix. Although the two native nurse species did not differentially affect outplant survival, D. viscosa is the most widespread and easily propagated and is thus more likely to be useful as an initial nurse species. The outplanted species showed variable responses to nurse habitats that we attribute to resource requirements resulting from their typical successional stage and nitrogen fixation capability.
Highlights
Numerous studies have documented priority effects whereby established plants suppress newly establishing individuals (e.g., D’Antonio et al 2001; Corbin and D’Antonio 2004)
M. faya was larger than the other nurse species, having greater height, width, basal diameter, and biomass (Table 1)
We found that nurse plants may be a useful tool for enhancing plant establishment, but their role depended on the outplanted species
Summary
Numerous studies have documented priority effects whereby established plants suppress newly establishing individuals (e.g., D’Antonio et al 2001; Corbin and D’Antonio 2004). These same incumbents can have positive effects on establishing species by increasing shade, nutrients, and protection from herbivores (Chapin et al 1994; Callaway and Walker 1997). Planting focal species under nurse species can increase survivorship and growth if facilitative effects outweigh competitive ones (GomezAparicio et al 2004; Padilla and Pugnaire 2006). A facilitative nurse effect can result from aboveground processes including shading from harsh sun, protection from wind, temperature extremes and herbivores, lowered water stress, and increased pollinator visitation
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