Nutrient‐use strategy and not competition determines native and invasive species response to changes in soil nutrient availability

Abstract

Non-native invasive plants often outcompete native species under high resource availability. Restoration techniques that lower resources may, therefore, create favorable conditions for resource conservative native species over resource exploitative invasive species. Research on this topic has focused on temperate grass and forb-dominated ecosystems and has rarely been tested for woody species or tropical vegetation. We evaluated growth, resource-use efficiency (RUE), ecophysiology, and competition (i.e. a relative interaction index based on biomass) of four woody native and two dominant invasive species from Hawaiian wet and dry tropical ecosystems in a greenhouse experiment. Density of plants was constant and species were grown with either a conspecific or the invasive species from that ecosystem type across a gradient of soil nutrient availability. Instantaneous photosynthetic rates varied minimally across nutrient availability. However, both invasive and one native species increased leaf area as nutrients increased, providing more photosynthetic area and increasing total biomass. Nitrogen RUE decreased with increasing nutrient availability for all but one native species, while phosphorus RUE remained constant for all but one native species. Competitive interactions were weak, variable, and not significantly impacted by soil nutrients. Overall, plants categorized as invasive or resource exploitative had larger changes in response variables with increasing soil nutrients compared to those categorized as native or resource conservative. These results suggest that manipulating soil nutrient availability is a potentially viable restoration tool for at least some woody species in tropical ecosystems. However, predicting restoration success requires understanding species-specific ecophysiological traits determining response to altered environmental conditions.

Restoration benefits of soil nutrient manipulation and weeding in invaded dry and wet tropical ecosystems in Hawaiʻi

“…Although the nutrient concentrations that drove these effects are significantly higher (approximately 20-fold) than typically occur in these plots after feral ungulate disturbance (Long et al 2017), this is still significant given that regeneration is a key aspect of restoration and a critical filter for invasive species success and community assembly. This finding is also consistent with previous observations that M. polymorpha, a dominant native overstory tree, showed greatly reduced growth and survival under high nutrient conditions in our greenhouse study (Knauf et al 2021), as well as with fertilizer additions applied in situ in N-limited wet montane forests (Ostertag & Verville 2002). These results collectively support the idea that resource manipulation can have complex effects on ecosystems, with elevated nutrient supply negatively affecting some dominant native species adapted to low-resource environments.…”

“…The reasons for the apparent disconnect between strong growth reductions of non‐native invasives in response to C amendment observed in a greenhouse setting (Knauf et al 2021) and the field results reported here are not entirely clear. However, there are several potential explanations.…”

“…Although other studies testing soil nutrient reductions have seen positive outcomes for native species within 1 year, these responses have been strongest in forb and graminoid‐dominated temperate ecosystems (Prober et al 2005; Eschen et al 2006). Results from our greenhouse study, where plants were grown under highly favorable conditions, showed that three of the four native species tested benefited from nutrient reductions relative to the invasive species with which they were competing within 6–12 months (Knauf et al 2021). It is possible that the current study did not confirm the greenhouse‐based findings simply because it was not of a long enough duration to detect competitive benefits for native versus non‐native species under field conditions.…”

“…Carbon amendments in a restoration context will be most effective where native species are resource-conservative and invasive species are resource-exploitative. In a prior greenhouse experiment (Knauf et al 2021) using woody tropical species from Hawaiʻi, we demonstrated that C amendments effectively altered competitive dynamics between native and non-native invasive species when resource use efficiencies of these groups differed. For example, the native N-fixing tree Acacia koa maintained relatively constant biomass over a broad range of nutrient availabilities while the biomass of the invasive non-native tree Psidium cattleyanum decreased by 10-fold at low compared to high soil nutrient availability.…”

“…In this study, we examined how altering soil nutrient availability, non‐native plant removal, and their combination affect growth and reproduction of dominant native and non‐native plants in both Hawaiian montane dry shrubland and wet forest ecosystems. We hypothesized that: (i) survival, growth, and reproduction of dominant native species would show little response to experimental manipulation of soil nutrient availability, whereas dominant non‐native invasive species would respond strongly (Knauf et al 2021); (ii) removal of non‐native plants would positively affect growth of native species (D'Antonio et al 1998), and (iii) native and non‐native species growth responses would be larger in wet compared to dry ecosystems, where other limitations (e.g. water availability) may exert strong limitations on plant growth.…”