Alpine Plant Communities and Ecosystem Services Under Drivers of Global Ecosystem Change

Anthropogenic climate warming is a significant driver of global ecosystem change. High-altitude alpine habitats with low annual mean temperatures warm proportionally faster than the global average. These ecosystems contain cold-tolerant, alpine-specialist plants that are disproportionately sensitive to the impacts of climate warming. Climate warming is predicted to facilitate upward shifts in species ranges, increasing invasion pressure and disrupting species interactions. Plant functional trait responses can provide insights into the possible impacts of climate warming on species that may alter alpine communities, which are providers of valuable and unique ecosystem services. Research on plant functional trait responses to warming has focused on easily quantified morphological traits. Knowledge of plant chemical traits is less common, limiting understanding of coordinated shifts in plant function in response to drivers of global change. My thesis aimed to understand how alpine plants respond to global drivers of ecosystem change, such as warming and shifts in plant community composition, by altering their morphological traits and tissue chemistry and how this affects changes in ecosystem service provision.

I measured the responses of 18 plant functional traits, including those related to leaf morphology, leaf and root tissue chemistry, aboveground biomass, and plant height, to warming and dominant species removal. Traits were measured across six species and two experiments based in Tongariro National Park, New Zealand: Warming and Removal in Mountains (WaRM) and Warming and Invasion (WIN). I hypothesised that trait responses would primarily indicate species benefit from warming and dominant species removal treatments. Plant functional trait responses were highly species-specific, with native species being the most responsive overall and often indicating a shift towards more acquisitive resource strategies, particularly at high-elevation sites. Under warming, high-alpine species Poa cita and Rytidosperma setifolium exhibited positive trait responses. The generative height of Poa cita increased under warming, and Rytidosperma setifolium demonstrated increases in leaf mass, length, and area, as well as vegetative height and aboveground biomass when the dominant species was also removed. Gaultheria colensoi had mixed responses, with decreased leaf size but increased tissue nutrients. Dracophyllum subulatum was measured in both WaRM and WIN experiments; leaf size increased under warming at the WIN site but decreased in the WaRM site, indicating higher stress levels. These trends were inversed at each site when warming and removal treatments were combined. Surprisingly, the traits of invasive species, Calluna vulgaris and the asters were largely unresponsive to treatments. Axes 1 and 2 from principal component analyses were also largely unresponsive to treatments despite individual trait responses, indicating a possible decoupling of trait correlations under warming and species removal.

Overall, my findings indicate that warming may alleviate temperature limitation to more acquisitive growth strategies in native alpine species, but that competition from invasive species may limit growth responses. These effects may be modulated by plant functional and mycorrhizal type, elevational range, site characteristics, and the length of the warming and invasion period. Alpine ecosystems will need careful management to navigate trade-offs in native plant survival and invasive species removal. An awareness of these trade-offs in future management and conservation plans may enhance outcomes for the culturally, scientifically, aesthetically, and recreationally valuable indigenous and endemic species found in Tongariro National Park.