Moss Matters: Unravelling the intricate web of interactions between mosses, microclimate, and seed germination in New Zealand's alpine ecosystems

Mosses are abundant throughout the world in a variety of ecosystems and help regulate water balance by mediating water storage and release. The effects of vascular vegetation are relatively well-known and their ability to alter temperature and moisture conditions at a fine scale has been well-studied. However, limited research has been undertaken, particularly in New Zealand, into the ecological relationships and effects of any bryophytes, including mosses. In grasslands, including New Zealand alpine tussock, mosses are highly important for ecosystem functioning, aiding in water balance regulation. As water availability is a crucial factor for many ecosystem processes, including the germination and growth of seedlings, the presence, absence, or species of moss may have an influence on microclimate, and thus, vascular plants. The degree to which a given moss affects microclimate has been linked to quantifiable traits, which may vary highly both within and among species. My thesis aimed to understand how the functional traits of bryophytes can inform their role in microclimate mediation and the germination of vascular plants in New Zealand tussock grasslands. In addition, I sought to examine how moss traits varied across climatic gradients, especially levels of precipitation.

I measured soil, ground surface, and air temperature, and soil moisture at Cardrona Alpine Resort, Central Otago, using 50 data loggers placed across 13 sites. I then compared how these abiotic factors varied with the presence of a moss (Polytrichum juniperinum), proximity to snow tussock (Chionochloa), and with topography (ridge, valley), over a summer growing season. I also collected and measured morphological and functional traits of 249 specimens of 11 moss species collected from grasslands throughout Canterbury and Otago, focussing on traits important for water relations. I then tested the effects of mosses with different traits on the germination of the exotic pest plant, Pilosella officinarum, using seven moss species as substrates compared to bare soil. Water holding capacity, colony density, and water retention ability were highly variable within and among moss species. Some specimens were able to take up ~175 % of their dry mass (Polytrichum juniperinum), others over 2000 % (Hypnum cupressiforme), and desiccation time (to 50 % water capacity) ranged from 6.4 to 271 hours. Colony density varied over two orders of magnitude. There were distinct differences in all functional traits among species and some among groups of species delimited by growth form; wefts were the least dense and had the greatest water holding capacities, while cushions had the shallowest colonies compared to both wefts and turfs.

I demonstrated significant differences in ground surface and soil temperatures in microsites that were near or far from individual tussock grasses and those that were on a ridge compared to in a valley. Being near a tussock or in a valley reduced fluctuations and extremes of temperature at the ground surface and in the soil. Moss did not have a detectable effect on microclimate. This may be due to the species present at the site I tested, P. juniperinum, which was a low turf with potentially little ability to affect water storage and temperature insulation, compared to other species. P. juniperinum had very high colony density, a trait linked to poor thermal insulative properties, and was the least able to take up and retain substantial amounts of water. This likely reduces its efficacy as a regulator between the weather and the microclimate.

Germination rate of Pilosella officinarum seeds was highest on the bare soil control compared to on any moss species. Seedling growth (per pot biomass) was lower in moss in general compared to bare soil and varied among moss species. Smaller, shallower colonies, such as those of Polytrichum juniperinum, were better substrates for seeds than deep turfs, such as Polytrichastrum alpinum. One relatively large species, Sanionia uncinata, was also nearly as good a substrate as the bare soil control and did not differ significantly in germination rate.

My results support the hypothesis that mosses can influence the germination of vascular plants, and that the effects may be mediated by quantifiable traits. They also show that tussock grassland microclimates vary at a scale of metres, mediated by vascular vegetation and topography. I demonstrated both intra- and inter-species-level trait variability in mosses, but findings did not suggest broad patterns at either level when considered across climatic gradients. My findings, in addition to previous ecological studies, indicate that mosses have important ecological functions and their removal when habitats are disturbed likely has impacts on ecosystem function, particularly with respect to seed germination.