Discussion

Soil Moisture and Temperature

This study showed that H. cupressiforme influences soil moisture and temperature within kānuka dryland shrubland soils by reducing annual and diurnal fluctuations. This is probably due to higher heat conduction capacity of water compared to the air which fills spaces within the moss (McLaren and Cameron 1996) producing less heat transfer in dry summer months and increased transfer in wet winter months. The soil was wetter in summer under moss layers probably due to high water holding capacity of the moss moderating transfer of water to the soil, run-off and evaporation (Bu et al. 2015; Michel et al. 2012). Moss may also have harvested water vapour from the atmosphere. Deeper moss probably provides a physical buffer to ground frost, reducing heat transfer capacity in winter. In boreal forests it has been reported that moss cover can intercept 23% of total rainfall although this is thought to be much less in New Zealand forests (DeLucia et al. 2003; Price et al. 1997). Moss is metabolically active during the wetter winter months (Proctor et al. 2007) potentially further reducing soil moisture due to evapotranspiration.
Mediation of soil temperature and moisture may benefit vascular plants on these exposed sites on dry, well-drained soils by improving water availability in the dry summer months. Vascular plants in the Lismore soil are known to reach wilting point at around 10-15% soil moisture (Drewitt 1979); in this study soil under a moss layer deeper than 6 cm was maintained above this range. These effects may become more significant with time since climate change is predicted to increase drought frequency and exacerbate soil moisture deficit in these habitats. However, competitive exclusion of adventive weeds in these dry habitats may be diminished by the retention of water availability (van der Wal et al. 2005).

Impact on Nitrogen

The main process likely to be governing available soil nitrogen under moss ground cover in these habitats is interception and use by mosses. Soil nitrate and ammonium concentrations were lower under moss than in bare soil, and soil nitrate decreased with increasing moss cover. In cooler climates this has been attributed to the effect of the moss cover on temperature and moisture, in turn affecting the microbes which facilitate ammonification and nitrification (Gornall et al. 2007). No extreme temperature variation was observed in this study and, althoughH. cupressiforme extracts may have anti-microbial properties (Altuner et al. 2014).
H. cupressiforme assimilates ammonium more readily than nitrate which explains a decrease in NH4+-N during the winter months when the moss is metabolically active. In earlier studies, H. cupressiforme and other mosses have been shown to be effective at absorbing nutrients, including NH4+-N in significant quantities, from wet and dry deposition, acting as a barrier to the soil and the rooting zone (Turetsky 2003).
Soil ammonium was most reduced beneath moss at intermediate depths of 3 cm where it is possible that lack of the thick, dead plant layer underneath the moss allows for NH4+-N uptake from the soil in addition to that deposited on the surface (Wang et al. 2014). An increase of moss depth increases the distance of actively growing shoots form the soil surface making soil NH4+-N unattainable potentially explaining the increase in ammonium concentrations beneath deeper moss layers (Bates 1994). The present study also showed a reduction in nitrate soil concentrations under moss layers, probably due to nitrate formation being a product of microbial ammonification.
In addition to preventing NH4+-N, and therefore NO3--N, reaching the soil by interception and utilisation, mosses are also thought to recycle N within tissues, sequestering N for long periods of time and delaying release to soil and the vascular plant rooting zone (Turetsky 2003). A study of feather mosses within a boreal forest found that mosses had been sequestering 1.8 kg N ha-1year-1 for the past 5000 years (Lagerström et al. 2007). This is equivalent to approximately 1% of the annual fertiliser applications of N to dairy pasture in New Zealand (Chapman et al. 2018).

Interactions with Vascular Plants

Although exotic grasses and weeds are responsive to additions of nitrate and phosphorus (Blackshaw et al. 2004) an inhibitory effect was observed on moss cover. It is likely that moss reduction was a function of the competitive, shading and smothering presence of the exotic grass and associated litter facilitated by increased nutrients (van der Wal et al. 2005).
Moss cover in the present study also negatively affected germination of the native species. Dormancy of seeds and inhibition of germination can be a response to far-red light conditions altered by the moss cover thereby inhibiting germination (Van Tooren and Pons 1988). NeitherK. serotina nor P. amoena germinate as effectively in dark conditions (Burrows 1996; Haines et al. 2007) and it is likely that the small seeds of each species dropped through the moss carpet following initial sowing, limiting the exposure to light. C. australis can germinate in dark conditions (Grüner and Heenan 2001) but germination rates were lower in the moss treatments; the moss may have acted as a barrier, preventing the radicle from reaching the soil (Jeschke and Kiehl 2008). Although some mosses boast allelopathic substances that can inhibit vascular plant germination (Michel et al. 2011), it is not reported in H. cupressiforme . Those C. australis andP. amoena plants that established in the moss treatments resulted in higher biomass; reduced fluctuation in soil moisture and temperature under the moss layer may have influenced growth (Ren et al. 2010).
Mosses may use and sequester nitrogen restricting transfer to the vascular plant rooting zone, thereby constricting invasion. This was implied in the glasshouse study; nitrate can be inhibitive to nodulation for nitrogen fixation (Brewin 1991) and C. australis only produced nodules in the moss layers, indicating less nitrate. Alteration of soil nutrition may also indirectly shape soil microbial communities (Delgado‐Baquerizo et al. 2017) which are central to biogeochemical cycles (Philippot et al. 2013). There are also suggestions that H. cupressiforme may have plant extracts which further modify microbial communities, potentially altering soil chemistry (Altuner et al. 2014).
Nutrient addition in the form of spillover into the remnants from the neighbouring pasture was observed in the field study with regard to mineral nitrogen and Olsen P. The alteration of soil chemistry and potential influence on microbial communities was negatively correlated with moss cover and positively correlated with exotic grass cover species which are adapted the more fertile soils (Meurk and Swaffield 2000). The effects of the moss carpet particularly on soil moisture and temperature may have been beneficial to those individuals that established promoting smothering of mosses and a further alteration in soil chemistry (Hobbie 2015). Therefore, although the moss layer may provide an effective tool in preventing spread of invasive plants by retaining a nutrient poor substrate, in the presence of increased nutrient deposition from nutrient spillover exotic weeds may initially benefit from abiotic conditions within the moss to further encroach and alter the habitat.

Conclusions

The effect of the terricolous moss cover in this low rainfall, typically nutrient poor environment is clearly significant but it is complex and involves hydrology, nutrient cycling and biotic interactions (Chamizo et al. 2016). Moss may be important in maintaining low nutrient soil conditions which promote native species regeneration rather than exotic species encroachment. This importance should drive efforts to conserve its presence in existing remnant habitats and indicate a requirement to incorporate it into ecological restoration schemes where ecosystem functioning is vital (Michel et al. 2013). However, where competitive exclusion has been mediated by nutrient spillover, the beneficial effects of the moss on the hydrological cycle may increase exotic species encroachment and alter soil chemistry further exasperating deterioration of the habitat. Therefore, ecological restoration in areas with soils of low nutrition should thoroughly consider and mitigate for the effects of nutrient spillover which could facilitate encroachment of weed species and decline of moss flora.

Acknowledgements

We thank Ngai Tahu Farming Ltd and the Forest and Bird Stocker Scholarship for providing research funding. The Department of Conservation the Spencer-Bower family provided permission to access the remnants.