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.