Recent and projected alterations to Earth’s climate have raised concerns about the impacts on biological processes, especially those pertaining to carbon (C) cycling and sequestration. It is still unclear whether plant responses will result in positive or negative feedback loops to climate change. Ecotypic differentiation of Chionochloa species (perineal tussock grasses) can occur across altitudinal gradients in montane tussock grasslands of New Zealand (NZ). This provides unique opportunities to assess the influence of climate on these species’ responses to C cycling and sequestration. To that end, we asked is productivity or decomposition of Chionochloa pallens more greatly altered across a climate gradient and how might this influence C sequestration? Productivity and decomposition of C. pallens (the dominant grassland species) was investigated on Mount Mangaweka, Ruahine Range, NZ via full-reciprocal translocation of living plants and leaf litter decomposition bags across all five plots incrementally spaced every ≈100m in altitude. A mean annual lapse rate of 0.6°C/100m in elevation provides a temperature change of 2.4°C over this elevational gradient, covering global circulation models’ predicted range of increases in surface temperature by the end of this century.
Results/Conclusions
Average productivity of in situ C. pallens over the 2011-2012 growing season was 184 and 136% greater at the two lowest plots in elevation compared to upper plots (P<0.001) and decreased with increasing altitude. Productivity of transplants differed between plots based on location of transplant origin (P<0.05) and destination (P<0.001); there was also an interaction (P<0.001). Translocation of C. pallens plants from all plots to the lowest altitude plot yielded the greatest productivity. Decomposition of C. pallens’ leaf litter based on location of origin did not differ between plots within years. However, after 2 years of decomposition at the two lowest elevation plots, the mass loss was (g/g) 29.6% greater than at the uppermost plot (P<0.001). Based on the annualized ratio of aboveground productivity to decomposition (i.e. amount of C sequestered), our findings indicate productivity, rather than decomposition, is the primary driver influencing C. pallens aboveground C sequestration. Warmer climates associated with projected climate change will likely impact decomposition more than productivity and ultimately reduce the C sequestration potential of C. pallens’ in NZ tussock grasslands, a least in the short term.