Abstract
We aim to assess net primary productivity (NPP) and carbon cycling in Andean tropical alpine grasslands (puna) and compare it with NPP of tropical montane cloud forests. We ask the following questions: (1) how do NPP and soil respiration of grasslands vary over the seasonal cycle? (2) how do burning and grazing affect puna productivity? (3) if the montane forest expands into the puna, what will be the resulting change in productivity? The study sites are located at the South-eastern Peruvian Andes; one grassland site and the forest sites are in Wayqecha biological station, and another grassland site in Manu National Park. At each grassland site, we selected a burnt and an unburnt area, installed unfenced and fenced transects in each area, and monitored above-ground productivity (NPPAG), below-ground productivity (NPPBG) and soil respiration (Rs) for 2 yr. In the forest, we monitored NPPAG, NPPBG and Rs for 2–4 yr. Grassland NPP varied between 4.6 ± 0.25 (disturbed areas) to 15.3 ± 0.9 Mg C ha−1 yr−1 (undisturbed areas) and cloud forest NPP was between 7.05 ± 0.39 and 8.0 ± 0.47 Mg C ha−1 yr−1, while soil carbon stocks were in the range of 126 ± 22 to 285 ± 31 Mg C ha−1. There were no significant differences on NPP between the puna and forest sites. The most undisturbed site had significantly higher NPP than other grassland sites, but no differences were found when relating grazing and fire at other sites. There were lower residence times of above-ground biomass compared to below-ground biomass. There was a strong seasonal signal on grassland NPPAG and NPPBG, with a shift on allocation at the beginning of the austral summer. High elevation tropical grasslands can be as productive as adjacent cloud forests, but have very different carbon cycling and retention properties than cloud forests.
Highlights
We found that undisturbed sites were more productive than disturbed sites, we did not find a strong effect of fire and grazing over net primary productivity (NPP) and the disturbed sites may have lost some above-ground biomass though grazing
We show the importance of tropical alpine Andean grasslands in terms of soil carbon stocks and productivity, which is comparable with their neighbouring cloud forests
There were no significant differences in NPP or soil carbon stocks between grasslands and forests
Summary
Alpine grasslands in the tropical Andes extend from the treeline (around 2500–3500 m elevation) to more than 4800 m a.s.l., (Román-Cuesta et al 2014), occupying around 470 000 km (Tovar et al 2013). The physiognomy of this tropical alpine vegetation varies within and between regions, Environ. 9 (2014) 115011 but the different physiognomies share some features, like similar topography and similar growth forms of the dominant tussocks, usually from the genera Calamagrostis, Scirpus and Festuca Another common feature is their long history of human disturbance. If conservation strategies promoting upward migration of forest tree species were successful, Andean grasslands would still be threaded by a progressive encroachment of montane forest tree species
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