Biomass production, N:P ratio and nutrient limitation in a Caucasian alpine tundra plant community

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Abstract
Questions: 1. To what extent is biomass production of a Cauca- sian alpine tundra plant community limited by soil nitrogen and/ or phosphorus? 2. Can the foliar N:P ratio predict the nutrient limitation pattern of alpine vascular plant communities? Location: Lichen-rich tundra on Mt. Malaya Khatipara in the NW Caucasus, Russia (43°27' N, 41°42' E; alt. 2800 m a.s.l.). Methods: We conducted a 4-year fertilization experiment (N, P, N+P, lime additions and irrigation) on the alpine tundra in the northwestern Caucasus, Russia. We determined responses of biomass, tissue nutrient concentrations and nutrient pools of the above-ground component of the plant community. Results: Total plant community biomass did not respond to fertilization. However, lichen biomass strongly decreased in response to the N- and N+P treatments, whereas vascular plant biomass increased in response to the N- and even more to the N+P treatment, but not to P or lime addition or irrigation. P- concentrations in vascular plant species were very low, but their biomass production was not principally P-limited, suggesting adaptation to low soil P-availability. The N-limitation of vascu- lar plant biomass production in the community, which in low- lands usually occurs at N:P ratios below 16, could not be predicted from the mean foliar N:P mass ratio in the control (N:P = 29). Conclusions: This Caucasian alpine plant community is an example of N- and P-co-limitation of vascular plant biomass production, with N being the principal and P the secondary limiting nutrient. Critical N:P ratios as determined for lowland communities are not applicable here.