Supplementary Discussion
The reliability of N:P ratios in above-ground standing total vascular plant biomass for
determining nutrient limitation has been discussed in literature. The diagnostic value of N:P
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ratios is regarded as much more reliable than that of single nutrient critical values and holds
for wetlands1, upland ecosystems2 and terrestrial ecosystems in general3. N:P ratios are
recommended to be used for assessing which nutrient limits the biomass production at the
vegetation level and only when factors other than N or P are unlikely to be limiting1. In such
cases biomass N:P ratios do reflect the relative availability of N and P and may indicate the
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degree of N or P deficiency experienced by a plant population, sometimes even more reliably
than fertilization experiments4.
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defined critical N:P ratios at: N:P > 16 indicating P-limitation, < 14 indicating N-
limitation, and between 14 and 16 N& P co-limitation. Following a recent evaluation of the
N:P ratio methodology1, we slightly reduced the lower value from 14 to 13.5, and note that
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‘N&P co-limitation’ based on this method should be interpreted as true co-limitation by N and
P together, or at least as no clear single limitation by N or P.
The obvious patterns and trends visible in Fig. 1a, b, d and f of the present paper show
that our approach is robust and not sensitive for small deviations in the critical values of N:P
ratios. Moreover, the clear positive trend of an increasing proportion of endangered species in
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the total species number with increasing N:P ratios (Fig 1f) suggests that the stronger the Plimitation is the more do endangered species dominate the species composition of a site.
1. Güsewell, S. & Koerselman, W. Variation in nitrogen and phosphorus concentrations of
wetland plants. Perspectives in Plant Ecology, Evolution and Systematics 5, 37-61 (2002).
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2. Tessier, J.T. & Raynal, D.J. Use of nitrogen to phosphorus ratios in plant tissue as an
indicator of nutrient limitation and nitrogen saturation. J. Appl. Ecol. 40, 523-534 (2003).
3. Güsewell, S. N:P ratios in terrestrial plants: variation and functional significance. New
Phytologist 164, 243-266 (2004).
4. Güsewell, S., Koerselman, W. & Verhoeven, J.T.A. Biomass N:P ratios as indicators of
nutrient limitation for plant populations in wetlands. Ecol. Applic. 13, 372-384 (2003).
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5. Koerselman, W. & Meuleman, A.F.M. The vegetation N:P ratio: a new tool to detect the
nature of nutrient limitation. J. Appl. Ecol. 33, 1441-1450 (1996).