Discrepancies in Growth Measurement Methods of Mosses: An Example from Two Keystone Species Grown under Increased CO₂ and N Supply in a Restored Peatland

Bryophytes dominate northern peatlands. Obtaining reliable measurements of moss-growth and how it may be affected by global changes are therefore important. Several methods have been used to measure moss-growth but it is unclear how comparable they are in different conditions and this uncertainty undermines comparisons among studies. In a field experiment we measured the growth and production of <i>Sphagnum fallax</i> (<i>Sphagnum</i>) and <i>Polytrichum strictum</i> (<i>Polytrichum</i>) using two handling methods, using cut and uncut plants, and three growth-variables, heightgrowth, length-growth, and mass-growth. We aimed “benchmarking” a combination of six methodological options against exactly the same set of factorial experiments: atmospheric CO₂ enrichment and N addition. The two handling methods produced partly different results: in half of the cases, one method revealed a significant treatment effect but the other one did not: significant negative effects on growth were only observed on uncut plants for elevated CO₂ and on cut plants for N addition. Furthermore, the correspondence between measurements made with various growth-variables depended on the species and, to a lesser extent, treatments. <i>Sphagnum</i> and <i>Polytrichum</i> growth was inhibited under elevated CO₂, and correlated to higher ammonium values. <i>Sphagnum</i> was however less affected than <i>Polytrichum</i> and the height difference between the two species decreased. N addition reduced the P/N ratio and probably induced P-limiting conditions. <i>Sphagnum</i> growth was more inhibited than <i>Polytrichum</i> and the height difference between the two species increased. Our data show that such a problem indeed exists between the cut and uncut handling methods. Not only do the results differ in absolute terms by as much as 82% but also do their comparisons and interpretations depend on the handling method—and thus the interpretation would be biased—in half of the cases. These results call for caution when comparing factorial studies based on different handling methods.