TITLE:
Discrepancies in Growth Measurement Methods of Mosses: An Example from Two Keystone Species Grown under Increased CO2 and N Supply in a Restored Peatland
AUTHORS:
Andy Siegenthaler, Alexandre Buttler, Philippe Grosvernier, Jean-Michel Gobat, Edward Mitchell
KEYWORDS:
Mosses, Growth-Measurement Methods, Elevated CO2 and N Deposition, Polytrichum, Sphagnum, Peatland Restoration, Nutrient Availability
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.5 No.15,
July
10,
2014
ABSTRACT: 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 Sphagnum fallax (Sphagnum) and Polytrichum strictum (Polytrichum)
using two handling methods, using cut and uncut plants, and three growth-variables,
height-growth, 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. Sphagnum and Polytrichum growth
was inhibited under elevated CO2, and correlated to higher ammonium
values. Sphagnum was however less
affected than Polytrichum and the
height difference between the two species decreased. N addition reduced the P/N
ratio and probably induced P-limiting conditions. Sphagnum growth was more inhibited than Polytrichum 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.