Background
Condensed tannins are naturally-occurring secondary carbon compounds produced in the leaves of a number of different plants that sometimes act to deter herbivorous insects. In addition, scientists with New Zealand's AgResearch Grasslands institute have found that sheep and cattle feeding on forage containing condensed tannins may emit less methane, which is the next most important atmospheric greenhouse gas behind carbon dioxide.

What was done
The authors grew, from seed, well-watered silver birch (Betula pendula) plants in small containers filled with peat that were supplied with nitrogen (N) at low, moderate and high rates equivalent to 0, 150 or 500 kg N per hectare per year, respectively, and maintained within climate-controlled closed-top chambers located out-of-doors at the University of Joensuu in Finland at either ambient or elevated air temperature (T or T + 2°C) at either ambient or elevated air CO2 concentrations (360 or 720 ppm) from mid-June 1999 to the end of the 2000 growing season, after which they harvested their leaves and determined their insoluble condensed tannin concentrations.

What was learned
As best we can estimate from the graphical presentations of Huttunen et al.'s results, the doubling of the atmospheric CO2 concentration they imposed on the tree seedlings led to the following increases in insoluble condensed tannin concentrations: 52% (low N), 17% (moderate N), 99% (high N) under the ambient air temperature regime, and 61% (low N), 67% (moderate N), 20% (high N) under the elevated air temperature regime.

What it means
With all air temperature and soil nitrogen treatments showing CO2-induced increases in insoluble condensed tannin concentrations in silver birch leaves, one could expect that this phenomenon would help to protect the trees' foliage from predation by voracious insect herbivores, as well as reduce methane emissions from ruminants that might nibble on birch-tree foliage produced in CO2-enriched air.