Paper Reviewed
Park, H.J., Lim, S.S., Yang, H.I., Lee, K.S., Park, S.I., Kwak, J.H., Kim, H.Y., Oh, S.W. and Choi, W.J. 2020. Co-elevated CO2 and temperature and changes water availability do not change litter quantity and quality of pine and oak. Agricultural and Forest Meteorology 280: 107795.

Writing as background for their work Park et al. (2020) discuss how leaf litter plays an important role in the cycling of carbon and nutrients in the world's forests. And yet, although much information has been gathered with respect to how changes in atmospheric CO2, temperature, and water supply impact those cycles, very few (if any) studies have examined the predicted co-occurring impact of these three abiotic factors. Thus, this team of nine Korean researchers set out to fill this knowledge gap.

The work was conducted in controlled-environment field chambers at Chonnam National University in Gwangju, South Korea. Over a period of two years Park et al. subjected two-year old pine (Pinus densiflora) and oak (Quercus variabilis) to a full-factorial combination of different CO2 (330 or 620 ppm), temperature (ambient and ambient +3°C) and soil water (low or high, corresponding to daily means of 0.109 and 0.164 cm³ H2O per cm³ of soil) regimes. At various times data were collected to ascertain the individual and combined effects of these variables on leaf litter production, chemistry and decomposition.

And what did the study reveal?

With respect to litter production, the authors write that "taking the effects of the individual and multiple environmental factors on litter production into consideration together, our results suggest that the projected increase in [CO2] may increase litter production for both pine and oak particularly when water availability is not limited. However, concomitant warming may hamper the elevated [CO2]-enhanced litter production, resulting in insignificant changes in litter production regardless of soil water availability."

With respect to litter chemistry, Park et al. report that "elevated [CO2] may increase lignin/N, but co-elevation of air temperature will negate such increase, leading to unchanged lignin/N under global warming with elevated [CO2] regardless of soil water availability." And with regard to litter decomposition, the researchers found little evidence of change in CO2 emissions from soils amended with the litters in the combined-parameter treatment.

Consequently, in light of all their findings, Park et al. conclude that "the co-elevation of [CO2] and air temperature, which is the most probably expectation in the future, may not change the litter parameters for both pine and oak regardless of soil water availability due to the counterbalancing effect of elevated [CO2] and air temperature on dry matter accumulation and on the concentration of carbohydrate compounds and elements." The results of this work should be encouraging news to those concerned that CO2-induced global warming may increase litter decomposition rates and release large amounts of sequestered carbon back into the atmosphere, causing a warming-induced temperature feedback. The data presented by Park et al. do not support such a thesis.