The effects of climate change on fine root dynamics in a Norway spruce forest

Abstract

Because of methodological restrictions, exact quantifications of the carbon sink strength of fine root systems are not available for mature forests. However, for assessing the effects of climate change on root-derived carbon fluxes to the soil, this knowledge is urgently needed. In my thesis, I applied sequential soil coring and minirhizotron observations to investigate the impacts of experimental summer drought and winter frost on fine root dynamics of Norway spruce by replicated throughfall exclusion and snow removal experiments in Southeast Germany. In addition, I estimated fine root longevity distribution with root diameter, root C/N ratio and soil depth under natural conditions using sequential soil coring, minirhizotrons and radiocarbon analyses as three independent methods. I tested the hypotheses that (i) soil drought and soil frost increase fine root mortality leading to a higher input of root-born organic matter into the soil and that (ii) drought- and frost-induced fine root mortality is compensated by higher rates of fine root production. Excluding throughfall for a period of six weeks reduced average soil moisture from 20 to 12 vol. % in the upper mineral soil, while snow removal induced soil frost with temperatures down to -5.5 °C in the organic layer. Sequential coring showed that soil drought and frost increased fine root mortality in the organic layer of the studied spruce stand by 61 and 29 %, respectively. However, enhanced fine root production compensated for the root losses caused by water shortage and low temperatures. Minirhizotron observations revealed that drought stress was mainly restricted to the organic layer, while frost stress was similar in the organic layer and the upper 25 cm of the mineral soil. We calculated that experimental soil drought and frost led to additional carbon inputs to the soil of about 28 and 47 g m-2. Under natural conditions, the carbon age of fine roots increased with depth from 5 years in the organic layer to 13 years in 40-60 cm mineral soil. Similarly, the C/N ratios of fine root samples were lowest in the organic layer and increased with depth. Roots > 0.5 mm in diameter tended to live longer than roots < 0.5 mm in diameter. Furthermore, the results on mean root longevity were strongly influenced by the method of investigation. Radiocarbon analyses yielded with 5.4 years much higher estimates than sequential coring (0.9 years) and minirhizotron observations (0.7 years). I conclude that even periods of relatively mild drought and frost have considerable potential to increase fine root mortality and the associated input of root-derived carbon to the soil organic matter pool in temperate Norway spruce forests. In addition, I propose that sequential coring and minirhizotron observations are likely to underestimate fine root longevity, while radiocarbon analyses may lead to overestimations.