Potential of above- and below-ground coarse woody debris as a carbon sink in managed and unmanaged forests

With respect to climate warming, carbon (C) sequestration is of important public and political interest. Forests represent important terrestrial C sinks. Their management can have direct and indirect influence on forest characteristics, including to some extent C sequestration. One direct effect of forest management is an increase in the stock of coarse woody debris (CWD). CWD represents a short- to middle term C sink that is of particular importance in natural and old-growth forests. Its impact on the soil organic carbon (SOC) stock is largely unknown. To investigate the impact of management, a case study was conducted in three adjacent managed and unmanaged forests with similar geological and micrometeorological conditions as well as similar tree species composition. In each forest, the C pools of the forest floor, the mineral soil and the CWD as well as their turnover times (TTs) or disappearance times (in case of CWD) were investigated. The unmanaged forests were withdrawn from management 40-100 years ago. The dominant tree species of temperate forests, European beech, Sessile oak and Norway spruce were considered. The experimental set-up permits to estimate how the C pools of a forest evolve within decades following its withdrawal from forest management. In each forest, the above-ground CWD stocks were inventoried. The volume and the decay class of each CWD piece was determined on an area of 1 ha. For each decay class, a representative number of samples of logs was sampled to measure wood density and C concentration. In addition, radiocarbon analysis and dendrochronological cross-dating were used to determine the time of tree death for CWD logs. From these data, disappearance times were calculated for the three tree species. In the unmanaged forests, the C stocks in the CWD accumulated to 10 Mg ha-1 in the spruce forest and to 24 – 30 Mg C ha-1 in the beech-oak forests. As such, the C stock in the CWD was 2 to 6 times greater in the unmanaged forests than in the managed forests where the C stocks in the CWD were around 5 Mg C ha-1 at all study sites. Average disappearance times of 30 and 70 years were calculated for beech and spruce CWD respectively. Oak CWD yielded a great variability of time since tree death at similar C densities of individual CWD pieces. The calculation of a decay function was thus not possible. However, the time since tree death of the dated oak CWD pieces indicated that oak CWD has the potential to remain in forests for more than 70 years. In addition to the field study, CWD samples of the three tree species and of three decay classes were incubated in a laboratory experiment under controlled conditions for a period of 380 days. In regular intervals, the CO2 production was measured and a leachate was produced to estimate the C fluxes from CWD in the gaseous and in the liquid phase. The yearly C loss was specific to the tree species and decay class. Beech CWD had the greatest C loss followed by oak and by spruce CWD. C loss generally increased with decay class for all tree species. The CO2 release represented the most important pathway of C loss, however, dissolved organic C (DOC) contributed between 1 and 25% of the total C loss. The DOC production was most important for oak CWD and for heavily decayed wood of all tree species. The C stock of below-ground CWD was estimated by uncovering stumps of known age. For each tree species, five stumps were sampled in their entity for two or three different times of tree death. Total mass and volume as well as C concentration of a representative number of sub-samples were measured. The great differences in volume, wood density and C concentration expressed the variability in the properties of below-ground CWD. For this reason, a calculation of the below-ground CWD mass in relation to the stump diameter was not possible. The number of stumps and snags was multiplied by an average C mass per stump to calculate below-ground CWD stocks. As a result of regular thinning and felling of trees and the resulting higher number of stumps, the below-ground CWD stocks in the managed forests were greater than in the unmanaged forests. The C stocks in the below-ground CWD ranged from 0.3 to 1.4 Mg C ha-1 in the managed and from 0.1 to 0.4 Mg C ha-1 in the unmanaged forest in one of the beech-oak forest. This corresponded to 16 % of the total CWD C stock in the managed forest and to 1 % of the total CWD C stock in the unmanaged forest. Soil samples were taken at 30 points on a regular raster plot on an area of 2 ha. The forest floor samples were separated by horizon in the field. The mineral soil was sampled up to a soil depth of 100 cm and separated by depth into 4 sub-samples. Of each sample, the organic C concentration was measured. Density fractionation in three fractions (<1.6 g cm-3, 1.6-2.0 g cm-3, >2 g cm-3) was carried out for one mixed sample of each soil depth. Radiocarbon signatures were measured of the mixed samples as well as of each fraction. The SOC stocks showed greater differences among the study sites than between the management forms. The SOC stocks ranged between 4.3 and 15.9 Mg C ha-1 in the forest floor and between 50 – 260 Mg C ha-1 in the mineral soil down to a depth of 1 m. At all study sites, the radiocarbon signatures of the Oe horizon indicated a shorter TT of SOC in the unmanaged than in the managed forests. The difference is attributed to a change in the decomposing community induced by the enhanced CWD stocks. Differences between managed and unmanaged forests in TT of SOC in the Oa horizon and the bulk mineral soil were not consistent across all study sites. Either potential management influences are overshadowed by other effects or the time since withdrawal from management is not sufficient to result in significant changes. Of the density fractions, only the light fraction <1.6 g cm-3 exhibited consistent differences across soil depths between management forms. No consistent patterns were found for the denser fractions. In dependence of tree species, CWD has the potential to substantially contribute to the C stocks of forest ecosystems. A withdrawal from management results in a significant increase in the CWD stocks within decades. However, SOC stocks did not increase as a result of enhanced CWD C stocks. A potentially greater input of C from CWD to the forest floor was compensated by a shorter TT of SOC in the Oe horizon. CWD and forest management had no effect on the SOC stocks or TT of the mineral soil. Most C from CWD is probably lost to the atmosphere as CO2 before it reaches the soil.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Krüger, Inken

Contributors dc:contributor

• Borken, Werner

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1686/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1686