Changes in decomposition rates and Collembola densities during the forestry cycle in conifer plantations.
Conifers have a tendency to produce detrimental changes in the soil that suppress decomposition. It is also possible that clear-felling and the short plantation rotation (40-50 years), usual in Britain, will eventually cause nutrient depletion. Decomposition rates are important in determining the level of nutrients available, to both the crop and the soil invertebrate community, and this study has compared them at three stages during the plantation cycle of a coniferous forest (mainly Picea sitchensis) in Hamsterley Forest in north-eastern England. Cotton cloth buried in the soil of open areas decomposed at twice the rate of cloth buried in the soil of closed canopy plantations. The rate of decomposition in the soil layer 0-80 mm from the surface was 1.5 times that in the 81-160 mm layer. In young (9-18 yr) plantations, the decomposition rate of cloth decreased with an increase in the age of the plantation. It is suggested that the decrease in decomposition rate with plantation age is associated with increased shading and a decrease in the amplitude of the diurnal temperature cycle. Springtail (Collembola) densities were high in spring on an open area and decreased in summer. The opposite occurred in the closed canopy plantation. Drying on the exposed open area is suggested as the cause of the decreased summer densities. On the site where Collembola densities were monitored, decomposition rates were higher in the upper soil layer of the closed canopy plantation than in an open area. It is probable that this was due to the upper layers on the exposed area drying, rather than to the high Collembola densities checking fungal decomposition. The nutrient release that accompanies decomposition has potential importance for the rate of growth of the conifer crop. This preliminary study suggests that the decomposition rate is highest in the years immediately after felling, probably favouring the establishment of the next rotation. This study has not detected suppression of decomposition on open or clear-felled areas in Hamsterley Forest, suggesting that present planting and harvesting methods are appropriate and should continue. However, further investigation of the interactions between fungi, meso- and macrofauna is needed to understand both nutrient cycling and the invertebrate community structure in plantation forestry.