Fast recovery of Norway spruce trees after thinning from above on a drained peatland forest site

Continuous cover forestry (CCF) has been promoted as an environmentally sustainable option for drained peatlands. The CCF management has also been challenged due to potential lower tree growth compared to traditional even-aged management, especially with suppressed trees that are released during thinning.

Our objective was to quantify the time lag of stem growth response after CCF thinning with suppressed Norway spruce trees. We also tested if carbon uptake of these trees increases immediately after thinning.    

We used tree increment cores from suppressed Norway spruce trees to estimate the impact of thinning from above on the delay and quantity of diameter growth and photosynthesis. The study was conducted in Lettosuo experimental site, where part of the stand was thinned according to CCF principles, and the other part was left as a control. We used δ13C measurements to quantify how reduced between tree competition alters the photosynthetic rate and stomatal conductance ratio of sampled trees. Those sampled trees from the thinned area were then compared to those from the control area.

Our results show that there was an average delay of 2 years with a moderate increase in the diameter increment of the suppressed trees after thinning, while during years 3 and 4 after thinning, the increase in the diameter was much greater. In contrast to that delay, we found that the photosynthetic rate increased immediately after thinning, as shown by the instant 2.5‰ change in δ13C during the following summer.