Open access publications in the SLU publication database

Abstract

A heterogeneously structured forest stand with pine overstory and naturally regenerated
spruce and oak trees in different size classes was documented. The effect of target diameter
cutting on stand structure and growth was analyzed as a case-study. Both, systematic
sample plots and forest gaps were used to describe the stand structure after cutting. Target
diameter cutting in different treatments reduced the standing volume from ca. 320 to 180
m³/ha. Forest canopy gaps were created on more than 15% of the stand area. The seedling
number of advanced natural regeneration was low (less than 500-1000 individuals per ha).
Based on the advanced regeneration in gaps, three different scenarios for future ingrowth
into the tree layer were defined. The extreme minimum ingrowth scenario assumed about
10 cm annual height growth and rather high mortality reported in literature from other
experiments (resulting in one tree annually reaching 5 cm DBH during the next 50 years).
Two other scenarios assumed 20 and 30 cm annual height growth. While no mortality was
presumed within the latter scenario, moderate mortality rates (reported in literature) were
chosen for the intermediate scenario. The maximum scenario postulates ten trees per year
and ha reaching 5 cm DBH (equal to ingrowth reported from boreal single-tree selection
forests). The moderate scenario assumes four new trees per year and he. Additional
scenarios after soil preparation in gaps were used, defined on the base of shelterwood
experiments.
To describe the future basal area growth and continued target diameter cutting in the next
50 years, a single-tree growth model was applied, using stand age-independent estimations
of the age of single trees. Thereby, a mean annual increment of 0.53-0.64 m2/ha was
projected, similar to 5.6-6.8 m³/ha volume. Some errors to estimate the standing volume in
multi-layered stands were detected during the simulation process. Compared to an evenaged
spruce stand planted on the same site, the expected growth of the study stand during
the next 50 years was one third lower. In average, about 120 m3/ha standing volume was
removed in 20-25 years-cutting cycles. To continue without longer harvest intervals after the
50 years-simulation period, soil preparation seems necessary to achieve a sustainable
number of small trees. Beside timber production, profitability was also lower by selective
cutting. But, the important advantage of target diameter cutting can be more equally
distributed income over time, with investments costs that can be covered by profit from
timber harvest at the same time. A regular income of 17000-28000 SEK per ha every 20-25
years seems possible from today´s perspective.
An additional treatment with alternative target diameters to promote particular tree species
did not affect the amount of removals and the length of cutting intervals substantially. But
simulations with 5 cm reduced target diameters caused very heavy removals and 35-40 years
to reach 300 m³/ha standing volume again. The study includes discussions of tree species
composition and development as well as a sensitivity analysis of the applied growth model.