Simulated transformation of even-aged Norway spruce stands to multi-layered forests: an experiment to explore the potential of tree size differentiation
Lars Dro ssler
1
Urban Nilsson
1
Lars Lundqvist
0
0
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences
, Umea,
Sweden
1
Southern Swedish Forest Research Centre, Swedish University of Agricultural Sciences
, Alnarp,
Sweden
The study investigated the possibility to transform normal young Norway spruce (Picea abies (L.) Karst) forests to develop more heterogeneous stand structures, aiming for multi-layered forest in the long run. On three sites in central and south Sweden, 60 per cent of stand basal area was removed by thinning mainly medium-sized trees and leaving the smallest and the largest trees. Over the next 50 years, future stand development was simulated using a single-tree growth model. Simulations were run with and without ingrowth. An additional ingrowth scenario after soil preparation was tested. Basal area was kept between 10 and 20 m2 ha21 during simulations. Projected stem wood production for the next 50 years was one-third lower compared with conventional thinning regimes. After 50 years, a multi-layered forest structure was indicated for the boreal sites in central Sweden, but not for the site in south Sweden.
Introduction
There is an increasing interest in alternatives to the clearfelling
system in Sweden (Anonymous, 2010, 2011). Different groups of
the society discuss and demand more heterogeneously structured
forest stands, for ecological and recreational purposes, and claim
that such forests are more natural. This is supported by Shorohova
et al. (2009) and Angelstam and Kuuluvainen (2004) who have
shown that pristine boreal forests were characterized by larger
variation in tree size than earlier assumed.
The most common practice to harvest mature forests in Sweden
is clearfelling, or the seed-tree method, whereas shelterwood and
other alternative harvest systems only account for 4 per cent
(Anonymous, 2009; Axelsson and Angelstam, 2011). Lundstro m
(2008) showed that 6 12 per cent of Swedish forests had ,80
per cent of standing volume in one single 20-year age class,
indicating presence of a second stand layer or a more heterogeneous
stand structure. The extent of single-tree selection forests, i.e.
uneven-aged forests with an inversely J-shaped diameter
distribution, was earlier estimated to be about one million hectare, of
which 600 000 ha had .150 m3 ha21 standing volume,
corresponding to 2 per cent of the total forest land in Sweden ( SOU,
1992).
Schu tz (2001b) stated that to transform even-aged stands into
multi-layered stands, (1) enough time should be allowed for the
process, and (2) a decision has to be made whether to attempt
transformation on the present generation of trees or on the
subsequent one. Depending on the stand structure and risk of wind
damage, a differentiation of tree sizes may be necessary as a
first step (OHara, 2001; Schu tz, 2001b). On sites where wind
throw is not deemed to be a problem, the current generation of
trees can be partially harvested and the stand regenerated,
whereas on sites prone to receive wind throw, the stand should
be thinned to promote single-tree stability, before beginning
transformation (Schu tz, 2001a).
The risk of wind damage depends mainly on wind speed, but
also on exposition, height of trees and soil type (Redde,
2002;Olofsson, 2006). Old, homogeneously structured spruce stands run a
high risk of storm damage, especially after thinning or creating
gaps to establish regeneration. Furthermore, dense stands with
poorly developed tree crowns are often supposed to be prone to
damage after thinning. For spruce forests in Germany, Spathelf
(1999) suggested that crown length should exceed one-third to
one-half of total tree height for transformation to be initiated
in the present tree generation. For dense spruce stands with
shorter crowns, and exposed to the dominating wind direction,
direct transformation was not recommended. In Sweden,
Wikberg and Lundmark (2008) reported wind damage levels of
20 30 per cent of standing volume during a 10- to 15-year
period after partial cuttings in mature pine spruce stands. In
spruce shelterwoods, damage levels varied from no damage up
to 70 per cent damage (Holgen and Ha nell, 2006; Holgen and
Sundstro m, 2007). In more heterogeneously structured, mature
coniferous forests, diameter limit cutting can be an option to
develop multi-layered stands (e.g. Duda, 2006; Dro ssler et al.,
2012).
Stands
Mesic
Table 1 Initial stand and site characteristics
Latitude
Numbers within parentheses is standard deviation.
aDominant height at the age of 100 years (Hagglund, 1973).
We recently established long-term field experiments in three
Norway spruce (Picea abies (L.) Karst.) stands at the age of first
thinning, with the objective to transform typical even-aged Norway
spruce stands to multi-layered stand structures. Adapted to
current Swedish mechanized management practice, the
transformation was initiated by very heavy, early thinnings. The aim of
the thinnings w (...truncated)