Optimising the yield of Douglas-fir with an appropriate thinning regimeEuropean Journal of Forest Research

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Authors
Jean-Philippe Schütz, Peter Lukas Ammann, Andreas Zingg
Year
2015
DOI
10.1007/s10342-015-0865-3
Subject
Forestry / Plant Science

Text

ORIGINAL PAPER

Optimising the yield of Douglas-fir with an appropriate thinning regime

Jean-Philippe Schu¨tz • Peter Lukas Ammann •

Andreas Zingg

Received: 16 October 2014 / Revised: 12 December 2014 / Accepted: 12 January 2015 / Published online: 28 January 2015  Springer-Verlag Berlin Heidelberg 2015

Abstract Aim of this study was to determine the longterm effects of thinning regime with different intensities on the stand and tree social classes development, particularly regarding the old question of compensative effect between stocking and girth improvement using the example of

Douglas-fir. Data provide from a thinning experiment in one site of Swiss Central Plateau, from age 11 until 41 years containing six treatment variants (three planting densities and two level of stand density) repeatedly measured six times, as well as from seven permanent yield plots times series aged between 17 and 127 years. We present as well the real observed characteristics in term of stem numbers distribution, diameter increment per d.b.h. categories, as results from simulation over the whole production time until 105 years with an appropriate growth simulator, in terms of net value increment (MAIv). Our results show that the effect of thinning works mainly on the social medium categories, in terms of number and girth improvement. Social dominant trees are more or less untouched by thinning, demonstrating that there are in every stand a quite important number of self-dominating trees which do not need particular silvicultural help to maintain their dominant status in the upper storey. The expected compensation between stem number reduction and girth improvement shows that the remaining stand density is determinant on the net value level. The so-called mass effect seems really more effective. The best way to improve a stand value appears to be artificial pruning.

Because a thinning regime should not consider only the economic productivity but take in account risk, stability and resilience and particularly the way of renewal, we suggest a way for optimising the thinning regime combining effect of biological rationalisation and risks.

Keywords Optimal thinning  Yield productivity 

Rotation  Douglas-fir  Pruning  Plenterring

Introduction

Regularly applied thinning is one of the most common silvicultural interventions, at least in temperate Central

Europe. Historically, thinning was first conceived as the advance harvesting of trees which would otherwise have been likely to die out because of overcrowding. This led, particularly in German silviculture to so-called thinning from below concepts, where essentially socially low individual trees are removed. Another thinning concept, top thinning, where taller trees are removed, coupled with vigourous interventions, was proposed in the wake of the

Reventlow (1801) trials, particularly for broadleaved tree species, like beech, and has been practised since then by the French silviculture school (Broillard 1901). Thus the way to apply thinning has long varied in terms of both the kind and degree of intervention. Some favour early heavy thinning (Bastien 1997; Wilhelm and Rieger 2013), whereas others prefer moderate thinning to maintain enough whole stand biomass (Utschig 2002) and use a soCommunicated by Aaron R Weiskittel.

J.-P. Schu¨tz (&)

Department Environmental Systems Science, ETZ Zu¨rich,

Bru¨gglia¨cker 37, 8050 Zurich, Switzerland e-mail: jph.s@bluewin.ch

P. L. Ammann

Swiss Silviculture Competence Center, 3250 Lyss, Switzerland

A. Zingg

Swiss Federal Research Institute WSL, 8093 Birmensdorf,

Switzerland 123

Eur J Forest Res (2015) 134:469–480

DOI 10.1007/s10342-015-0865-3 called mass effect. The dilemma of thinning is to find a balance between maintaining enough stumpage and creating added value by selecting the best timber and encouraging diameter growth by regulating the spacing between trees.

A paradigm change in thinning principles came with

Scha¨delin’s (1926, 1934) selective thinning proposal. Here thinning is considered as an intervention to improve stand development coupled with selective effects to enrich the yield by improving the quality. Thus thinning aims at ensuring the growth of the best trees. From a biological and silvicultural point of view, such nurturing interventions should be applied often and moderately, from an early stage on, in order to make use of the ability of young trees to adapt to more space. Such operations, e.g. in the early pole stage, are, however, very expensive even when performed mechanically. Whether such investment is profitable in the long run can only be assessed by comparing a model of the stand development with a growth simulator sensitive to spacing. As a reaction to increasing tending costs, ways of reducing these costs have been proposed based on so-called biological rationalisation (Schu¨tz 1999a, b, 2003, 2006;

Ammann 2004, 2013), where natural dynamic processes like social self-differentiation are promoted through very spatially specific (situative) operations in which competition is only controlled around the future crop trees. Such socalled situative selective thinning could be performed with very simple methods like girdling (Roth et al. 2001) or simple slant cuts of the competitors, where they are then left standing and not extracted. These methods should be used preferentially in the phase before commercial thinning is possible. They allow to substantially reduce costs about 5–10 times in comparison with the more usual motor-manual tending of the full area (Schu¨tz 2006).

Nowadays, it is wise to enlarge the thinning concept, taking into consideration its effects on stability, particularly during the trees’ sensitive phase of development when they can be damaged by lasting snow (Ivanov 2007) or by storms in the early phase of regeneration (Vanomsen 2006;

Knoke 1998). Thinning can also improve ecological amenities through allowing more light to penetrate the stand (Bailey and Tappeiner 1998). A thinning regime should not only consider the development of one cohort, but also of the following generation and the strategic objectives for a stand’s renewal, considering that clear cut is no more justifiable as a general silvicultural system.