Search

International audience; AbstractKey messageUsing the three characteristic points of a forest stand, dg (mean quadratic diameter), dmin(diameter of the smallest tree) anddmax (diameter of the largest tree), appears informative enough to determine the parameters of the whole diameter distribution and, hence, the standing volume, with an accuracy of 2–3%. This is related principally with a particular feature of the Weibull distribution function, and the empirical dependency of the main scale parameter α + β from the mean quadratic diameter (dg): This allows the prediction of the parameter β with an unexpectedly high likelihood. This feature could be used for growth modelling as well as inventory purposes, at least for monospecific and even-aged stands and, maybe more, because this feature is proper to the function itself.ContextOne of the most appealing applications of diameter distribution functions is to predict compliant stand diameters without needing to tally all stems, but in determining the function parameters only on the base of simple stand characteristics. This can be applied for yield model construction or inventory purposes.AimsThe aim of this paper is to compare different methods of estimating the Weibull distribution parameters, partly based on parameter recovery method (PRM). They use a remarkable, empiric property of the Weibull function. Their performances are assessed in comparison to real distributions from a wide database of permanent Swiss yield plots repeatedly measured (time series) for Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.).MethodsThe Weibull distribution offers the advantage of simple but reliable estimation procedures. One of these is the main (scale) parameter β being given at a remarkable point of the function free, i.e. independent from the shape parameter γ. Because dg lies very close to this point, it correlates empirically very tightly with this parameter and thus allows for a trustful simple estimation. We compare, with appropriate statistic tests, real distribution with such obtained with the usual maximum likelihood estimation (MLE) of the Weibull parameters and those obtained with these new procedures.ResultsThe results obtained from a set of 800 yield plots of regular spruce stands and 596 of beech in Switzerland illustrate the good performances of the two much simpler procedures. The accuracy of estimating the standing volume is about 1.4% for beech and 2.8% for spruce when the site index (SI) is known.ConclusionThe three considered characteristic stand attributes (dg, dmin and dmax) appear robust enough for determining the diameter distribution with a respectable accuracy. This is enough reason for a revival of the old, but very ingenious, method of angle count sampling of Walter Bitterlich (1947).

Aim of this study was to determine the long- term 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 mea- sured 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 pro- duction 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 com- bining effect of biological rationalisation and risks.

Our approach to thinning has changed over the last 50 years: why? (Essay) Thinning methods and principles have always fluctuated between an approach based on moderate interventions, aimed at maintaining a sufficient stocking level (the so-called “volume effect”) and the liberation of crowns of a limited number of trees. Nowadays Schädelin's concept of selective thinning focused on trees with high potential for the future, based on the efficient removal of those competitors which cause the most interference, is widely accepted. However, as regards the intensity and strength of thinning there are different suggestions in terms of number of trees to be removed and the extent of liberation of the crowns. Moreover, there is no consensus on the transition between the phase of compression in the young stand and the first thinnings, nor on the optimal final stand density to be aimed at. After the dramatic fall in income from forests, it was necessary to revisit the principles of tending young stands, as the costs were excessively high especially because of the high number of trees. It now appears more efficient to use the natural, and free, processes of social differentiation. Interventions founded on the so-called biological rationalisation make possible a significant reduction in costs. The results of the economic analysis of the various thinning regimes, measured by the net average increment in value, net of harvesting costs, show clearly the advantage of the volume effect and of maintaining sufficient stand density. However, it is also necessary to bear in mind stand stability, especially towards the end of the rotation period. Stand stability makes it possible to keep open, in both space and time, options for natural regeneration, which corresponds best to our goals of a multifunctional silviculture in the most efficient fashion.

International audience; The specific goals of this paper are the following:1. To determine, using beech-dominated pristine forests as anexample,themaindrivingforcesofgenerationrenewal intermsoflifecycledurationandspatialrenewalpatterns; 2. Toillustratecontrastinglightrequirementsduringrenewal by comparing pristine oak and beech forests; 3. To elucidate the lower limits of stocking and canopy openness for silvicultural renewal in small clusters based on pristine forest research; 4. Topresentanapproachforimplementingthesefindingsin a liberal and versatile forest management