Vorläufige Ergebnisse eines Pappel-Durchforstungs-Versuches in Südafrika
The present article deals with the response ofP. deltoides to thinning. In 1965 an experiment was laid out in an 8-year old poplar plantation, site class, 0.5 in the mist belt of Natal. The S% defined as the ratio mean distance between the trees mean height of the stand = 100 was used to express stand density and to characterize the degree of thinning. Three thinning degrees, designated as A, B and C, with an S% of 24,31 and 36 respectively, were tested in a completely randomized design wth four replications. The plot areas were 0.144 hectare and each plot was surrounded by a 5.4 m. wide buffer zone. In 1965, immediately after the thinning of the plots, the mean basal areas per hectare of the A, B and C plots were: A: 23.0 m2(= 100) B: 15.5 m2(= 66) C: 11.3 (= 49) The relationship between basal area at the beginning of the growing season and the basal area increment during the first growing season was expressed by the equation: basal area increment (m2/hect./annum) = -10.417+21.9258 (log basal area) - 8.3256 (log basal area)2 the multiple correlation coefficient being equal to 0.860. During the second growing season there was a weakly significant negative correlation between basal area and basal area increment. During the third year stand density and growth were not related. An analysis of the relationship between periodic current volume growth during the 3-year growth period and the mean stand density during this period, the latter defined as the average of the stand densities at the beginning and end of the period, revealed that basal area is more suitable to express stand density than S%. In the regression analysis, the residual variation with S% as explanatory variable was 8.3% of the total variation, as against 2.0% with mean basal area as explanatory variable. There was a linear relationship between basal area and periodic current volume increment. For a mean basal area during the growth period of 28.0 m2 per hectare, the annual volume increment was 49 m3, for a basal area of 14 m2, it was 35.5% below this maximum. During the third growing season the A-plots reached the “natural stocking density”, defined as the maximum basal area which the stand can carry on a certain site and at a given age. In the financial calculations, the indicating per cent was defined as: $$w = 100 \cdot \left[ {\sqrt[n]{{\frac{{2(A_{x + n} - A_x ) - n(O \cdot op)V}}{{Hk_x + Hk_{x + n} + 2B}} + 1 - 1}}} \right]$$ where: Ax, Ax+n = net-value of the stand at x, x+n years n = length of period in years V = capitalized annual general cost Hkx, Hkx+n = cost value of stand at x, x+n years B = market value of land p = rate of interest. The indicating per cent as calculated for each of the 12 plots and the following equation fitted: Ind.% = -2.25 + 1.967 (mean basal area) - 0.04498 (mean basal area)2 The optimum basal area therefore was 22 m2 per hectare, i.e. 71-73% of the natural stocking density.
Influence of tree parameters and stand density on diameter growth of Pinus radiata
SUMMARY This study deals with the growth distribution within the annual ring and with the influence of tree characteristics and stand density on diameter growth in two stands of P. radiata. An analysis of crown dimensions showed a curvilinear increase of crown length with DBH, a linear relationship between crown width and DBH and a slight decrease of the height above ground of the crown base with increasing DBH. The relative position of the point of the greatest crown width was not related to DBH. The South-Eastern winds, prevailing during summer, cause crown deformations. The smallest crown radius occurs on the South-Eastern and the largest radius on the North-Western side of the trees. Within the tree, diameter growth is directly related to crown radius, but for the same crown radius, the growth on the North-Western side of the stem exceeds that on the South-Eastern side. This might be explained in terms of a reduced photosynthetic output per unit leaf surface on the windward side or alternatively in te...
The Intuitive Physics of the Equilibrium of the Lever and of the Hydraulic Pressures: Implications for the Teaching of Elementary Physics.
The research field of intuitive physics focuses on discrepancies between 
theoretical and intuitive physical knowledge. Consideration of these 
discrepancies can help in the teaching of elementary physics. However, 
evidence shows that theoretical and intuitive physical knowledge may also 
be congruent. Physics teaching could further benefit from understanding the 
reasons for this congruence. The present study explored these reasons by 
investigating the intuitive physics of the equilibrium of the lever and of the 
hydraulic pressures. It was found that the intuitive-physics law of the lever 
was multiplicative for all participants while the intuitive-physics law of the 
hydraulic pressures differed among participants. Since these laws are 
equally simple and the layman probably has had extensive experience with 
the lever and scarce or no experience with the hydraulic lift, these findings 
support the general hypothesis that physical laws and corresponding 
intuitive-physics laws are congruent when people have had experience with 
the respective phenomena. The results and theoretical considerations suggest 
two strategic principles for teaching elementary physics.