Download Full PDF - 28.09 MB - The Society of Irish Foresters
Download Full PDF - 28.09 MB - The Society of Irish Foresters Download Full PDF - 28.09 MB - The Society of Irish Foresters
Crop Structure and Productivity for Unthinned Sitka Spruce 45closely related to volume productivity. Finally it was necessary to seeif the main remaining crop variable likely to influence production,i.e. stems per ha. (S) might improve volume/height and basal area/ht.relationships. Some comparisons were made.To see which of the two productivity categories, basal area ordominant diameter, was more closely related to volume at any topheight, the residuals of the Y estimates from observed values inequation (1) were compared with those in (3) and (4). (1) and (3)volume-basal area, compared best.To determine the best relationship of volume to basal area, to topdiameter and to top height in terms of more than one variable aregression of volume on basal area was first calculated giving thefollowing equation:(5) Vs= -833.8008+54.0466-1.9267G2+.0074G3 (F=167)Basal area was then combined with Rd and G. The best fit was themultiple linear equation:(6) Vs= -46.05122+1.5803G+.3472 RdG (F=1069)Combination of G with Dd 2 were less promising the best of thesebeing:(7) V8= 132.1615+.0123 Dd2G-.3953Dd2 (F= 475)and with Dd and Rd.(8) V 8= 17.8757+ 11.4782Rd+.01693 Dd 2 (F=275).Number of stems (S) was combined with Rd, G and Dd but itsinclusion with these variables made no improvement on equations(6) to (8). Equations relating to V t to the above variables weresimilar to those mentioned above, differing slightly in intercepts andcoefficients.It can be seen then that a production class range of volumes, basalareas, and diameters for top height can be calculated on the basis ofa range of curves within which 95 % of the crops sampled would fall.Total stemwood volume is of the order of20m. 3 greater than volumeto 8 cm. top diameter over the range of top heights sampled. Basalarea is more closely related to volume than is dominant diameter andis a better index by which to determine productivity class. Bestcalculations for volume involve a linear combination of basal areaand top height. The variation in volume per hectare for a top heightranges from 160 m. 3 to about 200 m 3 .YIELD CLASSESConsiderable problems were encountered when trying to estimateyield classes or classes of potential maximum mean annual incrementfrom the stands sampled. The main limitation was the sparsity ofcrops at or near rotation age.From the data assembled, however, an attempt has been made to
46 Irish Forestryfind out if volume production (from fig. 2) could be equated with ayield class. This required some comparison with the Forest ManagementTables of Hamilton and Christie (1971).The first problem was to assemble plots in order of their volumeproductivity. This was done by classifying each plot according to aratio of volume (V s)/Age 2 or Mean annual increment/age. Thisratio lay between 0.1 and 1.2 in plots sampled. An arbitrary classificationof five categories according to this ratio was imposed i.c.0.1-0.2, 0.3-0.4, 0.5-0.6, etc. The height age curves were thencalculated from height analysed sampled trees (600 in all) for plotsin each of the height categories. Again polynomial curves were used.Though these curves are not very systematically distributed and arcweak at their higher values they do indicate a range of yield classesplaced in ascending order according to mean annual incrementcapacity at any given age. Only the two lower curves are continuedto anything near rotation age, so deductions are tentative (Fig. 3).Equation (1) would suggest a range of mean annual increments by'Yield Classes' as shown in Table 4.TABLE 4-Approximate Yield Classes (M3/ha/An) for unthinned Sitka spruce,compared with Forest Management Table (1971) ClassesAge yrsM3/Ha./Annum10 7 5 ( 4)* -20 16 14 (14) 12 (10)30 I -- -40-- -IY.c. (24) (20)*(4) Forest management table classes (1971).- -9 ( 7) 5 ( 4)10 (12) 8 ( 6)14 (15) 9 (10)(16) (12)On this basis the 5 classes can be approximated to intervals of4m 3 /ha/an. when compared with those of Hamilton and Christie.Again deductions for older classes are only as good as the volume!height curve at these points.OTHER COMPARISONSAs all the crops dealt with were un thinned it was feIt desirabletc investigate natural mortality. Mortality expressed as a percentageof the number of all stems is up to 3 times that expressed as apercentage of basal area. Age and number of stems are the mostimportant influencing factors; top height to a lesser degree. Therelationship of % basal area dead against top height is shown onfig. 4. Mortality increases beyond 14m. top height. The highest
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Crop Structure and Productivity for Unthinned Sitka Spruce 45closely related to volume productivity. Finally it was necessary to seeif the main remaining crop variable likely to influence production,i.e. stems per ha. (S) might improve volume/height and basal area/ht.relationships. Some comparisons were made.To see which <strong>of</strong> the two productivity categories, basal area ordominant diameter, was more closely related to volume at any topheight, the residuals <strong>of</strong> the Y estimates from observed values inequation (1) were compared with those in (3) and (4). (1) and (3)volume-basal area, compared best.To determine the best relationship <strong>of</strong> volume to basal area, to topdiameter and to top height in terms <strong>of</strong> more than one variable aregression <strong>of</strong> volume on basal area was first calculated giving thefollowing equation:(5) Vs= -833.8008+54.0466-1.9267G2+.0074G3 (F=167)Basal area was then combined with Rd and G. <strong>The</strong> best fit was themultiple linear equation:(6) Vs= -46.05122+1.5803G+.3472 RdG (F=1069)Combination <strong>of</strong> G with Dd 2 were less promising the best <strong>of</strong> thesebeing:(7) V8= 132.1615+.0123 Dd2G-.3953Dd2 (F= 475)and with Dd and Rd.(8) V 8= 17.8757+ 11.4782Rd+.01693 Dd 2 (F=275).Number <strong>of</strong> stems (S) was combined with Rd, G and Dd but itsinclusion with these variables made no improvement on equations(6) to (8). Equations relating to V t to the above variables weresimilar to those mentioned above, differing slightly in intercepts andcoefficients.It can be seen then that a production class range <strong>of</strong> volumes, basalareas, and diameters for top height can be calculated on the basis <strong>of</strong>a range <strong>of</strong> curves within which 95 % <strong>of</strong> the crops sampled would fall.Total stemwood volume is <strong>of</strong> the order <strong>of</strong>20m. 3 greater than volumeto 8 cm. top diameter over the range <strong>of</strong> top heights sampled. Basalarea is more closely related to volume than is dominant diameter andis a better index by which to determine productivity class. Bestcalculations for volume involve a linear combination <strong>of</strong> basal areaand top height. <strong>The</strong> variation in volume per hectare for a top heightranges from 160 m. 3 to about 200 m 3 .YIELD CLASSESConsiderable problems were encountered when trying to estimateyield classes or classes <strong>of</strong> potential maximum mean annual incrementfrom the stands sampled. <strong>The</strong> main limitation was the sparsity <strong>of</strong>crops at or near rotation age.From the data assembled, however, an attempt has been made to
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