The effect of planting density on Pinus patula stem form, wood properties and lumber strength and stiffness

Erasmus, Justin (2016-03)

Thesis (MScFor)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Faster growth and reduced harvesting age are causing a reduction in the stiffness of structural lumber from South African-grown pine plantations. A number of studies have shown the positive effects of high planting densities as a tool to improve the mean modulus of elasticity (MOE) of structural lumber. The objective of this study was to investigate the effect of planting density on stem form, wood properties and the strength and stiffness of structural lumber of young Pinus Patula – the most important and extensively planted commercial softwood in South Africa. In the first part of this study, four different planting density treatments (403, 1097, 1808 and 2981 stems/ha) from an 18-year old P. patula spacing trial located in Mpumalanga, South Africa were sampled non-destructively. Stem slenderness, stem curvature, and the dynamic modulus of elasticity (MOEfak) were measured on 171 standing trees. Increment cores were removed from 40 trees for measurement of density, microfibril angle (MFA) and ring width using the Silviscan 3 technology. Planting density had a significant effect on stem curve with the lowest planting density having the highest mean stem curve. Planting density also had a highly significant effect on stem slenderness. The MOEfak increased greatly with increases in planting density. MFA was significantly influenced by both planting density and year ring number and the interaction between them. The mean MFA at similar ring numbers decreased significantly from the 403 stems/ha treatment toward the higher planting densities (1808 and 2981 stems/ha). Planting density had a limited effect on wood density. MFA seems to be the mechanism through which the tree compensates for the instability caused by a high slenderness ratio. Density, on the other hand, did not correlate with slenderness at all and was probably mostly influenced by environmental and growth factors. In the second part of this study, a total of 37 trees from two commercial compartments, planted at different densities, were processed into 71 logs, cant-sawed into lumber, and tested for static MOE, modulus of rupture (MOR), density, and warp. The first compartment was 18 years old, planted at 1334 stems/ha and thinned to 827 stems/ha at age 11. The second compartment was 17 years old, planted at 1667 stems/ha and was unthinned. Lumber from the 1667 stems/ha compartment had a mean MOE of 8967 MPa compared to a mean MOE of 7134 MPa for the 1334/827 stems/ha compartment. Based on this evidence and results from previous studies, it seems as if planting density has a large effect on the stiffness of young P. patula lumber and that planting density may be used as a practical management intervention to increase the stiffness of lumber.

AFRIKAANSE OPSOMMING: Vinnige groei en ‘n afname in rotasie-ouderdomme van SA dennehoutplantasies het ‘n afname in die styfheid van strukturele planke veroorsaak. Bestaande navorsing dui op ‘n positiewe verwantskap tussen hoë plantdigtheid en die modulus van elastisiteit (MOE) van hout. Die doelwitte van hierdie studie was om die effek van plantdigtheid te ondersoek op stam vorm, houteienskappe en die sterkte en styfheid van jong Pinus patula – die mees aangeplante naaldhoutspesie in SA. In die eerste deel van hierdie studie is vier verskillende plantdigtheidbehandelinge (403, 1097, 1808 en 2981 stamme/ha), vanaf ‘n 18 jaar oue P. patula spasiëringsproef, geleë in Mplumalanga, SA, nie-destruktief gemeet. Stamslankheid, stamkurwe en die dinamiese modulus van elastisiteit (MOEfak) is op 71 staande bome gemeet. Met behulp van die Silviscan 3 apparaat was inkrementboorsels verwyder vanaf 40 bome vir die meting van digtheid, mikrofibrilhoek (MFA) en jaarringwydte. Plantdigtheid het ‘n beduidende effek op stamkurwe getoon, met die hoogste gemiddelde stamkurwe gevind in die laagste plantdigtheidbehandeling. Plantdigtheid het ook ‘n hoogs beduidende effek op stamslankheid getoon met MOEfak wat baie toegeneem het met ʼn verhoogde plantdigtheid. Beide plantdigtheid en jaarring-nommer en die interaksie tussen die twee, het ‘n beduidende invloed gehad op MFA. Die gemiddelde MFA van soortgelyke jaaring-nommers het ‘n beduidende afname getoon van die 403 stamme/ha behandeling teenoor die hoër plantdigtheidbehandelings (1808 en 2981 stamme/ha). Plantdigtheid het ‘n klein invloed op houtdigtheid getoon. Dit wil blyk asof die MFA die meganisme is waardeur die boom kompenseer vir die onstabiliteit wat veroorsaak word deur ʼn hoër slankheid. In teenstelling, het digtheid nie ʼn korrelasie getoon met slankheid nie, maar was waarskynlik beïnvloed deur omgewings -en groeifaktore. In die tweede deel van hierdie studie was ‘n totaal van 37 bome vanaf twee kommersiële kompartemente, geplant teen verskillende digthede, verwerk in 71 saagblokke, gekantsaag tot planke en getoets vir statiese MOE, breekmodulus (MOR), digtheid en deformasie. Die eerste kompartemente was 18 jaar oud, geplant teen 1334 stamme/ha en uitgedun na 827 stamme/ha op 11-jarige ouderdom. Die tweede kompartement was 17 jaar oud, geplant teen 1667 stamme/ha en was nie gedun nie. Planke vanaf die 1667 stamme/ha kompartement het ʼn gemiddelde MOE van 8967 MPa gehad in teenstelling met ‘n gemiddelde MOE van 7134 MPa vir die 1334/827 stamme/ha kompartement. Dus, gebaseer op hierdie bewyse en uitslae van vorige studies, blyk dit dat plantdigtheid ‘n groot effek het op die styfheid van jong P. patula hout en dat plantdigtheid as ‘n praktiese bestuursingryping gebruik kan word om die styfheid van hout te verbeter

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