Modelling the effect of stand density management and environmental variables on Pinus patula wood properties

Thesis (PhDFor)--Stellenbosch University, 2020.ENGLISH ABSTRACT: Approximately one million hectares of Pinus patula has been planted worldwide, mainly in southern and eastern Africa and accounts for roughly half of the total softwood plantation area in South Africa. Improved growth rates and shorter rotation ages of these forest resources have caused an increase in the proportion of juvenile wood and a decrease in the stiffness of lumber, which often does not comply with the requirements for structural use. The growing space of trees has been shown to influence wood properties and may be a useful management intervention to improve stiffness properties. The financial b enefits of th ese sh ort ro tation systems also means that they are likely to persist into the future. An understanding of the properties of wood within the juvenile zone is therefore increasingly important. The objectives of this study were thus (1), to examine the effect of tree spacing expressed as stand density, particularly at stand establishment, on the stiffness of wood and important wood properties which are known to influence wood stiffness and (2), to study the development of wood in young trees as affected by selected environmental factors. The study was based on four experiments. In the first e xperiment wood increment cores were non-destructively removed from a total of 171 trees from four different planting density treatments from an 18-year old Pinus patula spacing trial. The wood density, microfibril a ngle (MFA) a nd r ing width were measured using Silviscan3 technology. In a second experiment, two commercial Pinus patula stands which were subjected to different stand density management regimes, were destructively sampled and 37 trees were processed into lumber of which the modulus of elasticity (MOE) and modulus of rupture (MOR) were measured. In the third experiment, 46 trees from a spacing trial was also destructively sampled and processed into lumber of which MOE and MOR were measured. The last experiment was a controlled greenhouse potting trial where the temperature, water supply and leaf nitrogen/potassium ratio (N/K) were measured and compared to the MFA and density of 168 trees over their first/second year of growth. Bending test results on lumber from trees from commercial stands showed that, compared to a number of stands with typical stand density regimes, only lumber processed from a higher stand density (1667 stems ha1) conformed to the requirements for structural use. The MOR values were however adequate across all management regimes. The MOE of lumber from a spacing trial showed that only the most closely spaced trees (2981 stems ha1) had lumber which conformed to requirements for structural grades. MFA, varied from roughly 30° at the pith to 7° at the bark, and along with wood density and knot characteristics, was able to explain over 70% of the variation in lumber MOE. The increase in lumber MOE with closer spacing was due to a combined effect of decreases and increases in both MFA and density respectively, and a restriction of the juvenile core. MFA and density were both significantly influenced by tree spacing, decreasing and increasing with closely spaced trees respectively. The differences in growth rate due to tree spacing could not fully capture the effects of spacing, which had an independent effect on wood properties. Environmental variables also had a significant effect on growth and cell properties of Pinus patula. The stiffness of the young saplings was significantly influenced by leaf mass and by water supply and N/K. Based on these results it seems as if wood stiffness is linked to foliar biomass. Stand density seems to have great potential as a management intervention to improve the cell and wood properties controlling the stiffness of South Africangrown Pinus patula lumber at final harvest. Low levels of water supply and N/K can also increase the stiffness of wood within early cambial ages.AFRIKAANSE OPSOMMING: Ongeveer een miljoen hektaar Pinus patula is reeds wêreldwyd aangeplant, hoofsaaklik in Suid- en Oos-Afrika, en beslaan die helfte van die sagtehoutplantasiearea in Suid-Afrika. Verbeterde groeikoerse en korter rotasietydperke van hierdie boshulpbronne het ’n toename in die hoeveelheid jeughout en ’n afname in die styfheid van hout veroorsaak, wat dikwels nie aan die vereistes vir strukturele gebruik voldoen nie. Daar is bewyse dat die groeiruimte van bome die houteienskappe beïnvloed en dit kan ’n nuttige bestuurs-intervensie wees om styfheidseienskappe te verbeter. Die finansiële v oordele van h ierdie k ort r otasiestelsels b eteken o ok dat dit waarskynlik in die toekoms sal voortduur. ’n Goeie begrip van die eienskappe van hout in die jeugsone word dus toenemend belangrik. Die doelstellings van hierdie studie was dus (1) om die effek van boomspasiëring te ondersoek, veral by vak-vestiging, op die styfheid van hout en belangrike houteienskappe met ’n invloed op houtstyfheid en (2), om die ontwikkeling van hout in jong bome te bestudeer, wat beïnvloed word deur geselekteerde omgewingsfaktore. Die studie is gebaseer op vier eksperimente. In die eerste eksperiment is inkrementboorsels uit ’n totaal van 171 bome verwyder uit vier verskillende plantdigtheidsbehandelings in ’n 18-jarige Pinus patula-spasiëringsproef. Die houtdigtheid, mikrofibrilhoek ( MFA) e n r ingwydte i s m et S ilviscan3-tegnologie g emeet. I n ’n tweede eksperiment is twee kommersiële Pinus patula-vakke, wat aan verskillende vakdigtheid behandelings onderwerp is, vernietigend bemonster en 37 bome verwerk tot hout waarvan die modulus van elastisiteit (MOE) en die modulus van die skeuring (MOR) gemeet is. In die derde eksperiment is 46 bome uit ’n spasiëringproef ook vernietigend bemonster en verwerk tot hout waarvan MOE en MOR gemeet is. Die laaste eksperiment was ’n beheerde kweekhuisproef waar die temperatuur, watertoevoer en die blaar stikstof/kalium verhouding (N/K) gemeet is en vergelyk word met die MFA en digtheid van 168 bome gedurende hul eerste/tweede jaar van groei. Die resultate van die buigtoets op hout van bome van kommersiële vakke het getoon dat, in vergelyking met ’n aantal vakke met tipiese vak-digtheidstelsels, slegs hout wat verwerk is van ’n hoër vakdigtheid aan die vereistes vir strukturele gebruik voldoen. Die MOR waardes was egter voldoende in alle bestuursregimes. Die MOE van hout uit ’n spasiëringsproef het getoon dat slegs die bome met die kleinste afstand tussen bome aan die vereistes vir strukturele gebruik voldoen. MFA, het gewissel van ongeveer 30° by die pit tot 7° by die bas, en tesame met die houtdigtheid en die kwas-kenmerke, kon meer as 70% van die variasie in die MOE van hout verklaar word. Die toename in die MOE van hout met ’n nouer spasiëring was te wyte aan ’n gesamentlike effek van afnames en toenames in onderskeidelik MFA en digtheid, en ’n beperking van die jeugkern. MFA en digtheid is beide aansienlik beïnvloed deur die boomafstand, neem af en neem toe met kleiner spasiëring, onderskeidelik. Die verskille in groeitempo as gevolg van boomafstand kon nie die gevolge van die spasiëring ten volle vasvang nie, wat ’n onafhanklike uitwerking op die houteienskappe gehad het. Omgewingsveranderlikes het ook ’n beduidende effek op die groei en seleienskappe van Pinus patula gehad. Die styfheid van die jong boompies was beduidend beïnvloed deur blaarmassa en die watervoorsiening en N/K. Op grond van hierdie resultate lyk dit asof blaarfunksie die houtstyfheid beïnvloed. Vakdigtheid blyk groot potensiaal te hê as bestuursintervensie om die sel- en houteienskappe te verbeter wat die styfheid van die Suid-Afrikaans-gekweekte Pinus patula-hout tydens die finale oes beheer. Lae vlakke van watervoorsiening en N/K kan ook die styfheid van hout binne die vroeë kambiaaltydperk verhoog.Doctora