Thesis (PhD(For))--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Pinus patula is the most intensively planted conifer in the tropics and sub‐tropics. In South Africa
Pinus patula plantations are the main saw‐log resource for structural lumber production. Improved
intensive silvicultural practices and tree breeding have resulted in marked increases in the rate of
growth. To reap the financial benefits of the faster growth, plantation managers are more and more
inclined to reduce rotation ages, which inevitably results in the production of higher proportions of
juvenile wood at final harvest, and lumber which often does not meet the minimum requirements
for stiffness for structural lumber. Knowledge of the variation and the accurate prediction of the
mechanical properties of the timber of standing trees can have various benefits for growers and
processors of trees. It can be used for tree allocation to different processing facilities, for processing
production planning, and to assist tree breeders to screen and select for superior breeding material.
The objectives of this study were (1), to examine the within‐ and between‐tree variation in wood
properties of young South African grown Pinus patula trees known to have important impacts on the
suitability of sawn lumber for structural purposes and (2), to develop empirical prediction models for
the flexural lumber properties from standing Pinus patula, based on variables that could be assessed
non‐destructively from standing trees.
Sample material was obtained from 170 trees (16‐20 years old) established in 17 compartments
along the Mpumalanga escarpment of South Africa. A large number of variables which could be
obtained non‐destructively from the trees while they were still standing, were measured. The trees
were subsequently felled and two logs, 2.1 m in length, were extracted from each tree at two height
positions. The 340 logs were processed into 1402 pieces of lumber for further measurements and
destructive testing. Results showed that the mean modulus of elasticity measured on edge (MOEedge) was far below the
limits set for structural grade softwood timber in South Africa. All the desirable properties for
structural lumber improved with distance from the pith with the exception of the 5th percentile value
for modulus of rupture (MOR), which was higher at the pith than for the boards processed adjacent
to the pith. Boards processed from the lower part of the stem were superior in most of the
important properties compared to those higher up in the stem.
Separate multiple regression models for predicting the average dynamic MOE (MOEdyn) of individual
boards, trees and compartments were developed. The models managed to explain 68%, 60% and
95% of the variation in MOEdyn respectively. The models developed for MOR explained 40% and 42%
of variability at board and tree level respectively. At compartment level, 80% of the variation in the
5th percentile MOR value could be explained by the model. Sensitivity analyses showed that site
index at base age of 10 years, acoustic time‐of‐flight, wood density and ring width were the most
influential variables in the MOE models. The models indicated that tree slenderness during early
growth seems to play a major role in determining the dynamic MOE and MOR of lumber. This is in
agreement with Euler’s buckling theory and the bending stress theory.
Microfibril angle (MFA) and density were measured on radial strips taken from a sub‐sample of trees
with the Silviscan 3 technology. The mean microfibril angle per year ring in Pinus patula varied
between 7o and 29o. In general MFA decreased with distance from the pith and height above ground
level. A multiple regression model including microfibril angle, density and ring width explained 71%
of the variation in the dynamic MOE of boards. Sensitivity analysis on the model showed that
microfibril angle and density had roughly equal influences on predicting the MOEdyn of Pinus patula
boards. / AFRIKAANSE OPSOMMING: Pinus patula is die mees aangeplante naaldhoutspesie in die tropiese en sub‐tropiese areas van die
wêreld. Dit is die grootste bron van saagblokke vir die produksie van strukturele hout in SA.
Intensiewe boskultuurpraktyke en boomteling het gelei tot ‘n merkbare verhoging in die groeitempo
van die spesie. Plantasiebestuurders is gevolglik geneig om rotasie‐ouderdomme te verlaag, wat lei
tot ‘n groter persentasie jeughout wat nie aan die minimum styfheidvereistes van strukturele hout
voldoen nie. Kennis van die variasie en die akkurate voorspelling van die meganiese eienskappe van
staande bome kan voordele inhou vir beide die verbouers en verwerkers van bome. Dit kan
byvoorbeeld van hulp wees met die toewysing van bome aan verwerkingsfasiliteite, vir
produksiebeplanning, en vir ondersteuning met die keuse van teelmateriaal vir boomtelers.
Die doelwitte van hierdie studie was (1), om die binne– en tussenboomvariasie in die
houteienskappe, wat ‘n bepalende invloed het op die geskiktheid van jong Suid Afrikaanse Pinus
patula bome vir strukturele hout produksie, te ondersoek en (2), om empiriese modelle vir die
voorspelling van die buigeienskappe van planke te ontwikkel, gebaseer op veranderlikes wat niedestruktief
op staande Pinus patula bome ge‐evalueer is.
Monsters vir die studie is verkry vanaf 170 bome (16‐20 jaar oud), geplant in 17 vakke op die
Mpumalanga platorand van Suid Afrika. ‘n Groot aantal veranderlikes is nie‐destruktief gemeet op
die staande bome waarna die bome gevel is en twee saagblokke, 2.1m in lengte, is op twee hoogte
posisies uit elke boom verwyder. Die 340 blokke is verwerk tot 1402 planke vir verdere metings en
destruktiewe toetse.
Resultate het getoon dat die gemiddelde modulus van elastisiteit gemeet op die dwarskant
(MOEedge) aansienlik laer was as wat vereis word vir strukturelegraad hout in Suid Afrika. Al die
gewenste eienskappe het toegeneem met afstand vanaf die murg behalwe die 5de persentiel
breekmodulus (MOR), wat hoër was vir murgplanke as vir aangrensende planke. Planke afkomstig
van die laer dele van die stam het oor die algemeen beter eienskappe gehad as planke afkomstig van
die hoër dele. Veelvuldige regressiemodelle kon 68%, 60% en 95% van die variasie in die gemiddelde dinamiese
MOE (MOEdyn) op die vlak van onderskeidelik individuele planke, bome en vakke verklaar. Die
modelle vir MOR kon 40% en 42% van die variasie op onderskeidelik plank‐ en boomvlak verklaar.
Die model vir 5de persentiel MOR van vakke kon 80% van die variasie verklaar. ‘n
Sensitiwiteitsanalise het aangetoon dat groeiplekindeks op ouderdom 10, akoestiese vlugtyd,
digtheid en jaarringwydte die belangrikste veranderlikes was wat MOEdyn beïnvloed het. Die modelle
het aangetoon dat die slankheid van bome tydens vroeë groei vermoedelik ‘n belangrike invloed op
die MOEdyn en MOR van planke het. Dit is in ooreenstemming met Euler se knikteorie en die
buigsterkteteorie.
Die mikrofibrilhoek en digtheid van ‘n steekproef van die bome is gemeet met die Silviscan 3
apparaat. Die gemiddelde mikrofibrilhoek per jaarring het tussen 7 o en 29o varieer. Hierdie variasie
was hoofsaaklik afhanklik van boomhoogte en aantal jaarringe vanaf die murg. ‘n Veelvuldige
regressiemodel wat mikrofibrilhoek, digtheid en jaarringwydte insluit, kon 71% van die variasie in
MOEdyn verklaar. ‘n Sensitiwiteitsanalise op die model het aangetoon dat mikrofibrilhoek en digtheid
ongeveer ewe belangrik was wat betref hulle invloed op die voorspelde MOEdyn van Pinus patula
planke.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86246 |
| Date | 04 1900 |
| Creators | Wessels, Coenraad Brand |
| Contributors | Rypstra, T., Malan, F. S., Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 113 p. : ill. |
| Rights | Stellenbosch University |
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