The effect of fire damage on the growth and survival mechanisms of selected native and commercial trees in South Africa

Odhiambo, Benedict Oithe (2015-04)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Surface fires are known to affect trees of different species differently, depending on the capacity of the bark to shield the cambium from heat. Tree bark characteristics differ among species and thus potentially influence the protective ability against cambium damage. The objectives of this study were to compare the protective role of bark against fire for selected indigenous and exotic species in the Western Cape, South Africa, and to investigate post-fire growth impacts following surface fire damage on Pinus radiata. In the first part of the study, trees were felled and billets of 25 cm height harvested from different heights along the trunk. Bark thickness, compass direction, stem diameter at breast height, bark moisture content and relative height of the sample in the stem were tested for their effect on heat insulation capacity of bark. Heating experiments were conducted at 400°C on the fresh billets with intact bark. Time to heat the cambium to lethal 60°C was determined. The second part of the study investigated the role of bark surface topology, bark density and bark chemical composition for its fire resistance. The same size billets were harvested from the lower trunk section of selected species. Surface topology was characterised by means of x-ray based computer tomography, density by moisture saturation method and bark chemical composition by thermo-gravimetrical analyses (TGA). The third part of the study investigated the impact of high intensity surface fires on growth of an 18 year old Pinus radiata plantation which was exposed to a ground fire 5 years prior to the analysis. Tree ring measurements were done on cores obtained by non-destructing coring method and various growth indices, based on yearly basal area increment (iBA) used to quantify growth response to the fire damage. Statistical analysis based on correlation, multi-model inference and multiple regression revealed no significant influence of compass direction and diameter at breast height. Heat resistance was mainly determined by bark thickness and to a lesser degree by moisture content. In several species relative height at the stem modulated the bark thickness effect. Higher up the stem bark of the same bark thickness offered less protection against heat. The results also suggest that in particular bark topology plays a role, while the correlations with bark density and chemical composition could not be secured statistically. A main finding was that fissures in the bark play a significant role. A regression model showed a significant influence of fissure width, fissure frequency and the minimum bark thickness to the cambium, which is a function of fissure depth. The results show that structural bark parameters are a necessary addition to explain heat resistance of bark. Statistical analysis employing one-way Anova and incorporating Tamhane’s T2 Post Hoc test revealed significant growth reductions following high intensity surface fire damage on Pinus radiata in the fire year with the impact being passed on to the following year. The recovery phase extended a two year period. During this time the trees showed increased diameter growth probably due to increased water availability.

AFRIKAANSE OPSOMMING: Dit is bekend dat oppervlakvure in bos-ekostelsels verskillende boomspesies verskillend affekteer, afhangende van die vermoë van bas om die kambium van hitte te beskerm. Baseienskappe verskil tussen spesies en het dus 'n potensiële invloed op die beskermende vermoë teen kambiumskade. Die doelwit van hierdie studie was om die beskermende rol van bas teen vuur te vergelyk tussen inheemse en uitheemse spesies in Wes-Kaapland, Suid Afrika asook om die na-vuur impak op groei te ondersoek met brandskade aan Pinus radiata. In die eerste deel van die studie is bome geoes en stompe van 25 cm lengte van verskillende hoogtes verwyder. Basdikte, kompasrigting, stompdiameter by borshoogte, basvoggehalte en die relatiewe hoogte van die stomp in die stam is getoets vir hul invloed op hitte-isolasiekapasiteit van bas. Verhittingseksperimente is gedoen teen 400 °C op die vars stompe wat steeds bas opgehad het. Die tyd om die bas tot by 'n skadelike 60 °C te verhit is bepaal. Die tweede deel van die studie het die rol van basoppervlaktopologie, basdigtheid, en die bas chemiese samestelling ondersoek in vuurweerstand. Dieselfde grootte stompe is geoes van die laer dele van die stam van uitgesoekte spesies. Oppervlaktopologie is bepaal deur middel van X-straal rekenaartomografie, digtheid deur die versadigingsvoggehaltemetode, en chemiese samestelling deur termo-gravimetriese analise (TGA). Die derde deel van die studie het die impak van hoë intensiteit oppervlakvure op groei van 18-jaar oue Pinus radiata ondersoek. Jaarringmetings is gedoen op inkrementboorsels wat nie-destruktief bekom is en verskeie groei-indekse, gebaseer op jaarlikse basale oppervlak aanwas, is gebruik om die groeireaksie op brandskade te kwantifiseer. Statistiese analise gebaseer op korrelasie, multi-model inferensie, en veelvuldige regressie het gewys dat kompasrigting en deursnee op borshoogte nie 'n beduidende invloed gehad het nie. Hitteweerstand was hoofsaaklik bepaal deur basdikte, en in 'n mindere mate basvoggehalte. By verskeie spesies het die relatiewe hoogte die basdikte-effek gemoduleer. Hoër in die stam het dieselfde dikte bas minder beskerming gebied as bas van laer in die stam. Die resultate impliseer dat basoppervlaktopologie ook 'n rol speel in hitteweerstand terwyl basdigtheid en chemiese samestelling nie 'n statisties beduidende rol gespeel het nie. 'n Belangrike bevinding was dat gleuwe of openinge in die bas 'n beduidende rol speel. 'n Regressiemodel wys dat 'n beduidende invloed deur gleufwydte, gleuffrekwensie en die minimum basdikte na die kambium. Die resultate wys dat strukturele basparameters 'n belangrike bykomende rol speel om hitteweerstand van bas te verduidelik. Die statistiese analise waar eenrigting ANOVA met Tamhane se T2 Post Hoc toets gebruik is toon dat ‘n beduidende groeivermindering teweeg gebring is in die Pinus radiata as gevolg van skade veroorsaak deur hoë intensiteit oppervlakvure waarvan die impak eers in die jaar na die vuur sigbaar was. Die herstelfase het oor twee jaar gestrek. Gedurende hierdie tyd het die bome 'n toename in deursneegroei getoon, waarskynklik as gevolg van verhoogde waterbeskikbaarheid.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96924
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