The impact of repeated prescribed burning in semi-mature pine plantation forests of Mpumalanga on fuel loads, nutrient pools and stand productivity

Gresse, Lodewyk Christoffel (2016-03)

Thesis (MScFor)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Wild fires in South Africa destroy vast areas of plantations annually and a growing need exists to reduce the fire risk effectively, economically and sustainably. Periodic prescribed burning is viewed by many researchers and managers as the only cost effective method to reduce fire damage and risk, and as such is being implemented on a large scale in SA. This thesis documents the effects of repeated prescribed burning operations on fuel load reduction, tree damage and stand growth rate, as well as nutrient dynamics in the system. Five field trials in semi-mature P. elliottii and P. patula plantations were laid out in Northern Mpumalanga during January 2014 to determine the effect of under-canopy burning on fuel loads, nutrient pools and stand productivity. Each trial consisted initially out of three control, three first burn and three second burnt plots. Fuel reduction. Under-canopy burning treatments were of low to moderate intensity with an average predicted FLI ranging between 44 and 602 kW m-1. With burning, the forest floor (FF) was significantly reduced in all trials (6.3 t ha-1 and 12.9 t ha-1 in P. elliottii and P. patula trials per burning event, respectively). Small quantities of additional litterfall (that would counter the initial goal of FF reduction) were noted for a single month in 3 out of 5 trials, but the short duration of this effect meant that the cumulative effect was insignificant. Average litterfall rates per species were 5.52 and 8.82 t ha-1 yr-1 for P. elliottii and P. patula, respectively. In general, the highest litterfall rate occurred during the winter months and the lowest during the summer months. The goal of FF reduction to values around 10-15 t ha-1 can be achieved with two prescribed burns in many P. elliottii stands (with smaller FF’s), but it may require at least 4 burning events in some P. patula stands with FF loads of more than 50 t ha-1. A significant reduction in understorey vegetation was still evident 8 months after treatment implementation except for one trial but species abundance returned to pre-burn levels 14 months after treatment implementation with no changes in species composition. Effects on stand condition and growth. Crown damage was restricted to one trial whilst root combined with cambium damage occurred in two trials, a result attributed to the smouldering effect of the FF. Among burning treatments, only Non-significant dbh and BA increment differences were evident, 4 and 12 months after treatment implementation but a highly significant seasonal effect could be seen in all trials: dbh increment was 61% higher in summer than in winter. Nutrient dynamics. FF nutrient loss was directly related to FF consumption by fires, with N loss varying from 33 – 119 kg ha-1; P, K, Ca and Mg losses averaging 3.8, 12.6, 72.4 and 17.2 kg ha-1. An increase in foliar macro nutrient concentration of freshly fallen needles was observed, but only during the first month after treatment implementation. Most soil nutrients showed modest changes that were not significantly different from other burning treatments. Modest increases in soil pH was observed with one in 5 trials recording a significant increase in pH (KCl) after the second burn. Total N mostly decreased with increasing number of burning treatments, but this effect was only of a significant magnitude in one out of 5 trials. Exchangeable Mg showed a small increase in all trials after burning, but the magnitude of this effect was only significant in one out of 5 trials. In conclusion: The evidence collected across 5 replicated experiments showed that significant fuel reduction (in FF and understorey) is attainable with 2 to 4 low intensity, repeated prescribed burning events under pine tree canopies. There appears to be minimal damage to trees and no significant short term response in diameter increment after repeated prescribed burns. The effect on nutrient dynamics in the system is modest and non-significant in most cases.

AFRIKAANSE OPSOMMING: Bosbrande vernietig jaarliks groot plantasie areas in Suid-Afrika en daarom bestaan daar ʼn groeiende behoefte om die brandrisiko op ʼn effektiewe, ekonomiese en volhoubare wyse te verminder. Periodieke beheerde brande word deur baie navorsers en bestuurders geag as die enigste koste-effektiewe metode in die vermindering van vuur skade en brandstof ladings en daarom word dit grootskaals geïmplementeer in SA. Die tesis dokumenteer die effek van herhaalde voorgeskrewe brand operasies op brandstof vermindering, boom skade en opstandsgroei, as ook nutriënte dinamika in die sisteem. Vyf veldproewe is uitgelê in semi-volwasse P. elliottii en P. patula plantasies in die Noordelike gebied van Mpumalanga gedurende Januarie 2014 om die effek van onderboom-brande op brandstofladings te bepaal so ver dit nutriënte poele en opstandsproduktiwiteit betref. Die navorsingspersele het elk aanvanklik bestaan uit drie kontrole, drie eenmalig gebrande en drie tweemalig gebrande persele. Brandstoflading vermindering. Onder-boom brande het ʼn lae tot matige intensiteit gehad met ʼn voorspelde vuurfront intensiteit wat gewissel het tussen 44 en 602 kW m-1. Deur middel van beheerde brand is die bosvloer-ladings beduidend verminder in al die navorsingspersele (6.3 t ha-1 en 12.9 t ha-1 per enkele brand in P. elliottii en P. patula persele onderskeidelik). Klein addisionele verskille in naaldeval (wat die oorspronklike doelwit van bosvloer lading-vermindering sou teenwerk) is aangeteken gedurende die eerste maand na behandeling in 3 van die 5 persele, maar die kort tydsduur van hierdie tendens het beteken die kumulatiewe effek nie beduidend was nie. Die gemiddelde naaldeval tempo per spesie was 5.52 en 8.82 t ha-1 yr-1 vir P. elliottii en P. patula onderskeidelik. Daar is bykomend gevind dat die hoogste natuurlike naalde-val tempo gedurende die wintermaande voorkom en die laagste naalde-val in die somermaande. Die doelwit van brandstoflading vermindering na waardes tussen 10-15 t ha-1 kan behaal word in twee beheerde brande in meeste P. elliottii opstande (met kleinerige bosvloer ladings), maar minstens 4 brande mag benodig word in sekere P. patula opstande met aanvanklike bosvloer ladings groter as 50 t ha-1. ʼn Beduidende afname in kreupelbosgroei was merkbaar 8 maande na implementering van behandelings (met uitsondering van een perseel), maar die aantal spesies per behandeling het vinning teruggekeer na voor-brand hoeveelhede: teen 14 maande na behandeling was daar geen verskil meer bemerkbaar in spesie samestelling nie. Effek op boom kondisie en groei. Kroondakskade was beperk tot een navorsingsperseel en ʼn kombinasie van wortel en kambium skade het in twee persele voorgekom – ʼn resultaat toegeskryf aan die smeulende effek van die bosvloerbrand. Die verskille in dbh en basale oppervlak groei-tempo tussen behandelings was nie beduidend op tydstip 4 en 12 maande na implementering van behandelings nie, maar ʼn hoogs beduidende seisoenale effek is waargeneem in al die persele: die aanwas in dbh was 61% hoër in somer as winter. Nutriënte dinamika. Bosvloer-nutriënte inhoud was sterk gekorreleer met die afname in bosvloer lading, en die verlies aan N was tussen 33 en 119 kg ha-1; P, K, Ca en Mg verliese was gemiddeld 3.8, 12.6, 72.4 en 17.2 kg ha-1 oor al die persele. ʼn Toename in makro nutriënt-konsentrasies van vars – gevalde dennenaalde is gevind gedurende die eerste maand na behandeling. Die vlakke van nutriënte in die bogrond het meestal matige veranderings getoon wat nie beduidend verskil het van ander brand persele nie. Matige toenames is gevind in grond-pH, met ʼn beduidende toename in pH (KCl) merkbaar in slegs een van die 5 tweevoudig-gebrande persele. Totale N in die bogrond het meestal afgeneem met toenemende brand behandelings, maar die effek was sleg beduidend in een van die vyf persele. Uitruilbare Mg in die bogrond het ʼn klein toename getoon oor al die persele na brande, maar die omvang van die effek was slegs beduidend in een van die vyf persele. Ten slotte: Die bewyse versamel oor al 5 gerepliseerde eksperimente wys dat ʼn beduidende afname in brandstoflading (bosvloer en kreupelbos) haalbaar is met 2 tot 4 lae intensiteit, voorgeskrewe brande onder denneboom kroondakke. Daar blyk minimale skade op bome te wees en geen beduidende kort termyn reaksie in deursnee groei na herhaalde beheerde brande is gevind nie. Die effek op nutriënte dinamika in die sisteem is beskeie en nie-beduidend in meeste gevalle nie.

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