Growth responses to fertilizer application of thinned, mid-rotation Pinus radiata stands across a soil water availability gradient in the Boland area of the Western Cape

Thesis (MscFor)--Stellenbosch University, 2011.

ENGLISH ABSTRACT: The purpose of the study was to investigate the effect of mid rotation fertilizer application on leaf area index (LAI), basal area and volume increment in thinned Pinus radiata stands on the most common soils of the Boland region in the Western Cape. The study was conducted on a range of sites in the Boland region of MTO Forestry Company, chosen to reflect the two most common soil types and a water availability gradient in each soil type. A factorial combination of fertilizer treatments with three levels each for nitrogen (N) at 0, 100 and 200 kg ha-1 and phosphorus (P) at 0, 50 and 100 kg ha- 1 was used. This design was replicated four times across a gradient of water availability for each of the two common soil groups, forming a complete trial series. All replications were laid out in P. radiata stands that had received their mid-rotation thinning prior to treatment implementation. LAI, diameter at breast height and height measurements as well as foliar analysis were determined before the implementation of the study in 2008 and then subsequently at predetermined intervals in 2009 and 2010. Leaf area index and stem volume increment were measured in order to evaluate the influence on growth efficiency. LAI was estimated using the gap fraction method with the use of a ceptometer. Volume increment was calculated using diameter and height measurements and basal area was calculated by means of diameter measurements. The abovementioned growth responses were then used to determine the effect of increased nutrient availability on stand growth. There were no significant interactions detected between any of the factors, N, P and water availability class in their effect on LAI, basal area, volume increment and growth efficiency. LAI increment responded significantly to N and P in the first year but only to P in the second year after treatment. Significant basal area responses to N and P were recorded in the second but not the first year. This might have been due to the fact that trees had to re-build their canopies after thinning before a basal area response could be obtained. For the variables where an analysis of total growth response over the two year period was done, basal area increment and volume increment significantly responded to the application of nitrogen but not to phosphorus. Growth efficiency was not significantly influenced by either nitrogen or phosphorus over the full two year monitoring period. Water availability class consistently and significantly influenced basal area increment, volume increment and growth efficiency over the two year period as well as during year one and year two. The best responses generally occurred as a result of the additive effects of N and P. The growth response did not remain the same across the water availability classes. The wetter sites tended to have greater responses than the drier sites. Although these are still early results, the growth responses could be attributed to an increase in LAI. Nutrient analysis through vector analysis indicated that the additional N and P from fertilizer application were taken up by the trees thereby resulting in greater LAI and increased stem wood production.

AFRIKAANSE OPSOMMING: Die studie het ten doel gestel om die effek van mid-rotasie bemesting op blaar oppervlak indeks (BOI), basale oppervlakte- en volume aanwas te ondersoek in gedunde opstande van Pinus radiata op die mees algemene grondtipes van die Bolandstreek, Wes-Kaapland. Eksperimente is uitgelê oor 'n reeks van groeiplekke in die Bolandstreek wat gekies is om 'n water beskibaarheidsgradient te verteenwoordig oor elk van die twee mees algemene grondtipes. 'n Faktoriaal-kombinasie van kunsmisbehandelings met drie vlakke elk van stikstof [(N) teen 0, 100 en 200 kg ha-1] en fosfor [(P) teen 0, 50 en 100 kg ha-1] is toegedien. Hierdie ontwerp is vier maal herhaal oor 'n gradient van grondwater beskikbaarheid, oor elk van die twee mees algemene grondtipes, om sodoende 'n volledige eksperimentele reeks te vorm. Elke herhaling is uitgelê in 'n P. radiata opstand wat reeds 'n mid-rotasie dunning ondergaan het voor implementering van die kunsmis behandelings. Metings van BOI, deursnee op borshoogte, boomhoogte asook blaarmonsters is geneem voor implementering in 2008 en daarna met vooraf bepaalde tussenposes in 2009 en 2010. Die BOI en stam volume aanwas is bepaal om die effek van behandelings op groeieffektiwiteit te evalueer. Die gaping fraksie tegniek is gebruik om BOI te skat met behulp van 'n sonvlek septometer. Volume aanwas is bereken vanaf deursnee en hoogtemetings en basale oppervlak aanwas vanaf deursnee-metings. Metings van al bogenoemde groeireaksies is gebruik om die effek van verhoogde voedingstof beskikbaarheid op opstandsgroei te evalueer. Daar was geen betekenisvolle interaksies tussen enige van die faktore N, P of water beskikbaarheidsklas met betrekking tot reaksies op BOI, basale oppervlak- en volume aanwas of groei-effektiwiteit nie. Die BOI het betekenisvol gereageer op N en P in die eerste jaar, maar slegs op P in die tweede jaar na behandeling. Basale oppervlakte aanwas is betekenisvol verbeter deur N en P in die tweede jaar maar nie in die eerste jaar nie. Dit is waarskynlik as gevolg van die feit dat opstande eers hul kroondak moes herstel (na dunnings) voordat 'n reaksie in basale oppervlak verkry kon word. Vir die veranderlikes waar 'n analise van die groeireaksie oor die volle twee jaar moniteringsperiode gedoen is, het basale oppervlak- en volume aanwas betekenisvol gereageer op stikstof maar nie op fosfor nie. Groei-effektiwiteit is nie betekenisvol geaffekteer deur N of P oor die volle twee jaar moniteringsperiode nie. Water beskikbaarheidsklas het basale oppervlak en volume aanwas asook groei-effektiwiteit betekenisvol en voortdurend beïnvloed in die eerste en tweede jaar, asook gedurende die volle twee jaar moniteringsperiode. Die beste groeireaksie is oor die algemeen verkry waar N en P gesamentlik toegedien is en waar dus aanvullende reaksies verkry is. Groeireaksies het betekenisvol verskil na gelang van water beskikbaarheidsklas, met die grootste reaksie op die natste groeiplekke. Hoewel hierdie vroeë resultate is, kan ons die meganisme van die reaksie primêr toeskryf aan 'n toename in BOI. Vektor analise van blaar voedingstof vlakke het aangedui dat addisionele N en P na kunsmis toediening opgeneem is, wat die weg gebaan het vir 'n toename in BOI en verhoogde volume aanwas.

Masters