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

[摘要] ENGLISH ABSTRACT: The purpose of the study was to investigate the effect of mid rotation fertilizer applicationon leaf area index (LAI), basal area and volume increment in thinned Pinus radiata standson 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 ForestryCompany, chosen to reflect the two most common soil types and a water availabilitygradient in each soil type. A factorial combination of fertilizer treatments with three levelseach 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 availabilityfor each of the two common soil groups, forming a complete trial series. All replicationswere laid out in P. radiata stands that had received their mid-rotation thinning prior totreatment implementation.LAI, diameter at breast height and height measurements as well as foliar analysis weredetermined before the implementation of the study in 2008 and then subsequently atpredetermined intervals in 2009 and 2010. Leaf area index and stem volume incrementwere measured in order to evaluate the influence on growth efficiency. LAI was estimatedusing the gap fraction method with the use of a ceptometer. Volume increment wascalculated using diameter and height measurements and basal area was calculated bymeans of diameter measurements. The abovementioned growth responses were thenused 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 wateravailability 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 recordedin the second but not the first year. This might have been due to the fact that trees had tore-build their canopies after thinning before a basal area response could be obtained. Forthe variables where an analysis of total growth response over the two year period wasdone, basal area increment and volume increment significantly responded to theapplication of nitrogen but not to phosphorus. Growth efficiency was not significantlyinfluenced 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 yearone and year two.The best responses generally occurred as a result of the additive effects of N and P. Thegrowth response did not remain the same across the water availability classes. The wettersites tended to have greater responses than the drier sites. Although these are still earlyresults, the growth responses could be attributed to an increase in LAI. Nutrient analysisthrough vector analysis indicated that the additional N and P from fertilizer applicationwere taken up by the trees thereby resulting in greater LAI and increased stem woodproduction.

[发布机构] Stellenbosch University