Project Code: MW-01-074
Sponsor:Millar Western Forest Products Ltd.

The study examines the physiological basis for stagnation and the growth response to release in high-density, fire-origin lodgepole pine stands by investigating the role of nitrogen in the physiology of photosynthesis and growth in suppressed lodgepole pine stands. This study has resulted in the development of a process model relating growth to physiological processes and efficiency of nitrogen use.

Study sites were located in stagnating lodgepole pine stands originating in 1956 burns in north-central Alberta. The Windfall burn, northwest of Whitecourt was thinned in May 1995. The sites located within the Gregg burn, south of Hinton were fertilized with 400 kg/ha N in fall 1998. Tree and physiological measurements were collected at each site following treatment.

It appears that the ability of individual trees to capture adequate amounts of nitrogen to support photosynthetic and growth activities is a key process, and the total nitrogen pool of the site is of less importance. Intraspecific competition within the stand, and the processes by which that competition is played out, are likely to be key determinants of stand productivity. The efficiency with which stands utilize nitrogen was found to be an important factor in nitrogen economy. Not only do the fastest growing trees have more nitrogen in their crowns, they also appear to use it more efficiently than the slower growing trees. Thinning treatments appear to be having the desired effect of reducing intraspecific competition for site nutrients and allowing greater needle mass to be supported on the remaining trees. Fertilization could provide similar responses by increasing site resources rather than reducing the number of individuals competing for those resources. In both treatments the amount of resources per individual tree increases and ultimately, needle mass and growth increase. A model relating net carbon assimilation for whole tree crowns, in treated and untreated stands of differing productivity, to physiological process, morphology and gross crown structure was developed using Microsoft Excel.