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Effects of Sitka Alder Retention and Removal on the Growth of Young Lodgepole Pine in the Central Interior of British Columbia (EP 1185)
Sitka alder is a common shrub species in lodgepole pine forests in the interior of British Columbia. In regenerating stands, alder may increase in height and cover to a level where competition for light and soil resources and physical damage caused by whipping may negatively affect growth of the planted or naturally regenerated conifer crop species. However, interior forests are commonly N deficient and often respond positively following N fertilization. As an N-fixing species, Sitka alder can benefit site fertility through N additions, with accretion estimates from coastal BC ranging from 20 to 35 kg N/ha. Clear guidance is lacking on where the balance of advantage lies, that is at what alder density are the likely benefits of N addition offset by competition with crop trees for light and soil resources and by the possible detrimental effects on foliar nutrient balance.
In 1995, the BC Ministry of Forests, Research Branch and the Prince George Forest Region implemented a cooperative project to study the effects of different levels of Sitka alder retention of the growth of young, naturally regenerated lodgepole pine. The study site is located southwest of Prince George within the Stuart Dry Warm variant of the SBS biogeoclimatic zone. The previous mature forest was clear-cut harvested in 1987 and subsequently naturally regenerated to a mixture of lodgepole pine and Sitka alder. At the time of trial establishment, the density of the 7-year-old lodgepole pine was approximately 10,500 trees per hectare, while the alder density averaged 4,100 clumps per hectare, representing a mean alder cover of 51%.
To determine the effects of different densities of Sitka alder retention on the growth and development of naturally regenerated pine and alder.
The experiment compares the growth of alder and pine at four alder retention levels: (1) pine with no alder (i.e., complete eradication of alder clumps), (2) pine and low alder (500 clumps per hectare retained), (3) pine and moderate alder (1000 clumps per hectare retained), and (4) pine and high alder (2000 clumps per hectare retained). A total of twelve 0.08-ha rectangular treatment plots were established. Each of the four treatments was randomly assigned to three of the treatment plots. The alder clumps within each treatment plot were thinned to the specified density at the time of plot establishment. The lodgepole pine within each treatment plot was manually thinned to a density of 1000 stems per hectare.
A 0.036-ha assessment plot was established within each treatment plot to monitor the growth and development of retained lodgepole pine and alder clumps. Each assessment plot has 36 lodgepole pine trees and a number of alder clumps proportional to the specified alder retention density (i.e., 18, 36, and 78 clumps for the 500, 1000, and 2000 alder densities, respectively). Within each plot, the growth of lodgepole pine and alder is assessed every 3 years. Lodgepole pine foliar nutrient status is also assessed at each remeasurement.
Brockley, R.P. and P. Sanborn. 2003. Effects of Sitka alder on the growth and foliar nutrition of young lodgepole pine in the central interior of British Columbia. Can. J. For. Res. 33: 1761-1771. [PDF]
Brockley, R. and P. Sanborn. 2007. Assessing the effects of Sitka alder on the growth of young lodgepole pine in the central interior of British Columbia (SBSdw3): 9-year results. B.C. Ministry of Forests and Range, Victoria. Extension Note 79.
Sanborn, P., R. Brockley, and C. Preston. 2001. Effects of Sitka alder retention and removal on the growth of young lodgepole pine in the central interior of British Columbia. B.C. Ministry of Forests, Victoria. Working Paper 60.