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UBC Theses and Dissertations

Nutrient and establishment density effects on structural development and growth processes in juvenile western redcedar and western hemlock stands on northern Vancouver Island Negrave, Roderick William

Abstract

Conifer plantations established on sites formerly occupied by old-growth cedarhemlock (CH) forests have exhibited poor growth compared to plantations established on sites previously occupied by early-seral hemlock amabilis fir (HA) stand on northern Vancouver Island. Poor growth on CH sites has been associated with low nutrient supply capacity, compared to HA sites, and competition from the ericaceous shrub salal. Fertilization with N and P has been shown to improve conifer growth and increasing plantation establishment density has been suggested as a method to competitively exclude salal from CH sites. A trial was established in 1988 to examine the effects of fertilization and establishment density on the growth of western redcedar and western hemlock plantations established on CH and HA sites. In this thesis, I examine the effects of these silvicultural treatments on the growth and structure of juvenile conifer stands 14 years after establishment and attempt to determine structural and functional mechanisms by which these treatments have affected community growth and site productivity. I also used the trial to examine current theories regarding the influence of soil resource availability on competition processes. Fertilization improved the height and basal area growth of both species on both sites 10 years after its application. Fertilization increased total stand mass and the masses of wood, foliage, bark and branches. Hemlock responded more strongly to fertilization than cedar. Elevated foliar concentrations of P were associated with fertilization treatments on both CH and HA sites but N concentrations were only elevated on HA sites. Other nutrients, not applied in fertilizer, also had elevated foliar concentrations associated with fertilization. Increasing establishment density was associated with greater stand basal area and mass. However, individual component masses of wood, foliage, bark and branches different in their response to increasing establishment density on each site. The magnitude of all mass components increased with establishment density on HA sites. On CH sites, wood and foliage mass declined with increasing establishment density while stand masses of bark and branches tended to increase. Fertilization increased stand mass proportion in wood and decreased mass proportion in foliage. Increasing establishment density increased stand mass proportion in foliage and decreased mass proportion in wood. Hemlock tended to have greater mass proportion in wood while cedar tended to have greater mass proportions in bark and branches and, on HA sites, in foliage. The presence and influence of competition on mass structure of stands was greatest on the least productive areas associated with CH sites and unfertilized treatments and increased with site productivity. Fertilization decreased or did not influence foliar efficiency. Increasing establishment density decreased foliar efficiency. Hemlock generally had greater foliar efficiency than cedar. Foliar efficiency varied with tree size and was positively correlated with tree size in cedar but negatively correlated with size in hemlock. Light interception was increased by fertilization and increasing establishment density. Light use efficiency was not influenced by fertilization and declined with increasing establishment density. Light use efficiency increased exponentially with stand light interception for growth of foliage, wood and total dry mass. Nutrient use efficiencies of N, P, K, S, Ca, Mg, Zn and B were increased by fertilization. Nutrient use efficiencies tended to be greatest in cedar on CH sites and greater in hemlock on HA sites. Establishment density generally did not influence nutrient use efficiencies. Growth of cedar and hemlock stands on both sites showed a consistent positive linear relationship with stand foliage mass. Size hierarchies were reduced by fertilization and were not influenced by establishment density or absolute biological density. Cedar had greater structural inequality on HA sites and structural inequality of hemlock was greater on CH sites. Degree of structural inequality tended to be negatively associated with growth response to treatments and site: stands with greater growth had lower structural inequality. Development of structural inequality over the experimental period varied with site and species. Reduction in structural inequality was associated with increased relative growth rates of smaller versus larger individuals in stands. Stands developed positive skewness that was reduced by fertilization. It was concluded that fertilization continued to increase stand growth 10 years after treatment. In structural or functional terms, the only growth mechanism positively influenced by fertilization was increased nutrient use efficiency. Symmetric competition was likely the dominant mode on these sites and increasing site productivity and fertilization decreased its presence and influence. No evidence for increased asymmetric competition development associated with increased site productivity and fertilization was found.

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