Forest Fertilization: Some Guiding Concepts

Drawing on results from studies of nutrient cycling and fertilizer response in pine, three concepts are proposed: (1) Fertilizers are generally of benefit to the trees, not the site, and measureable permanent site improvement is only likely if the amount of nutrient applied is large in relation to the soil capital. (2) In the absence of any site improvement, fertilizer response is best explained, and costed, by using the simple analogy of an acceleration through time, leading to a reduction in rotation length. (3) Consideration of the variations with tree age in the patterns of nutrient uptake, cycling and immobilization, and in the capture and retention of atmospheric nutrients, leads to the suggestion of three distinct nutritional stages. During the years prior to canopy closure (Stage I) tree growth is very dependent on soil nutrient concentrations and response to a number of nutrients can be expected. Thereafter, (Stage II) responses are unlikely unless foliage biomass has to be reconstructed, e.g after thinning However, on low nitrogen capital sites, immobilization of nitrogen in biomass and humuus as trees age can lead to the progressive development of deficiency (Stage III), a deficiency that may eventually disappear as tree demands fall with age. It is suggested that together these three concepts provide a rational framework that will aid in the explanation and extrapolation of results from empirical fertilizer trials