Improvement of nutritional site quality 13 years after single application of fertiliser N and P on regenerating cedar-hemlock cutovers on northern Vancouver Island, B.C.

Abstract

Post-clearcut silvicultural treatments, to improve tree growth and reduce salal (Gaultheria shallon) competition, were established in five different forest blocks on northern Vancouver Island, in 1984. Plots were either left untreated, brushed of competing salal vegetation, fertilized [(250 kg N + 100 kg P) ha-1], or brushed + fertilized. Three of these blocks were revisited 13 years later, in the summer of 1997, and various chemical, biochemical and microbial parameters were measured in forest floor humus samples to determine long-term effects of treatments on nutritional site quality. Brushing resulted in lower humus pH and extractable base cations, whereas fertilization increased Bray-extractable P. Over a 20-week aerobic incubation, significantly more N was mineralised in humus from fertilized plots than from brushed plots. Over a 14-d anaerobic incubation, significantly more N was mineralised in humus from the fertilized treatment than other treatments. Similarly, gross transformation rates of NH4+and NO3-, measured by 15N-dilution, were higher in humus from the fertilized treatment than other treatments. Ecophysiological indices of microbial communities (basal respiration, specific death rate, metabolic quotient, and energy deficiency index), derived by humus respirometry, suggested that there was higher available C in fertilized and brushed + fertilized treatments than in the brushed and control treatments. Total microbial biomass was equal to C-limited microbial biomass, which further confirmed that available C was the growth-limiting factor for microbial communities in all treatments. The prokaryotic fractions of microbial biomass in all treatments were approximately equal (≈ 65%). PCA ordination of microbial communities, based on C source utilisation patterns, showed a distinct clustering of humus samples taken from one of the sites. Within the cluster of samples taken from the other two sites, samples from fertilized plots scored separately from those from control plots. In salal foliage, concentrations of condensed tannins were higher in brushed and control plots than in fertilized and brushed + fertilized plots. In spite of other studies that have reported increased tree height following fertilization and/or removal of salal, results of the present study suggest improvement in nutritional site quality occurs only with fertilization, whereas brushing may in fact be detrimental. The long-term growth of hemlock observed in fertilized plots may be the result of changes to key ecosystem structures and processes brought about by increased speed of succession and accelerated canopy closure.