Factors limiting early growth of western redcedar, western hemlock and Sitka spruce seedlings on ericaceous-dominated clearcut sites in coastal British Columbia

Abstract

A 3 year field and pot study was conducted to determine the effects of several biotic and abiotic factors on the early growth of western hemlock ( Tsuga heterophylla (Raf.) Sarg.), western redcedar ( Thuja plicata, Donn) and Sitka spruce ( Picea sitchensis, Bong. Carr.) seedlings established on 2- and 8-year-old clearcut sites previously occupied by old-growth western hemlock and western redcedar forests (referred to as younger and older CH, respectively), and on adjacent 2-year-old clearcut sites previously occupied by second-growth western hemlock and amabilis fir ( Amabilis amabilis (Dougl.) Forbes) forests (referred to as younger HA) in coastal British Columbia. The objective of the study was to determine which factors are associated with the poor growth characteristic of the salal ( Gaultheria shallon, Pursh) dominated CH clearcut sites. No soil moisture deficits were measured on any of the three types of clearcut sites at any time of year. The best seeding growth was on the younger HA sites followed by the younger CH sites and then the older CH sites. The better growth on the younger HA sites was associated with a higher availability of N and P in the first 20 cm depth of the forest floor. No differences in matric soil water potential and pH, and only small differences in soil temperature were measured between the three types of clearcut sites. Complete removal of the competing vegetation on the younger and older CH sites resulted in an increase in conifer seedling growth and in the availability of N (22–40%) and P (15–32%); however, it did not affect cellulose decomposition and matric soil water potential, and increased soil temperature only slightly. Both western hemlock and Sitka spruce seedlings were very responsive to differences in nutrient availability measured between types of clearcut sites and planting treatments. In contrast, western redcedar was not responsive. All three conifer species had very high mycorrhizal colonization on the younger CH sites, and this was not altered by the removal of the competing vegetation (mainly salal). These results suggest that the nutritional stress and poor growth of conifers on salal-dominated CH clearcut sites in coastal British Columbia can be explained by:(1) inherently low forest floor nutrient availability; (2) competition between salal and conifer seedlings for scarce nutrients and nutrient immobilization in salal; (3) declining nutrient availability a few years after clearcutting and slashburning.