Factors influencing size inequality in peatland black spruce and tamarack:evidence from post-drainage release growth

1 We used tree ring analysis to determine stem radius and thus examine size variation over time in two even-aged (approximately 40-year-old) mixed pop ulations of black spruce and tamarack established on peatlands in a boreal forest. We also followed the response of one of these populations to improv ed edaphic conditions over 8 years following drainage. 2 Populations of trees in undrained areas showed a decline in size variabil ity over time until age 20-25 years, after which size heterogeneity was rel atively stable. 3 For trees in undrained areas there was a relationship between age and siz e for the first 20-25 years, but this relationship then broke down due to a period where relative growth rate and size were inversely related. 4 For the population of trees in the drained area, smaller trees (i.e. thos e that had been growing more slowly prior to drainage) showed significantly greater drainage-induced release growth, while larger trees (those growing faster prior to drainage) showed an initial reduction in growth following drainage and, overall, less release growth. 5 The response of tamarack to drainage was more dramatic than for black spr uce. 6 Despite extensive variation in tree size, drainage dramatically reduced v ariability in growth rate among trees of each species, such that size varia bility in the populations declined. 7 We postulate that the heterogeneity of microsites with respect to edaphic conditions, perhaps associated with the hummock to hollow microtopographic gradient, has a major influence on growth variation, and hence size inequa lity, in peatland populations of black spruce and tamarack.