Discordance in spatial patterns of white pine (Pinus strobus) size‐classes in a patchy near‐boreal forest

Abstract

Summary White pines (Pinus strobus) in a c. 1 ha near‐boreal aspen‐red maple stand in the western Great Lakes region, USA, were allocated to five size‐classes that were tested for discordance of their spatial patterns (i.e. within‐size‐class spatial distribution of individuals and spatial associations among different size‐classes). The size‐classes were 0.5–1 m tall, 1–2 m tall, 2–4 m tall, > 4 m tall but < 20 cm diameter at breast height (d.b.h.), and > 20 cm d.b.h. The size‐classes also differed in age. Their spatial patterns were analysed using formulae for spatial autocorrelation with categorical data at neighbourhood (0–14 m) and stand (0–70 m) scales. Size‐classes clustered in neighbourhoods that differed in size ranging from 6 m in diameter (size‐class 2) to 40 m (size‐class 3), suggesting that the pattern of each size‐class is due to a different set of environmental factors. Similar size‐classes tended to be randomly or positively associated at neighbourhood scales, while the most dissimilar size‐classes occupied different neighbourhoods. Size‐classes 1 and 2 were located farther away from seed trees than size‐class 3, which was farther away than size‐class 4. Thus periodic seedling invasion of a deciduous matrix may be followed by the gradual retraction of older size‐classes into a refuge near the mature seed source where competition from shrubs and overstorey trees is lower. Although the lack of fine‐scale clustering suggests density‐dependent mortality within the refuge, few older individuals were found outside this area. Two regeneration bottlenecks may structure white pine populations. First, recruitment into size‐class 2 is restricted to small patches (possibly in understorey gaps) within neighbourhoods with abundant size‐class 1. Relatively unrestricted recruitment to size‐class 3 follows, and after that recruitment to size‐class 4 is again limited, this time to refugia with sparse canopy near adult trees.