Spatial structure of semi-natural and plantation stands of Scots pine (Pinus sylvestris L.) in northern Scotland
Mason, W.L.; Connolly, T.; Pommerening, A.; Edwards, C.
Forestry (Oxford) 80(5): 564-583
ISSN/ISBN: 0015-752X DOI: 10.1093/forestry/cpm038
The success of current initiatives to maintain and enhance the area of and the special habitats provided by the remnant semi-natural pinewoods of northern Scotland will depend upon foresters' ability to foster more natural structures in even-aged plantations through stand manipulation. However, there is little information on the structures and spatial patterns that can be found in Scottish pinewoods; such knowledge could be used to design appropriate silvicultural regimes. A study was carried out to compare spatial structure in three 0.8-1.0 ha plots in the Cairngorms National Park; one plot was a 78-year-old plantation stand, the other two were semi-natural stands with trees up to 300 years old. Basic mensurational data showed that the semi-natural stands were characterized by a wider range of tree sizes and more large (> 50 cm d.b.h.) trees. Spatial structure was evaluated with a range of different indices: the aggregation index of Clark-Evans (CE), the uniform angle and diameter differentiation indices, Ripley's L function of tree spatial distribution, pair and mark correlation functions and experimental variograms of tree diameter. The CE revealed a regular distribution in the plantation with the semi-natural stands having a random pattern. Further analysis of the latter stands indicated that, in each case, the older trees in the stand were regularly distributed while the younger ones were clustered. There was little difference in uniform angle values between the stands while the diameter differentiation distributions suggested greater variety in diameter within the semi-natural stands than in the plantation. The Ripley's L function showed that trees in the plantation were regularly distributed at close distances but clustered over wider distances. There were differences in pattern between the semi-natural stands; in one, trees were clustered because the positions of the younger trees were influenced by past regeneration trials, whereas in the other stand a random pattern was observed. Similarly, the variogram indicated widespread homogeneity in diameter within the plantation, while the semi-natural stands showed high variation at close spacing because of competition followed by spatial autocorrelation up to similar to 20 m distance. Thereafter, one of these stands had a very different pattern because of a more intensive regeneration history. All the indices, apart from uniform angle, were able to discriminate between the plantation and the two semi-natural stands, but only the more detailed spatial indices were capable of identifying differences within the latter. The implications of these results for management strategies in plantations are discussed.