Plant species richness in managed boreal forests-Effects of stand succession and thinning

Disturbance is a key factor affecting plant species richness and it has been hypothesized that species richness reaches a peak some time after disturbance occurs. Managed boreal forests are characterized by large scale disturbance in the form of clear-cuttings, which partly resembles natural disturbance regimes. The young stands however, which may be important to many plants, are often homogenous and dense compared to naturally regenerated stands. We examined previously unstudied effects of disturbance, succession and management practices on plant species richness in boreal forests. Survey data covering the northern and central parts of Sweden (4465 plots) were used to compare species richness and composition in four stand maturity classes-cutting, young, pre-mature and mature, and two fertility classes-rich and poor. In addition, the effect of thinning was examined in young and pre-mature forests. In both pine and spruce forests species richness was higher in the cutting and young classes than in the pre-mature and mature classes. In pine forests, the difference in species richness between young and mature forests was greater in rich than in poor fertility classes. Spruce forests grow on somewhat richer sites than pine forests, but there was no significant effect of site fertility within these stands. A few Ericaceous species were dominant, although this trend is slightly relaxed in favor of pioneer species in the cutting and young classes, which results in higher species richness. Plant communities were also affected by thinning. Stands thinned in the young maturity class exhibited higher species richness in both the young and pre-mature classes than did unthinned stands. Thinning in the pre-mature class had no effect on species richness. We conclude that a peak in plant species richness was recorded in early forest succession stages, especially in fertile sites. Moreover, early pre-commercial thinning seems to promote and sustain this diversity. Hence, young stands and their management may play a central role in the preservation of plant diversity in boreal production forests. We suggest that thinning practices could be developed to support plant diversity, for example the creation of larger gaps in lush young forests. (C) 2008 Elsevier B.V. All rights reserved.