Stand Structure Beats Age for Ground Cover Vegetation in Ageing Hemiboreal Scots Pine and Norway Spruce Stands

Intensifying forest management and a reduction in the rotation period necessitates the development of intensive biodiversity conservation strategies, such as the triad concept, which aims at ensuring habitat connectivity. Such an approach depends on the relationships between biodiversity components and manageable stand characteristics. Mostly, the biological value of stands has been associated with age, although stand structures, which are often intercorrelated with age, might be of primary importance. The relationships between ground cover vegetation, which is a principal component and indicator of the biological value of temperate conifer forests, and stand characteristics were assessed in pre-harvesting/harvesting age and old-growth coniferous stands in the eastern Baltic region (Latvia). The old-growth stands were nearly two times older than the pre-harvesting/harvesting age stands. Both stand groups showed generally similar ground cover flora, though ground cover vegetation showed higher variability in the old-growth stands. The principal gradients of ground cover vegetation were related to light, site fertility, and structural diversity, as well as the degree of deciduous (particularly Betula spp.) admixture in a tree stand. Considering the explicit contrasts, stand age did not affect ground cover vegetation, implying the principal effects of stand structure, which are manageable characteristics. This implies the potential for specific management to aid the ecological connectivity of stands in commercial forest landscapes.