Old-growth forests differ from even-aged managed forests, for example, in the age or size of canopy trees. In contrast, understory trees in managed and old-growth forests may appear superficially similar, but they may still differ in ways that would impact species communities associated with them, such as epiphytes. To address this, we surveyed epiphytic lichens on 270 rowans across managed, urban, and old-growth forests in southern Finland. The sampled trees were components of the understory and represented a size gradient, mostly in the range of 1–15 cm in diameter at breast height. Forest type and tree diameter affected both lichen species richness and composition significantly. A few species were associated with small rowans, but many more with larger ones. Thus, tree-level species richness increased with diameter. Tree-level diversity was higher in old-growth forests, independently of tree size, compared to managed and urban forests. Species composition was divergent in urban forests, which is apparently related to nitrogen pollution and the urban heat island effect, although these effects were relatively light. Ruderal species traits were more prevalent on small rowans and rowans in urban forests, and competitive traits on larger rowans and rowans in old-growth forests. Our results reflect differences in understory dynamics and structures between old-growth and managed (including urban) forests, which also affect associated species communities. The assumed main driver of our results was that, despite similar sizes, understory rowans may have been older in old-growth forests than in managed and urban forests. Understory trees can significantly contribute to epiphyte diversity, especially when their substrate properties differ from those of canopy trees. When that condition is met, as with rowans in spruce forests, understory trees should not be overlooked in epiphyte inventories, and retaining them during thinning and in clearcuts can support epiphyte diversity in managed forests.
Nirhamo et al. (Sun,) studied this question.