Lichen communities on Populus tremula are affected by the density of Picea abies

Abstract

Questions Aspen (Populus tremula) is declining in the old‐growth forests of boreal Fennoscandia. This threatens the numerous taxa that are dependent on old aspens, including many epiphytic lichens. Potential methods to aid epiphytic lichens on aspen are centered around treatments which affect the density of Norway spruce (Picea abies). In this study, we investigated how epiphytic lichen communities on aspen are affected by the variation of spruce density in the immediate vicinity of the focal aspen. Location Southern boreal forests in Finland. Methods We recorded the occurrence of lichens from 120 aspens in 12 semi‐natural forest sites. We used spruce basal area as the measure for spruce density. The selected aspens represented a gradient in spruce basal area in the vicinity of the aspen from 0 to 36 m2/ha. We also measured other tree‐ and stand‐level variables that are known to influence lichen occurrence. Results Lichen communities on aspen were affected by spruce density, stand age and bark pH. Both lichen species richness and the richness of red‐listed species were highest at an intermediate spruce density, and both increased with stand age. Lichen species richness was higher when bark pH was lower. Additionally, community composition was influenced the most by spruce density, followed by bark pH. Conclusions Our study highlights the detrimental effects of high spruce density on lichen diversity on aspens. This is caused by high spruce density resulting in low light availability. Lichen diversity on aspens was highest when spruce density was intermediate. Spruce thinning in aspen‐rich old‐growth forests can be helpful in ensuring the long‐term persistence of old‐growth lichens on aspen in protected forests.

Habitat associations of red-listed epiphytic lichens in Finland

“…Long-term changes in canopy cover have been only scarcely studied, but it has been shown to have increased between 1959 and 2011 in unmanaged landscapes in northern Finland (Kulha et al 2020). Studies have shown that lichen richness is lower in dense stands with low light availability (Marmor et al 2012;Klein et al 2020;Nirhamo et al 2021). According to our results, over 80% of red-listed epiphytic lichens prefer high or intermediate light availability.…”

“…Our data suggests that some species prefer an unusual combination of high humidity and high light availability (see also Nilsson et al 2022). Our results highlight that dense forests are inhospitable to many red-listed lichens (see also Gauslaa et al 2006;Gauslaa et al 2007;Klein et al 2020;Nirhamo et al 2021). Instead, they require a sufficient availability of light, often in combination with relatively high humidity.…”