Lichens facilitate seedling recruitment in alpine heath

Abstract

Questions How do mat thickness, physical structure and allelopathic properties of terricolous mat‐forming lichens affect recruitment of vascular plants in dwarf‐shrub and lichen heath vegetation? Location The mountains of Dovrefjell, central Norway. Methods In autumn, seeds of ten vascular plant species were collected and sown in a common garden experiment with mats of six lichen species and bare soil controls as experimental treatments. We recorded growing season soil temperature and moisture, and seedling recruitment and growth after one year. The effect of lichen secondary compounds on germination was tested in a growth chamber experiment and compared to the lichen–plant interactions detected under field conditions. Results The lichen mats buffered extreme soil temperatures and soil drying in dry weather, with soils below the thickest mats (Cladonia stellaris and C. rangiferina) experiencing the lowest temperature fluctuations. Seedling recruitment and seedling growth in the field and seed germination in the lab were species‐specific. Seedling recruitment rates were overall higher within lichen mats than on bare soil, but the c. 6.5‐cm‐thick mats of C. stellaris reduced recruitment of many species. The lab experiment suggested no overall strong effect of lichen allelopathy on seed germination, and effects on seed germination were only moderately correlated with the lichen–plant interactions observed for seedling recruitment in the field. Conclusions In harsh environments like alpine dwarf‐shrub and lichen heaths, the presence of lichens and the resulting amelioration of the microclimate seem more important for vascular plant recruitment than are allelopathic effects often reported in lab experiments. We might therefore expect most terricolous lichens, depending on the plant species in focus, to facilitate rather than hamper the early stages of plant recruitment into lichen‐dominated arctic‐alpine heath vegetation.

Herbivores reduce seedling recruitment in alpine plant communities

“…Although clonal species can persist for long periods, long‐term vegetation dynamics depend increasingly on changes in community composition and hence successful seedling recruitment. Seedling recruitment is expected to depend on microclimatic conditions (Walck et al 2011) and on species interactions (Fayolle et al 2009, Milbau et al 2013, Nystuen et al 2019), both of which are likely to be affected by ongoing environmental changes. While seed production and germination in cold biomes are often found to respond positively to warmer temperatures, at least when sufficient soil moisture is available (Milbau et al 2009, Klady et al 2011, Walck et al 2011), seedling survival and establishment are in turn thought to be sensitive to droughts associated with warm weather (Moles and Westoby 2004, Fenner and Thompson 2005, Leck et al 2008).…”

The decline of alpine lichen heaths generates atmospheric heating but subsurface cooling during the growing season

“…Nevertheless, our study does show that the high albedo of lichens is not leading to lower soil temperatures below lichens than below shrubs during the growing season. Since recent studies have shown that differences in color, and therefore albedo, did not even cause a difference in soil temperature between lichen species (Nystuen et al, 2019;Van Zuijlen et al, 2020), it is unlikely that albedo is an important factor determining the difference in soil temperature between the two different vegetation types. Instead, the marked differences in canopy structure between lichens and shrubs are a more essential factor.…”

“…Multiple studies have discussed the cooling capacity of lichens on the underlying soil, and have argued that this cooling capacity is a result of their high albedo and the insulating properties of lichens due to their low conductivity. However, most studies measured or modelled lower soil temperatures below lichens in relation to bare soil or disturbed lichens (Beringer et al, 2001;Gold et al, 2001;Porada et al, 2016;Nystuen et al, 2019;Van Zuijlen et al, 2020), but the comparison with another vegetation type has rarely been made. Our study shows that the cooling capacity of lichens does not lead to a lower soil temperature compared to shrubs.…”

“…Most of the field measurements on the influence of lichens on the subsurface microclimate are based on differences between lichens and bare soil or disturbed lichens (e.g. Fauria et al, 2008;Nystuen et al, 2019;Van Zuijlen et al, 2020). However, constructive field measurements on the difference in soil temperature between lichens and shrubs are lacking and therefore it is uncertain how the observed shift from lichen-dominated areas to shrub-dominated areas will change the micro-and large-scale climate in alpine and arctic areas.…”

Determinants of tree seedling establishment in alpine tundra

“…Small-scale disturbances, by removing or reducing the abundance of competitors, generate new microhabitats suitable for seedling emergence and establishment (Milbau et al, 2013;Nystuen et al, 2014;Lembrechts et al, 2016). However, severe disturbances can counteract the benefits of reduced competition, for instance when it leads to too hot and too dry soils that can be detrimental for tree seedlings (Kambo and Danby, 2018;Nystuen et al, 2019).…”