Bryophyte traits explain climate‐warming effects on tree seedling establishment

Abstract

Summary Above the alpine tree line, bryophytes cover much of the tundra soil surface in dense, often monospecific carpets. Therefore, when climate warming enables tree seedling establishment above the tree line, interaction with the bryophyte layer is inevitable. Bryophytes are known to modify their environment in various ways. However, little is known about to which extent and by which mechanisms bryophytes affect the response of tree seedlings to climate warming. We aimed to assess and understand the importance of bryophyte species identity and traits for tree seedling performance at tree line temperatures and their response to warmer conditions. Seedlings of two common, tree line‐forming tree species (Betula pubescens and Pinus sylvestris) were planted into intact cushions of eight common tundra bryophyte species and bryophyte‐free soil and grown for 18 weeks at current (7·0 °C) and near‐future (30–50 years; 9·2 °C) tree line average growing‐season temperatures. Seedling performance (biomass increase and N‐uptake) was measured and related to bryophyte species identity and traits indicative of their impact on the environment. Tree seedlings performed equally well or better in the presence of bryophytes than in bryophyte‐free soil, which contrasts to their usually negative effects in milder climates. In addition, seedling performance and their response to higher temperatures depended on bryophyte species and seedlings of both species grew largest in the pan‐boreal and subarctic bryophyte Hylocomium splendens. However, B. pubescens seedlings showed much stronger responses to higher temperatures when grown in bryophytes than in bryophyte‐free soil, while the opposite was true for P. sylvestris seedlings. For B. pubescens, but not for P. sylvestris, available organic nitrogen of the bryophyte species was the trait that best predicted seedling responses to higher temperatures, likely because these seedlings had increased N‐demands. Synthesis. Climatically driven changes in bryophyte species distribution may not only have knock‐on effects on vascular plant establishment, but temperature effects on seedling performance are themselves moderated by bryophytes in a species‐specific way. Bryophyte traits can serve as a useful tool for understanding and predicting these complex interactions.

The role of bryophytes for tree seedling responses to winter climate change: Implications for the stress gradient hypothesis

“…Therefore, while bryophyte species with different mat thicknesses may differ in their capacity to insulate the underlying soil, such differences do not seem to cause temperature differences at seedling rooting depth, or to be the driver of differences in the performance of these seedlings. Other properties of the bryophytes, such as their effects on nutrient availability (Lett, Nilsson, et al., ), their production of secondary compounds (Lett, Nilsson, et al., ; Soudzilovskaia et al., ) or their impacts on soil microbial community structure (Davey & Currah, ; Davey, Heegaard, Halvorsen, Ohlson, & Kauserud, ; Horton & van der Heijden, ) may thus be more important than insulating capacity for their winter impact on seedling survival.…”

“…on 27 August 2013 by cutting the sides and bottom with a serrated knife. Cutting bryophytes from below does not affect their performance as bryophytes have no roots and this approach has been successfully applied before (Lett, Nilsson, et al., ; Stuiver, Wardle, Gundale, & Nilsson, ). All cores were cut at 6 cm depth, as gravel and rocks below this depth underneath P. ciliare usually prevented deeper coring.…”

“…As such, positive effects of bryophytes on seedling establishment have been observed in alpine and polar bryophyte communities (Casanova‐Katny & Cavieres, ; Sohlberg & Bliss, ; Wheeler, Hermanutz, & Marino, ). However, in line with the SGH, the impact of bryophytes on plant growth depends on the environmental context (Doxford, Ooi, & Freckleton, ; Lett, Nilsson, Wardle, & Dorrepaal, ), and bryophytes often suppress seedling establishment in milder environments, such as in boreal forests and at lower elevations and latitudes (Gough, ; Soudzilovskaia et al., ; Wardle, Lagerström, & Nilsson, ; Zamfir, ). Moreover, different bryophyte species differ in their insulating capacity (Soudzilovskaia, van Bodegom, & Cornelissen, ) and therefore potentially differ in their ability to affect seedling establishment.…”

Global drivers of tree seedling establishment at alpine treelines in a changing climate

“…In line with this, seedling growth responded less to warming in the presence of shrubs potentially due to competition (Grau et al., ). Further, tree seedlings were more responsive to higher nutrient availability under elevated temperatures at the alpine treeline (Grau et al., ) and under controlled conditions (Hoch, ; Lett, Nilsson, Wardle, & Dorrepaal, ), suggesting that the demand for nutrients at the alpine treeline increases with warming.…”

Plant–plant interactions could limit recruitment and range expansion of tall shrubs into alpine and Arctic tundra

“…We found that the moss seedbed had intermediate rates of germination, which could be due to high water retention capacity compared to herbaceous vegetation. In stressful environments, mosses can facilitate recruitment by sheltering seedlings from herbivores and temperature extremes (Wheeler et al 2011;Lett et al 2017).…”