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

Abstract

When tree seedlings establish beyond the current tree line due to climate warming, they encounter existing vegetation, such as bryophytes that often dominate in arctic and alpine tundra. The stress gradient hypothesis (SGH) predicts that plant interactions in tundra become increasingly negative as climate warms and conditions become less harsh. However, for seedlings, climate warming might not result in lower winter stress, if insulating snow cover is reduced. We aimed to understand if bryophytes facilitate seedling survival in a changing winter climate and if these effects of bryophytes on tree seedlings comply with the SGH along elevational gradients under contrasting snow conditions. In the Swedish subarctic, we transplanted intact bryophyte cores covered by each of three bryophyte species and bryophyte‐free control soil from above the tree line to two field common garden sites, representing current and future tree line air temperature conditions (i.e. current tree line elevation and a lower, warmer, elevation below the tree line). We planted seedlings of Betula pubescens and Pinus sylvestris into these cores and subjected them to experimental manipulation of snow cover during one winter. In agreement with the SGH, milder conditions caused by increased snow cover enhanced the generally negative or neutral effects of bryophytes on seedlings immediately after winter. Furthermore, survival of P. sylvestris seedlings after one full year was higher at lower elevation, especially when snow cover was thinner. However, in contrast with the SGH, impacts of bryophytes on over‐winter survival of seedlings did not differ between elevations, and impacts on survival of B. pubescens seedlings after 1 year was more negative at lower elevation. Bryophyte species differed in their effect on seedling survival after winter, but these differences were not related to their insulating capacity. Synthesis. Our study demonstrates that interactions from bryophytes can modify the impacts of winter climate change on tree seedlings, and vice versa. These responses do not always comply with SGH, but could ultimately have consequences for large‐scale ecological processes such as tree line shifts. These new insights need to be taken into account in predictions of plant species responses to climate change.

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

“…In all but one of eight observations, soil moisture had a similar or more positive effect on seedling survival than warming. Snow addition had similar neutral or positive effects on survival in 12 observations (Bader et al., ; Lett, Wardle, Nilsson, Teuber, & Dorrepaal, ).…”

“…The effect of warming on seedling survival was only changed by that of another factor in three of the 32 observations where this was tested (Table ). As such, seedlings growing at colder elevations benefitted more from increased snow cover than those in a warmer environment in two observations (Lett, Wardle, et al., ) and moisture addition changed the effect of warming from negative to positive (Loranger et al., ). In summary, as warming had a relatively small negative or no effect on survival, other factors played a relatively larger role.…”

The ratio between bryophyte functional groups impacts vascular plants in rich fens

“…We initially supposed, in line with previous speculations (Clapham, ; Peterka et al., ) or studies from different ecosystems (Soudzilovskaia et al., ; Lett et al., ), that the species’ responses to the bryo‐ratio will coincide with the capability or matter of vegetative reproduction. The analysis of clonal traits indeed showed contrasting strategies for the vascular plants preferring either non‐sphagnaceous bryophytes or Sphagnum .…”

“…Therefore, it is important to understand the role of the relationship between vascular plants and bryophytes in the functioning of mires to guarantee effective protection of mire biodiversity. The previous studies testing the relationship between bryophytes and vascular plants in mires or similar habitats were mostly conducted on few sphagna-dominated bogs and acidic poor fens (Malmer, Svensson, & Wallén, 1994;Heijmans, Klees, & Berendse, 2002;Malmer et al, 2003;Pouliot et al, 2011), heath-woodlands (Soudzilovskaia et al, 2010) or tundra (Lett, Wardle, Nilsson, Teuber, & Dorrepaal, 2018). Udd et al (2015) studied co-occurrence patterns in Swedish rich fens and found mostly neutral and few positive and even fewer negative associations between vascular plant and bryophyte species.…”

Belowground interactions differ between sympatric desert shrubs under water stress

“…In the SGH proposed by Bertness and Callaway (), increased environmental stress leads to increased intensity and the importance of mutual benefits. The SGH has been confirmed by many studies (Mori, ), but some studies have shown that this hypothesis is not always tenable (Lett, Wardle, Nilsson, Teuber, & Dorrepaal, ). Under extreme stress conditions, facilitation can disappear or become competition.…”