Abstract
Climate warming enables tree seedling establishment beyond the current alpine treeline, but to achieve this, seedlings have to establish within existing tundra vegetation. In tundra, mosses are a prominent feature, known to regulate soil temperature and moisture through their physical structure and associated water retention capacity. Moss presence and species identity might therefore modify the impact of increases in temperature and precipitation on tree seedling establishment at the arctic-alpine treeline. We followed Betula pubescens and Pinus sylvestris seedling survival and growth during three growing seasons in the field. Tree seedlings were transplanted along a natural precipitation gradient at the subarctic-alpine treeline in northern Sweden, into plots dominated by each of three common moss species and exposed to combinations of moss removal and experimental warming by open-top chambers (OTCs). Independent of climate, the presence of feather moss, but not Sphagnum, strongly supressed survival of both tree species. Positive effects of warming and precipitation on survival and growth of B. pubescens seedlings occurred in the absence of mosses and as expected, this was partly dependent on moss species. P. sylvestris survival was greatest at high precipitation, and this effect was more pronounced in Sphagnum than in feather moss plots irrespective of whether the mosses had been removed or not. Moss presence did not reduce the effects of OTCs on soil temperature. Mosses therefore modified seedling response to climate through other mechanisms, such as altered competition or nutrient availability. We conclude that both moss presence and species identity pose a strong control on seedling establishment at the alpine treeline, and that in some cases mosses weaken climate-change effects on seedling establishment. Changes in moss abundance and species composition therefore have the potential to hamper treeline expansion induced by climate warming.
Highlights
Shifts in the position of alpine and arctic treelines are among the most dramatic climate-driven plant migrations globally, and they may cause substantial climate feedbacks through altering soil carbon content and albedo (Chapin et al, 2005; Hartley et al, 2012; Mayor et al, 2017; Parker, Subke, & Wookey, 2015; Wilmking, Harden, & Tape, 2006)
Positive effects of warming and precipitation on survival and growth of B. pubescens seedlings occurred in the absence of mosses and as expected, this was partly dependent on moss species
P. sylvestris survival was greatest at high precipitation, and this effect was more pronounced in Sphagnum than in feather moss plots irrespective of whether the mosses had been removed or not
Summary
Shifts in the position of alpine and arctic treelines are among the most dramatic climate-driven plant migrations globally, and they may cause substantial climate feedbacks through altering soil carbon content and albedo (Chapin et al, 2005; Hartley et al, 2012; Mayor et al, 2017; Parker, Subke, & Wookey, 2015; Wilmking, Harden, & Tape, 2006). Moss species have been shown to differentially modify responses of establishing tree seedlings to warming or precipitation under controlled conditions where climate settings were kept constant (Lett et al, 2017; Stuiver et al, 2014) These effects on seedling establishment at natural settings for example, at or near the treeline have not been investigated. We measured various abiotic variables in the seedling microenvironment throughout the experiment in order to understand the mechanisms underpinning our results Addressing these hypotheses in combination will allow us to understand how mosses may modify the effects of climate (and climate change) on tree seedling establishment and how this in turn may impact climate change-driven treeline expansion
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