Precipitation regime controls bryosphere carbon cycling similarly across contrasting ecosystems

Roger Grau‐Andrés,Paul Kardol,Marie‐Charlotte Nilsson,David A. Wardle

doi:10.1111/oik.07749

Roger Grau‐Andrés, Paul Kardol + Show 2 more

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https://doi.org/10.1111/oik.07749

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Abstract

In arctic and boreal ecosystems, ground bryophytes play an important role in regulating carbon (C) exchange between vast belowground C stores and the atmosphere. Climate is changing particularly fast in these high‐latitude regions, but it is unclear how altered precipitation regimes will affect C dynamics in the bryosphere (i.e. the ground moss layer including senesced moss, litter and associated biota) and the closely associated upper humus layer, and how these effects will vary across contrasting environmental conditions. Here, we set up a greenhouse experiment in which mesocosms were assembled containing samples of the bryosphere, dominated by the feather moss Hylocomium splendens, and the upper humus layer, that were collected from across a boreal forest chronosequence in northern Sweden which varies strongly in nutrient availability, productivity and soil biota. We tested the effect of variation in precipitation volume and frequency on CO2 exchange and dissolved organic carbon (DOC) export, and on moss growth. As expected, reduced precipitation volume and frequency lowered net CO2 efflux, DOC export and moss growth. However, by regulating moisture, the lower bryosphere and humus layers often mediated how precipitation volume and frequency interacted to drive C dynamics. For example, less frequent precipitation reduced moss growth only when precipitation volume was low. When volume was high, high moisture content of the humus layer helped avoid moss desiccation. Variation in precipitation regime affected C cycling consistently in samples collected across the chronosequence, despite large environmental variation along the sequence. This suggests that the bryosphere exerts a strong buffering effect on environmental variation at the forest floor, which leads to similar responses of C cycling to external perturbations across highly contrasting ecosystems. As such, our study indicates that projected increases in droughts and ground evapotranspiration in high‐latitude regions resulting from climate change will consistently reduce C losses from moss‐dominated ecosystems.

Highlights

Climate is changing fastest in high-latitude regions, where global models forecast an increase in the temporal heterogeneity of precipitation and in the occurrence of extreme precipitation events, as well as increased periods of prolonged droughts (Collins et al 2013)
Our study demonstrates strong effects of moisture dynamics on the C balance of the bryosphere and upper humus layers, and shows that joint consideration of precipitation volume and frequency is key to understanding moss-dominated forest floor function in boreal regions
This is important as future climate scenarios for boreal and arctic regions predict large changes in moisture regimes through increased precipitation and evapotranspiration, and more frequent extreme precipitation events and droughts (Collins et al 2013, Berg et al 2017)

Summary

Introduction

Climate is changing fastest in high-latitude (boreal and arctic) regions, where global models forecast an increase in the temporal heterogeneity of precipitation and in the occurrence of extreme precipitation events, as well as increased periods of prolonged droughts (Collins et al 2013). Rapid warming will increase evapotranspiration rates and contribute to declines in moisture content at and below the ground surface (Collins et al 2013, Berg et al 2017) These changes in moisture content can affect nutrient and carbon (C) cycling (Wu et al 2011, Öquist et al 2014, Sierra et al 2015), and impact the substantial amounts of C stored in high-latitude ecosystems, much of which is located at or near the ground surface (Jobbágy and Jackson 2000, Pan et al 2011). These high-latitude C stores are sensitive to climate change (Gauthier et al 2015, Koven et al 2017). The bryosphere contributes to soil C inputs by producing recalcitrant litter, which can represent a large proportion of the upper humus layer (Lang et al 2009, Hilli et al 2010, Jonsson et al 2015), and controls decomposition by regulating ground temperature and moisture (Jackson et al 2013, Sun et al 2017) and nitrogen inputs (Lindo et al 2013), making it an important driver of C cycling in highlatitude ecosystems

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