Delayed responses of an Arctic ecosystem to an extreme summer: impacts on net ecosystem exchange and vegetation functioning

D Zona,D A Lipson,J H Richards,M Ueyama,G K Phoenix,W C Oechel,C S Sturtevant,A K Liljedahl

doi:10.5194/bg-11-5877-2014

Abstract

Abstract. The importance and consequences of extreme events on the global carbon budget are inadequately understood. This includes the differential impact of extreme events on various ecosystem components, lag effects, recovery times, and compensatory processes. In the summer of 2007 in Barrow, Arctic Alaska, there were unusually high air temperatures (the fifth warmest summer over a 65-year period) and record low precipitation (the lowest over a 65-year period). These abnormal conditions were associated with substantial desiccation of the Sphagnum layer and a reduced net Sphagnum CO2 sink but did not affect net ecosystem exchange (NEE) from this wet-sedge arctic tundra ecosystem. Microbial biomass, NH4+ availability, gross primary production (GPP), and ecosystem respiration (Reco) were generally greater during this extreme summer. The cumulative ecosystem CO2 sink in 2007 was similar to the previous summers, suggesting that vascular plants were able to compensate for Sphagnum CO2 uptake, despite the impact on other functions and structure such as desiccation of the Sphagnum layer. Surprisingly, the lowest ecosystem CO2 sink over a five summer record (2005–2009) was observed during the 2008 summer (~70% lower), directly following the unusually warm and dry summer, rather than during the extreme summer. This sink reduction cannot solely be attributed to the potential damage to mosses, which typically contribute ~40% of the entire ecosystem CO2 sink. Importantly, the return to a substantial cumulative CO2 sink occurred two summers after the extreme event, which suggests a substantial resilience of this tundra ecosystem to at least an isolated extreme event. Overall, these results show a complex response of the CO2 sink and its sub-components to atypically warm and dry conditions. The impact of multiple extreme events requires further investigation.

Highlights

Global increases in mean temperature (Vavrus et al, 2012) and evapotranspiration are expected to lead to drier conditions in many regions of the world (IPCC, 2013)
In contrast to the previous published studies which mostly investigated the impact of flooding on CO2 and CH4 fluxes (Zona et al, 2009, 2012), this study focuses on the impact of a naturally occurring extreme event on net ecosystem exchange (NEE), gross primary production (GPP), ecosystem respiration (Reco), net Sphagnum CO2 exchange (NSE), leaf area index (LAI), microbial biomass C, and ammonium (NH+4 ) in soil pore water
Sphagnum mosses do not have roots and rely on capillary transport of water between and within the short structure of their photosynthetic tissues (Price et al, 2009), and are very sensitive to surface moisture conditions. These results suggest that a combination of unusually warm temperatures and high radiation input can negatively affect the functioning of a dominant vegetation component (e.g., Sphagnum spp.) of this wet-sedge tundra ecosystem if precipitation is low

Summary

Introduction

Global increases in mean temperature (Vavrus et al, 2012) and evapotranspiration are expected to lead to drier conditions in many regions of the world (IPCC, 2013). Heat waves and warm spells are increasing in frequency and duration in most of the world (Perkins et al, 2012), including at high latitudes (Tingley and Huybers, 2013). Large parts of Alaska and Canada have been subjected to a substantial drying over the last 50 years (as shown by the increase in the Palmer Drought Severity Index using the Penman–Monteith equation, PDSI-PM, Sheffield et al, 2012). It is very important to understand extreme weather events such as drought and warming as they may have a disproportionate impact on ecosystem structure and function compared to climate change trends, exceeding the lethal thresholds for organisms more rapidly (Marchand et al, 2006a; Jentsch et al, 2007). Zona et al.: Delayed responses of an Arctic ecosystem to an extreme summer

Methods

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Discussion

Conclusion