Species-specific temporal variation in photosynthesis as a moderator of peatland carbon sequestration

Aino Korrensalo,Tomáš Hájek,Timo Vesala,Janne Rinne,Eeva-Stiina Tuittila,Lauri Mehtätalo,Ivan Mammarella,Pavel Alekseychik

doi:10.5194/bg-14-257-2017

Abstract

Abstract. In boreal bogs plant species are low in number, but they differ greatly in their growth forms and photosynthetic properties. We assessed how ecosystem carbon (C) sink dynamics were affected by seasonal variations in the photosynthetic rate and leaf area of different species. Photosynthetic properties (light response parameters), leaf area development and areal cover (abundance) of the species were used to quantify species-specific net and gross photosynthesis rates (PN and PG, respectively), which were summed to express ecosystem-level PN and PG. The ecosystem-level PG was compared with a gross primary production (GPP) estimate derived from eddy covariance (EC) measurements.Species areal cover, rather than differences in photosynthetic properties, determined the species with the highest PG of both vascular plants and Sphagna. Species-specific contributions to the ecosystem PG varied over the growing season, which, in turn, determined the seasonal variation in ecosystem PG. The upscaled growing season PG estimate, 230 g C m−2, agreed well with the GPP estimated by the EC (243 g C m−2).Sphagna were superior to vascular plants in ecosystem-level PG throughout the growing season but had a lower PN. PN results indicated that areal cover of the species, together with their differences in photosynthetic parameters, shape the ecosystem-level C balance. Species with low areal cover but high photosynthetic efficiency appear to be potentially important for the ecosystem C sink. Results imply that functional diversity, i.e., the presence of plant groups with different seasonal timing and efficiency of photosynthesis, may increase the stability of C sinks of boreal bogs.

Highlights

Northern peatlands are a globally important carbon (C) sink and storage of approximately 500 Gt C (Gorham, 1991; Yu, 2012) as a result of an imbalance between photosynthesis and decomposition
Cumulative growing season PG upscaled to the ecosystem level using the separate light response curves for species and months (Eq 1) was 230 g C m−2 (Julian days 121–273)
The similarity of the PG estimates calculated with species-wise and monthly light response curves and gross primary production (GPP) estimates derived from eddy covariance (EC) measurements (Fig. 1a) adds credibility to the methods used and indicates that the photosynthetic parameters measured under laboratory conditions are comparable with field measurements

Summary

Introduction

Northern peatlands are a globally important carbon (C) sink and storage of approximately 500 Gt C (Gorham, 1991; Yu, 2012) as a result of an imbalance between photosynthesis and decomposition. Boreal bogs are peatland ecosystems where photosynthetic productivity is limited by midsummer dry periods, light induced stress and, in particular, low nutrient availability (Frolking et al, 1998; Moore et al, 2002). Due to the low rate of photosynthesis, the annual C sink of boreal bogs is weak and sensitive to changes; even a small change in the environmental conditions that regulate the C cycle can turn the ecosystem into a C source (Waddington and Roulet, 2000; Lund et al, 2012). The rate by which CO2 enters the ecosystem through photosynthesis of all of the individual plants together is the definition of gross primary production (GPP). On the scale of individual plants, the same processes are called gross photosynthesis (PG), plant respiration (R) and net photosynthesis (PN) (Chapin et al, 2011)

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Conclusion