Carbon dynamics in arctic vegetation
View/ Open
Date
24/11/2011Author
Street, Lorna Elizabeth
Metadata
Abstract
Rapid climate change in Arctic regions is of concern due to important feedbacks
between the Arctic land surface and the global climate system. A large amount of
organic carbon (C) is currently stored in Arctic soils; if decomposition is stimulated
under warmer conditions additional release of CO2 could result in an accelerating
feedback on global climate. The strength and direction of Arctic C cycle - climate
feedbacks will depend on the growth response of vegetation; if plant growth increases
some or all of the extra CO2 emissions may be offset. Currently the Arctic is thought to
be a small net sink for CO2, the expected balance of terrestrial C sinks and sources in
the future is unknown. In this thesis I explore some of the critical unknowns in current
understanding of C cycle dynamics in Arctic vegetation.
Quantifying gross primary productivity (GPP) over regional scales is complicated by
large spatial heterogeneity in plant functional type (PFT) in Arctic vegetation. I use
data from five Arctic sites to test the generality of a relationship between leaf area
index (LAI) and canopy total foliar nitrogen (TFN). LAI and TFN are key drivers of
GPP and are tightly constrained across PFTs in Low Arctic Alaska and Sweden,
therefore greatly simplifying the task of up-scaling. I use data from Greenland, Barrow
and Svalbard to asses the generality of the LAI-TFN relationship in predicting GPP at
higher Arctic latitudes.
Arctic ecosystems are unique among biomes in the large relative contribution of
bryophytes (mosses, liverworts and hornworts) to plant biomass. The contribution of
bryophytes to ecosystem function has been relatively understudied and they are poorly
represented in terrestrial C models. I use ground based measurements in Northern
Sweden to fill an existing data gap by quantifying CO2 fluxes from bryophytes patches
in early spring and summer, and develop a simple model of bryophyte GPP. Using the
model I compare bryophyte GPP to that of vascular plants before, during and after the
summer growing season, finding that productive bryophyte patches can contribute up
to 90 % of modelled annual GPP for typical vascular plant communities at the same site, and that the relative magnitude of bryophyte GPP is greatest in spring whilst the
vascular plant canopy is still developing.
Understanding how GPP relates to plant growth is important in relating remotely
sensed increases in Arctic ‘greenness’ to changes in plant C stocks. I use a 13C pulselabelling
techniques to follow the fate of recently fixed C in mixed vascular and
bryophyte vegetation, with a focus on quantifying the contribution of bryophytes to
ecosystem carbon use efficiency (CUE). I show that bryophytes contribute significantly
to GPP in mixed vegetation, and act to increase ecosystem CUE. I highlight the
importance of including bryophytes, which do not have roots, in aboveground:
belowground partitioning schemes in C models.
To further explore C turnover in bryophytes, I use the results of a second 13C
labelling experiment to develop a model of C turnover in two contrasting Arctic mosses
(Polytrichum piliferum and Sphagnum fuscum). I find significant differences in C
turnover between Polytrichum piliferum which respires or translocates about 80 % of
GPP, while Sphagnum fuscum respires 60 %. This analysis is the first to explicitly
model differences in C partitioning between Arctic bryophyte species.
Finally, I discuss the implications of each chapter for our understanding of Arctic C
dynamics, and suggest areas for further research.