| dc.description.abstract | The tundra is currently warming twice as rapidly as the rest of planet Earth, which is
thought to be leading to widespread vegetation change. Understanding the drivers,
patterns, and impacts of vegetation change will be critical to predicting the future
state of tundra ecosystems and estimating potential feedbacks to the global climate
system. In this thesis, I used plant traits – the characteristics of individuals and
species – to investigate the fundamental structure of tundra plant communities and to
link vegetation change to decomposition across the tundra biome.
Plant traits are increasingly used to predict how communities will respond to
environmental change. However, existing global trait relationships have largely been
formulated using data from tropical and temperature environments. It is thus
unknown whether these trait relationships extend to the cold extremes of the tundra
biome. Furthermore, it is unclear whether approaches that simplify trait variation,
such as the categorization of species into functional groups, capture variation across
multiple traits. Using the Tundra Trait Team database – the largest tundra trait
database ever compiled – I found that tundra plants revealed remarkable consistency
in the range of resource acquisition traits, but not size traits, compared to global trait
distributions, and that global trait relationships were maintained in the tundra biome.
However, trait variation was largely expressed at the level of individual species, and
thus the use of functional groups to describe trait variation may obscure important
patterns and mechanisms of vegetation change.
Secondly, plant traits are related to several key ecosystem functions, and thus offer
an approach to predicting the impacts of vegetation change. Notably, understanding
the links between vegetation change and decomposition is a critical research priority
as high latitude ecosystems contain more than 50% of global soil carbon, and have
historically formed a long-term carbon sink due to low decomposition rates and
frozen soils. However, it is unclear to what extent vegetation change, and thus
changes to the quality and quantity of litter inputs, drives decomposition compared to
environmental controls. I used two common substrates (tea), buried at 248 sites, to
quantify the relative importance of temperature, moisture and litter quality on litter
decomposition across the tundra biome. I found strong linear relationships between
decomposition, soil temperature and soil moisture, but found that litter quality had the
greatest effect on decomposition, outweighing the effects of environment across the
tundra biome.
Finally, I investigated whether tundra plant communities are undergoing directional
shifts in litter quality as a result of climate warming. Given the importance of litter
quality for decomposition, a shift towards more or less decomposable plant litter
could act as a feedback to climate change by altering decomposition rates and litter
carbon storage. I combined a litter decomposition experiment with tundra plant trait
data and three decades of biome-wide vegetation monitoring to quantify change in
community decomposability over space, over time and with warming. I found that
community decomposability increased with temperature and soil moisture over
biogeographic gradients. However, I found no significant change in decomposability
over time, primarily due to low species turnover, which drives the majority of trait
differences among sites.
Together, my thesis findings indicate that the incorporation of plant trait data into
ecological analyses can improve our understanding of tundra vegetation change.
Firstly, trait-based approaches capture variation in plant responses to environmental
change, and enable prediction of vegetation change and ecosystem function at large
scales and under future growing conditions. Secondly, my findings offer insight into
the potential direction, rate and magnitude of vegetation change, indicating that
despite rapid shifts in some traits, the majority of community-level trait change will be
dependent upon the slower processes of migration and species turnover. Finally,
my findings demonstrate that the impact of warming on both tundra vegetation
change and ecosystem processes will be strongly mediated by soil moisture and trait
differences among vegetation communities.
Overall, my thesis demonstrates that the use of plant traits can improve climate
change predictions for the tundra biome, and informs the fundamental rules that
determine plant community structure and change at the global scale. | en |
