CHARTER post-doc Andrew Martin (Andrew is in Work Package 4, see a video interview with him here) of the University of Oxford has recently published a paper in which he was the lead author (along with Work Package 4 leader Marc Macias-Fauria and CHARTER project leader Bruce Forbes, among others) in the journal, New Phytologist (read full article here, open access).
The paper is titled ‘Common mechanisms explain nitrogen-dependent growth of Arctic shrubs over three decades despite heterogeneous trends and declines in soil nitrogen availability‘. Andrew’s academic background is in dendrochronology (the science that deals with the dating and study of the annual growth increments, or tree rings, in woody trees and shrubs).
Lead author, Andrew Martin
“The key message that we want to promote with this paper is that dendrochronology has largely been correlative in nature with little work being done towards gaining a more mechanistic understanding. What we have tried to do here is bring in some model fitting and model selection methods to interrogate the data using simple mathematical equations of how we think the theory works to actually tease out the ecological mechanisms that have caused growth patterns over recent decades; that is what is new about this paper”
The paper is derived from the study of 10 individual Salix lanata (the Woolly Willow), from the Yuribei river valley in the southern part of the Yamal peninsula, Russia. The Yuribei empties into the southwest Kara Sea.
Andrew analysed the stable nitrogen isotopes in wood rings of these individuals (for a total of 399 individual measurements) to reconstruct the availability of soil based nitrogen between 1980 and 2013.
Arctic ‘greening’ or ‘shrubification’ has been well established and increasingly pronounced according to multiple studies over the last decade, through increased shrub height, patch infilling and range expansion. The Arctic tundra is experiencing the fastest rate of warming on the planet coupled with dramatic reductions in sea ice extent. While this shrubification has been related to temperature and soil moisture, changes in ‘greenness’ appear to have become progressively decoupled from rising air temperatures, suggesting that other factors are involved. Warming is a major factor, but the situation is more complex and multiple drivers are present.
The paper focusses on examining soil nutrients specifically the presence of nitrogen, which is generally the most limiting nutrient in Arctic soil environments. The paper found that plants are not just passive organisms but they actually engineer their environment. Aside from analysing each plant’s growth rates and local nitrogen signals over time, Andrew undertook mechanistic modelling to test the degree to which plants were able to control their own environments and soil nutrients and found that plant controls on the environment appeared to have been actually quite pronounced.
Andrew found that as the Willow shrubs grew larger, the increasing amount and quality of the litter was such that plants were indeed actively influencing the nitrogen levels of their immediate environment.
Next steps for Andrew in the project include extending the modelling approach in this paper further backwards in time, as well as applying to other Arctic shrub species. Andrew is also leading the creation of a pan-Arctic database of variability in Arctic biodiversity over the last 10,000 years, as a key part of CHARTER Work Package 4.