Thermodynamic basis for the demarcation of Arctic/alpine treelines

Abstract:

At the edge of alpine and Arctic ecosystems all over the world, there is a region in which it is either infeasible or unfavorable for trees to exist, colloquially identified as the treeline. There are many theories attempting to explain this phenomenon, indicating possible drivers such as excessive light, high winds or low temperatures. We explore the possibility of a thermodynamic basis behind this demarcation in vegetation composition comprising of functional groups (FGs) to supplement the scientific understanding of these physical mechanisms by considering ecosystems as open thermodynamic systems. Recent research¹ points to the utility of this approach in characterizing the thermodynamic advantage – defined as increased work efficiency (WE) and entropy flux – associated with the coexistence of multiple FGs, such as trees and shrubs in a forest. Given local availability of energy, water and nutrients, ecosystems have a higher probability of naturally self-organizing to this multiple FG state.

If we consider a meadow or tundra as an ecosystem with only one FG and a forest as one with multiple, we can utilize a similar approach to deduce whether the existence of trees continues to have a thermodynamic advantage beyond a treeline. To study this we construct counterfactuals in which trees exist on an alpine meadow or Arctic tundra. Using eddy covariance data from the Italian Alps, Colorado Rocky Mountains and Taiga Plains of Canada, we model and compute ecosystem entropy flux, work and WE – the ratio of outgoing latent and sensible heat to incoming energy flux – at sites on both sides of each treeline with and without trees. Results indicate that the observed meadow/tundra sites have greater WE than the same sites with simulated trees. This demonstrates that the inability of an ecosystem’s environmental conditions to support the existence of trees in turn causes this structure to have a thermodynamic disadvantage. Thus, we assert that an ecosystem will self-organize towards the most advantageous vegetation structure, which is inherently driven by its available energy, nutrients and water but made possible by thermodynamic feasibility.

¹ Richardson and Kumar (2020) Discerning the thermodynamic feasibility of the spontaneous coexistence of multiple functional vegetation groups. Under revision for Nature Scientific Reports