Scientific Reports, Volume 12, Issue 1

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Springer Science and Business Media LLC
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Thermodynamic basis for the demarcation of Arctic and alpine treelines
Meredith Richardson Martin | Praveen Kumar | Oliver Sonnentag | Philip Marsh

At the edge of alpine and Arctic ecosystems all over the world, a transition zone exists beyond which it is either infeasible or unfavorable for trees to exist, colloquially identified as the treeline. We explore the possibility of a thermodynamic basis behind this demarcation in vegetation by considering ecosystems as open systems driven by thermodynamic advantage-defined by vegetation's ability to dissipate heat from the earth's surface to the air above the canopy. To deduce whether forests would be more thermodynamically advantageous than existing ecosystems beyond treelines, we construct and examine counterfactual scenarios in which trees exist beyond a treeline instead of the existing alpine meadow or Arctic tundra. Meteorological data from the Italian Alps, United States Rocky Mountains, and Western Canadian Taiga-Tundra are used as forcing for model computation of ecosystem work and temperature gradients at sites on both sides of each treeline with and without trees. Model results indicate that the alpine sites do not support trees beyond the treeline, as their presence would result in excessive CO[Formula: see text] loss and extended periods of snowpack due to temperature inversions (i.e., positive temperature gradient from the earth surface to the atmosphere). Further, both Arctic and alpine sites exhibit negative work resulting in positive feedback between vegetation heat dissipation and temperature gradient, thereby extending the duration of temperature inversions. These conditions demonstrate thermodynamic infeasibility associated with the counterfactual scenario of trees existing beyond a treeline. Thus, we conclude that, in addition to resource constraints, a treeline is an outcome of an ecosystem's ability to self-organize towards the most advantageous vegetation structure facilitated by thermodynamic feasibility.

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Earlier snowmelt may lead to late season declines in plant productivity and carbon sequestration in Arctic tundra ecosystems
Donatella Zona | Peter M. Lafleur | Koen Hufkens | Barbara Bailey | Beniamino Gioli | George Burba | Jordan P. Goodrich | A. K. Liljedahl | Eugénie Euskirchen | Jennifer D. Watts | Mary Farina | John S. Kimball | Martin Heimann | Mathias Göckede | Martijn Pallandt | Torben R. Christensen | Mikhail Mastepanov | Efrèn López‐Blanco | Marcin Jackowicz-Korczyński | Han Dolman | Luca Belelli Marchesini | R. Commane | Steven C. Wofsy | Charles E. Miller | David A. Lipson | Josh Hashemi | Kyle A. Arndt | Lars Kutzbach | David Holl | Julia Boike | Christian Wille | Torsten Sachs | Aram Kalhori | Xingyu Song | Xiaofeng Xu | Elyn Humphreys | C. Koven | Oliver Sonnentag | Gesa Meyer | Gabriel Gosselin | Philip Marsh | Walter C. Oechel

Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations in the Arctic has limited our understanding of the impact of covarying environmental drivers on the carbon balance of tundra ecosystems. In this study, we address some of these uncertainties through a novel record of 119 site-years of summer data from eddy covariance towers representing dominant tundra vegetation types located on continuous permafrost in the Arctic. Here we found that earlier snowmelt was associated with more tundra net CO2 sequestration and higher gross primary productivity (GPP) only in June and July, but with lower net carbon sequestration and lower GPP in August. Although higher evapotranspiration (ET) can result in soil drying with the progression of the summer, we did not find significantly lower soil moisture with earlier snowmelt, nor evidence that water stress affected GPP in the late growing season. Our results suggest that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing season length may not materialize if tundra ecosystems are not able to continue sequestering CO2 later in the season.