Continental-scale consequences of tree die-offs in North America: identifying where forest loss matters most, Swann 2018

Compendium Volume 2 Number 1 July 2018

Vegetation cover affects the amount of solar energy a land area absorbs and/or releases, thus altering local temperatures and precipitation. Plants regulate local temperatures through shading, albedo and evapotranspiration, which releases latent[9] heat.

The ability of a surface to shed energy through latent or sensible heat is key to determining that surface’s temperature – shifts in the relative balance between the two can lead to increases in surface temperatures (where sensible heat is relatively higher) or decreases (where latent heat is relatively higher) [Swann 2018: 2].

This study shows that changes in vegetation cover in a given place affect not only the local climate, but also the climate system at a continental scale. The results are temperature and precipitation changes in remote parts of the continent relative to where the tree loss occurred, leading to changes in ecosystem productivity in those remote parts. This phenomenon is called ‘ecoclimate teleconnections.’

Plants profoundly influence local climate by controlling the exchange of energy and water with the atmosphere. Changes in and/or losses of plant type or plant functioning can alter the local climate, but also potentially large scale climate by modifying atmospheric circulation. … the potentially global impact of plant cover change on other ecosystems as communicated by the atmosphere has been under-appreciated and is only beginning to be evaluated [Swann 2018: 2].

Researchers simulated tree die-offs in their model by replacing all trees in a given domain[10] with grass.

Domain-scale tree loss led to changes in local (within same domain) surface properties and fluxes including albedo and evapotranspiration. These changes in surface properties modified local surface climate (e.g., precipitation and temperature), as well as impacted atmospheric circulation. The atmospheric circulation response connects the direct forcing of tree loss on the local atmosphere to other regions, impacting climate and thus resulting in altered Gross Primary Productivity (GPP) across North America [Swann 2018: 3-4].

Furthermore, the severity of the remote effects of tree loss depends not only on the scale of the tree loss, but also on the location of the tree loss. The study found, for example, that tree loss in an area covering most of California had greater effect on GPP in other parts of the continent than did tree loss of a similar scale elsewhere.

Thus, in addition to the magnitude of forest loss, the location of forest loss plays an outsized role in determining the continental scale impact [Swann 2018: 6].

Swann et al., 2018, Continental-scale consequences of tree die-offs in North America: identifying where forest loss matters most, Environmental Research Letters 13 055014: 

[9] Latent heat is energy released or absorbed in a constant-temperature process. For example, evaporation releases latent heat from a surface through the transformation of water into vapor, where the vapor carries energy off the surface. By contrast, sensible heat can be felt and directly affects the temperatures on the body where it resides.

[10] ‘Domain’ refers here to each of the 13 most densely forested bioclimatic regions in the US, as identified by the US National Ecological Observatory Network [Swann 2018: 2].

For the full PDF version of the compendium issue where this article appears, visit Compendium Volume 2 Number 1 July 2018