Ph.D Thesis


Ph.D StudentRohatyn Shani
SubjectEvaluating the Potential Benefits of Large-Scale Forestation
Action in Drylands as a Climate-Change Mitigation
Strategy
DepartmentDepartment of Civil and Environmental Engineering
Supervisors PROF. Yohay Carmel
PROF. Dan Yakir


Abstract

Forests have complex interactions with the climate system, with implications for the Earth's radiation balance. Forest growth affects the climate in opposite ways. Through its biogeochemical processes, it accumulates large amounts of carbon and thus mitigates the greenhouse effect. Forests are often darker than their surroundings. Thus, forest growth lowers surface reflectance (albedo), increases radiation absorption, and adds heat to the surface. These contrasting effects are not well understood or quantified at present, particularly in dry regions.

Drylands are defined as areas where the ratio of annual precipitation to potential evapotranspiration (aridity index) is below 0.65. Drylands cover ~41% of the global land surface. Forestation actions (afforestation and reforestation) in drylands are widely used to combat desertification, and as a means for climate mitigation. However, the net climatic effect of forestation actions is unknown, and in some areas, the net effect may even be warming.

In the first part of the research, I calculated the climate-related benefits of forestation across the short-distance and steep aridity gradient of Israel (~200km). This provided insights regarding small-scale variations of forestation climatic benefits, which were previously only considered at large spatial scales. I showed that the balance between cooling biogeochemical effects and warming biogeophysical effects in forested and non-forested ecosystems can vary dramatically across this gradient. Interestingly, the time required for the forestation cooling effects to compensate for its warming effects decreased from >200 years in the driest site to ~70 years in the intermediate site, and ~40 years in the wettest site. Afforestation was therefore found to be less climatically beneficial in drier conditions. 

Next, I integrated the variations found in the small-scale analysis into an assessment of climatic benefits from large-scale forestation, based on remote-sensing tools and metrics previously established in the literature. I developed an approach to identify suitable land for forestation in drylands, and then to quantify its climate mitigation potential, by estimating its Net Equivalent Stock Change (NESC) over 80 years of the forest lifetime, accounting for both carbon sequestration and albedo changes.

I evaluated this approach first as a regional case study for Queensland, Australia, and then implemented it for the entire global semi-arid and dry sub-humid biomes. The results from the case study indicated a relatively high potential of dryland forestation to offset Queensland's greenhouse gas emissions; NESC was equivalent to 15% of the projected carbon emissions. In the global analysis, the results were different; almost 50% of the forested land was found to have a net warming effect, even after 80 years of forestation, and the remaining 50% would offset approximately 1% of the projected global emissions. These results are considerably lower than previously estimated in the literature, where the albedo effect was not accounted for.

These findings highlight the importance of robust, spatially explicit assessment of large-scale forestation projects in drylands. Specifically, the albedo warming effect must be accounted for when assessing forestation value to climate mitigation.