07/10/2026
By Ali Fallahmaraghi

The Kennedy College of Sciences, Department of Environmental, Earth, and Atmospheric Sciences, invites you to attend a doctoral dissertation defense by Ali Fallah on “The Impact of Vegetation Changes on Climate Extremes: A Modeling Investigation of Physiological Forcing and Phenological Shifts."

Candidate Name: Ali Fallah
Degree: Doctoral
Defense Date: Wednesday, July 29, 2026
Time: 2 – 5 p.m.
Location: Room 212, Olney Science Center, North Campus

Thesis/Dissertation Title:
The Impact of Vegetation Changes on Climate Extremes: A Modeling Investigation of Physiological Forcing and Phenological Shifts

Committee:

  • Advisor Christopher Skinner, Department of Environmental, Earth and Atmospheric Sciences, University of Massachusetts Lowell
  • Mathew Barlow, Department of Environmental, Earth and Atmospheric Sciences, University of Massachusetts Lowell
  • Jeffrey Basara, Department of Environmental, Earth and Atmospheric Sciences, University of Massachusetts Lowell
  • Gabe Kooperman, Department of Geography, University of Georgia

Brief Abstract:

Vegetation is undergoing widespread changes across the globe, with shifts in phenology, structure, and function affecting growing season length, vegetation greenness, and plant water use efficiency. These changes, in turn, have far-reaching consequences for climate extremes by altering surface energy fluxes, air temperature, drought patterns, and wildfire risk. These vegetation shifts are driven by rising atmospheric carbon dioxide (CO₂) through two pathways: the direct effects of CO₂ fertilization and altered stomatal conductance, and the indirect effects of warming-driven changes in phenology. Understanding the relative contributions of these vegetation-driven changes versus CO₂ radiative effects is essential for improving our mechanistic understanding of vegetation-climate interactions and reducing uncertainties in future climate projections produced by Earth system models (ESMs).

Using climate model experiments with the Community Earth System Model (CESM), this dissertation investigates the complex role of vegetation in shaping high-impact weather and climate extremes, with the goal of constraining vegetation-driven climate uncertainty in models. Chapter 1 analyzes how vegetation responses to rising CO₂ influence soil moisture and the likelihood and characteristics of future flash droughts in the Northern Hemisphere mid-latitudes. By isolating the influences of CO₂ fertilization and CO₂ stomatal conductance effects from CO₂ radiative forcing, it finds that CO₂-induced changes to plant characteristics are of sufficient magnitude to modify flash drought characteristics; that CO₂ fertilization effects counteract the CO₂ stomatal conductance effects on projected flash drought occurrence; and that the combined influence of the vegetation response to rising CO₂ can either amplify or counteract CO₂ radiative-driven flash drought changes depending on location. Chapter 2 uses an ensemble of idealized experiments with CESM to isolate the climate response to an imposed one-month advance in spring vegetation phenology, independent of CO₂ radiative and physiological forcing. 

The simulations show that earlier phenology enhances spring transpiration across much of the mid-latitudes, cooling mean temperatures by roughly 1°C and reducing the frequency of occurrence of extreme heat by 2 to 3 days each spring. The same early-season water use depletes soil moisture, increasing anomalously dry soil days in spring and summer by as much as 4 to 8 days per month, so that earlier leaf-out mitigates one extreme while exacerbating another. Chapter 3 examines how these phenological shifts influence fire weather and wildfire risk in the western U.S., coupling the phenology-shifted simulations to an observationally trained burned area model. 

It finds that earlier leaf-out significantly dries the fire-season land surface, intensifying spring evapotranspiration and depleting soil moisture, but that this signal weakens as it propagates toward the fire season, leaving fire-season vapor pressure deficit, the dominant control on burned area, essentially unchanged. As a result, the regional mean change in burned area is not statistically significant, with the strongest candidate response confined to the Northern Rockies, indicating that the wildfire response to earlier phenology is muted relative to the soil moisture response because the land surface signal weakens before it reaches fire-season vapor pressure deficit. Overall, this work highlights the important contribution of phenology and physiology to the climate system response to CO₂ change and underscores the need to account for vegetation dynamics when projecting future high-impact climate events in Earth system models.