How does elevated tropospheric ozone exposure affect plant health over time?

Tropospheric ozone forms via photocatalysis of a free radical mechanism between NOx (NO and NO2) hydrocarbons and Volatile Organic Compounds (VOCs)1, resulting from industrial and transport pollution. Tropospheric ozone pollution has increased globally since 1950s2, and although ozone precursor emissions have plateaued in Europe and North America since the 1990s, long-range transport of ozone pollution produced in Asia and South America continues to raise European baselines3. Ozone exposure poses a major risk to both human4 and plant health5, as well as having a radiative forcing effect6.
Ozone exposure causes oxidative stress in vegetation, leading to the increased release of VOCs by plants7 which in turn increase ozone production via a positive feedback effect8. Other key impacts of ozone on vegetation are early leaf drop (defoliation) and reduced growth. Ozone exposure affects critical growth stages, increases drought sensitivity and reduces the positive impact of nitrogen on root biomass accumulation9. At an ecosystem level, ozone exposure decreases primary productivity by limiting transpiration10 via stomatal closure. Ozone damage in forests can be monitored remotely via crown defoliation, discoloration and foliar injury.
The Joint UK Land Environment Simulator (JULES) is a land surface model developed as a component of the Met Office and NERC Earth System Modelling Strategy. JULES currently uses the impact of stomatal conductance on growth as the single ozone linked parameter (ozone exposure reduces conductance), however the effects of ozone pollution on vegetation are complex11. The impact of chronic versus acute ozone exposure is poorly understood. Additionally, questions remain about long-term tree responses to changing ozone exposure. For example, to what degree do trees adapt to rising ozone baselines in terms of growth? There is debate as to whether ozone exposure limits plant CO2 uptake, or whether decreased stomatal flux under enhanced CO2 protects vegetation from ozone exposure.
A better understanding of how changing ozone exposure affects vegetation in ecosystems would allow Earth system models, such as JULES, to better represent how plants respond to ozone fluxes across temporal and spatial scales. This improved representation of near surface ozone pollution would allow Earth system models to better predicts future carbon balances in a changing climate12.

Chapter questions:
1). What changes in global vegetation health can be attributed to ozone exposure via satellite data analysis?
2). How does ozone exposure translate to physiological changes in trees? Do trees acclimate to chronic ozone exposure?
3). How has changing ozone exposure over long time scales affected growth patterns in trees?
4). How can the vegetation response to ozone in Earth System Models be improved to include longer term growth responses, as well as differences in susceptibility to ozone exposure potentially linked to past exposure?