Description

Background:

Tropospheric ozone forms via photocatalysis of a free radical mechanism between NO_x (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. Our research will also help to inform reforestation management by providing insight into the susceptibility of natural woodland to ozone damage.

This project is in collaborative partnership with Centre for Ecology and Hydrology (CEH) and Rutherford Appleton Laboratory (RAL) and aims to answer the following key questions:

1). What changes in global vegetation health can be attributed to ozone exposure via satellite data analysis? (in collaboration with RAL)

We will perform preliminary analysis of global trends in tropospheric ozone patterns and vegetation indices over 1995-present from satellite data newly made available by our partners at RAL Space. Normalized Difference Vegetation Index (NDVI) will be used as a measure of vegetation greenness to assess health. Solar Induced Fluorescence (SIF) is a novel remote sensing signal which will be used to directly assess photosynthetic rates. We then will use fingerprint attribution analysis to assess the impact of tropospheric ozone pollution on plant health indices, by comparing observed data to JULES predictions with and without the ozone damage element. Attribution analysis will reveal how strong the global signal of ozone damage is, and how well the JULES model currently captures ozone impact on vegetation.

2). How does ozone exposure translate to physiological changes in trees? Do trees acclimate to chronic ozone exposure? (in collaboration with CEH)

We will test the potential physiological mechanisms underpinning the observed trends in ozone exposed vegetation by investigating tree growth and gas exchange within paired UK sites of high and low ozone damage. High ozone damage sites will display the impacts of acute ozone exposure. To compare the effects of chronic ozone exposure, we will also take physiological measurements from trees under artificially enhanced ozone concentrations and controls at the Bangor free air manipulation site via collaboration with our partners at CEH. Trees from the paired sites and the ozone manipulation site will be investigated using leaf level measurements and growth analysis.

At leaf level, stomatal conductance and photosynthetic rate will be measured on living leaves using porometry and fluorometry; stomatal density will also be assessed. Leaf spectroscopy will also be used to link field measurements to the absorbance spectra in satellite data¹³.  These measurements will help explain how observed changes in annual growth link to the stomatal responses to ozone as currently represented in earth system models.  Linking these two timescales, quantitative wood anatomy will reveal detailed wood biomass accumulation throughout a growth season for trees under high and low ozone stress. Stable carbon isotopes in this wood will be analyzed to quantify the balance between stomatal conductance and photosynthetic rate over the growth season¹⁴.

3). How has changing ozone exposure over long time scales affected growth patterns in trees?

We will examine vegetation responses to changing ozone exposure over decadal to century timescales using dendrology records. Tree ring width records annual growth, and stable carbon isotopes within rings record the past balance between stomatal conductance and photosynthetic rate per growth season¹⁵. These measurements will be taken for tree rings from the paired sites. Ozone through time will be estimated using chemical atmospheric modelling by our partners at CEH.  The resultant time series of ozone and dendrology data will be used to test hypothesized relationships between growth, stomatal conductance and ozone exposure through time. This will be used to affirm or challenge the current stomatal model for ozone exposure in JULES.

Long lived species such as Oak can provide multi-century chronologies in dendrology. We will use long chronologies to expose trends in annual tree growth from pre-industrial era to present, a period of significant change in ozone concentration. We will compare the dendrology of trees in areas which transitioned from rural to industrial with areas which have remained rural, in order to isolate the effect of localized ozone pollution on tree growth.

4). How can the vegetation response to ozone in Earth System and/or Forestry Models be improved to include longer term growth responses, as well as differences in susceptibility to ozone exposure potentially linked to past exposure?

The combination of global satellite data and physiological investigations will allow broad conclusions to be made about the mechanisms by which ozone exposure impacts vegetation across varying spatial and temporal scales. This improved understanding will be formulated into new ways of representing the ozone-vegetation component of the JULES model and tested in the IMOGEN system before full implementation in the UK Earth System Model. The improved ozone-vegetation component will be tested by comparing the accuracy of the ozone damage signal in attribution analysis before and after this improvement. The model will allow the impact of continued ozone pollution on global and UK carbon sequestration potential to be estimated.

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