Current concentrations of tropospheric ozone pollution are decreasing the growth of trees in the northern and temperate mid-latitudes, according to a meta-analysis published today in the journal Global Change Biology. Tree growth, measured in biomass, is already 7% less than the late 1800s, and this is set to increase to a 17% reduction by the end of the century.
Ozone pollution is four times greater now than prior to the Industrial Revolution in the mid-1700s; if modern dependence on fossil fuels continues at the current pace, future ozone concentrations will be at least double current levels by the end of this century with the capacity to further decrease the growth of trees.
The northern hemisphere temperate and boreal forests currently provide an important carbon sink; however, current tropospheric ozone concentrations (O3) and O3 projected for later this century are damaging to trees and have the potential to reduce the carbon sink strength of these forests.—Wittig et al. 2008
The study is the first statistical summary of individual experimental measurements of how ozone will damage the productivity of trees, including data from 263 peer-reviewed scientific publications.
The study found that above- and below-ground productivity were equally affected by ambient O3.
Current ambient O3 (40 ppb on average) significantly reduced the total biomass of trees by 7% compared with trees grown in charcoal-filtered (CF) controls.
Elevated O3 of 64 ppb reduced total biomass by 11% compared with trees grown at ambient O3 while elevated O3 of 97 ppb reduced total biomass of trees by 17% compared with CF controls.
The root-to-shoot ratio was significantly reduced by elevated O3 indicating greater sensitivity of root biomass to O3.
At elevated O3, trees had significant reductions in leaf area, Rubisco content and chlorophyll content which may underlie significant reductions in photosynthetic capacity.
Trees also had lower transpiration rates, and were shorter in height and had reduced diameter when grown at elevated O3.
At elevated O3, gymnosperms were significantly less sensitive than angiosperms.
Taken together, these results demonstrate that the carbon-sink strength of northern hemisphere forests is likely reduced by current O3 and will be further reduced in future if O3 rises. This implies that a key carbon sink currently offsetting a significant portion of global fossil fuel CO2 emissions could be diminished or lost in the future.—Wittig et al. 2008
Wittig V.E., Ainsworth E. A., Naidu S.L., Karnosky D. F., Long S.P. (2008) Quantifying the impact of current and future tropospheric ozone on tree biomass, growth, physiology and biochemistry: A quantitative meta-analysis. Global Change Biology; doi: 10.1111/j.1365-2486.2008.01774.x