Although the main consideration of policy must be the reduction in pollution at source, there has been an increasing recognition that the biosphere is an important sink for many pollutants, with plant canopies being considered more effective than other land uses. Thus, the biosphere provides benefits additional to those associated directly with their aesthetic and wildlife characteristics. Plants facilitate the uptake, transport and assimilation or decomposition of many gaseous and particulate pollutants. Indeed, the layered canopy structure of trees, which has evolved to maximise photosynthesis and the uptake of carbon dioxide, provides a surface area of between 2 and 12 times greater than the land.
