Vegetation thickening in an ecological perspective: Significance to national greenhouse gas inventories

'Vegetation thickening' often refers to an increasing shrub and tree density on many grazed rangelands, woodlands and forests that may or may not have supported such woody plant populations in the past. It is one of several ecosystem changes, including post-clearing re-growth, afforestation and reforestation, which are variants of the same biological phenomenon - the recovery phase of disturbance/recovery cycles that all vegetation undergoes continuously. There are various levels of human influence over both phases. It is important as part of the global carbon cycle and potentially for its implications for implementation of the Kyoto Protocol. Vegetation thickening poses some inventory and carbon accounting challenges in this regard because of difficulties with quantification and attribution. The attribution of carbon sinks to natural, indirect or direct human influence is difficult because of the complex interactions of factors in determining woodland dynamics. The lack of clear scientific distinction of causation requires decisions to be made on how this is calculated in inventories of greenhouse gas emissions. Advantages, disadvantages, workability and dilemmas of five possible accounting approaches to dealing with these human-influenced biological components are discussed. These approaches range from accounting solely for emissions from clearing ignoring complementary re-growth sinks, to full emissions accounting including methane, nitrous oxide and CO₂ emissions from the managed animals and land.