**7. Discussion and conclusion**

Laser altimetry is currently the only technique capable of measuring tree heights in closed canopies and therefore offers a remote and non-destructive means of estimating vegetation volume, biomass or carbon content to account for vegetation distribution. This avoids difficulties posed by inaccessibility, time or cost-intensive field campaigns.

The replacement of current field-based methods is not contemplated as a realistic option, however, data collection in the field can be made more effective and targeted as a result of lidarbased inventories. This is already happening in Norway for example, where 90% of stand inventories are being made in relation to lidar surveys (E. Næsset, personal communication).

At present, the retrieval of stand and individual tree parameters is highly dependent on field data collection for the calibration and validation of a sensor's estimates. However, the most efficient use of lidar will require a deeper understanding of the phenomenology of tree interception of the laser hits and how this relates to the physical characteristics of the vegetation being monitored. This understanding can be improved using lidar simulation models. This may offer the possibility to construct more widely applicable height and diameter recovery models using current allometric relationships derived from models or by observations from a network of nationwide permanent sample plots.

The recognition of the importance of biomass mapping and the significant contribution of lidar data for this purpose are demonstrated by the investment and commitment by the US Congress to research in this field at both county and national scales through the NASA-led Carbon Monitoring System initiative (NASA, 2010). This project integrates the use of multiple datasets to generate national and county level biomass products. Elsewhere, the investment in airborne lidar by several governments for national scale campaigns further demonstrates the important role that this technology can play in forest inventory and monitoring.

Such means of identifying areas of forest biomass change can offer important contributions to efforts to inform and encourage practices of Reducing Emissions from Deforestation and forest Degradation in developing countries – REDD (Asner *et al.*, 2010; FAO *et al.*, 2008) and to report on Land Use, Land Use Change and Forestry – LULUCF (IPCC, 2003).

As emerging technologies such as photon counting or multispectral lidar sensors come into operation, the capacity for wider coverage and increasingly accurate lidar-derived applications for biomass assessment will further expand.
