Background

Our forests

According to the FAO (2010) approximately 30% of the emerged Earth is covered by forests, which represents 4.03 billion hectares. The definition of forest provided by the Global Forest Resource Assessment, is a land with tree crown cover (or equivalent stocking level) of more than 10 percent and area of more than 0.5 hectares (ha). The trees should be able to reach a minimum height of 5 meters (m) at maturity in situ (FAO 2006).

The distribution of forest is determined by climate variables such as temperature and precipitation, which have resulted in a clear latitudinal gradient of forest distribution.

Pan et al., (2013) The Structure, Distribution, and Biomass of the World’s Forests.  Annu. Rev. Ecol. Evol. Syst. 2013.44:593-622.

Figure 2: Pan et al., (2013) The Structure, Distribution, and Biomass of the World’s Forests.  Annu. Rev. Ecol. Evol. Syst. 2013.44:593-622.

Natural and human-induced disturbance factors, such as fires, insect-outbreaks, windstorms and logging activities, are important agents in shaping the distribution of forests. By continent, Asia, including Russia, holds the largest proportion of forest land, followed by South America, Africa, Central and North America, Europe and Oceania.

Fig 3. Forest distribution by continent

Forest ecosystems play a key role in the global carbon cycle, since carbon is exchanged naturally between forests and the atmosphere through photosynthesis, respiration, decomposition and combustion (IPCC, 2000). At a global scale, forest ecosystems hold approximately 80% of the carbon contained in the aboveground vegetation biomass and 40% of the carbon contained in roots, litter and soils (Dixon et al., 1993).

The projected increases in temperature, changes in rainfall patterns, the modification of atmospheric conditions like cloud cover, and the increase in frequency and intensity of disturbances such as wildfires, as a result of climate change, will also likely impact vegetation growth and regeneration as well as the natural rates of mortality (Chapin et al., 2010).

In addition to their role in the carbon cycle, forests are essential for human live, with approximately 300 M people living in forested areas and 1.6 billion depending on them for their livehoods. Forests are among the most important repositories of terrestrial biological diversity, which is the basis for a great variety of goods and services provided by forests. Forests represent sources of wood and non-wood products, as contributors to soil and water conservation, and as repositories of aesthetic, ethical, cultural and religious values (FAO, 2011).

Forest structure

Forest structure, the 3D arrangement of forest components, has a profound effect on ecosystem functioning and it has been proposed as an Essential Biodiversity Variable (EBV) for monitoring worldwide biodiversity [1]. Despite its importance there is still limited knowledge of the spatial distribution of forest structure and its dynamics. Therefore, in order to develop appropriate sustainable forest management practices and policies for climate change mitigation and adaptation, accurate spatially explicit information on the forest structure and its spatio-temporal dynamics is of paramount importance. Providing explicit information on the position, dimension, quantity and connectivity of forest components is almost an impossible task, especially over large areas. Consequently, forest structure is usually described by a set of variables like canopy height; fractional cover (FC); canopy gap size; clumping degree; biomass, an Essential Climate Variable (ECV); species composition or canopy bulk density (CBD), among others.

Remote sensing, which can be defined as the science of obtaining information of the Earth, or an object in general, without being in contact with it, has the potential to provide this critical information at the adequate temporal and spatial scales. Yet, methods have to be developed.

See the links on the Remote Sensing Tutorials section for online tutorials on remote sensing.

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