Forest Degradation

Forest Degradation in the Northern Hemisphere

As the Pleistocene glaciers began to retreat gradually about 18,000 years ago (BP), species of the boreal forest began to move northward in North America and Eurasia. Around 9,000 BP, milder climatic conditions favored expansion of dense forests in northern areas. Since then, the climate has changed, and the ecological conditions allowing northern expansion of forests no longer prevail. These trees were established during warmer climatic episodes a few hundred to a few thousand years ago and have persisted by vegetative reproduction. Presently, boreal forests occupy about 17% of Earth's total land surface in the Northern Hemisphere.

The North American boreal forest belt is represented by three zones: (1) the closed-crown forest [Page 1158]zone, (2) the lichen woodland zone (or taiga), and (3) the forest-tundra zone. The closed-crown forest zone is the southernmost portion of the boreal forest. It contains the greatest richness of species, the warmest soils, the highest productivity, and the longest-growing season. North of the closed-crown forest zone is the lichen woodland zone, represented by open, sparse forest. To the north of the lichen woodland is the forest-tundra zone, which occurs along the northern edge of tree growth or tree line. Patches of trees consisting of only a few species form a complex mosaic with the tundra. Boreal forest is characterized by a limited number of conifer species such as pine (Pinus), spruce (Picea), larch (Larix), and fir (Abies) and several deciduous trees such as birch (Betula) and poplar (Populus).

Figure 1 Model of stand disturbances causing the shift of the closed-crown forest to the lichen-spruce woodland in the closed-crown forest zone

Source: Girard, F., Payette, S., & Gagnon, R. (2009). Origin of the lichen-spruce woodland in the closed-crown forest zone of Eastern Canada. Global Ecology and Biogeography, 18, 291–303.

Notes: The continuous black arrows indicate good regeneration, whereas the black dotted arrows indicate poor regeneration of the closes-crown forest. The dashed black arrows represent a short interval between two disturbances (compounded disturbances). The dashed and dotted arrow indicates a possible reversion of the lichen-spruce woodland into a closed-crown forest.

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