Outside the tropics most of these ultra-flood-tolerant swamp forests usually have either a relatively short winter or no winter, and trees do not have the stress of preserving an extensive root system in anaerobic conditions throughout long, non-productive winter periods. In the cold and cool-temperate regions of the world, particularly where the climate is oceanic, flooding is generally unfavourable for tree survival unless the ground is frozen during the period of potential inundation. The few woody species that survive in wetlands in cool oceanic regions are generally bushes and scrub of a limited number of species in which the genera Salix and Alnus are probably the most successful. The alder (Alnus glutinosa) is a typical species of riverbanks and lake edges. A common feature of alder stands is the formation of floodline communities due to the seeds washing up on banks during periods of flooding where they subsequently germinate. Alder seedlings do not establish readily under water and the river or lake edge can provide a suitable refuge from the dangers of flooding during the vulnerable period of establishment. Once established, alders are tolerant of flooding although they develop differently and show the many stemmed (polycormic) form in areas that are frequently flooded as compared with the pole (monocormic) form which is more usual in unflooded sites (see below).
In relation to tissue aeration in trees it should be noted that several species (e.g. birch, poplar, alder) have greenish tissues below the outer peridermal or rhytido-mal layers. These chlorophyll-containing tissues within the stems are able to use the stem internal CO2 and the light penetrating the bark for photosynthesis. When the photo-assimilation process is examined quantitatively
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