Adaptations
The environmental factors affecting trees are climate, soils, topography, and biota. Each species of tree adapts to these factors in an integrated way—that is, by evolving specific subpopulations adapted to the constraints of their particular environments. As discussed above, the major factor is the decrease in temperature with increasing elevation or extremes in latitude. Each subpopulation adapts to this by modifying the optimum temperature at which the all-important process of photosynthesis takes place.
Many tree species that survive in unfavourable habitats actually grow better in more-favourable habitats if competition is eliminated. Such trees have a low threshold for competition but are very tolerant of extremes. For example, the black spruce (Picea mariana) is found in bogs and mountaintops in the northeastern United States but cannot compete well with other trees, such as red spruce (P. rubens), on better sites. Consequently, in the White Mountains of New Hampshire in the northeastern United States, red spruce is found at the base of the mountains and black spruce at the top, with some development of subspecies populations (hybridization) at intermediate elevations.
Competition within a species (and in some cases genus) is often most intense because the individuals compete for the same environmental resources. Since trees are unable to move in search of resources, competition for available space and resources can be important. Competition aboveground centres on light, space, and symbionts (largely pollinators), while that below ground is over water, space, nutrients, and symbionts (microorganisms such as mycorrhizae and nitrogen-fixers).
The ability of a tree to coexist with other members of the species in a given habitat may depend on the diversification of the space and resources they require. In extreme environments, such as are found on mountains and in the subarctic, survival depends on the physical factors of the environment, whereas in more-moderate habitats biotic factors become increasingly important. Flexibility and efficiency of resource use then become more important in determining survival and reproduction.
The concept of species’ niche relates the species or individual to the totality of its environment. The niche for a plant species is the set of environmental conditions that permits a given species to exist based on its morphological, anatomical, cytological, and physiological capacities.