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.

For a given species there are limiting values for each environmental factor; these define the niche. Habitats change over time, but changes in species are not as rapid or drastic as those of habitats. In addition to changes that take place within chronological time, tree species and forests change during developmental time—for example, seedlings of trees such as white pine (Pinus strobus) are generally more tolerant of shade than are the adult forms of the species.

Competition between trees is actually more severe under limiting conditions (water, nutrients, or light) than it is under toxic conditions. Under toxic pollution levels, the tree may be damaged by the surplus of a single toxic element or condition, and the species least susceptible will be the most successful. Plants that can most fully exploit a habitat tend to dominate it, and, since trees have evolved trunks that allow them access to the aerial environment and massive root systems that permit them to infiltrate the subterranean environment, they dominate much of the biosphere. Trees are at a disadvantage only in drier areas, in Alpine and Arctic environments, and in competition with humans.

The number of species of trees within a forest tends to increase as they approach the Equator. This is due to various environmental factors, including decreased stress in terms of light, temperature, water, and length of the growing season. The productivity and heterogeneity of the habitats also increase in these situations. Moreover, the frequency of disturbance (e.g., storms, floods, landslides, and fires) is greater, as is the response to the disturbance, which also contributes to species diversity in tropical forests.

Trees may respond to their environment in a number of ways, chiefly by morphological and physiological responses as well as by the reallocation of available nutrients and water to those organs in most need. There are usually both genotypic and phenotypic aspects to such physiological and morphological adaptations. Moreover, there is a dynamic equilibrium between genetic stability (the capacity of individuals to produce offspring adapted to the parental environment) and genetic variability (the capacity to produce offspring with requirements that are different from those of their parents). Genetic variability produces some offspring with a greater potential to adapt to new habitats and also to changes induced by the disturbance of the original habitat.

Phenotypic plasticity is a way in which organisms can harmonize the conflict between stability and variability—that is, the way in which the morphological expression of a given genotype varies under different environmental conditions. While forest species must maintain present adaptiveness to the current environment, the future of the species may depend on sufficient variability to adapt to future environments. Further, changes in the ability of a species to utilize the available resources of the environment can have major effects on coexisting species.

The shape of a tree is an ecological construct, since its form is dependent on the habitat and the stresses of the environment. Open-grown trees, such as those in gardens and parks, generally have foliage extending along the length of the trunk (bole) for a considerable distance. Forest trees, on the other hand, compete for growing space and generally have an expanse of foliage-free bole below a more limited tree crown. The aggregate of the tree crowns constitutes the canopy of the forest, and this may be displayed in a single layer or stratified into several layers, depending on the number and kinds of trees that make up the forest.

The ultimate goal of tree ecophysiology is to determine why a certain tree grows where it does. The complex answer includes the following elements: its seed or source; its fitness for survival, growth, and reproduction in that particular habitat; and its ability to compete favourably with other inhabitants of the habitat.

The growth, structure, and composition of a forest are a function of the intensity and quality of light streaming into it. Trees partition the light resource in time and space.

The time dimensions include seasonal, successional, and developmental time. In seasonal time, the time of leafing out and leaf fall and the time of flowering, seed formation, and germination are considered. In successional time, clearings in forests initiate growth in preexisting seedlings and new germinants, which causes progressive changes in the distribution of light and results in changes in species composition over time. In developmental time, changes take place in the physiology and morphology of the tree with age.

Trees can reach or approach adaptation to a specific habitat by different combinations of morphological, anatomical, and physiological traits. The more closely the trees use the same subset of adaptive features, the more strongly they compete with each other for habitat resources. For this reason, trees of the same species compete more strongly with each other on a site than they do with members of other species.