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.