This is a simple measure of the amount of forested land in a defined type of watershed. This could be a river basin or a smaller watershed unit, depending on how the term watershed is defined.
Non-plantation (even-aged) forests are typically composed of trees occupying a range of age classes, from seedlings, to trees many decades or centuries old. The number of trees in each age group varies with time since last disturbance, rate and distribution of logging, and competition among trees. Different types of forests will have different ranges of age classes and distribution of trees among age classes.
An important characteristic of forests is the proportion of a region that is composed of different types or groups of types of forest. Forest type is typically determined by dominant plant species or other taxonomic group.
Forests naturally vary in density (i.e. number of trees per unit area). The impact of this density will vary with climatic conditions, soils, tree age/size class, and tree type.
The maintenance of a wide range of size classes of trees and especially large trees can help to protect biodiversity in an area. Different tree species will vary in size compared to each other and may also vary naturally in the distribution of proportion of trees in each size class.
The total forested area in a region can be compared to historic totals in order to understand if forests are in decline, recovering, or becoming too dominant (e.g., in a former grassland) in a region of interest.
While the amount of US forest land has remained nearly unchanged since the beginning of the 20th century, regional changes both in amount and types of forest cover have occurred as a result of changing patterns of agriculture and development. The distribution of various forest cover types is a critical determinant of the condition of forest ecosystems.
Stand structure can be defined as the horizontal and vertical spatial distribution of trees of different diameters and heights in a forest ecosystem. The height differentiation index takes into account variations in tree heights in a forest area. Stand structure heterogeneity (horizontal and vertical) leads to a higher number of species and contributes to higher stability and forest integrity.
This indicator is a measure of the large snag production of an area. Large standing and fallen dead trees are critical habitat for birds and mammalian predators, store carbon, and contribute eventually to soil carbon and nutrient cycling.
Thick (or large) trees provide cover, habitat, carbon storage, high carbon sequestration rates (per tree) and aesthetic benefits. The number of thick (large) trees per acre will naturally vary by species, geomorphic/soil conditions, natural disturbance, and climate conditions.
