Chapter 1 What is Vegetation?

Vegetation is the spatial manifestation of biological communities on the earth’s surface, usually of primary producers (plants), which in terrestrial settings is stationary and is the majority of total community biomass. Vegetation components are mostly rooted in soil or attached to rock substrate, but epiphytes, plants growing on other plants, can be significant features in humid systems. Aquatic and marine systems may incorporate un-rooted (drifting) primary producers (e.g. floating aquatic plants and pelagic or limnetic algae) or large stationary biological structures created by reef building organisms (coral reefs and oyster beds). Dead biomass in the form of leaf litter, snags (dead standing trees), and logs (down wood) are vegetation byproducts that may inform developmental status and habitat suitability for other organisms.

There are two main attributes of vegetation: structure and composition. Structure is the spatial arrangement of the plant community in 3 dimensions. The horizontal arrangement (xy footprint) is expressed as cover, with is percentage of the ground surface occupied by a vertical projection of the outline of each plant. Plant taxa with greater cover can be said to have greater importance than plants with lower cover. The amount of light energy captured as net annual productivity is scaled by canopy cover with varying intensities depending on leaf area index or approximated by foliar cover. The vertical arrangement of plants is expressed as maximum plant height. Together, horizontal outline and height of each plant can be conceptualized as a cylinder, with total biomass scaled to this volume at varying intensities. Taller plants can intercept light before it can reach shorter plants, and are therefore said to be dominant.

For trees, biomass is more directly scaled with basal area (cross-sectional area of stems per unit land area) multiplied by height. Stem quadratic mean diameter or dbh (diameter at breast height) of all trees in a stand, scales with stand age, and is important for estimating stand stem density if basal area is known.

Height in vegetation inventory is organized into strata. The most basic is the distinction between overstory and understory (see figures 1 and 2). However, the US National Vegetation Classification (FGDC 2008) prescribes a tree stratum (5+ m), shrub stratum (0.5-5 m), and a herb or field stratum (0-0.5 m) for all terrestrial vascular plants, a non-vascular or ground stratum (0 m) for bryophytes and lichens, and a submerged stratum (< 0m) for aquatic plants. Stratum membership is based on the height of perennial buds or annual seeds, therefore a herbaceous plant is always considered to be in the field stratum regardless of the height of the plant, unless it is established as an epiphyte in the canopy of a woody plant. In contrast, woody plants, may occupy multiple strata, representing different age cohorts of the same taxon or species.

Figure 1. Overstory includes all vegetation taller than 5 m (16.4 ft), and can be subdivided into subcanopy and canopy strata at 15 m (±5 m)
Figure 1. Overstory includes all vegetation taller than 5 m (16.4 ft), and can be subdivided into subcanopy and canopy strata at 15 m (±5 m)
Figure 2. Understory includes all vegetation less than or equal to 5 m (16.4 ft), and is subdivided into a field or herb stratum at less than 0.5 m (1.6 ft) and shrub stratum at 0.5-5 m.
Figure 2. Understory includes all vegetation less than or equal to 5 m (16.4 ft), and is subdivided into a field or herb stratum at less than 0.5 m (1.6 ft) and shrub stratum at 0.5-5 m.

Composition is the taxonomic identity and abundance of the plant taxa making up the vegetation. Abundance can be rated as canopy cover, foliar cover, or biomass. According to the USNVC standard (FGDC 2008), canopy cover is the most universal way to rate vegetation, as it is most closely correlated to estimates of cover from remote sensing (aerial photos).

Functional attributes of plants such as maximum height, stem persistence, structural support needs, leaf shape, and leaf persistence, are classified as habits or growth forms. A given taxon or species is genetically predisposed to a certain habit under optimal conditions, regardless of its current developmental state. Thus, a tree species is always a “tree” regardless of its present small stature or shrub-like appearance due to young age or poor growth conditions. A tree does not get classified as an “herb” or a “shrub” based on its stratum, but should instead be referred to as tree “seedling” and tree “sapling”, respectively. A shrub is expected to only rarely attain the height of a tree. A liana (woody vine) is able to occupy any height stratum, but only as long as it has physical support as evident by trees or shrubs of matching height. A “herb” (forbs and graminoids) is still a herb, even if some grow nearly as tall as a tree.

References

FGDC, Federal Geographic Data Committee. 2008. “National Vegetation Classification Standard, Version 2.” https://fgdc.gov/standards/projects/vegetation/NVCS_V2_FINAL_2008-02.pdf.