Vegetation Classification and Mapping Project Report - USGS

Appendix E
Plant community and map class key
The plant community and map class key, developed to identify plant communities (NVC associations,
alliances and park specials) and map classes (base, group, management, and macrogroup)
was used in the field during accuracy assessment of the Petrified Forest National Park vegetation
mapping project. It can be used at Petrified Forest NP in areas of at least 0.5 ha in size (e.g.
a circle within a 40-m radius). Most base map classes key to one plant community, but some are
aggregations of more than one plant community and may appear in several places in the dichotomous
key.
Because the group, management, and macrogroup map classes are aggregations of base map
classes, they also appear multiple times in the key. The group class may represent aggregations
of the base map classes to the group level of the National Vegetation Classification Standard
(NVCS), version 2 (2008). Terrestrial ecological systems, as described by NatureServe, were used
as a first approximation of the group level. The macrogroup map classes represent aggregations
of base map classes to the macrogroup level of the NVCS Version 2, as approximated by the
project team. The management map classes were developed in consultation with park staff, with
the intent of preserving high map accuracy while maintaining base map classes that are of importance
to park managers.
The key begins with a lifeform key that separates the observed vegetation or land cover into one
of four lifeform types, each with a subkey. The subkeys consist of decision couplets that describe
the vegetation characteristics and typical geomorphic setting. Communities containing the same
species may be separated into different associations, or even lifeforms, due to differences in cover
and/or habitat. Because of this, both parts of the couplet should be considered before proceeding
through the key. As is the case with a botanical key, often it is important to follow the key a
few couplets beyond the point where you believe you may have correctly identified a map label
or association because you may find a better fit in subsequent steps, or you may gain increased
confidence that your initial choice was correct.
At Petrified Forest National Park, fluctuations in precipitation between drought and wet years
affects the cover of vegetation. Herbaceous perennial plants may be driven into dormancy and
show lower cover than expected. during droughts. Alternatively, a flush of annual forb growth
may follow a wet period. This fluctuations in cover needs to be considered when using a key
developed for average conditions.
Some map labels are based on landforms, such as “Barren Badlands”, where existing vegetation
is very sparse. However, it is important to remember that the vegetation at Petrified Forest
National Park can be very sparse (