California Rapid Assessment Method for Wetlands - State Water ...

California Rapid Assessment Method for Wetlands v. 5.0.2 – Chapter 4
4.4.2 Horizontal Interspersion and Zonation
A. Definition: Horizontal biotic structure refers to the variety and interspersion of plant
“zones.” Plant zones are plant monocultures or obvious multi-species associations that are
arrayed along gradients of elevation, moisture, or other environmental factors that seem to affect
the plant community organization in plan view. Interspersion is essentially a measure of the
number of distinct plant zones and the amount of edge between them.
B. Rationale: The existence of multiple horizontal plant zones indicates a well-developed plant
community and predictable sedimentary and bio-chemical processes. The amount of
interspersion among these plant zones is indicative of the spatial heterogeneity of these
processes. Richer native communities of plants and animals tend to be associated with greater
zonation and more interspersion of the plant zones.
C. Seasonality: This metric is not sensitive to seasonality.
D. Field Indicators: The distribution and abundance of horizontal plant zones, plus their
interspersion, are combined into a single indicator. For large wetlands, the prominent zonation
is evident in aerial photographs of scale 1:24,000 or smaller. For vernal pools and other
depressional wetlands that are essentially round in plan form, the plant zones might be more or
less concentric. For small wetlands, the zonation is apparent only in the field. The zones may
be discontinuous and they can vary in number within a wetland. Plant zones often consist of
more than one plant species, but some zones may be mono-specific. In some cases, one or two
plant species dominates each zone. In order to score this metric, the practitioner must evaluate
the wetland from a "plan view," i.e., as if the observer was hovering above the wetland in the air
and looking down upon it. Figure 4.7 through 4.10 can aid evaluating the degree of horizontal
interspersion (adapted from Mack 2001), which is rated using Table 4.19a-c.
Low
Medium
Low
Medium
Figure 4.7: Diagram of the degrees
of interspersion of plant zones for
Lacustrine, Depressional, Playas,
and Slope wetlands. Hatching
patterns represent plant zones
(adapted from Mack 2001). Each
zone must comprise at least 5% of
the AA.
High
High
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