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

California Rapid Assessment Method for Wetlands v. 5.0.2 – Chapter 4
4.4.3 Vertical Biotic Structure
A. Definition: The vertical component of biotic structure assesses the degree of overlap among
plant layers. The same plant layers used to assess the Plant Community Composition metrics
(see Section 4.4.2) are used to assess Vertical Biotic Structure. To be counted in CRAM, a layer
must cover at least 5% of the portion of the AA that is suitable for the layer (see Appendix II).
This metric does not pertain to Vernal Pools, Vernal Pool Systems, or Playas.
B. Rationale: The overall ecological diversity of a wetland tends to correlate with the vertical
complexity of the wetland’s vegetation. For many types of wetlands in California, overlapping
layers of vegetation above or below the water surface contribute to vertical gradients in light and
temperature that result in greater species diversity of macroinvertebrates, fishes, amphibians, and
birds. In riparian areas, the species richness of birds and small mammals tends to increase with
the density and number of well-developed, overlapping plant layers. Many species of birds that
nest near the ground or water surface in wetlands commonly require a cover of vegetation at
their nest sites. Multiple layers of vegetation also enhance hydrological functions, including
rainfall interception, reduced evaporation from soils, and enhanced filtration of floodwaters.
In many depressional wetlands and some wet meadows, the detritus of above-ground growth of
low and medium layers of herbaceous plants and emergent monocots tends to get entrained
within the layers as an internal canopy below the maximum height of the upper plant layer.
These “entrained canopies” serve as cover for many wildlife species.
In estuarine wetlands, the entrained canopies entrap debris including coarse plant litter that is
lifted into the canopies by rising tides. As the tide goes out, the material is left hanging in the
plant cover. Over time, these entrained canopies can gain enough density and thickness to
provide important shelter for many species of birds and small mammals that inhabit estuarine
wetlands. Most passerine birds and rails that nest in estuarine wetlands choose to nest below an
entrained canopy because it protects them from avian predators, including owls and harriers.
C. Seasonality: This metric should be assessed late during the growing season.
D. Field Indicators: Vertical structure must be assessed in the field. The vertical component of
biotic structure is commonly recognized as the overall number of plant layers, their spatial
extent, and their vertical overlap relative to the expected conditions.
Plant layers for this metric are determined in the same way as defined in the Plant Community
Metric. Only the maximum height of any vegetation should be used to determine the plant layer
to which it belongs. For example, although a tall tree might span the entire range of all the layers,
it can only represent one layer, based on its overall height. Standing (upright) dead or senescent
vegetation from the previous growing season can be used in addition to live vegetation to assess
the number of plant layers present. However, the lengths of prostrate stems or shoots are
disregarded. I other words, fallen plants are not lifted into a vertical position as evidence of
plant height.
Once the plant layers in the wetland are identified, use the following worksheet and figures to
assess the Vertical Biotic Structure metric. Note that plant layers in perennial saline estuarine
wetlands can be difficult to distinguish, and assessing the entrained canopy in an estuarine
wetland requires close examination of the low plant layers.
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