Methods for Collection, Development, and Use of Vegetation Response Variables
Kari F. Cretini1, Jenneke M. Visser2, Ken W. Krauss3, and Gregory D. Steyer1
1U.S. Geological Survey, National Wetlands Research Center, Coastal Restoration Field Station, Baton Rouge, LA 70803
2Department of Renewable Resources and Institute for Coastal Ecology and Engineering, University of Louisiana Lafayette, Lafayette, LA 70506
3U.S. Geological Survey, National Wetlands Research Center, Lafayette, LA 70506
In 2003 the Coastwide Reference Monitoring System (CRMS), a network of 392 monitoring sites along the coast of Louisiana, was implemented under the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA; Steyer and others, 2003, 2006). The CRMS monitoring sites characterize the Louisiana coastal wetland landscape as they are located within all marsh types (fig. 1). The CRMS network also represents the coastal Louisiana restoration project area as sites are located inside and outside of other CWPPRA projects (fig. 1). Sites located outside projects are used as references for project effectiveness to be assessed against. Comparisons at other spatial scales including marsh type and hydrologic basin are also possible through CRMS (Steyer and others, 2003, 2006).
To assess performance of restoration projects associated with CRMS it is important that we use existing knowledge to establish appropriate restoration targets (that is, goals). The existing knowledge includes descriptions of community and plant responses from published literature and the extensive dataset maintained by the CRMS analysis teams and their collaborators. These sources can help us define how a wetland community type is expected to respond to natural variations in major environmental drivers and help us develop a suite of indices to assess wetland condition. Individual indices for vegetation, hydrology and soils will be developed to assess the condition of these parameters at a CWPPRA project or group of projects. The indices along with other community reponse variables can be used to assess restoration project effectiveness.
In this document, we focus on emergent vegetation. Emergent vegetation, sampled within CRMS marsh and forested wetland sites, provides a description of the community types of coastal Louisiana over time (Folse and others, 2008). Several vegetation response (that is, performance) variables (table 1) are used to document subtle changes in vegetation assemblage associated with either natural degradation or restoration projects. A current conceptual model (fig. 2) of the environmental drivers of plant performance identifies flooding, salinity, and nutrient availability as the key components in coastal Louisiana ecology. The first two environmental drivers, flooding and salinity, are monitored through CRMS. The prevailing idea among project managers is that restoration projects can affect flooding and salinity by (1) altering the amount of water exchange between the project and surrounding area and/or (2) changing the project area elevation through sedimentation, nutrient gains, or nutrient deficits. Although nutrient availability is not currently monitored, it may have nutrient feedbacks that can indirectly alter wetland elevation gain (fig. 2; McKee and others, 2007).
The purpose of this document is to provide to those associated with CRMS (that is, Federal and State sponsors, project managers, landowners, data users etc.) with the following: (1) the collection and development methods for the vegetation response variables and (2) the ways in which these response variables will be used to evaluate CRMS project and program effectiveness. New response variables may be added, or current response variables may be removed, as data become available and as our understanding of restoration success indicators develops.
Figure 1. Louisiana coastal area with CRMS sites and CWPPRA project areas depicted. CRMS sites are displayed according to the 2008 marsh type. Table 1. Overview of vegetation response variables and applicable wetland type for CRMS assessment
A complete description of the methods and sampling design for collecting vegetation data in emergent marsh and forested wetlands can be found in Folse and others, (2008). Summaries of these methods and designs for emergent marsh and forested wetland sites are given below.
Sampling is conducted within ten 2 m × 2-m vegetation stations along a 282.8-m transect within each 200 m × 200-m CRMS site (Folse and others, 2008). The same stations are sampled on repeat visits unless the station is lost or destroyed by a natural or human disturbance. Within each vegetation station the cover of each plant species is visually estimated near the end of the growing season (August 1 to September 30) by following the Braun-Blanquet cover scale (table 2). Total vegetation cover of each station and cover of each layer (that is, tree, shrub, herbaceous, carpet) is estimated between 0 and 100 percent (Folse and others, 2008). The sum of each vegetation layer may exceed 100 percent because of overlapping canopies. The average height of the dominant (that is, greatest percent cover) species is measured in each vegetation station. Plant species nomenclature follows the USDA PLANTS Database (USDA, NRCS, 2008).
Table 2. Vegetative cover values for the Braun-Blanquet method (reproduced from Folse and others, 2008).
Sampling within forested wetland sites is conducted within three 20 m × 20-m forest stations along a 282.8-m transect within each 200 m × 200-m CRMS site (Folse and others, 2008). Within each station the herbaceous, understory, and canopy layers are sampled during the fall (August 1 to October 31) by using a nested sampling design (Folse and others, 2008). In each forest station there are three 6 m × 6-m understory stations, each of which contains one 2 m × 2-m herbaceous station. Canopy and understory layers are sampled every 3 years while herbaceous species layers are sampled each year. Vegetation sampling within the nine herbaceous stations is identical to the sampling procedure within marsh sites (see previous section). For understory layers, the number and height of all woody shrubs and trees <5 cm diameter at breast height (DBH) within each of the nine understory stations are recorded. For canopy layers, tree species >5 cm DBH are identified and the DBH (137 cm above the forest floor) is measured within each of the three canopy (that is, forest) stations. Canopy cover is also collected with a spherical densiometer by averaging cover in the four cardinal directions at the center of each CRMS plot. Hemispherical photography will also be used beginning in spring 2009 to provide data on percent cover and structure of overstory vegetation.