Goals & Methods
One of the main goals of the Inventory and Monitoring Project is to have a “good” plant list for all Mass Audubon properties and to periodically update that list. A “good” plant list means that a competent observer has spent enough time on the property at different times during the growing season to be reasonably sure that over 95 percent of the species have been found. A second goal is to be able to detect changes in the vegetation community over time.
The vegetation monitoring on Mass Audubon wildlife sanctuaries includes:
- Species lists to determine presence/absence at a wildlife sanctuary
- Quantitative surveys to detect changes in abundances
Observers record the species they find and the date and amount of time spent on the wildlife sanctuary. A plant list indicates whether a species is present at that wildlife sanctuary but does not give any information on abundance.
Quantitative sampling is more sensitive than plant lists to changes in a natural community of interest. It involves the use of transect lines and plots or quadrats. Quadrats enclose predefined areas so that the abundances of different species can be estimated.
We have used several methods to quantitatively survey vegetation on our wildlife sanctuaries. A brief summary of each is provided.
a. Permanent rectangular plots
Developed in the 1980s by former Mass Audubon plant ecologist Peter Dunwiddie, it involves the creation of a 33x66 ft (20x10m) permanent plot in a natural community of interest.
- The corners of the plots are marked with pieces of rebar that are sunk into the ground so that their top is flush with the surface. Plots are relocated in subsequent years by using a metal detector to find the rebar used to mark the corners.
- Trees within the plot are identified, and the additional trees are identified and measured at set distances along transects outside the plot (“point-centered quarter method”).
- Diameter at breast height (dbh) is measured and canopy heights estimated.
- The species in the shrub and herb layers are identified within 10.7 ft2 (1 m2) quadrats along three transect lines. Nested quadrats of 2.2 ft2 0(0.2 m2) within the 10.7 ft2 (1 m2) quadrats are used for estimating percent cover of each species.
- Covers are recorded as the Braun-Blanquet cover classes. This is a system of categorizing the amount of cover of different species of plants within the quadrat into different cover classes based on a visual estimate of % cover (from 1 to 6 with 1 being 1% cover and 6 being 75-100%.
b. Circular plots
Developed in the 1990s by former Mass Audubon botanist Tom Rawinski, 1,076 ft2 (100m2) plots are used for herbaceous communities and 4,306 ft2(400 m2) for forests.
- The center of an area of interest in marked with rebar or PVC and the plots set up by using a length of rope (18.5 ft for the smaller plots, 37.0 ft for the larger plots).
- Within the plots, percent covers of different plant species are recorded at different strata.
c. Quadrats along transects
Used where the impact of ecological management measures (e.g, prescribed fire, mowing) is being evaluated or where there is an obvious environmental gradient in the community, such as in a salt marsh from the edge of a marsh creek to the upland edge.
- Plants are sampled in 10.7 ft2 (1 m2) quadrats, which have been positioned in intervals (typically 33 ft) along a transect line.
- Percent covers and vegetation height are estimated within the nested quadrats as described in method (a).
d. Hybrid Method
Developed in 2008 by Rebecca Weaver, a University of Massachusetts graduate student who examined Mass Audubon plant community data as part of her master’s thesis.
- Contains elements of both methods (a) and (b).
- Trees are identified and their dbh’s measured in 4,306 ft2 (400 m2) circular plots.
- Seedling, sapling, and shrub abundance are quantified in two circular 108 ft2 (10 m2) plots located 16.5 ft (5 m) from the plot center along the east-west axis of the plot.
- Eight nested quadrants arranged along the north-south axis of the plot are used to quantify the herb layer.
- The percent cover of each species is estimated using cover classes within each of the quadrats.
- Nesting provides a much more reliable estimate of percent frequency of occurrence for future comparisons.
How do we decide which of these methods to use?
All methods provide useful quantitative information about vegetation. The historical record of vegetation captured in methods (a) and (b) above is extremely valuable, so we will continue to use those methods for plots that are already set up.
New vegetation sampling to document the current status and future trends in priority natural communities will use the hybrid method. As in the past, communities with environmental gradients will be sampled with transects (method c above).
Number of existing vegetation plots or transects on Mass Audubon wildlife sanctuaries
# of plots on sanctuaries
|"Dunwiddie" 20 x 10m Plots||83|
|Weaver Hybrid Plots||25|