What is Phytosociology?

       Phytosociology is the study of the characteristics, classification, relationships, and distribution of plant communities (The American Heritage Dictionary, 3rd ed). It is useful to collect such data to describe the population dynamics of each species studied and how they relate to the other species in the same community. Subtle differences in species composition and structure may point to differing abiotic conditions such as soil moisture, light availability, temperature, exposure to prevailing wind, etc. When tracked over time, species and individual dynamics can reveal patterns of response to disturbance and how the community changes over time.
 

Methods of Phytosociology Study

In this phytosociological study, we took a census of all the woody plants that had a dbh (diameter at breast height) greater than 4.6 cm in our study area on Saltonstall Ridge. We ventured to the site on a few early spring weekends and worked in crews, recording the data and marking measured trees with a lumber crayon to prevent duplicate measurements.
 
 

Our study area consisted of 4 adjacent transects running approximately west to east across the Saltonstall Ridge (See Site Description).  Each transect had 30 10x10meter plots (See Ridge model).  For every woody stem encountered in this study area, we recorded the species, dbh (diameter at breast height: the diameter of the trunk at 137cm from the ground) and the vigor code (where 0=live, 3=sick, 4=dead and 5=snag). (DATA 1999) Slope was also recorded for each plot. We used this information to calculate several descriptive statistics for the Ridge woody plant community.  We also examined the same data collected by our predecessors on the Ridge in 1991 (DATA 1991). Though our study included 4 more plots than the 1991 study (one more on the east end of each transect), we compared the community structure to see how the woody vegetation changed on the Ridge in the last eight years. For both the 1991 and 1999 data, the original data sheets from the field were compared to the raw data entered in the computer to check for entry errors. The computer files were changed as needed.

To assess the accuracy of our field data collection, we re-measured the trees on the first 12 plots on transect 1.  These plots were first examined in January 1999 and re-examined in march 1999.  Errors were extremely small, especially considering the noise created by rough bark or other lumps on the bark.  We were assured that our field data collecting methods were accurate and constant for the study.  For the results, click here.

Most of our summary and descriptive analyses were performed on uncorrected data only. These data are displayed in the graphs. The total basal area per hectare and total density of stems per hectare were also calculated with data corrected for slope and the woods road running along the top of the Ridge. When the data was collected in the field, we ran a 10-meter tape up and down the slopes. If the descriptive statistics were to correspond to a flat, square area (i.e. one square hectare), they would need to be calculated using a different area for each plot (the plots, on a flat scale, were not 10x10 meters square). Also, no woody plants can grow on the woods road. This area was subtracted from the calculations to give area-based statistics without this barren gap.
 
 

	Total basal area (m^2/ha)	Total density (#/ha)
Uncorrected data	20.12	759
Corrected for the road	20.50	789
Corrected for slope	21.37	823

Basal area, relative basal area, density, relative density, frequency, relative frequency, importance value and two diversity indices were calculated (See equations).
 

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Saltonstall Ridge Species

Our study area on the Saltonstall Ridge is a mixed-species hardwood forest with elements of the oak-hickory forest and the northern hardwoods. The trees form a closed canopy which shades most of the ground in the summer. The thin soil on the rocky slopes also supports many understory and non-woody species, such as black jetbead (Rhodotypos scandens), some Rubus species and multiflora rose (Rosa multiflora). For a complete list of the woody species found in our site in 1991 and 1999, click here. 
 

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Results and Discussion
Forest Structure and Composition on Saltonstall Ridge

The expanse of mature woody vegetation in our site spreads across the ridge, broken only by the narrow dirt road that serves as an access route to the Water Authority’s stands. Forest structure (i.e. the distribution of size among trees) and composition (i.e. the species present in the forest) vary greatly on a small (e.g. 10x10meter) scale, but are fairly uniform when the study area is examined as a whole. 

Sugar maple was found on almost all of our study plots, and white ash occurred on more than half. All other species were found on fewer than half of the plots, but none were clustered in any particular area.
 
 
 
 
 
 
 
 
 
 
 
 
 

The distribution of basal area per hectare for each plot in the study area can be seen here.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

To see if any differences in basal area exist between years or across the ridge, we calculated an average basal area for each group of four plots (corresponding plots across the four transects). The slight reductions in basal area from 1991 to 1999 on the western slope of the ridge are due to logging that occurred there between the last study and this year’s study. The slight increases in basal area on the eastern slope show that the trees in this undisturbed section of the forest have grown.

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An examination of tree size (specifically, dbh) reveals how each of the dominant species on the site contributes to the forest structure. Since size is also a fair indicator of tree age, we can see the population dynamics of each species and how the individuals have changed over time.

Fewer live American beech were found in almost all of the size classes in 1999 than were found in 1991. During our study this spring, we noticed many dead and dying beech on the ridge. Since beech is a shade tolerant species and often occupies a dominant position in a mature forest canopy, it is not dying due to the micro-climatic changes associated with forest succession. We also ruled out beech bark disease as the culprit and are at a loss to explain its decline on the ridge.
 
 
 
 
 

The diameter distribution of sugar maple shows the typical "reverse-J" curve of an all-aged distribution. Sugar maple is present in the forest in all size (and thus, age) classes and there is ample regeneration. 

Many of the young black birch found in 1991 have died, but the rest of the trees have simply grown bigger, as seen by the upward shift in the diameter distribution in 1999. The absence of many stems in the smaller size classes in 1999 indicates that black birch is not replacing itself in our study area and a decline in the total density of the species should be seen in the future.
 
 
 
 
 
 
 

The white ash and the hickories, like the black birch, are getting bigger, but also self-thinning, so there are fewer in the small size classes. Neither species is being replaced by younger individuals.
 
 
 
 
 
 
 
 
 
 

No structural pattern is easily discerned from the diameter distribution of the oaks. This may be due to the low number of stems in the study site (small sample size).
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

The remaining woody species have been pooled together and show that these smaller or more infrequent trees, are holding there own in the forest. They are being thinned out through competition as they reach larger sizes, but are constantly being replaced by younger, smaller individuals.
 
 
 
 
 
 
 
 
 
 
 
 
 

Smoothed curves of the diameter distributions of all the species found in 1999 can be seen below. The sugar maple dominates most of the size classes, but white ash, the hickories and the oaks dominate, in turn, over the maples.
 
 
 
 
 
 
 
 
 
 
 

Phytosociology page text and graphs by:
Mariana Upmeyer

Other phyto graphs by:
Tom Siccama

Illustrations by:
Anne Osborn
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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