METHODS OF ANALYZING VEGETATION CHANGE PART TWO:
Methods relevant to assessing historic range of variability of vegetation
The focus here will be on methods that are potentially useful in documenting changes in
vegetation patterns and identifying the causes or processes involved in these changes at time
scales of a few decades to several centuries. Although fossil records (both macro-fossils and
micro-fossils) are important in documenting vegetation changes, the typical millennial time scale
of fossil studies is beyond the scope of this course. An introductory level source on all of these
methods is: D. Egan and E.A. Howell. 2001. The Historical Ecology Handbook: A
Restorationist’s Guide to Reference Ecosystems. Island Press.
A. Short-term historical methods
1. Written documentary (textual) sources include 1) early accounts of European explorers
(typically 16th to 18th centuries); 2) archival documents from colonial authorities (both
civilian and religious); 3) pre-20th century travelers; 4) Native American sources (e.g.
“captivity narratives” from early colonists; transcriptions of Native American oral traditions)
notes and reports of 19th century ethnologists; 5) early scientific and surveying expeditions;
and 6) local histories.
Useful for gross changes, but reliability for subtle change is doubtful.
Discuss: How can the reliability of historical sources be evaluated? How can reporter
bias be assessed?
2. Historical maps of vegetation
Quality and utility are highly variable.
Issues that must be addressed:
a. Spatial scale-- is the map to scale?
b. Positional accuracy-- how can that be assessed?
c. Classification comparability— are the vegetation types comparable in level of detail to
modern classifications?
d. Classification accuracy— how can reliability of the historical map be assessed?
3. Terrestrial photographs
Only available after c. 1850.
If available, excellent for documenting gross changes and generating hypotheses to
explain the changes (especially when combined with other data).
Problems:
a. Locations may not be representative (e.g. along roads, railroads)
b. Changes are not easily quantified
c. Both maps and photographs are only “snapshots” in time and may not be
representative of landscape conditions over longer time scales of decades or centuries.
4. Aerial photographs/remote sensing images
Excellent for quantifying changes in cover type.
Problems:
a. Aerial photographs older than c. 1935 are rare.
b. Rarely allow identification of individual species.
5. Re-measurement of previously surveyed areas
a. Old vegetation surveys for earlier ecological studies or timber inventories.
b. Use of General Land Office surveys
B. Monitoring of permanent plots (including experimental plots such as exclosures)
Considerations in installing permanent plots:
1. High cost—labor intensive, maintenance costs.
2. Location:
representative sites;
objective vs. subjective placement of plots to test particular hypotheses;
accessibility and ease of re-location by future works;
long-term protection of the site.
3. Potential parameters to be measured or recorded:
tree species identification and permanent markers on trees;
diameter at breast height (dbh; accuracy);
tree locations (x and y coordinates);
tree ages and ring width samples for growth measurements;
tree heights;
understory composition, tree seedling populations (often with smaller plots);
importance of clear measurement instructions (e.g. size class definitions of
seedling vs. tree, measurement of bifurcated trunks, how to measure dbh of trees
with buttresses, etc.)
4. Experimental permanent plots
if the cost of using permanent plots is justified, then maximum value is usually
obtained by making the permanent plots part of an experimental design (e.g.
trenching experiments, silvicultural treatments, animal exclosures).