A Brief Explanation of the Vegetation Map
The most critical data on these forms was the collection of a complete list of plant species and their corresponding ground covers. The covers were recorded for each species as a relative percentage of the entire surface area of the polygon. This means that the listed covers would add up to a value close to 100% for stands completely covered with dense vegetation, but in the typically sparser vegetation found in the sample area the total was frequently significantly less than 100%. These cover values are the principal characteristic of the stand’s vegetation used to assign a vegetation type.
Vegetation types polygons were determined through field observations and intensive data collection. An experienced vegetation field crew would determine the outer boundaries of a homogeneous patch of vegetation. Then within that patch a complete list of species and their corresponding percent of ground cover was recorded for each species as a relative percentage of the entire surface area of the vegetation patch. This means that the listed covers would add up to a value close to 100% for stands completely covered in vegetation. But it was more typical to find areas far more sparse and most totals were less than 100%. These cover values for every surveyed patch on the Point Loma peninsula were entered into a statistical program that lumps similar data together. The result was a series of vegetation “alliances”, that is a group of species often found together in similar settings. The alliances were typically named after the common one or two dominant species that make up the group. For example, the Chamise-Mission Manzanita alliance is dominated by these two species, Chamise (Adenostoma fasciculatum) and Mission Manzanita (Xylococcus bicolor). But within each alliance there are many more species that occur than just the primary named ones.
Last revised 09-Jun-13