An Investigation of the Environmental Relationships of Selected Forest Habitat Types in Northern Utah
It was the purpose of this study to examine an assumption basic to the forest habitat type classification system. Included in each habitat type is all land capable of supporting a single climax plant community type. In practice, land is grouped based on species composition, relative abundance, and successional trends of the vegetation supported by the land. Land units of the same habitat type are assumed to represent similar environments. No previous critical evaluation of this assumption has been done. Land in the study area had been previously classified under the habitat type system. Relationships between vegetation and environment were studied in the Abies lasiocarpa/Pedicularis racemosa, Abies lasiocarpa/Osmorhiza chilensis, Abies lasiocarpa/Berberis repens, Pseudotsuga menziesii/Physocarpus malvaceus, Pseudotsuga menziesii/ Berberis repens, and Pseudotsuga menziesii/Cercocarpus ledifolius habitat types. Environmental variables potentially important in determining the vegetation characteristics defining the habitat types were measured over two summers (1977 and 1978) in stands representative of these types. These measurements showed these habitat types to occupy significantly different environments for most environmental variables studied. Environments were more variable between than within the habitat types. Two-dimensional direct gradient analyses for single and multiple environmental variables were compared to ordination results to find which of the environmental variables measured might determine the vegetation gradients indicated by the ordination. Gradients of elevation, maximum and minimum air temperature, and estimated annual incident solar radiation did not correlate well with ordination axes. Best correlation, 0.78 and 0.74 respectively, resulted for summer soil temperature measured at 50 cm for one ordination axis and, for the second ordination axis, a linear combination of soil moisture percentage at 20 cm, estimated percent volume of coarse rock fragments in the soil, and available soil water storage capacity estimated from soil texture class and percent rock. These temperature and moisture variables are felt to be important through their influence on plant moisture stress. Direct measurement of predawn plant moisture stress on conifer saplings did not differentiate between habitat types. Results were highly variable. This was attributed to morphological and microhabitat influences such as disease, rooting pattern and shading which may obscure larger scale environmental differences between stands. It is hypothesized that vegetation in these habitat types responds to environmental gradients that determine the availability of soil moisture to plant roots to meet transpirational demand and atmospheric influences on that demand. Hypotheses of the relationships of the habitat types to these environmental gradients were developed. It is tentatively concluded that the habitat type classification system is effective in stratifying the physical environment in terms of environmental factors which are physiologically meaningful to the vegetation characteristics defining the habitat type classes.
Lawton, Penelope Morgan (1979). An investigation of the environmental relationships of selected forest habitat types in Northern Utah. MS. 102p