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Statistical analysis of tree growth and some environmental factors of plant communities in a selected area of the coastal western hemlock zone Eis, Slavoj

Abstract

A study of productivity and environment of forest plant communities was carried out in a selected area of the Coastal western hemlock zone. This study is a part of the composite ecological project on this zone, which includes investigations of soils and vegetation by George Lesko and Laszlo Orloci respectively. In the present investigation, climate, site productivity and environmental characteristics of the associations were statistically evaluated using correlation and regression analyses. The purpose of the study was to assess the degree to which the productivity and the plant community are influenced by individual environmental factors as well as by groups of factors. It was found that almost all the stands investigated were severely affected by fire and that most stands in lower altitudes have developed following destruction of the previous stands by fires. The history of major fires was traced back at least 500 years. The pattern of ecosystem forest communities has been used as a basis for the separation of biologically equivalent forest habitats. Microclimates of seven of the most important associations were studied in detail over a period of twelve months. It was concluded that topography is the primary factor influencing soil and water conditions within a given macroclimatic region. This results in the development of a certain microclimate and an accompanying association. The greatest differences among the plant communities were in temperature maxima and relative-humidity minima. Temperature means and minima, humidity means and maxima and rainfall all differed substantially only between the two subzones. Forest stand statistics were compared with conventional stand tables. Site-index curves for Douglas-fir, western hemlock, western red cedar, amabilis fir and Sitka spruce indicate differences between associations, and show typical trends which reflect site quality. It was concluded that a set of physiographic characteristics is typical for each association as well as for each productivity class. However, wide standard deviations and large overlaps indicate that similar physiographic locales may be occupied by different associations and by stands of different productivity. Topographical features were found to be more closely correlated with plant communities than with productivity. Many significant correlations between site index and sixteen environmental factors were found within individual associations. General trends in the site index - environmental factor relationships were also studied. The results of the investigations have shown that it is possible to use many combinations of environmental factors for an estimation of forest productivity and of plant community. However, it was found that, due to high correlations among environmental factors, only two or three characteristics need be used for estimation of either productivity or plant community with an accuracy approaching cases in which many environmental factors were considered. Almost all of the variability of the plant communities studied can be accounted for by differences in the soil and moisture regime. Similar correlation of site index with soil and moisture was found to be substantially lower. Seepage water and soil permeability were found to be the two most important characteristics of both plant community and site index.

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