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Site Index estimates by Site Series (SIBEC) - second
approximation Relationships
Between Site Index And Site Factors The biogeoclimatic ecosystem classification (BEC) system organizes our knowledge of ecosystems and serves as a framework within which to manage resources. It helps to divide landscapes into ecologically uniform segments at various levels of integration (regional, local, and chronological) and generalization (climatic, or "zonal," vegetation, and site). At the local level of integration, landscapes are stratified using vegetation and soils information into vegetation and site units (i.e., site series and site associations). A narrow range of environmental conditions, or an "environmental gradient" distinguishes each site series. This gradient will usually provide distinct growth potentials for a given tree species over a fixed time period. Tree growth is influenced by various site factors, which taken together, determine the site’s quality. These growth, or site, factors include climate (light and temperature) and soil moisture, nutrients, and aeration. In general, the potential tree growth, or site index, is greatest on moist sites and increases with soil fertility. It is possible to use a simple equation to highlight the functional relationships between site index and site factors:
On sites where these site factors are in balance, trees grow well and site index is higher; on sites where these factors are in excess or are deficient, trees grow less well and site index is lower. However, the site index of a species varies between biogeoclimatic units and the precise relationship is not always clear based on any single variable. The influence of climate or soil moisture or soil nutrients on site index is most likely a result of the interaction of site factors. In this section, some relationships between site index and site factors are examined. The presence of these relationships has been documented for several species and sites (e.g., Green et al. 1989; Carter and Klinka 1990; Klinka and Carter 1990; Wang et al. 1994a, 1994b; Wang and Klinka 1995; Wang and Klinka 1996; Kayahara and Pearson 1996; Splechtna 2001).
3.1 Biogeoclimatic Units and Site Index Biogeoclimatic unit-biogeoclimatic subzones and their variants-represent groupings of contiguous ecosystems influenced by a discrete regional climate. Because climate is the most important determinant of plant growth, the site index for a given species will most likely vary from climate to climate or from subzone to subzone. The effect of climate on site index is best detected by comparing site index on zonal sites between different subzones. For example, Douglas-fir site index shows a reasonably positive relationship with increasing precipitation in the selected sequence of Coastal Douglas-fir (CDF) and Coastal Western Hemlock (CWH) biogeoclimatic units; however, the relationship to the selected temperature variables is not as strong (see Table 1). Table 1. Summary of selected climate data (average of stations) and estimated Douglas-fir (Fd) site index on zonal sites for some southern coastal biogeoclimatic subzones
3.2 Soil Moisture and Site Index In general, site index increases from water-deficient to moist sites and decreases from moist to wet sites. This trend is demonstrated for lodgepole pine and interior spruce using results from Kayahara et al. (1996) in Figure 2. In this example, statistical analysis indicated the presence of a strong productivity gradient that coincided with the assumed soil moisture gradient. The mean site index of the study species increased from very dry to fresh sites, reached a plateau on moist sites, and then decreased from moist to wet sites (Figure 2). The highest mean site index of any study species always occurred on sites with no water deficit or surplus during the growing season. The lowest site index always occurred on very dry or wet sites.
Figure 2. Categorical plots of lodgepole pine (A) and interior spruce (B) mean site index for Sub-Boreal Spruce zone stands on nutrient poor and medium sites according to actual soil moisture regimes. Vertical bars represent ±1 standard error.
3.3 Soil Nutrients and Site Index Site index usually increases with an increasing supply of available soil nutrients, particularly nitrogen. This trend is demonstrated for lodgepole pine and interior spruce using results from Kayahara et al. (1996) in Figure 3. In this example, a strong productivity gradient was evident that coincided with the assumed soil nutrient gradient. The mean site index of all study species increased consistently from very poor to very rich sites (Figure 3). The highest site index for all species always occurred on sites very rich in nitrogen.
Figure 3. Categorical plots of lodgepole pine (A) and interior spruce (B) mean site index for Sub-Boreal Spruce zone stands on slightly dry and fresh sites according to soil nutrient regimes. Vertical bars represent ±1 standard error.
3.4 Integrating Site Factors and Site Index The combined effect of soil moisture and nutrients on site index for a given climate or group of similar climates (subzones, or a group of climatically similar subzones) is most effectively displayed on the edatopic grid (Figure 4). Each cell (edatope) on the grid represents a group of sites with a very narrow range in soil moisture and nutrient conditions. Under any soil nutrient conditions, the site index of most species generally increases from very dry to moist sites and then decreases from moist to wet sites. Under any soil moisture conditions, site index generally increases from very poor through very rich sites. This general trend is demonstrated for western hemlock in Figure 4. The site index estimates presented are for the most common, diagonal sequence of edatopes-slightly dry to moist and poor to rich.
Figure 4. Edatopic grid for the Submontane Very Wet Maritime variant of the Coastal Western Hemlock zone. Mean site index is given for western hemlock (Hw).
3.5 Tree Species and Site Index Broad similarities exist in the relationships between average site index and biogeoclimatic unit and edatopic grid position among tree species. However, within this broad, general pattern of similarities, important differences are also evident between species. These differences arise because species respond differently to a given combination or level of growth factors. A typical example is provided in Figure 2. Average site index for both lodgepole pine and interior spruce increases with increasing soil moisture. However, the site index for pine is greater than for spruce on very dry to moist sites. On very moist and wet sites, the difference is less pronounced. Figure 3 shows the average site index of both species responding similarly to soil nutrients-again with pine showing a consistently higher site index.
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2008 Approximation
