Foliar nutrient levels in the needles of trees are not "site factors" in themselves but rather are a reflection of a number of different factors, including soil levels of individual elements, influences of site factors where trees are growing, and, ultimately, in many cases the overall vigor of the trees themselves. As discussed earlier, average foliar nutrient levels in needles of trees on sample plots were generally at or above those levels needed for good growth and foliage characteristics of trees of most Christmas-tree species, including balsam and Fraser fir, with levels on individual plots from well below to well above desirable levels. To look more closely at influences on foliar nutrient levels, correlation and multiple regression analyses were used in attempts to relate foliar levels of five elements - nitrogen, phosphorus, potassium, calcium, and magnesium - to various combinations of soil, site, and tree characteristics. Summaries of those analyses are shown in Tables 4 and 5.
For nitrogen, potassium, and calcium, there were significant, positive correlations between foliar levels and most tree characteristics, reflecting higher levels in more vigorous trees. The correlations with foliar potassium were particularly strong. As discussed previously, specific information is not available for desirable soil nutrient levels for "Canaan" fir, although average soil and foliar potassium levels were generally adequate for most Christmas tree species, including Fraser fir. From data in this study it is not clear if the strong relationship between foliar potassium and growth and foliage quality of trees is reflective of higher potassium requirements (Table 4).
For soil physical factors, correlations with foliar element levels were generally strongest for factors which would be indicative of better drainage conditions. Nitrogen levels showed relatively low correlations, with only A horizon potassium (positive) and magnesium (negative) being statistically significant at the 10 percent probability level. This is not particularly surprising since foliar nitrogen may be more reflective of soil organic matter levels and/or atmospheric input rather than soil elemental levels (Table 4).
Foliar phosphorus and potassium showed very strong relationships with soil levels of the two elements, particularly in the A horizon; this is encouraging because it may indicate that recommendations for phosphorus and potassium fertilization based on soil tests will be reflected in higher levels in the foliage. There were also statistically significant relationships between foliar phosphorus and soil potassium levels in the A and B horizons and magnesium in the B horizon and foliar potassium with soil phosphorus in both the A and B horizons. Conversely, statistical relationships between foliar calcium and magnesium with soil levels were very low (Table 4).
Using the correlations discussed previously, a series of multiple regression equations were developed to test relationships between foliar nutrients as dependent variables and tree, soil physical, and soil chemical factors. As shown in Table 5, only the equations for phosphorus and potassium accounted for over half of the total variation in foliar levels; for potassium, 35 percent of that variation was associated with total tree height, the only statistically significant tree factor in any of the equations, which, as discussed previously, is in itself a measure of tree vigor. As a result, as discussed for the regressions between tree and site factors, it would be meaningless to use the equations in an attempt to develop guidelines for improving foliar nutrient levels of trees.
| Table 4. Simple Correlations Between Foliar Nutrient Levels and Tree and Soil Factors for Plots Having "Effective" Chemical Weed Control. | |||||
|---|---|---|---|---|---|
| Site/Tree Factor6 | Foliar Nutrient | ||||
| Nitrogen | Phosphorus | Potassium | Calcium | Magnesium | |
| Simple Correlation Coefficient, r | |||||
| Tree Factors | |||||
| Total Tree Height | 0.355 | 0.111 | 0.605 | 0.283 | 0.011 |
| 4-Year Height | 0.161 | 0.111 | 0.575 | 0.111 | -0.031 |
| Needle Length | 0.595 | -0.131 | 0.355 | 0.344 | 0.001 |
| Foliage Color | 0.314 | -0.011 | 0.605 | 0.041 | -0.041 |
| Foliage Weight | 0.375 | -0.181 | 0.222 | 0.252 | 0.011 |
| Soil Physical Factors | |||||
| Depth to Mottling | 0.385 | -0.091 | 0.212 | 0.375 | 0.061 |
| A Horizon Sand | 0.081 | 0.365 | 0.385 | 0.051 | 0.041 |
| A Horizon Clay | -0.212 | -0.395 | -0.181 | -0.222 | -0.111 |
| B Horizon Sand | 0.181 | 0.192 | 0.304 | 0.121 | 0.091 |
| B Horizon Clay | -0.273 | -0.131 | -0.091 | -0.324 | -0.111 |
| Soil Chemical Factors | |||||
| A Horizon Phosphorus | 0.081 | 0.615 | 0.283 | 0.304 | 0.233 |
| A Horizon Potassium | 0.212 | 0.202 | 0.385 | 0.111 | -0.263 |
| A Horizon Calcium | -0.111 | 0.041 | -0.051 | -0.091 | -0.021 |
| A Horizon Magnesium | -0.212 | 0.041 | -0.021 | -0.212 | 0.091 |
| B Horizon Phosphorus | -0.131 | 0.334 | -0.051 | 0.061 | 0.131 |
| B Horizon Potassium | 0.181 | 0.263 | 0.273 | 0.041 | -0.131 |
| B Horizon Calcium | -0.101 | 0.171 | 0.031 | -0.061 | 0.061 |
| B Horizon Magnesium | -0.111 | 0.233 | -0.031 | -0.161 | 0.061 |
|
1 Not statistically significant. 2 Statistically significant at the 10 percent probability level. 3 Statistically significant at the 5 percent probability level. 4 Statistically significant at the 1 percent probability level. 5 Statistically significant at the 0.1 percent probability level. 6 Only those variables for which correlations were statistically significant with one or more tree or site factors are included. | |||||
| Table 5. Summary of Multiple Regression Analyses of Relationships Between Foliar Nutrient Levels (Nitrogen, Phosphorus, Potassium, Calcium, and Magnesium) and Soil Physical, Soil Chemical, and Total Tree Height. | |||||
|---|---|---|---|---|---|
| Site/Tree Factor | Foliar Nutrient | ||||
| Nitrogen2 | Phosphorus2 | Potassium2 | Calcium2 | Magnesium2 | |
| Total Tree Height | + | + | + | ||
| Depth Soil Mottling | + | + | + | ||
| A Horizon Sand | + | + | |||
| A Horizon Clay | - | - | - | + | |
| B Horizon Sand | + | + | |||
| B Horizon Clay | - | - | + | + | |
| A Horizon Phosphorus | + | + | - | ||
| A Horizon Potassium | + | + | + | ||
| Total Variation Accounted For, %3 | 31 | 53 | 54 | 22 | 12 |
| Variation Accounted for by Tree Height, %4 | 4 | 3 | 35 | 5 | 1 |
| 1 Only those tree or soil factors that were statistically significant in one or more multiple regression equations were included in the table. 2 A + or a - sign indicates positive or negative relationships between individual tree or soil factors and foliar nutrients in multiple regression equations. 3 Percent of total variation (100%) accounted for by one to seven factors in individual equations, including total tree height. 4 Percent of total variation accounted for by total tree height only. | |||||