A BASE LINE ASSESSMENT OF THE NUTRITIONAL REQUIREMENTS OF NOBLE FIR  (Abies procera Rehd.) FOR CHRISTMAS TREES[1]

Brian Mahoney

Department of Crop Science, Horticulture and Forestry, UCD

A.      Field fertiliser trial

The objective of the fertiliser trial was to examine the impact of nitrogen at six levels upon the growth and development of noble fir (Abies procera (Rehd.)) Christmas trees.

The experiment was established in December, 1997.  It was set up as a single tree plot experiment with nine replications.  One hundred and eight sample trees were selected, scattered throughout the experimental area.  There were 12 treatments: two seasons of application (spring and autumn) and six levels of nitrogen (Table 1).  The spring fertilisation treatment applications were made in early April, prior to flushing (bud burst), while the autumn application took place in early October.

TABLE 1:  EXPERIMENT TREATMENTS

where: N is nitrogen; P is phosphorus; and K is potassium.

An initial site assessment took place in December, 1997.  The trees were reassessed in December, 1998.  The top height of each sample tree was measured as was the leader length.  The number of leader and terminal buds was counted.  A trees bud index was determined using leader length and bud number.  Foliage samples were collected from each sample tree to rate the foliage colour and for foliar chemical analysis.  The foliar samples were chemically analysed for their concentrations (% oven dry weight) of N, P K, Ca (calcium), and Mg (magnesium).  The mean values for foliage in December, 1997, were 1.44%, 0.17%, 0.71%, 0.57%, 0.08%, respectively, for N, P, K, Ca and Mg.  The corresponding values for tree height, leader length and the bud index were, 132 cm, 22.4 cm and 2.07 cm/bud, respectively.  In addition, nine soil samples were randomly collected within the experimental area and chemically analysed to determine the initial site fertility.  The experimental data were statistically analysed to examine the effects of season of application and treatment level and the interaction between these two factors on the response variables: tree height, leader length, bud-index, foliage colour and foliar concentrations of N, P, K, Ca and Mg.

Results

The field fertiliser trial

No single fertiliser treatment consistently gave the best response over the range of response variables (Tables 2 and 3).  Results indicate that the season of application had no significant effect for any of the response variables measured and that there was no significant interaction between season of application and fertiliser treatment level.  Fertiliser treatment level had a significant effect on foliar N concentration and foliage colour.  Surprisingly, foliar N levels decreased for all the treatments (including the control), with a significantly greater decrease for treatments 2 and the control, when compared to treatments 3 and 6.  Foliage colour improved for all the treatments, including the control.  There was a significant difference between treatment 3 and treatments 2 and 4, and the control, and between treatments 5 and 6 and the control.

TABLE 2: MEAN IMPACT OF FERTILISER UPON TREE GROWTH AFTER ONE GROWING SEASON

TABLE 3: IMPACT OF FERTILISER UPON MEAN FOLIAR NUTRIENT CONCENTRATION (% oven dry weight) AFTER ONE GROWING SEASON

Relationship between tree height growth and applied N

The pattern of growth indicates that there was a positive response in tree height to increasing levels of N.  The maximum response occurred at between 60 to 76 kg/ha (Figure 1).  Above these rates growth rapidly decreased.

FIGURE 1: PREDICTED AND ACTUAL CHANGE IN TREE HEIGHT INCREMENT IN RESPONSE TO APPLIED N

Relationship between leader length and N application

The predicted change in leader increment increased with increasing levels of N, reaching a peak increment at N applications of approximately 60 to 76k kg/ha

(Figure 2), thereafter, leader increment gradually decreased with increasing levels of N.

FIGURE 2: PREDICTED AND ACTUAL CHANGE IN LEADER LENGTH IN RESPONSE TO N APPLICATION

Relationship between bud index and N application

A decrease in the bud index is desirable, as it results in an increase in tree density.  The predicted trend indicated it was not until applications of approximately 76 kg/ha that the predicted bud index decreased (Figure 3).

FIGURE 3: PREDICTED AND ACTUAL CHANGE IN BUD INDEX IN RELATION TO N APPLICATION

Relationship between foliar N concentration and N application

Foliar N concentration decreased at each of the N application rates employed.  The decline in foliar N levels decreased with increasing levels of N (Figure 4).  At the N application rate of 152 kg/ha, the predicted decrease in foliar N concentration was negligible (-0.03% oven dry weight) and closely corresponds to a rate at which foliar N levels were being maintained.

FIGURE 4: PREDICTED AND ACTUAL CHANGE IN FOLIAR N CONCENTRATION IN RELATION TO RATE OF N APPLICATION

Relationship between foliar P concentration and N application

Clearly, foliar P levels decreased with increasing rates of N application (Figure 5).  These changes principally reflect a maximum response at N application rates ranging from 0 to approximately 38kg/ha (Figure 5).  Thereafter, the increase in foliar P concentrations declined with increasing levels of N.

FIGURE 5: PREDICTED AND ACTUAL CHANGE IN FOLIAGE P CONCENTRATION IN RELATION TO N APPLICATION

Relationship between Foliage K Concentration and N Applied

The foliar K concentration increased over the range of N applied in the experiment.  The predicted foliar K levels increased gradually with increases in the level of N applied (Figure 6), reaching a peak response at N application rates of approximately

76 kg/ha.  Further increases in N application above that point were associated with a decline in the predicted increase in foliar K levels.

FIGURE 6: PREDICTED AND ACTUAL CHANGE IN FOLIAGE K CONCENTRATION BY APPLIED

Relationship between foliage colour and N application

Foliage colour improved at each level of N application.  The effect levelled out at N application rates of between 76 to 152 kg/ha (Figure 7).

FIGURE 7: EFFECT OF FOLIAR N CONCENTRATION ON FOLIAGE COLOUR

The relationship between foliar N concentration and foliage colour was statistically significant.  The predicted foliage colour response gradually increased with increasing foliar N concentration (Figure 8) and peaked at approximately 1.53% (oven dry weight).  Thereafter, foliage colour gradually declined with increasing levels of foliar N.  The results indicate that the optimum foliage colour in noble fir Christmas trees was achieved at a foliar N concentration of approximately 1.45 to 1.55% (oven dry weight).

FIGURE 8: RELATIONSHIP BETWEEN FOLIAR N LEVELS AND FOLIAGE COLOUR

B.        Biomass and Nutrient Content of Noble Fir Christmas Trees

The objective of the biomass and nutrient content study was to determine the annual nutrient demands which noble fir Christmas trees place on a site.  Such information may be of use in deciding upon appropriate fertilisation practices.

In January of 1998, an evaluation began of the biomass and nutrient content of noble fir Christmas trees.  Two, well formed, good quality trees of similar height were selected from each of the following four sites: Kellehers’ Christmas tree farm; Glenealy Christmas tree farm; Emerald Group, Grangecon; and Kilmacthomas foliage farm (Table 4).  Sample trees were excavated and following measurement of tree height and trunk diameter (at 15 cm above the root collar), each sample tree was partitioned into stem, roots, branches and foliage (by whorl) components.  All tree components were ground in a Wiley mill to pass through a 20 mesh sieve.  The N, P, K, Ca and Mg concentration of the sample components were determined by chemical analysis.

TABLE 4: TREE HEIGHTS BY SAMPLE TREE

Results

Annual accumulation of biomass and nutrients

The net accumulation of biomass by the sample trees, ranged from 412 to 529 gm/yr.  At the various stocking densities used these values amounted to per hectare accumulations which ranged from 1.08 to 4.00 tonnes/yr.  Over an eight-year rotation, these values would amount to biomass accumulations of approximately 8.64 t/ha to 32.0 t/ha (Table 5).  The annual nutrient accumulation per tree, gave values ranging from 2.7 to 4.1gm/yr for N, 0.45 to 0.57 gm/yr for P, 2.26 to 2.89 gm/yr for K, 1.33 to 2.55 gm/yr for Ca and 0.30 to 0.40gm/yr for Mg.  These values amounted to per hectare accumulations which ranged from 9.4 to 30.6 kg, 1.25 to 4.30 kg,

6.04 to 20.0 kg, 4.36 to 19.3 kg and 0.88 to 2.78 kg, respectively, for N, P, K, Ca and Mg (Table 5).  Over an 8 year rotation, the whole tree harvesting of 1 hectare of noble fir Christmas trees, could potentially remove approximately 244.8 kg of N, 34.4 kg of P, 160.0 kg of K, 154.0 kg of Ca and 55.6 kg of Mg (Table 6).

TABLE 5: ESTIMATED ANNUAL BIOMASS (DRY WEIGHT) AND NUTRIENT ACCUMULATION PER TREE AND PER HECTARE) AT EACH OF THE SAMPLE SITES

Nutrient accumulations within individual sample trees was relatively similar, however, the quantity accumulated per hectare was largely dependent upon the stocking density.  Nutrient accumulation increased per hectare as the stocking density increased.  The Kelleher’s (7,576 trees/ha) and Emerald (6,944 trees/ha) sites had consistently higher accumulations when compared with the Kilmacthomas site (2,500 trees/ha).

Noble fir can retain its needles for several years and due to the short rotations (8-10 years) and, as a result of the removal of the full tree in harvesting, only negligible recycling of nutrients will occur from needle fall over the growth period.  The great majority of the accumulated nutrients were incorporated in the above-ground portion of the trees and harvesting Christmas trees removes this from the site.  Stump removal during complete cultivation would remove additional nutrients from the site.  Such nutrient removals indicate the potential for deficiencies in subsequent rotations, if the nutrients removed are not replaced.

TABLE 6: ESTIMATED TOTAL BIOMASS AND NUTRIENTS REMOVED FOLLOWING THE HARVESTING OF NOBLE FIR CHRISTMAS TREES ON AN EIGHT-YEAR ROTATION

Nutrient concentrations

For N and Ca the branches contained nutrient levels intermediate between the foliage and stems.  Generally, the P, K, and Mg levels were highest in the upper whorl branches and decreased in order of branches, foliage, roots and stems, except for P and Mg, where for the lower whorls, the foliage had higher concentrations than the branches, and for P, where stem levels exceeded root levels (Table 7).  From the results it was clear that branch and foliage nutrient concentrations varied with vertical crown position, therefore, when using foliage nutrient analysis to interpret the nutrient status of noble fir Christmas trees, the foliage samples should be composed of current year needles from the same level of the crown.  Repeat samples should also be collected from the same crown position.

TABLE 7: AVERAGE NUTRIENT CONCENTRATIONS IN THE DIFFERENT PLANT COMPONENTS OF NOBLE FIR CHRISTMAS TREES

The distribution of biomass and nutrients

The average distribution of biomass among the tree components was 23% in the stems, 24% in the roots, 18% in the branches and 34% in the foliage.  The foliage constituted on average only 34% of the biomass, but accounted for 58%, 49%, 38%, 55% and 43%, respectively, of the N, P, K, Ca and Mg accumulated.  In contrast, the stems contained on average 23% of the biomass, but only 12%, 15%, 16%, 14%, and 12%, respectively, of the N, P, K, Ca, and Mg accumulated (Table 8).  Clearly, the foliage represented the major sore of nutrients rich tissue.  On average, 12% of the N, 13% of the P, 23% of the K, 11% of the Ca and 22% of the Mg would remain behind in the tree roots after harvest, however, tree roots decompose slowly so the nutrients present would not readily be available to the next rotation.

TABLE 8: AVERAGE BIOMASS AND NUTRIENT DISTRIBUTION IN NOBLE FIR CHRISTMAS TREES

Discussion

The field fertiliser trial

The results for this experiment may be site specific and could differ considerable on other site types.  The results indicate that the optimum rate differed for several of the response variables examined, therefore, a balance must be struck to achieve the most effective overall response.  N application rates of approximately 60 to 76 kg/ha maximised the tree height and leader length response, however, the predicted response for foliage colour levelled out at N application rates of about 76 to 152 kg/ha.  The response of the predicted foliar N concentration and bud index appeared to improve with increasing levels of N applied.

Growth response

Trees on the experiment site grew well without receiving any fertiliser.  The predicted response patterns were similar for both tree height and leader length increment in that they followed curves of diminishing returns.  The results clearly illustrate the economics of fertilisation in that the cost per unit of response increased as the fertiliser application increased, thus, the efficiency of the treatment decreased.  The lack of a significant response for the trees bud index was expected, as the bud primordia are initiated in the previous growing season, thus the trees bud index would be unaffected by treatment until the second growing season after fertilisation.

Foliar nutrient concentrations

The foliar nutrient levels did not appear to be deficient at the start of the experiment.  Foliar N, Ca and Mg concentrations decreased and foliar P and K concentrations increased for all the treatments, including the control, indicating that factors outside of the experiment treatment had an effect.  The predicted responses for foliar P, K and Mg concentrations eventually declined with increasing levels of N.  This suggests that foliar nutrient levels were being gradually diluted as the N applications increased.

The decline in foliar N was unexpected.  It would appear that the N levels applied were insufficient to maintain the foliar N concentrations.  At the end of the experiment, all of the treatments were below the guideline value of 1.40% dry matter (Fletcher et al., 1998).  A continued decline in future years would be a cause for concern, as it would result in moderate to severe N deficiencies.  From an inspection of the predicted trend it would be reasonable to suggest that N applications in excess of 156 kg/ha would result in increases in the foliar N concentration. 

P and K rose dramatically in the foliage, for all of the treatments.  At the end of the experiment, all of the treatments had foliar P levels above the normal range of 0.13 to 0.17% dry matter (Fletcher et al., 1998), while the foliar K levels remained within the normal range of  0.73 to 0.93% dry weight and were close to the “satisfactory” value of 0.8% dry weight (Pfeifer, 1988).

Foliage colour

The results indicate that fertiliser applied in October produced a rapid change in the foliage colour.  Colour improved for all the treatments, including the control.  The reason for this improvement in colour for the control trees was not clear.  Perhaps it was due to the natural year to year variation with can occur with foliage colour. Essentially, the trees with the greatest improvement in foliage colour tended to be those treated with the higher levels of N.  Interestingly, applications of P and K in the absence of N (treatment 2) were not significantly different from the control.  Overall, colour was nearly adequate even without fertiliser applications.  Optimum foliage colour in noble fir Christmas trees was achieved at a foliar N level of approximately 1.45 to 1.55% dry weight.

References

Fletcher, R., C. Landgren., S. Webster., and M. Bondi. 1998.  Fertilizing Noble fir. Christmas Tree Lookout. Winter. Pp.21-28.

Pfeifer, A. 1988.  Noble fir for Christmas trees. Forest Service. Dept. of Energy. Report. 8p.