Growth and Nutrient Concentration in Flowering Dogwood After Nitrogen Fertilization and Dormant Root Pruning

Materials and Methods

In September 1986, a Hayesville clay loam soil (clayey, oxidic, mesic, typic hapludult) was fumigated with methyl bromide, 4.9 kg/100 m², and amended to meet the pH and fertility levels recommended for tree seedlings (29). Mature seeds of flowering dogwood were collected locally in Fletcher, N.C. during Fall, 1986 and planted Dec. 3, 1986 at a 7.6 x 7.6 cm spacing. Ammonium nitrate (33% N) was surface applied at 38 kg/100 m² on June 29,1987 and May 25,1988. Seedlings were dug by hand on March 5,1989. All soil was washed from the root system. Fresh weight of tops (above ground tissues) and roots (below ground tissues) were estimated using the technique of Young and Werner (33). A random sample of 40 seedlings was removed to determine actual fresh and dry weight (70°C for 96 h) of the top and roots. With these data the following regression equations were developed to predict the initial dry weight of the tops and roots of the seedlings to be used in the study:

1. Top dry weight (g) = −0.311 + [0.494 x top fresh weight (g)], R² = 0.94

2. Root dry weight (g) = 0.312 + [0.443 x root fresh weight (g)], R² = 0.91

To insure seedling uniformity, seedlings were selected by weight, height and trunk diameter before potting into 11.4 liter (#3) containers with a sand substrate. Seedlings were stored in the dark at 4°C until May 16, when the seedlings were moved out of the cooler and into the greenhouse under natural light and day/night temperatures of 24 and 15°C, respectively.

The experiment, a 3 x 4 factorial in a randomized complete block design with 10 replications, was conducted at the Mountain Horticultural Crops Research Station, Fletcher, NC. The two main factors were 3 concentrations of nitrogen (25, 75 or 150 mg/l) and 4 amounts of root pruning (0,25, 50 or 75% removed by weight). Roots were removed in an inverted cone to simulate root loss that might be experienced during typical transplanting. Budbreak (day 0) occurred when the tips of the leaves in the terminal bud were visible. Beginning at day 0, two liters of nutrient solution (Table 1) were applied to each tree on Monday, Wednesday and Friday mornings. Containers were watered in the afternoon on Monday, Wednesday and Friday, and twice daily on the remaining days. Plant height and trunk diameter were measured on day 0 and every 14 days thereafter.

At day 0, 10 seedlings were harvested to determine initial nutrient concentrations. Four replicates were harvested 45 days later (also referred to as harvest 1), with the remaining replicates (six) harvested at 90 days (also referred to as harvest 2). At each harvest, seedlings were washed free of sand and separated into roots and top. Each top was subdivided into new stem growth (formed during current growing season), old stem (referred to as stem) and leaves. Leaf area was measured with a LI-COR 3100 (LI-COR, Lincoln, Neb.) leaf area meter. All plant material was dried at 70°C for 5 days, weighed and ground to pass a 40-mesh sieve. Each tissue sample (1.25g)was combusted at 490°C for 6 hr. The resulting ash was dissolved in 10 ml 6 N HCI and diluted to 50 ml with distilled deionized water. Concentrations of P, K, Ca and Mg were determined by inductively coupled plasma emission spectroscopy. Nitrogen was determined using 10 mg samples in a Perkin Elmer 2400 CHN elemental analyzer.

Leaf, new stem, stem and root dry weights were used to calculate the following: top dry weight (sum of leaf, new stem and stem dry weights) and T:R ratio (top dry weight: root dry weight). At each harvest, nutrient content for leaves, new stem, stem and roots and relative growth rates (RGR) were calculated (20). Treatment effects were determined by analysis of variance and regression analysis.

Results and Discussion

Initial mean values of selected parameters are shown in Table 2. Height growth was not affected by root pruning or N concentration until 42 and 56 days after budbreak, respectively (data not shown). Seedling height increased curvilinearly with increasing N, with maximum height at 75 mg/l (data not shown). Nitrogen concentration did not affect trunk diameter growth, but root pruning reduced trunk diameter growth (P<0.01) by 42 days after budbreak. Trunk diameter decreased linearly with increasing root pruning (data not shown).

At harvest 1, N did not affect any measured parameter (Table 3). In contrast, N affected all parameters except stem RGR at harvest 2. Root pruning significantly affected all parameters except new stem dry weight at harvest 1 (Table 3). Due to the similar responses to root pruning at harvest 1 and 2, only data for harvest 2 will be presented, excluding relative growth rates. The nitrogen x root pruning interaction was not significant at either harvest.

Leaf area, leaf dry weight, new stem and stem dry weights increased’curvilinearly in response to N, with the maximum at 75 mg/l (Table 4). Root dry weight and root RGR decreased with increasing N. This is in agreement with Brouwers (3) and Ingestad (14) who showed that increasing N concentration decreased root growth. This is supported by the increase in T:R ratio with increasing N (Table 4). While an enriched nutrient environment does enhance root growth and density (5, 27), results from this study demonstrated that heightened N levels decreases root growth following root pruning. Nitrogen is still required to maintain plant processes (28) and N deficiency could be detrimental. However, these results suggest that N application should be minimized during the first season after transplanting.

Leaf area, dry weights of leaves, new stem, stem and roots decreased with increasing root pruning (Table 4). Ninety days after removing 25%, 50% or 75% of the root system, root dry weight was reduced 7%, 52% and 68% respectively, compared to the unpruned control. T:R ratio increased with increasing root pruning. After 90 days, the T:R ratio for 0% and 25% root pruned were identical, however, the T:R ratio for 50% and 75% root pruned trees had not recovered. Time needed to restore the plant’s balance varies greatly based upon percent root loss, age of plant and species. Richard and Rowe (22) reported that peach seedlings have similar T:R ratios 25 days after root pruning while Monterey pine (Pinus radiata) seedlings recovered in 80 days (26).

At harvest 1 (45 days), stem RGR decreased linearly with increasing root pruning (Table 5). Stem RGR decreased 307% at 75% pruning compared to 0% pruned. Root RGR increased linearly with increasing root pruning. Root RGR at 75% root pruned increased 185% compared to 0% root pruned. This is in agreement with other studies which have demonstrated that after root loss, growth is redistributed in favor of the roots (2,17, 21). In any given situation a functional equilibrium exists between the top and root, and when any external factor disturbs this equilibrium, the plant reacts to re-establish the balance (3). The greater the shift in T:R ratio the greater the growth enhancement of the removed part (22). Thus, root RGR increased with increasing root loss. Root growth appears to be at the expense of stem growth. In apple, early season root growth is made from carbohydrate reserves in the stem and roots (18). In this study, root pruning removed much of the carbohydrates stored in the roots which would leave the stem as the major source .of carbohydrates for new root growth. At harvest 2, stem RGR decreased with increasing root pruning, however, the stem was no longer losing dry weight (RGR was positive). Root RGR increased with increasing root pruning but, the 25% and 50% root pruning treatments were only 5% and 9%, greater than 0%, respectively, while the 75% root pruned was 44% greater than 0%. This illustrates that root growth rate returns to values similar to the unpruned control as the plant approaches a balanced T:R ratio.

Tissue nutrient concentration and content produced similar statistical trends so only nutrient concentration will be presented. In addition, N concentration and root pruning affected tissue nutrient concentration in each plant part similarly at harvest 1 and 2, so only data from harvest 2 will be presented. Table 6 indicates which factors induced significant responses for each nutrient in each plant part. Percent N increased in leaf, new stem, stem and root with increasing N (data not presented). Similar results were reported by Courts and Philpson (5) and Ingestad (15). Percent P in leaf; %K in new stem, stem and root; %Ca in leaf, new stem and root; and %Mg in the rootdecreased with increasing N (data not presented). Ingestad (15) reported similar results in leaves, stems and roots of birch, Betula verrucosa.

Percent leaf N, P and Cadecreased curvilinearly in response to root pruning, with the minimum at 25% root pruning (Table 7). Percent P in new stem and root; %K in root; %Mg in new stem and root increased with increasing root pruning. Similarly, Richards and Rowe (22) reported that root pruned plants tended to have higher levels of N, P, K, Ca and Mg than unpruned controls. Geisler and Ferree (9) speculated that as roots regenerate, uptake of some nutrients may increase. Uptake of P, Caand Mg takes place at or near the root tip (11). If root morphology is important in P, Ca and Mg uptake, then root pruning might affect their levels. In this study, %P and %Mg increased in new stem and root with increasing root pruning. Perhaps root pruning enhanced new root initiation, resulting in higher P and Mg levels. In contrast, %Ca in all plant parts, excluding leaf, and %P in leaf and stem was not affected by root pruning. Humphries (12) speculated that root pruning could lead to nutrient deficiencies due to reduced uptake. In this study, only foliar %K decreased with increasing root pruning, suggesting that growth reductions resulting from root pruning were not due to nutrient deficiencies created by root loss.

Most woody plants have the capacity to enhance root growth after root loss. The external N concentration can alter this response; as the external level of N increases, root growth decreases. While N is essential for proper plant performance results from this study suggest that N should be applied conservatively during the first year of newly planted trees since an elevated N environment decreases root growth. This decreased root growth may negatively impact transplant survival and prolong the establishment period.