For all studies, there were increases in average weights of foliage samples from trees growing on plots receiving N fertilization (Tables 3 and 6), but with no consistent benefits from applications above 100 pounds of N per acre. However, none of those differences were statistically significant, apparently because of wide variation in responses of trees in plots both between and within replications for individual studies. In part, this may be the result of variations in internal soil drainage conditions within individual study areas. For individual treatments, increases in weights ranged from as little as four to as great as 33 percent. As noted for previous traits, the greatest increases in weights of foliage samples occurred for trees in plots planted on better drained sites at the Pomerene Forest Laboratory and were least for the two sites having the poorest internal soil drainage, Wooster 2 and Pomerene 3.
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Table 3. Effects of Nitrogen Fertilization on Numbers of Lateral Limbs and Foliage Weights of Trees Established in 1993. | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Fert. Level lbs/ac | Lateral Limbs | Foliage Weight | ||||||||
| 1993 | 1994 | 1995 | 1996 | 1997 | 1993 | 1994 | 1995 | 1996 | 1997 | |
| ---no.--- | ---gms.--- | |||||||||
| Wooster 1: Established 1993 | ||||||||||
| 0 | 17 | 18 | 15 | 16 | 16 | 0.90 | 0.95 | 1.15 | 1.01 | 0.99 |
| 100 | 21 | 22 | 20 | 18 | 20 | 1.05 | 1.05 | 1.26 | 1.10 | 1.05 |
| 200 | 23 | 25 | 23 | 18 | 18 | 1.15 | 1.09 | 1.26 | 1.05 | 1.12 |
| 300 | 20 | 23 | 19 | 19 | 19 | 1.10 | 1.08 | 1.30 | 1.13 | 1.11 |
| Avg. | 20 | 22 | 19 | 18 | 18 | 1.05 | 1.04 | 1.24 | 1.07 | 1.07 |
| pF1 | 0.52 | 0.41 | 0.16 | 0.44 | 0.40 | 0.14 | 0.37 | 0.41 | 0.58 | 0.51 |
| LSD2 | - | - | - | - | - | - | - | - | - | - |
| Pomerene 1: Established 1993 | ||||||||||
| 0 | 30 | 28 | 18 | 23 | 21 | 1.25 | 1.20 | 1.46 | 1.30 | 1.21 |
| 100 | 30 | 29 | 20 | 24 | 20 | 1.44 | 1.41 | 1.55 | 1.38 | 1.28 |
| 200 | 32 | 29 | 21 | 27 | 21 | 1.46 | 1.42 | 1.57 | 1.41 | 1.36 |
| 300 | 33 | 31 | 22 | 25 | 22 | 1.40 | 1.40 | 1.56 | 1.46 | 1.32 |
| Avg. | 31 | 29 | 20 | 25 | 21 | 1.39 | 1.37 | 1.53 | 1.39 | 1.29 |
| pF1 | 0.95 | 0.21 | 0.22 | 0.55 | 0.82 | 0.32 | 0.38 | 0.34 | 0.42 | 0.40 |
| LSD2 | - | - | - | - | - | - | - | - | - | - |
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1pF: Probability of statistical significance for analysis of variance F test.
2LSD: Least significant difference at 5% probability level for comparing differences between treatment means. | ||||||||||
In earlier studies with Douglas-fir and Fraser fir (Brown, 1976; Brown and Vimmerstedt, 1976), weights of representative shoot samples from trees fertilized with N were consistently greater than for unfertilized trees. However, those differences were relatively small (4 to 5 percent), and benefits were greatest on better-drained, relatively infertile sites. In studies with established Douglas-fir plantings (Brown and Vimmerstedt, 1983), foliage weights increased significantly on trees that received N applications of 100 to 150 pounds per acre, without significant benefits from higher rates.