The impact of leaf damage on the growth of young silver birch seedlings with and without additional nutrient supply was investigated by simulating leaf-insect damage and applying different levels (25%, 50% and 75%) of artificial defoliation. Based on field-practical and cost-effective methods, we determined how fertilization practices compensate for foliage loss, and the combined effect on silver birch seedling growth. The mineral fertilizers applied to the 25-75%-defoliated silver birch seedlings reduced the growth in aboveground biomass compared to the fertilized but undamaged seedlings. Our results showed that when the birch seedlings received more nutrients they did not compensate for the loss of foliar mass. However, the seedlings loosing part of their foliar mass and receiving no additional fertilizers did compensate for the foliage loss and their root growth was not weakened, using soil nutrients more effectively. Mineral fertilization up to optimal nutritional balance could be a beneficial tool for increasing growth rate and biomass accumulation in the short-term period. However, our study demonstrated that additional fertilization does not necessarily lead to growth compensation of partly defoliated young birch trees.
Numerous studies have investigated the effects on vegetation growth of environmental stresses due to different combinations of abiotic and biotic agents (
Plant response to herbivores is affected by various environmental factors, including the soil nutrient status.
The capacity of trees to allocate elements within their tissues has been widely discussed (
The capability of birch species to exhibit compensatory growth and recover after severe defoliation was found to improve after increasing the nutrient supply (
The study by
Contrary to the widely discussed compensatory continuum hypothesis, which predicts lower tolerance under low soil nutrient supply (
This paper attempts to analyze the impact of simulated leaf damage, which was chosen to demonstrate leaf-insect damage and artificial defoliation up to 75% on the growth of young silver birch seedlings with and without additional nutrient (N, P, K and Mg) supply. We hypothesized that mechanical defoliation is an appropriate method for the compensatory growth studies of trees (
Throughout the vegetation season of 2015, silver birch (
The seedlings were randomly assigned into fertilized, which received 120 kg ha-1 nitrogen, 90 kg ha-1 phosphorus, 135 kg ha-1 potassium and 35 kg ha-1 magnesium, and unfertilized seedlings. The applied amount of NPKMg fertilizers corresponded to the optimal amount of fertilizers commonly used for one-year-old birch seedlings. The optimal fertilizer dosage for the particular substrate was experimentally defined at the Agrochemical laboratory of Lithuanian Research Center for Agriculture and Forestry. Granulated NPK fertilizer was mixed to substrate as raw material, leaving it to dissolve gradually during the vegetation season, while Mg fertilizer was dissolved in water. All fertilizers were applied once over a period of six-week after planting of birch seedlings.
In the middle of June 2015, fertilized and unfertilized seedlings were allocated to 25, 50 and 75% artificial defoliation, and three treatments simulating insect damage (three or six holes per leaf, or clipping one-third off each leaf) were conducted. The defoliation was conducted using scissors, each leaf was damaged with three or six non-overlapping holes (0.33 cm2) using a steel hole-punch, or one-third of each leaf was removed using scissors. Defoliation treatments were applied once. In total, seven treatments were conducted, including a non-defoliated control. No visible injury caused by biotic or abiotic agents was observed during the experimental period. Each treatment had 10 replicates, a total of 140 seedlings were grown during the experiment.
The potted trees were placed adjacent to each other in rows mixing the treatments, covering an experimental area of 130 × 360 cm. The artificial defoliation was designed to approximate the actual levels of insect defoliation during the active vegetation period in Lithuania. The mean temperature of the 2015 growing season in the study area was 14.4 °C, ranging from 7.3 °C in April to 20.1 °C in August (data from the meteorological station situated 0.5 km away from the study area). The mean precipitation was 23.8 mm, with 43.2 mm in April, 42.0 mm in May, 10.0 mm in June, 21.2 mm in July and 2.6 mm in August. These meteorological values were close to the standard climatic norm.
The height of birch seedlings was measured periodically 1, 5 and 10 weeks post-defoliation on June 29, July 30 and August 30, respectively.
The net photosynthetic rate of the remnant leaves was measured two weeks post-defoliation (July 10) with a portable photosynthetic system LCpro-SD® (ADC BioScientific Ltd, England). The measurements were conducted between 10:00 AM and 12:00 PM, having similar environmental conditions for all samples. Three seedlings were systematically selected and measured from all fertilized and unfertilized treatments. The photosynthetic rate was measured for three leaves from each seedling, yielding nine leaves per treatment. We measured mature, sunlit, naturally undamaged leaves in each treatment.
The diameter of the main stem of each individual seedling was measured at a distance of 2 cm above the stump base using an electronic digital caliper before final harvesting.
The above- and belowground biomass of seedlings was estimated at the end of the experiment,
The total aboveground biomass of each tree was calculated by summing the masses of dried shoots, branches and leaves. The total biomass of each seedling was calculated by summing the above- and belowground biomasses. Leaf, aboveground, and cumulative dry mass were calculated by summing the dry mass at harvest plus the leaf mass removed during defoliation. Root biomass was expressed in relation to the aboveground biomass, and the root-to-shoot ratio was calculated. Cumulative leaf dry mass in relation to the root was calculated, and the ratio of leaf production-to-root ratio was obtained.
Growth compensation of seedlings (
Normal distribution of the above variables was tested using the non-parametric Lilliefors and Kolmogorov-Smirnov tests. Due to non-normal distribution of data, we used the Kruskal-Wallis analysis of variance (KW-ANOVA) test to ascertain the significance of differences in dry mass between treatments. Treatment means are presented throughout the study with the standard error of the mean (±SE).
All statistical analyses were conducted using the software package STATISTICA® ver. 7.0 (StatSoft Inc., Tulsa, OK, USA) with α = 0.05 in all cases.
To compare the growth of fertilized and unfertilized one-year-old silver birch seedlings, the height increment was measured just before harvesting. The increment in height of fertilized seedlings was 1.5 times higher than that of unfertilized seedlings, decreasing from 32.1 ± 1.2 (the control) by 20-23% (25-50% defoliation). In the case of fertilized birch trees, 25-75% defoliated seedlings reached a lower height increment compared with the fertilized control (
Similar tendencies were recorded for the stem increment. The fertilization increased stem growth by 3-4 times compared to unfertilized seedlings (
Photosynthesis intensity of unfertilized seedlings varied depending on the damage intensity, with a tendency to increase with increasing defoliation (
At harvest, differences between the total production of fertilized and unfertilized seedlings were the most significant (
Because tree foliage was damaged - seedlings lost up to 75% of leaves - it could be expected that main direct changes were observed in the leaf or aboveground biomass. However, indirect effects could also occur in the roots or even in the total tree biomass. The total leaf mass of unfertilized seedlings that were artificially damaged by perforation with three holes per leaf (or 10% defoliated) exceeded the leaf mass of the control seedlings by 1.2 times, while leaf production was exceeded by 1.4 times. Seedlings defoliated by 75% were particularly able to compensate for leaf mass loss, damaged seedlings exceeding the control by 1.5 times (
The highest total production of fertilized seedlings was found in the control treatment (
Biomass growth and allocation to different tree compartments of fertilized and unfertilized silver birch seedlings were quite similar for both damaged and control seedlings (
This study clearly demonstrated that additional nutrient supply is the most important factor for the growth of birch seedlings, both in height and diameter. This is in agreement with most general assertions, which emphasize optimal nutrition as a limiting factor for vegetation. Several previous studies have confirmed the essential role of N as a key nutrient for growth (
We initially assumed that fertilized trees respond differently to various damage or foliage loss. To this purpose, we applied various defoliation levels (10-75%) to silver birch seedlings. In addition, 10, 20, and 27% defoliation was conducted by simulating leaf perforations and clipping part of all leaves per tree, corresponding to three and six holes per leaf and one-third clipped off each leaf, respectively. When the latter leaf damages (leaf holes and clipping) were compared with simple defoliation treatments, when 25, 50, and 75% of leaves were removed, it was obvious that no significant differences between the two groups of treatments were recorded.
In our study, we determined how lost foliage together with fertilization affects growth compensation in silver birch seedlings. Following
Nonetheless, the unfertilized birch seedlings under the possible synergetic influence of high defoliation and nutrient limitation showed relatively good growth potential. Indeed, the severe 50-75% defoliation resulted in higher photosynthesis, and the seedlings grown without additional fertilization showed higher compensation potential. In particular, this effect was significant for higher damage levels. However, we did not measure the impact of repeated damage during the next vegetation periods, which could cause quite different growth variations, especially if seedlings had severe damage during the previous growth season.
These patterns indicate that seedlings of fast growing tree species, such as silver birch, have relatively high potential to respond to various environmental disturbances. However, even in the case of severe damage, additional fertilization does not necessarily lead to more intensive growth. Mineral fertilization up to optimal nutritional balance could be a beneficial tool causing more intensive growth, higher biomass during the vegetation season, short-term response and partial compensation for lost foliage in the case of insect outbreak.
The mineral fertilizers applied for the 25-75%-defoliated silver birch seedlings reduced the growth of aboveground biomass compared with the fertilized but undamaged seedlings. Our results showed that when seedlings received more nutrients they did not compensate for the loss of foliar mass. Meanwhile, the seedlings that lost a part of their foliar mass but received no additional fertilizers, compensated for the lost foliage and have not weakened the root growth, using soil nutrients more effectively.
Relationship between the percentage change in height from control (0% is equal to control) and defoliation (%) in fertilized and not fertilized silver birch seedlings at harvest.
Relationship between the percentage change in stem diameter from control (0% is equal to control) and defoliation (%) in fertilized and non-fertilized silver birch seedlings at harvest.
Leaf photosynthetic rate (μmol m-2 s-1) of fertilized and unfertilized silver birch seedlings at increasing defoliation percentage.
Relationship between dry leaf mass (g) and defoliation (%) in fertilized and non-fertilized silver birch seedlings at harvest. Grey boxes represent the range of dry leaf mass values observed for control seedlings.
Relationship between the percentage change in leaf production from control (0% is equal to control) and defoliation (%) in fertilized and non-fertilized silver birch seedlings at harvest.
Final biomass allocation of fertilized (A) and unfertilized (B) silver birch seedlings at harvest.
Dry mass (g) of leaves, shoots, stems and roots of fertilized and non-fertilized
Variable | Fertilizers | Control | Artificial defoliation / simulated insect damage | Artificial defoliation | ||||
---|---|---|---|---|---|---|---|---|
10% / 3 holes per leaf | 20% / 6 holes per leaf | 27% /clipped 1/3 of leaf | 25% | 50% | 75% | |||
Leaf mass (g) | Yes | 6.03 ± 0.54 c | 4.57 ± 0.50 bc | 3.29 ± 0.36 abc | 3.77 ± 0.41 abc | 3.79 ± 0.61 abc | 2.41 ± 0.32 ab | 2.26 ± 0.23 ab |
No | 0.85 ± 0.32 ab | 1.06 ± 0.18 b | 0.62 ± 0.06 ab | 0.86 ± 0.09 ab | 0.71 ± 0.08 ab | 0.75 ± 0.13 ab | 0.48 ± 0.12 a | |
Leaf production (g)1 | Yes | 6.03 ± 0.54 b | 4.91 ± 0.50 ab | 3.92 ± 0.36 ab | 4.38 ± 0.41 ab | 4.44 ± 0.61 ab | 3.63 ± 0.32 a | 3.51 ± 0.23 a |
No | 0.85 ± 0.32 b | 1.19 ± 0.18 ab | 0.83 ± 0.06 ab | 1.03 ± 0.09 ab | 0.81 ± 0.08 ab | 1.28 ± 0.13 ab | 1.26 ± 0.12 a | |
Shoot mass (g) | Yes | 2.18 ± 0.27 b | 1.26 ± 0.19 ab | 0.85 ± 0.10 a | 1.02 ± 0.14 ab | 0.89 ± 0.13 a | 0.77 ± 0.14 a | 0.93 ± 0.13 a |
No | 0.11 ± 0.04 a | 0.16 ± 0.03 a | 0.11 ± 0.01 a | 0.13 ± 0.02 a | 0.09 ± 0.04 a | 0.15 ± 0.06 a | 0.10 ± 0.02 a | |
Stem mass (g) | Yes | 8.91 ± 0.54 b | 7.10 ± 0.69 ab | 5.80 ± 0.61 ab | 5.98 ± 0.52 ab | 5.95 ± 0.70 ab | 4.68 ± 0.38 a | 5.44 ± 0.46 a |
No | 2.43 ± 0.36 a | 2.59 ± 0.18 a | 2.29 ± 0.17 a | 2.47 ± 0.15 a | 2.71 ± 0.21 a | 2.58 ± 0.20 a | 2.18 ± 0.12 a | |
Aboveground mass (g) | Yes | 17.13 ± 1.31 b | 12.93 ± 1.33 ab | 9.95 ± 1.04 a | 10.77 ± 0.98 ab | 10.63 ± 1.42 ab | 7.85 ± 0.75 a | 8.63 ± 0.80 a |
No | 3.39 ± 0.72 a | 3.79 ± 0.34 a | 3.01 ± 0.22 a | 3.46 ± 0.26 a | 3.51 ± 0.28 a | 3.48 ± 0.36 a | 2.76 ± 0.23 a | |
Aboveground production (g) | Yes | 17.13 ± 1.31 b | 13.28 ± 1.33 ab | 10.57 ± 1.04 ab | 11.38 ± 0.98 ab | 11.28 ± 1.42 ab | 9.07 ± 0.75 a | 9.88 ± 0.80 a |
No | 3.39 ± 0.72 a | 3.39 ± 0.34 a | 3.22 ± 0.22 a | 3.63 ± 0.26 a | 3.61 ± 0.28 a | 4.01 ± 0.36 a | 3.54 ± 0.23 a | |
Root mass (g) | Yes | 10.34 ± 0.72 b | 7.84 ± 0.74 ab | 6.11 ± 0.72 a | 6.99 ± 0.53 ab | 6.54 ± 0.74 ab | 4.87 ± 0.46 a | 5.31 ± 0.50 a |
No | 2.23 ± 0.42 a | 2.75 ± 0.24 a | 2.19 ± 0.17 a | 2.41 ± 0.24 a | 2.69 ± 0.27 a | 2.71 ± 0.31 a | 2.01 ± 0.30 a | |
Total mass (g) | Yes | 27.47 ± 2.02 b | 20.77 ± 2.06 ab | 16.05 ± 1.73 a | 17.76 ± 1.48 ab | 17.18 ± 2.07 ab | 12.73 ± 1.18 a | 13.94 ± 1.28 a |
No | 5.62 ± 1.12 a | 6.54 ± 0.57 a | 4.96 ± 0.29 a | 5.87 ± 0.48 a | 6.20 ± 0.53 a | 6.19 ± 0.66 a | 4.77 ± 0.51 a | |
Total production (g)2 | Yes | 27.47 ± 2.02 b | 21.11 ± 2.06 ab | 16.68 ± 1.73 a | 18.37 ± 1.48 ab | 17.83 ± 2.07 ab | 13.95 ± 1.18 a | 15.19 ± 1.28 a |
No | 5.62 ± 1.12 a | 6.67 ± 0.57 a | 5.17 ± 0.29 a | 6.04 ± 0.48 a | 6.30 ± 0.53 a | 6.72 ± 0.66 a | 5.55 ± 0.51 a | |
Root/shoot ratio3 | Yes | 0.61 ± 0.02 a | 0.60 ± 0.02 a | 0.57 ± 0.04 a | 0.62 ± 0.02 a | 0.59 ± 0.03 a | 0.54 ± 0.02 a | 0.53 ± 0.02 a |
No | 0.67 ± 0.05 a | 0.70 ± 0.02 a | 0.64 ± 0.09 a | 0.66 ± 0.03 a | 0.74 ± 0.04 a | 0.67 ± 0.04 a | 0.56 ± 0.04 a |