The resistance to stress of seedlings during the initial phases after planting is fundamental for assuring fast establishment and long-term survival of artificial regeneration. Although needing less storage space and handling during their production and planting, small seedlings are considered to be less efficient in terms of water uptake and more sensitive to a water deficit than bigger seedlings. The responses to a water deficit produced by a suspension of irrigation for 14 days were assessed in black spruce (
Quick and successful post-cutting regeneration ensures a continuous and sustained productivity of the stands as well as the ecological functions and ecosystem services of the forest to be maintained over time. Forest management activities thus have to guarantee an appropriate regeneration of harvested stands. In Canada, between 60 and 70% of the harvested area (mainly conifer stands) is regenerated by artificial means, mainly by planting and seeding to efficiently achieve the desired density and species composition of the future stands (
Artificial regeneration is a useful but expensive component of forest management, with 80% of the plantation costs being directly related to the seedlings (
Reduced growth is generally observed in seedlings of boreal species during the 2-3 years following planting. Seedlings have roots enclosed in a small rootball that reduce or inhibit the ability of plants to respond to the water requirements for evapotranspiration (
The aim of this paper was to assess the response to a severe water deficit of seedlings of different sizes growing under controlled conditions simulating extreme post-planting conditions. The hypothesis that the small size makes seedlings more sensitive to water stress was tested. Such relationships have been in part analyzed previously, but comparisons were made only for seedlings of big sizes and, to our knowledge, no study is available on the responses to water stress of small seedlings (
The investigation was conducted on 2- and 3-year-old black spruce seedlings of different sizes and named C25, C50, C110 and C350, according to the cavity volume of the containers in which they were grown (
On 24 July 2007, the seedlings were transplanted in 10 raised beds (240 cm × 120 cm × 14 cm) arranged in 5 rows (blocks) and filled with sand (<1% in loam). Therefore, the spacing between the seedlings was 10 × 12 cm, which assured no interaction or competition between the developing roots of seedlings. The raised beds were located in a plastic tunnel equipped with a forced ventilation set at 25 °C. Eighty seedlings were planted along 10 rows per raised bed, and each row received two seedlings per size according to a random pattern. A row of C50 was planted along the perimeter of the raised bed to minimize the edge effect. Each block involved two raised beds randomly representing the control and the treatment. The raised beds were fertilized twice with 10 g of NPK (20-20-20) during two weeks of acclimatization.
Overall, the experiment lasted 25 days. The seedlings were irrigated every two days to maintain a volumetric soil water content of between 6 and 8%, which was evaluated daily in each container by using a TDR probe (model MP-917, ESI Environmental Sensors Inc., Victoria, BC, Canada). Treated seedlings were submitted to a water stress beginning on 6 August during which irrigation was withheld for 14 days. After that period, irrigation was restored at the same frequency as the control, with measurements being maintained for the successive 11 days. Maximum and minimum air temperature and relative humidity were measured daily using two digital hygrometer-thermometer recorders (Fisher Scientific, Pittsburgh, PA, USA) located close to the seedlings.
Physiological measurements were taken during and after the treatment for a total of 9 sampling days and consisted of pre-dawn (
Data were analyzed as split-split-plot with 5 replicate blocks, where each block contained control and treated seedlings. Overall, 720 seedlings were measured (4 sizes × 2 water regimes × 5 blocks × 9 sampling days × 2 periods of measurements). The variables related to physiological measurements were compared with analysis of variance (three-way ANOVA in a block-split-plit-plot design) and Restricted Maximum Likelihood Method REML procedure of the JMP® statistical software (version 10, SAS Institute Inc., Cary, NC, USA) by using seedling size as a categorical variable. The homogeneity of variance was graphically verified on residual plots of the linear models (
Analysis of covariance (ANCOVA) was used to assess the relationships between gas exchange and water potential, and to compare data of root biomass growth among seedling sizes. The models were applied on transformed data to respect the assumption of linearity. Goodness-of-fit of each model involved the significance of F-values, the proportion of variation accounted for (R2) and the distribution of residuals.
During the overall period of monitoring, minimum and maximum temperatures were on average 13.6 and 30.6 °C, respectively (
The control seedlings of all sizes maintained a similar and constant water potential during the experiment, with average
The effect of treatment on gas exchange was evident particularly for
The relationships between
At the beginning of the treatment, root biomass ranged between 194 to 7820 mg (
This experiment investigated some physiological responses of seedlings of different sizes to a water stress produced under controlled environmental conditions. Before the resumption of irrigation on day 14, treated seedlings attained
The stomatal conductance to water vapor of needles showed a higher sensitivity to water potential, with a clear reduction observed at
The experiment produced two contrasting water conditions during the treatment period, with volumetric soil water content decreasing quickly after the suspension of irrigation and reaching 1.4% within one week. This was related to the use of a sandy substrate that allowed a rapid water loss. Such characteristics only partially represent the conditions experienced by planted seedlings on typical post-cutting soils in black spruce stands, which are deep and rich in organic material (
C25 had the lowest root density compared to seedlings of the bigger sizes, indicating that in proportion, a higher volume of soil was available for the roots of smaller seedlings, which have lower needle biomass and consequently need less water for the evapotranspiration process (
The growth of new roots is a key factor for the successful establishment of seedlings after planting. The definitive establishment allows a sufficient rate of water and nutrient uptake to be sustained in the long term or during stressful events, which can occur early and frequently in boreal forests (
The slow growth of black spruce prevents sizes suitable for planting to be attained within short periods. As a result, in seedlings of this species the size is intrinsically dependent on age. Moreover, production of the experimental seedlings carefully respected the standard criteria adopted by the forest tree nurseries in Quebec to make the results representative and effective for practitioners. Thus, in this investigation, the factor age covariated with seedling size, as either 2- or 3-year-old seedlings were used for small or big seedlings, respectively. This prevented identification of the main factor triggering the physiological performances of seedlings. Bigger seedlings have a less branched root system and lower proportions of unsuberized fine roots, and could be less efficient in water and nutrient uptake (
Before the experiment, C25, the smaller seedlings, were transferred to tunnels for two weeks to receive a short-day treatment. This was not provided for the seedlings of the other sizes. Such a treatment is contemplated by the protocol of forest tree nurseries of Quebec and currently used to induce growth cessation and bud set in seedlings, mainly for those planted in summer, which should be better enabled to withstand summer environmental conditions (
Another factor possibly affecting the results was related to the characteristics of containers. As shown in
Post-cutting regeneration is a critical period for successful and effective forest management, and is costly in the case of seeding or planting. The resistance to stress of seedlings during their first phases after planting is fundamental for avoiding a growth check and assuring fast establishment and long-term survival. There is increasing interest in seedlings of small sizes for artificial regeneration because they need less storage space and handling. However, the resistance of small seedlings to water deficit is questioned. In this work, the response to a severe water deficit produced by a suspension of irrigation for 14 days was assessed in black spruce seedlings of different sizes. Smaller seedlings exhibited similar or higher water potential and gas exchanges than bigger seedlings both during and after the treatment. The root growth of both small and big seedlings was not affected by the water stress. The initial hypothesis that small seedlings are more sensitive to a water stress was thus definitely rejected. The performances observed in smaller seedlings were attributed to higher ability of roots to sustain the evaporative needs of needles under water deficit and to maintain higher rates of carbon assimilation. However, the smaller seedlings were also necessarily younger, thus the question whether the differences observed were connected to their size or age remained unanswered.
The results of this experiment suggested that the performance in terms of tolerance to water stress and maintenance of root growth during a water deficit was similar in seedlings of small and big size, even under temperatures warmer than those generally occurring in typical boreal forests dominated by black spruce. Moreover, delayed stomatal closures were observed to occur in smaller seedlings, which allowed higher rates of photosynthesis to be maintained than in bigger seedlings. Given the financial and logistic advantage of using smaller seedlings in artificial regeneration, their utilization should be seriously considered and tested in boreal stands experiencing occasional drought events or non-optimal soil moisture conditions.
The four containers used to produce the seedlings for the experiment.
Environmental conditions and water content measured in black spruce seedlings submitted to water stress. Grey area represents the treatment period. Error bars indicate 95% confidence interval (n=5).
Physiological responses of black spruce seedlings of different sizes submitted to a water stress treatment. (
Relationships between
Root biomass of black spruce seedlings of different sizes submitted to a water stress treatment. Vertical axis is scaled at common logarithm. Grey area represents the treatment period.
Characteristics of the seedlings (mean ± standard deviation, n=20) and their growing containers (C25, C50, C110, C350 - see also
Item | Characteristics | C25 | C50 | C110 | C350 |
---|---|---|---|---|---|
Containers | Number of cavities | 113 | 67 | 45 | 25 |
Cavity depth (cm) | 7.5 | 8 | 12 | 12.5 | |
Cavity volume (cm3) | 25 | 50 | 110 | 350 | |
Seedlings | Seedling age (yrs) | 2 | 2 | 3 | 3 |
Seedling height (cm) | 12.82 ± 1.04 | 20.28 ± 2.53 | 31.81 ± 3.77 | 71.32 ± 4.38 | |
Diameter at collar (mm) | 1.9 ± 0.16 | 2.54 ± 0.20 | 3.71 ± 0.71 | 8.18 ± 0.68 | |
height:diameter ratio (cm/mm) | 6.80 ± 0.85 | 8.01 ± 1.10 | 8.80 ± 1.57 | 8.76 ± 0.84 | |
Needles (DW g) | 0.23 ± 0.05 | 0.53 ± 0.11 | 1.27 ± 0.34 | 7.02 ± 1.70 | |
Stem (DW g) | 0.19 ± 0.05 | 0.39 ± 0.09 | 0.95 ± 0.28 | 9.40 ± 1.59 | |
Roots (DW g) | 0.22 ± 0.06 | 0.49 ± 0.12 | 0.93 ± 0.35 | 6.67 ± 1.89 | |
Shoot:root ratio | 1.90 ± 0.31 | 1.91 ± 0.35 | 2.56 ± 0.63 | 2.61 ± 0.70 | |
Root density (cm3 cm-3) | 2.56 ± 1.40 | 6.60 ± 2.45 | 4.24 ± 1.00 | 7.00 ± 3.63 |
ANOVA comparisons performed on the physiological measurements collected from black spruce seedlings of different sizes (S) submitted to a water stress treatment (T). (
Source ofvariation | ndf | ddf |
|
|
|
|
||||
---|---|---|---|---|---|---|---|---|---|---|
F-ratio | p | F-ratio | p | F-ratio | p | F-ratio | p | |||
Treatment (T) | 1 | 4 | 8.36 | <0.05 | 23.28 | <0.01 | 14.61 | <0.05 | 50.53 | <0.01 |
Day (D) | 8 | 64 | 17.25 | <0.0001 | 23.14 | <0.0001 | 28.05 | <0.0001 | 64.99 | <0.0001 |
T × D | 8 | 64 | 11.54 | <0.0001 | 20.31 | <0.0001 | 8.96 | <0.0001 | 9.67 | <0.0001 |
Between T at D0 | 1 | 64 | 0.01 | ns | 0.32 | ns | 0.26 | ns | 0.01 | ns |
Between T at D2 | 1 | 64 | 0.03 | ns | 0.05 | ns | 0.00 | ns | 1.07 | ns |
Between T at D4 | 1 | 64 | 0.06 | ns | 3.77 | ns | 2.08 | ns | 9.03 | <0.01 |
Between T at D7 | 1 | 64 | 2.24 | ns | 17.11 | <0.001 | 25.26 | <0.0001 | 58.93 | <0.0001 |
Between T at D9 | 1 | 64 | 8.05 | <0.05 | 10.09 | <0.01 | 4.02 | ns | 54.46 | <0.0001 |
Between T at D11 | 1 | 64 | 25.78 | <0.0001 | 36.94 | <0.0001 | 33.92 | <0.0001 | 19.20 | <0.0001 |
Between T at D14 | 1 | 64 | 55.21 | <0.0001 | 124.02 | <0.0001 | 34.85 | <0.0001 | 34.46 | <0.0001 |
Between T at D21 | 1 | 64 | 0.01 | ns | 0.24 | ns | 0.26 | ns | 1.70 | ns |
Between T at D25 | 1 | 64 | 0.07 | ns | 1.08 | ns | 0.14 | ns | 3.65 | ns |
Size (S) | 3 | 215 | 1.80 | ns | 8.29 | <0.0001 | 148.06 | <0.0001 | 198.94 | <0.0001 |
T × S | 3 | 215 | 14.92 | <0.0001 | 3.66 | <0.05 | 1.63 | ns | 8.93 | <0.0001 |
S × D | 24 | 215 | 3.02 | <0.0001 | 1.14 | ns | 5.26 | <0.0001 | 7.20 | <0.0001 |
T × S × D | 24 | 215 | 2.79 | <0.0001 | 2.20 | <0.01 | 1.17 | ns | 1.86 | <0.0001 |
P-values of the orthogonal contrasts calculated for the days with significant effect of the treatment according to the ANOVA shown in
Contrasts | Day |
|
|
|
|
---|---|---|---|---|---|
(C25, C50) |
4 | ns | <0.01 | <0.0001 | <0.0001 |
7 | <0.01 | <0.001 | <0.0001 | <0.01 | |
9 | <0.0001 | ns | <0.0001 | <0.0001 | |
11 | <0.0001 | <0.01 | ns | ns | |
14 | <0.0001 | <0.01 | <0.05 | ns | |
C25 |
4 | ns | ns | ns | ns |
7 | ns | ns | ns | ns | |
9 | <0.05 | ns | ns | ns | |
11 | ns | ns | ns | ns | |
14 | ns | ns | ns | ns | |
C110 |
4 | ns | <0.001 | <0.0001 | ns |
7 | <0.01 | ns | <0.01 | ns | |
9 | <0.01 | ns | <0.01 | <0.05 | |
11 | <0.0001 | ns | ns | ns | |
14 | ns | ns | ns | ns |
Results of the ANCOVA models performed between
Dependentvariable | Source of variation | Regressors | Model | ||||
---|---|---|---|---|---|---|---|
Type III SS | F-value |
|
F-value |
|
R2 | ||
|
|
0.034 | 81.11 | <0.0001 | 21.73 | <0.0001 | 0.70 |
seedling size | 0.013 | 10.42 | <0.0001 | ||||
0.005 | 3.91 | <0.05 | |||||
|
|
6.887 | 112.94 | <0.0001 | 23.57 | <0.0001 | 0.72 |
seedling size | 0.629 | 3.44 | <0.05 | ||||
0.134 | 0.74 | ns |
ANCOVA comparisons performed on root biomass of black spruce seedlings of different sizes submitted to a water stress treatment. The analysis was performed on log-transformed data to respect the assumption of linearity. (ns): not significant.
Source of variation | ndf | ddf | F-ratio | p |
---|---|---|---|---|
Treatment (T) | 1 | 4 | 0.66 | ns |
Day (D) | 8 | 64 | 15.83 | <0.0001 |
D × T | 8 | 64 | 1.80 | ns |
Size (S) | 3 | 215 | 1237.71 | <0.0001 |
T × S | 3 | 215 | 1.51 | ns |
S × D | 24 | 215 | 1.20 | ns |
T × S × D | 24 | 215 | 0.71 | ns |