New Forests (2016) 47:815–827 DOI 10.1007/s11056-016-9546-4
Impact of planting container type on growth and survival of three hybrid poplar clones in central Alberta, Canada Barb R. Thomas1
•
Stefan G. Schreiber1 • David P. Kamelchuk1,2,3
Received: 7 December 2015 / Accepted: 8 July 2016 / Published online: 15 July 2016 Ó Springer Science+Business Media Dordrecht 2016
Abstract We compared growth performance and survival of three hybrid poplar clones (Walker, Northwest and Okanese) planted as cuttings into five different StyroblockÒ containers (412A, 415D, 512A, 515A, 615A) with increasing cavity volume and decreasing cavity density under commercial growing conditions at two nurseries in central Alberta, Canada. After 175 days of growth, our results showed considerable variation in growth traits among container types while survival was generally high with an overall average of 89 %. Initial cutting diameter appeared to be an important predictor of survival and our results showed that a cutting diameter of C7.5 mm increased survival rates of the tested hybrid poplar planting stock. Furthermore, containers with larger cavity volume and lower cavity density had a strong positive influence on growth and survival across nurseries (R2 = 0.96). Growth trait interactions with container type showed that container 512A (cavity volume: 220 ml; cavity depth: 12 cm) resulted in more diameter growth across clones. Cavities with a depth of 15 cm (415D, 515A, 615A) resulted in higher root:shoot ratios than cavities with a depth of only 12 cm (412A, 512A), irrespective of cavity volume or cavity density. Lastly, our study identified Okanese as a well-rounded clone with great growth potential both above and below ground. From an operational standpoint, we found container types 512A and 515A the most cost-effective choices under the assumption that nursery space and budgets are limiting factors.
Barb R. Thomas and Stefan G. Schreiber have contributed equally to the study. Electronic supplementary material The online version of this article (doi:10.1007/s11056-016-9546-4) contains supplementary material, which is available to authorized users. & Barb R. Thomas
[email protected] 1
University of Alberta, 442 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada
2
Little Creek Agroforestry, Box 32, Ellscott, AB T0A 1B0, Canada
3
Alberta-Pacific Forest Industries Inc., PO Box 8000, Boyle, AB T0A 0M0, Canada
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Keywords Boreal forest Cavity density Cavity volume Nursery production Okanese Styroblock
Introduction Intensively managed hybrid poplar plantations remain a viable alternative for short term fibre production under scenarios of both a changing climate (i.e.: short rotation times) and reduced land availability (i.e.: increased fibre production per unit area). In particular, in Alberta, Canada, the pressures from energy sector expansion and climate change are palpable and are beginning to support a shift in land management strategies (Schneider 2002; Schneider et al. 2009). This shift includes a change in policy from 100 % extensive management of forests to a land use planning framework supporting the concept of intensively managed fibre production (i.e.: to reduce shortfalls in timber supply) on a restricted land-base on government stewardship lands (LUF 2013). In addition, private land development, where there are no restrictions on species use (SRD 2009), has resulted in investigations of alternative cropping scenarios, including consideration of hybrid poplars as a crop (Saurette et al. 2008). Throughout the Prairie Provinces of Canada, the primary hybrid poplars used for fast growth, traditionally as wind-breaks and shelterbelts, have historically come from crosses between the cottonwoods, black poplars (Aigeiros section) and balsam poplars (Tacamahaca section) (Cooke and Rood 2007; Talbot et al. 2011; Zalasky et al. 1968). Through selection and clonal propagation of the most vigorous progeny, growth rates greater than six times that of the native deciduous species (2 m3 ha-1 yr-1) can be achieved (Anderson and Luckert 2007; Jarvis 1968). Most poplar and willow species and their hybrids are also relatively easy to propagate from stem cuttings allowing for mass propagation of individual clones through use of stooling beds (DesRochers and Thomas 2003; Schroeder and Walker 1990; Stanturf et al. 2001; Verwijst et al. 2012; Vigl and Rewald 2014). Although a considerable number of studies have been completed over many decades on optimizing seedling production over a range of conifer species (e.g. Bayley and Kietzka 1997; Duryea and Landis 1984; Grossnickle et al. 1994; Nelson and Lavender 1979; Switzer and Nelson 1963), less effort has been spent on poplars and aspens, and hardwoods in general (DesRochers and Thomas 2003; Landha¨usser et al. 2012; Schroeder and Walker 1990; Wilson and Jacobs 2006). Yet, the opportunity to plant vast tracks of unimproved pasture lands to plantations across Saskatchewan alone has been estimated at 260,000 ha within 100 km of existing mills (Johnston et al. 2001, 2002). Across western Canada, van Kooten et al. (1999) determined there is seven million ha of agricultural land available for afforestation plantations. The options for nursery stock production, once cuttings are harvested from stooling beds, are either to produce cuttings to be rooted in the nursery (rooted plugs or bare-root stock) or to directly plant unrooted cuttings into field sites, reducing overall costs. However, direct planting of unrooted cuttings in the relatively dry regions of the boreal-prairie transition zone can greatly reduce initial survival (Wang et al. 2014). Work by Block et al. (2009) found a survival difference of 51 % between rooted versus unrooted cuttings of Walker poplar, a widely planted hybrid poplar clone, after 1 year of field growth in Saskatchewan. Although rooted cuttings are initially more expensive to grow, Riemenschneider (1997) reports that hybrid poplar cuttings rooted in the nursery can outperform
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unrooted cuttings of the same genotype and offset the additional cost associated with the nursery propagation step. When choosing rooted cuttings over unrooted cuttings, a decision has to be made regarding the optimal container type (i.e.: when bare-root stock production is not an option) in order to achieve the highest growth performance and survival while minimizing costs. In an extensive literature review of pot size on plant growth, Poorter et al. (2012) found on average, that biomass increased by 43 % when container volume doubled. They further suggest that pot size should be chosen so that the total plant biomass does not exceed 1 g/L. These recommendations however, are in the context of scientific research in which a high biomass to pot size ratio can influence the relative differences of applied treatments and hence prevent generalization of those results and their reliability for practical use. Under operational conditions, this recommendation is economically not feasible since commercial nurseries generally charge based on a per m2 area of bench space and not on production size, although minimum stock size specifications typically also have to be met. A typical container for hybrid poplar cuttings at a commercial nursery in Alberta, Canada is the 415D Styroblock (Pers. Obs. B. Thomas), which has cavities with a diameter of 4 cm and a depth of 15 cm resulting in a volume of 164 ml and a planting density of 364 cavities per m2. These containers however, while economically efficient, may not represent the optimal choice in order to also maximise plant growth and survival. Initial cutting diameter for hybrid poplars has also been identified as a proxy for growth performance and survival with large diameter cuttings usually performing best. Dickmann et al. (1992) suggest using hybrid poplar cuttings with diameters greater than 6 mm, while Hansen and Tolsted (1981) found that cuttings of a difficult-to-root Populus alba hybrid should be larger than 9 mm. Yet another study found no strong relationship between cutting diameter of hybrid poplars with growth performance or survival (Robison and Raffa 1996). As pointed out by Wilson and Jacobs (2006), the overwhelming amount of nursery performance studies are dealing with conifer seedlings and although work has been done, hardwoods in general are underrepresented in the literature. In order to further address issues associated with cutting diameter, container sizes and clonal performance under commercial growing conditions, a study was conducted with the goal of evaluating the growth and survival of three operationally important hybrid poplar clones (Walker, Northwest and Okanese) at two commercial nurseries. Our main objectives were to: (a) compare growth and survival of three selected hybrid poplar clones planted in five different StyroblockÒ container types after a single growing season, (b) assess the effect of initial cutting diameter as a proxy for growth performance and survival, and (c) provide recommendations on cutting size and StyroblockÒ container type for optimizing growth and survival in the nursery under operational conditions.
Materials and methods Plant processing and planting One-year old dormant whips were harvested from 2-year old stool beds located at the Smoky Lake Forest Nursery, in central Alberta, from three hybrid poplar clones: Walker (Populus deltoides 9 P. x petrowskyana), Northwest (P. balsamifera 9 P. deltoides), and Okanese (Walker 9 P. x petrowskyana) (Lindquist et al. 1977; Schroeder et al. 2013; Talbot et al. 2011). Whips were cut into approximately nine cm long cuttings (DesRochers
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and Thomas 2003) with one healthy bud near the top of each cutting and kept in plastic bags at -3 °C in storage facilities at Smoky Lake Forest Nursery. The following spring, each of two commercial nurseries (A and B) located 127 km apart in east central Alberta received 957 cuttings of each clone (for cultural practices provided by the nursery see Supporting Information Table S1). At each nursery, cuttings were randomly assigned to five differently sized StyroblockÒ containers (from here on referred to as Styroblocks or container) as described in Table 1 and planted flush with the soil surface. A total of three containers of each treatment size and clone per Styroblock type were used at each nursery (5 Styroblocks 9 3 replicates 9 3 clones 9 2 nurseries = 90 containers total) and a total of 5742 cuttings were planted (see also Supporting Information Figure S1 for the experimental layout). In order to compare clonal responses within the same Styroblock container type (size), each Styroblock type (e.g. 412A, 415D etc.) was grouped together with border rows of empty blocks surrounding each group. Groups consisted of three replicate Styroblocks of each clone for a total of nine Styroblocks per group. Clones and replicates were randomized within each grouping of Styroblocks. Complete randomization of Styroblock types, clones and replicates to allow for true blocking, would have resulted in confounding effects on growth patterns, no longer representing the operational realities of how large commercial nurseries would grow stock of different sizes (i.e.: by Styroblock sizes). To minimize the inclusion of three clones within each grouping, only the inside 15 trees per Styroblock container were measured. Furthermore, to avoid ‘loafing’ at the edges of each group, Styroblocks were re-randomized twice during the growing season when trees were measured for height and caliper at days 55 and 105 (data not shown). Therefore, with 45 containers in total, there were five groups (treatment types) of nine containers per nursery. Depending on the bench size and configuration, each nursery determined the best orientation for the Styroblocks within their respective facilities and within their ‘regular’ commercial growing stock. The total area occupied by the Styroblock trial representing only a fraction of the space in the nursery and therefore considered relatively homogeneous although not measured.
Measurements Growth measurements (shoot height and shoot diameter) were conducted on an ‘inside’ group of 15 trees per Styroblock, leaving, depending on the container type (Table 1), one or more outside rows of border trees. The diameter of the initial cutting was measured at the soil surface on each cutting immediately after planting. At the end of the experiment (day 175), two trees per Styroblock were harvested from the 15 measurement trees and Table 1 StyroblockÒ container types and sizes Container typea
Cavity top diameter (cm)
Cavity depth (cm)
Volume per cavity (ml)
Cavities per m2
412A
4.3
11.7
125
364
415D
4.3
15.2
164
364
512A
5.1
11.9
220
284
515A
5.1
15.2
250
284
615A
6.1
15.2
336
213
a
Container 412 for example stands for a diameter of 4 cm and a depth of 12 cm
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placed into the freezer until they were processed. Processing included washing the soil away from the roots and then drying the roots for 48 h at 80 °C to a constant weight before root and shoot weights were measured and root:shoot ratios calculated. All measurements were collected by the same personnel at both nurseries throughout the study.
Data analysis All data exploration, analysis and graphics were carried out using the R programming environment 3.1.3 (R Core Team 2015). The following additional packages were used: plyr (Wickham 2011), ggplot2 (Wickham 2009), lme4 (Bates et al. 2014), and lsmeans (Lenth 2015). For the dependent variables shoot diameter and root:shoot ratio, a linear mixedeffects model was fitted using the lmer() command of the lme4 package. For the binary outcome of survival (dead/alive) a generalized linear mixed-model was fitted using the glmer() command and the logit link function. The independent variables in all models were Nursery, Container and Clone as fixed effects. Initial cutting diameter was added as a covariate to test for interactions with the fixed effects (except for Clone because average cutting diameter was correlated with clone as stool beds varied in size by clone). This allowed us to test whether initial cutting diameter had an influence on the measured growth traits. Since the experimental unit was each Styroblock container (45 containers per nursery), and not the 15 cavities within each container, we specified ContainerID nested in Nursery (90 unique combinations representing each container) as random to account for multiple measurements in each container (Supporting Information Table S2). The model equation for both the linear-mixed effect model and the generalized linear mixed-effects model is: Yijklm ¼ l þ Ni þ Cj þ CLk þ ðN CÞij þ ðN CLÞik þ ðC CLÞjk þ Dl þ ðN DÞil þ ðC DÞjl þ Cid ðN Þmi þ 2ijklm where Ni is the ith Nursery (fixed effect), Cj is the jth Container type (fixed effect), CLk is the kth Clone (fixed effect), Dl is the lth initial cutting diameter (covariate) and Cid ðN Þmi the mth ContainerID nested in the ith nursery (random effect). Variables in parentheses represent interactions between these variables. Pairwise comparisons were conducted using the lsmeans() command of the lsmeans package and P values were adjusted using Tukey’s HSD test with a family-wise confidence level set at 0.95.
Results Growth traits were quite variable with the most significant differences due to container type and clone (Fig. 1; Table 2, Supporting Information Table S2). Initial cutting diameter appeared to be a key predictor for survival (P \ 0.001) even though survival was generally very high with an average of 89 % across both nurseries. An initial cutting diameter C7.5 mm emerged as a threshold at which survival reached its highest levels (Fig. 2). Neither container type nor clone or nursery had a significant influence on survival. However, an interaction between nursery and container was significant (P \ 0.001) and can also be seen by the contrasting survival rates (fitted lines) for container 512A and 515A between nursery A and B (Fig. 2) (see also Table S2). When pooled by container type, a regression analysis between shoot height, shoot diameter and survival showed strong positive relationships (R2 = 0.96, P = 0.002;
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b Northwest
Okanese
Walker
Okanese
4 2 0
Nursery B
Nursery B
4 2
41 41 2A 515D 5 12 A 61 5A 5A 41 41 2A 515D 512A 61 5A 5A 41 41 2A 515D 5 12 A 615A 5A
41 4 1 2A 515D 5 12 A 6 1 5A 5A 41 2 41 A 515D 51 2A 6 1 5A 5A 41 41 2A 515D 51 2A 615A 5A
0
c
d Okanese
0.5
0.5
41 41 2 A 515D 5 1 2A 615A 5A 41 2 41 A 515D 51 2A 615A 5A 41 2 41 A 515D 51 2 A 615A 5A
0.0
Okanese
Nursery B
1.0
Nursery B
0.0 1.5
Northwest
100 75 50 25 0 100 75 50 25 0
Nursery A
1.0
Walker Nursery A
1.5
41 41 2 A 515D 5 12A 615 A 5A 41 2 41 A 515D 51 2 A 6 1 5A 5A 41 41 2A 515D 512A 615A 5A
Northwest
Survival (%)
Walker
Root : shoot ratio
Northwest
Nursery A
50 40 30 20 10 0 50 40 30 20 10 0
Nursery A
Shoot height (cm)
Walker
Shoot diameter (mm)
a
Container
Fig. 1 Overview of mean growth trait measurements for shoot height (a), shoot diameter (b), root:shoot ratio (c) and percent survival mean values (d) for the tested StyroblockÒ container types summarized by clone and nursery. Error bars represent the 95 % confidence interval
R2 = 0.98, P = 0.001, for height and diameter respectively) (Fig. 3a, b). This appears to be primarily driven by decreasing cavity density, and only marginally as a result of cavity volume (ml). In fact, container 512A with a lower volume compared to 515A had increased shoot height and shoot diameter although not significant (Table 2). An interaction plot of shoot height and shoot diameter with container type at the clone level also supports the positive influence of cavity density on growth traits (Fig. 4a, b). However, this relationship tapers-off at 284 cavities/m2 for the Okanese and Northwest clones. The Walker clone on the other hand, benefited from the reduced cavity density (and increase in cavity volume) by further significantly increasing shoot height and shoot diameter. Figure 4c shows that variation in root:shoot ratio is driven by container depth (12 vs. 15 cm for containers with the same density) and not container volume. Furthermore, Fig. 4c illustrates that the Okanese clone had the highest root:shoot ratios when compared to the Northwest and Walker clones (see also Table 2).
Discussion As the fibre demand for forestry or bioenergy use continues to increase and alternative cropping options for farmers develop, the expansion of plantation forestry in the prairie provinces of Canada may become far more widespread. Many aspects of successful programs are being tackled through studies ranging from social acceptance and carbon
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Table 2 Multiple mean comparisons of growth traits (shoot height, diameter, survival and root:shoot ratio) and survival after a 175 day growing period for nursery, container and clone Nursery
Mean
Container
Mean
412A
27.7 (0.6)A
415D
A B
Clone
Mean
Walker
33.6 (0.5)A
Northwest
33.9 (0.5)B
Okanese
30.0 (0.5)B
Shoot height (cm) 1
33.4 (0.4)A
2
B
31.6 (0.4)
512A
28.1 (0.6) 35.1 (0.6)
B
515A
33.0 (0.6)
615A
38.4 (0.6)C
412A
3.11 (0.06)A
Walker
3.84 (0.04)A
415D
B
Northwest
3.47 (0.04)A
C
Okanese
3.71 (0.05)B
Shoot diameter (mm) 1
3.84 (0.03)A
2
B
3.50 (0.03)
512A
3.36 (0.06) 3.93 (0.06)
C
515A
3.71 (0.06)
615A
4.25 (0.05)D
412A
0.52 (0.05)A
Root:shoot ratio 1 2
0.67 (0.02) 0.72 (0.02)
415D 512A
Walker
0.59 (0.03)A
CD
Northwest
0.61 (0.03)A
AB
Okanese
0.89 (0.03)B
0.77 (0.04) 0.55 (0.04)
BC
515A
0.71 (0.03)
615A
0.92 (0.05)D
Survival (on logit scale) 1
2.32 (0.16)
412A
2.43 (0.30)A
Walker
2.30 (0.18)
2
2.50 (0.17)
415D
2.02 (0.20)A
Northwest
2.48 (0.20)
512A
2.41 (0.25)A
Okanese
2.46 (0.22)
515A
2.40 (0.24)A
615A
2.80 (0.28)A
All mean comparisons are based on least squares means (lsmeans) and were adjusted using Tukey’s HSD test with a family-wise confidence level set at 0.95. Significant differences are indicated with different letters. If no letters are present, the main effect was not significant at P \ 0.05. Standard error of the mean is in parentheses
accounting to silvicultural management strategies (Cai et al. 2011; DesRochers et al. 2006, 2007; Henkel-Johnson 2013; Neumann et al. 2007). An area still requiring research however, is the optimization of nursery culture specifically on hybrid poplar production, where millions of trees are required and where unrooted cuttings are of limited use (e.g. Block et al. 2009; DesRochers and Thomas 2003; Navratil and Rochon 1981). Understanding the trade-offs between growth performance and survival of hybrid poplar clones between planting container type and nurseries are critical components for optimized commercialization and maintaining economic viability given the annual production of millions of rooted cuttings used for operational deployment in hybrid poplar plantations (Anderson et al. 2014; Anderson and Luckert 2007). As is typical of hybrid poplar plantations, when only a relatively small number of selected clones are being deployed, every step in the production system is critical for economic success (Larocque et al. 2013). In northern climates, where rooted cuttings are the most reliable form of stock type, ensuring that greenhouse bench space is optimized for production of the highest quality stock is critical. This study compared the overall performance and survival of three operational
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Fig. 2 Probability of survival (dead = 0/alive = 1) as a function of initial cutting diameter summarized by container type and nursery. The first digit of the container type represents the cavity diameter and the following two digits the cavity depth (both in cm). The grey-shaded areas around the general linear model fit represent the 95 % confidence interval
hybrid poplar clones planted in five different commercially used StyroblockÒ container types at two different operational nurseries. One clone in particular, Walker poplar, is widely used and represented between 35 to 50 % of the stock planted by Al-Pac in their hybrid poplar plantations covering an area of more than 10,000 ha (Pers. Obs. B. Thomas). Therefore, it is of no surprise, that Walker, and the more recently released hybrid poplar clone Okanese (both represented in the present study), have received considerable attention in the scientific literature in recent years (Kalcsits et al. 2009; LeBoldus et al. 2007; Schreiber et al. 2016, 2011; Schroeder et al. 2013; Silim et al. 2009). This trial has shown that both commercial nurseries are capable of achieving survival rates close to 90 % with production of rooted hybrid poplars in most container types. Our study showed that the initial cutting diameter is also an important pre-selection tool to further maximize survival rates (Fig. 2; Table 2), which is consistent with other studies (DesRochers and Thomas 2003; Hansen and Tolsted 1981). Neither container type, nursery nor clone could significantly explain differences in survival (Table 2), although we did find a significant interaction between nursery and container type. However, since nursery was considered as a replicate in our study and not a treatment per se, we cannot explain the underlying mechanisms that resulted in this interaction. The interaction between nursery and container type can be seen in Fig. 2 (cf. survival in container 512A vs. 515A between nurseries). In general, we suspect that larger cutting diameters may contain more nonstructural carbohydrates and therefore provide a longer lasting energy reserve. With respect to rooting ability, Veierskov (1988) showed that a higher content of stored carbohydrates in cuttings improved rooting success if other physiological conditions were favourable. Other evidence suggests that carbohydrate allocation and distribution within the cutting may be more important than just carbohydrate content (Druege 2009). Yet other studies were not able to find these relationships (e.g. Haissig 1982, 1984). It therefore appears that the influence of carbohydrate content on rooting ability remains controversial and could also be species or clone specific.
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a
823
40
Shoot height (cm)
R 2 = 0.96, P = 0.002
35
●
412A 415D
30
512A 515A ●
84
615A
88
92
96
Survival (%)
b Shoot diameter (mm)
R 2 = 0.98, P = 0.001 4.0
3.6
●
412A 415D 512A
3.2
515A ●
84
615A
88
92
96
Survival (%) Fig. 3 Regression of shoot height (a) and shoot diameter (b) with survival. The first digit represents the cavity diameter and the following two digits the cavity depth (both in cm). Error bars represent the 95 % confidence interval. Symbols represent the tested StyroblockÒ container types
With respect to container types, cavities/m2 seemed to be a key variable ensuring diameter growth and survival when all data were pooled at the container level and across nurseries (Fig. 3). Containers with lower cavity densities showed the highest survival rates and growth performance. However, when comparing shoot diameter across container types at the clone level, our study also highlights that the container with the lowest cavity density and the largest cavity volume (615A) is not necessarily a better choice than containers 512A and 515A, respectively (Fig. 4a). It appears that there is a trade-off, where more volume and less cavity density, does not result in an increase in survival and growth performance. From an economic perspective this is also quite important information since higher cavity densities allows for growing more trees on a given m2 area of bench space. Our study also showed that cavity depth, irrespective of volume, is an important driver of root:shoot ratios (Fig. 4b). This is an interesting and useful finding, as it could indicate which clone container combination results in healthy clones given that shoot height and diameter is not compromised. It is also important to note, that the Okanese clone showed
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a
Vol: 336 Den: 213
45
Shoot height (cm)
Fig. 4 Interaction plot of container type and clone for shoot height (a), shoot diameter (b) and root:shoot ratio (c). The first digit of the container type represents the cavity diameter and the following two digits the cavity depth (both in cm). Vol = cavity volume in ml and Den = number of cavities per m2. The values in parentheses represent the numbers of containers per m2 (=cavity density). Error bars represent the 95 % confidence interval
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●
Vol: 220 Den: 284
Vol: 250 Den: 284
40
35 Vol: 125 Den: 364
30
Vol: 164 Den: 364
● ●
●
● Walker
●
Northwest
25
Okanese
412A
415D
512A
515A
615A
Container type
b
Vol: 336 Den: 213
Shoot diameter (mm)
5.0
Vol: 164 Den: 364
4.0 3.5
●
Vol: 220 Den: 284
4.5
Vol: 125 Den: 364
Vol: 250 Den: 284
●
●
●
●
● Walker
3.0
Northwest Okanese
412A
415D
512A
515A
615A
Container type
c
Root : shoot ratio
1.25
1.00
Vol: 336 Den: 213
Vol: 164 Den: 364 Vol: 220 Den: 284
Vol: 125 Den: 364
Vol: 250 Den: 284
0.75
● 0.50
●
●
●
● ● Walker
Northwest Okanese
412A
415D
512A
515A
615A
Container type
both the highest root:shoot ratios, produced the tallest trees in the trial (cf Fig. 4a, b) and might therefore replace Walker poplar as the dominant clone in operational hybrid poplar plantations. In long-term field performance trials, Okanese has also shown itself to be
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superior from an ecophysiological and pathological perspective (LeBoldus et al. 2007; Schreiber et al. 2016). Since this experiment did not follow up on outplanting performance, we can only speculate, based on the literature, which traits may be useful for forecasting purposes. In general, references regarding rooted hybrid poplar cuttings are sparse (Wilson and Jacobs 2006). There is however, a wealth of information available on seedlings, specifically conifers, and generally shoot height and particularly shoot diameter emerge as the key traits (Grossnickle et al. 1994; Thompson 1985). Until more detailed information is available to forecast outplanting success in rooted cuttings of hybrid poplars, nursery operators should aim for shoot height and shoot diameter as the main proxies. Furthermore, root mass and root:shoot ratios also represent two additional traits which could be considered in forecasting outplanting success (Landha¨usser et al. 2012). Given that container 415D is the current operational choice for hybrid poplar cuttings in Alberta, our results demonstrate that the 512A or 515A containers should be considered as a viable alternative. From a purely growth and survival standpoint, changing to a container with a larger cavity warrants consideration. Whether or not the larger cavity size can be justified under operational conditions will depend on the number of trees needed for planting, the available budget and space at the nursery and ultimately, if survival and growth performance after outplanting is improved. In light of our results, an economic analysis is the next logical step to determine the optimum StyroblockÒ size to use and if using container 512A or 515A is a viable alternative in producing healthier, stronger trees, in combination with a minimum cutting diameter of [7.5 mm for outplanting and operational deployment. Any forestry operator needs to understand the production capabilities of the nursery they are using for their production stock to ensure the best clone, culture and ultimately survival is achieved. Acknowledgments The authors would like to thank Alberta-Pacific Forest Industries Inc., (Al-Pac) for financial support of this work and Joanna Ramsum from Al-Pac and Dan McCurdy and Larry Lafleur from Coast to Coast Reforestation for their time and effort on this trial. We would also like to acknowledge all the summer students employed by Al-Pac who assisted with harvesting and washing of all plant materials, as well as two anonymous reviewers who provided helpful comments on a previous version of the manuscript. Compliance with ethical standards Conflict of interest None.
References Anderson JA, Luckert MK (2007) Can hybrid poplar save industrial forestry in Canada?: a financial analysis in Alberta and policy considerations. For Chron 83:92–104 Anderson JA, Long A, Luckert M (2014) A financial analysis of establishing poplar plantations for carbon offsets using Alberta and British Columbia’s afforestation protocols. Can J For Res 45:207–216 Bates D, Ma¨chler M, Bolker B, Walker S (2014) Fitting linear mixed-effects models using lme4. arXiv: 1406.5823 Bayley AD, Kietzka JW (1997) Stock quality and field performance of Pinus patula seedlings produced under two nursery growing regimes during seven different nursery production periods. New For 13:341–356 Block RMA, Knight JD, Booth NWH, van Rees KCJ (2009) Nursery stock type, nitrogen fertilization and shoot pruning effects on the growth of juvenile hybrid poplar in Saskatchewan. Can J Plant Sci 89:289–301 Cai T, Price DT, Orchansky AL, Thomas BR (2011) Carbon, water, and energy exchanges of a hybrid poplar plantation during the first five years following planting. Ecosystems 14:658–671
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