Restricted rooting space in response to soil compaction and belowground competition with herbaceous plants are two main limiting factors for successful reforestation after surface mining. Fine-textured, nutrient-rich soils with adequate soil moisture are particularly susceptible to both of these concerns and while there are recognized ways to manage competition, attempts to alleviate soil compaction through mechanical means have produced varying results. While roots of some herbaceous plants may penetrate compacted soil layers, possibly offering an alternative means to overcome physical restrictions, these potential benefits need to be weighed against negative effects from competition with planted trees. We examined the individual and combined impact of soil decompaction (deep tillage) and management of competing vegetation (herbicide) on soil properties, resource availability, and above- and below-ground growth of aspen (Populus tremuloides Michx.) seedlings on a reconstructed mine soil affected by severe subsoil compaction. Our findings suggest that although deep tillage reduced bulk density, this did not increase resource availability and had limited effect on seedling growth. In contrast, competition with smooth brome grass (Bromus inermis Leyss.) drastically reduced aspen belowground growth because the grass rapidly occupied available rooting space, while simultaneously lowering the availability of water and nutrients, in particular nitrogen.

Key message Root systems of aspen seedlings display limited architectural plasticity in response to below-ground competition, but seedlings compensate for restricted rooting space and reduced root system size, by optimizing water uptake. Abstract Below-ground competition with grasses often plays a critical role during tree seedling establishment, but many underlying mechanisms are not well understood. We used a controlled field experiment to study how trembling aspen (Populus tremuloides Michx.) seedlings compete with smooth brome grass (Bromus inermis L.) for space during the first 3 years of seedling establishment and how it affected aspen seedling development. Our study showed that competition with grasses had a limited impact on architectural plasticity of aspen seedlings. Seedlings faced with competition from smooth brome were overall smaller and most architectural parameters, with the exception of shoot height, appeared simply scaled down proportionally. Shoot height changed less than other parameters, because aspen competing with grass allocated relatively more carbon to shoots than roots and adopted a slender shoot morphology to quickly overtop the competition. Aspen growing with grass competition had significantly smaller root systems. Both lateral extent and maximum rooting depth were reduced by ~ 50%. In response to the restricted rooting space, roots of aspen seedlings faced with grass competition had a lower specific root length. Root carbohydrate reserves were not affected by competition; however, aspen roots growing with grass competition had higher soluble sugar concentrations which may be associated with the observed three times higher water uptake efficiency per unit root biomass. Our findings suggest that aspen seedlings have limited capacity for architectural plasticity in response to root competition, but are at least temporarily able to compensate for reduced root system size and rooting space, by optimizing water uptake efficiency.

Over 18 million hectares of lodgepole pine forests have been affected by a massive mountain pine beetle outbreak in western Canada. The ensuing need for reforestation compels forest managers to deploy more drought-resistant stock in anticipation of a hotter and drier future climate. Recent studies suggest that naturally occurring lodgepole pine × jack pine hybrids may be more drought-resistant than pure lodgepole pine. We used single nucleotide polymorphism (SNP) markers to determine the level of jack pine introgression present in the region J lodgepole pine seed orchard, in northwest Alberta, and identified six hybrid genotypes with 10-18% jack pine introgression for further study. We compared the performance of open-pollinated seedlings of lodgepole, jack and hybrid pines under well-watered and drought conditions in a greenhouse experiment. Seedlings were subjected to repeated drought cycles, withholding water for either two or three weeks. We found that hybrid pines grew significantly faster than lodgepole pine under well-watered conditions while retaining the high tolerance to prolonged drought exhibited by lodgepole pine seedlings, a particularly beneficial combination of traits during the critical stage of establishment. We therefore suggest the deployment of hybrid lodgepole pine as a strategy to promote adaptation to hotter and drier future climates.

Seed orchards play a critical role in reforestation, but often struggle to consistently produce enough seed. A mixture of the less polar gibberellins, GA 4 and GA 7 , has been shown to promote flowering in Pinaceae species, but the responses vary significantly based on species, genotype and provenance, application method, timing and dosage. Previous studies with lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) suggest that the response to exogenous GA 4/7 varied among genotypes. Gibberellins also play a critical role in seed development and germination but it remains unclear how exogenous GA 4/7 will affect seed quality. We used 25 lodgepole pine genotypes in a commercial seed orchard in Alberta, Canada to explore the impact of genotype and geographic origin on the efficacy and optimal timing of GA 4/7 stem injections to promote female cone bud initiation, cones and seed production. Overall, we found that GA 4/7 stem injections promoted female conelets in 40% of genotypes included in this study. For these 10 genotypes, we found that the latitude of their provenance was positively correlated with later effective application times. Of the remaining 15 genotypes, five were hypersensitive to exogenous GA 4/7 showing needle discolouration and loss, and 10 genotypes showed no response. In general, steminjected GA 4/7 did not affect the number of seeds per cone while two genotypes showed a reduced seed yield and one genotype showed increased seed yield per cone. Seed weights were not affected by the stem-injected GA 4/7 , while the July application date resulted in higher germination rates compared to control trees.

Tree improvement programs in Alberta have been primarily based on selections from open-pollinated wild stand trees, without systematic breeding of elite parents through controlled pollination. This strategy has resulted in relatively slow advancements in genetic gain in most programs, given their age. Our objective was to compare seed yield, genetic parameters, and economic implications of three different breeding strategies to advance Alberta’s white spruce (Picea glauca (Moench) Voss) improvement programs. Eighteen genotypes, identified as ‘elite’ based on height breeding values (BV), were selected from a first-generation white spruce orchard and controlled crosses (CC) and controlled polymix (PM) crosses performed to compare seed yield and genetic parameters with open-pollinated (OP) seedlots from the same parent. Results show, that on average, OP seedlots had 29 seeds/cone and weighed 2.7 grams/1000 seeds, significantly larger than the CC seedlots with a mean of 10 seeds/cone and 2.2 g/1000 seeds. Polymix crosses produced intermediate mean values with 19 seeds/cone and 2.5 g/1000 seeds and were not significantly different from the other two breeding strategies. No statistical differences were found for cone length or germination percentage among the CC, PM, and OP breeding strategies. Significant phenotypic variability was found for seeds/cone (yield) and seed weight among families from CC, and these traits had moderate narrow-sense heritabilities of 0.49 (±0.11) and 0.29 (±0.13), respectively. There was no significant correlation between the general combining ability (GCA) of seed yield and BV for seedling growth, but females F132, F138, and F927 and males M966, M1002, and M1045 showed the best performance in seed yield and growth and would be good candidates to include in future controlled crosses. If CC breeding is considered a strategy for operational seed production, as conducted elsewhere, the highest land expectation value (LEV) was $141 CAD/ha at age 60, and the best net present values (NPV) were for both discount rates (2%, 4%) with log prices ($90 CAD/m3, $200 CAD/m3) tested. However, this advantage of CC vs PM and OP only occurred on the highest quality sites (i.e., Site Index (SI) 13 and 18). On the low-productivity site (SI = 6), with the various scenarios, improved material did not show any financial benefit. Our results show that seeds produced from the controlled crosses breeding strategy were inferior to those produced from the controlled polymix and open-pollinated strategies. However, with sufficient investment and company support, the CC breeding strategy could be used as a cost-effective method to increase genetic gain and advance white spruce breeding programs in Alberta.