Somatic embryogenesis (SE) is a laboratory-based method that allows for cost-effective production of large numbers of clonal copies of plants, of particular interest for conifers where other clonal propagation methods are mostly unavailable. In this study, the effect of l-glutamine as an organic nitrogen source was evaluated for three contrasted media (containing NH₄ + NO₃ without glutamine, or glutamine + NO₃, or glutamine without inorganic nitrogen) during proliferation and maturation of Norway spruce somatic embryos through analyses of activities of the key enzymes of nitrogen metabolism: nitrate reductase (NR), glutamine synthetase (GS) and arginase. A major change in nitrogen metabolism was indicated by the increased activity of GS from zero in the proliferation stage through maturation to high activity in somatic embryo-derived plantlets; furthermore, NR activity increased from zero at the proliferation stage to high activity in maturing embryos and somatic-embryo derived plantlets. In contrast, arginase activity was high at all stages. In accordance with the GS and NR data, proliferation was fastest on the glutamine media, the largest number of mature embryos developed on the glutamine medium without inorganic nitrogen, and the ammonium content of proliferating cultures was much higher than that of mature somatic embryos. The best developed embryos, judged by germination, were from the glutamine + NO₃ medium. These insights into nitrogen utilization will help to improve culture medium composition. Full article

Plantation-grown Fraser fir (Abies fraseri (Pursh) Poir.) Christmas trees were subjected to nine shearing regimes over nine years in a plantation in western North Carolina (USA). Treatments differed in the year of onset as well as the length of the residual leader (25 to 46 cm). Long leaders (36 to 46 cm) yielded trees that were taller than trees sheared with short leaders (25 to 30 cm), but the gain in height was offset by a lower average U.S. Department of Agriculture grade. Late in the rotation, trees sheared with 36–46-cm leaders were 17–34% heavier than those with 25–30-cm leaders. Average wholesale price of trees sheared with long leaders was 57% greater than that of trees sheared with short leaders. Assuming good bud density on leaders and branches, and considering other factors as well, the optimum leader length for Fraser fir Christmas trees in western North Carolina appears to be 30 to 41 cm (12 to 16 inches). Depending on site quality and variation in bud density and vigor among individual trees, leader length can be reduced, if necessary, to increase crown density. Full article

Heavy cone production by Fraser fir Christmas trees requires significant labor inputs to remove nuisance cones. We conducted two field trials in collaboration with operational Christmas tree farms to evaluate the effectiveness of post-emergent herbicides to stop the development of newly emergent cone buds. In the first trial (2016), we applied six products (two conventional herbicides and four herbicides labeled for organic production) to trees using back-pack sprayers in operational plantations at four farms in Michigan. Three products; Scythe, Axxe, and Avenger, provided better cone kill than the other products but resulted in phytotoxicity at two locations. In 2017, we applied the three most effective products from the earlier trial at three farms either as single applications or as two applications approximately one week apart. We also evaluated a hand-held mechanical de-coning device at two farms. For all the products and the mechanical device, cone control in the 2017 trial was high (>80%). Phytotoxicity to foliage was low (mean rating, <0.3; 0 = none, 2 = severe) for single applications of the herbicides. Repeated applications increased cone control slightly but also increased risk for phytotoxicity. The mechanical device caused significant damage to shoots and foliage. We attribute the increased product effectiveness and reduced phytotoxicity between the 2016 and 2017 studies to improved coverage and earlier spray timing. Based on the current retail product cost, chemical cone control can be cost-effective compared to handpicking cones if trees have high numbers of cones that can take several minutes to remove. The effect of using surfactants and reducing product rates should be investigated along with mechanized application. Full article

Fraser fir (Abies fraseri [Pursh] Poir.) is an important Christmas tree species in the United States, and understanding its phenology is important for managing Fraser fir trees in plantations or forests. Many management decisions are informed by and dependent on shoot phenology, from late spring frost protection to shearing, and from timing pesticide sprays to managing cone production. The ability to predict important phenological stages will become increasingly important as the climate warms, as is predicted for the primary regions where Fraser fir is grown for Christmas trees. Here, we report on the development of a model of shoot phenology in Fraser fir, and present one example of how this model may be applied to the problem of managing cone production. We surveyed shoot phenology at nine Christmas tree plantations in Michigan over three years, and used the data obtained to develop a phenology model of shoot growth. Derived from the beta sigmoid function and based on growing degree days, this phenology model offers a high predictive power and is robust to extremes of temperature and precipitation. When applied to cone production, our model provides guidance for timing practices that influence cone bud formation, both for reducing nuisance cones in Christmas tree plantations and for enhancing cone production in seed orchards. In addition, the model may assist with timing other practices tied to shoot phenology. The performance of our model under extreme heat and drought conditions suggests a role for this and other phenology models in predicting and mitigating the effects of climate change on tree growth and development. Full article

Grafting provides a means to clonally produce Fraser fir (Abies fraseri (Pursh) Poir.) Christmas trees that have desirable traits such as faster growth, greater crown density, increased pest resistance, or more desirable foliage attributes than seedling stock. Grafting Fraser fir to disease resistant rootstocks also provides a means to ameliorate the impact of root rot, predominantly caused by Phytophthora cinnamomi Rands. The influence of ortet age on growth and market value of grafts has not been studied for Fraser fir Christmas tree production. A field trial was established in 2004 near Independence, Virginia (USA), with the objectives of assessing (1) the effect of ortet age (stock plants = 6 to 8, 10 to 12, and 18 to 20 years) and (2) shearing regimes (fixed leader length versus variable leader length) on growth, quality, and market value of Fraser fir Christmas trees. Commercial height, Christmas tree grade (based on U. S. Dept. of Agric. standards), and net present value (US dollars) were assessed at the time of harvest. Cone damage to quality was rated after 8 years in the field. Scions from Fraser fir Christmas trees 2 m or taller produced grafts that expressed maturation, resulting in lower tree quality, heavier cone damage, and decreased market value compared to seedling stock. In contrast, the quality and market value of grafts was similar to that of seedlings when scions were collected from young Fraser fir Christmas trees. For Christmas tree production, scions should be collected from the upper whorls of trees no older than 2 to 3 years in the field (6 to 8 years from seed). The effect of age on Fraser fir clones varies so that pre-screening might identify some older selections suitable for use as scion donors. Fixed versus variable shearing regimes had little effect on tree value, although some individual clones responded better to one regime or the other. Full article

Subalpine fir (Abies lasiocarpa (Hooker) Nuttall), which is native to western North America, is of considerable interest for Christmas tree production in northern Europe. Seedlings are usually grown from seeds under combined nursery greenhouse/outdoors conditions, but commonly show early growth cessation in the nursery, resulting in small plants for field transplanting. This increases the production time and makes the seedlings vulnerable to stressors at the planting site. Day extension with far-red (FR) light was shown to enhance elongation and delay bud set in seedlings of some woody species, but such information is limited for Abies. Here, we investigated the effects of day extension with FR, red (R), different R:FR-ratios or blue (B) light from light emitting diodes on subalpine fir seedlings grown at different temperatures. Day extension with FR or combined R-FR light, in contrast to R or B light, increased shoot elongation significantly as compared to short days without day extension, often with more growth at 18 °C than 24 °C. The FR treatments delayed terminal bud development, although bud set was not completely prevented. These results demonstrate that larger seedlings of subalpine fir seedlings for Christmas tree production can be obtained by employing day extension with FR or combined R:FR light, preferably under cool temperature. Full article

Deployment of genetically resistant Eastern white pine (Pinus strobus L.) planting stock could reduce economic losses to root rot caused by Phytophthora cinnamomi Rands in Christmas tree and forest plantations. This study aimed to determine the degree of genetic control of resistance to P. cinnamomi in Eastern white pine and secondarily, to compare the aggressiveness of two P. cinnamomi isolates derived from different host species. Phytophthora isolates from Fraser fir (Abies fraseri (Pursh) Poir.) and Eastern white pine were used in a main and supplemental study, respectively, including 83 and 20 open-pollinated families. In each study, two-year-old seedlings were inoculated twice each of two consecutive years and mortality was assessed biweekly for 16 weeks each year. During the first year, mortality increased over time to 18.6% and 40.4% while family variation in mortality ranged from 1.3% to 60.0% and 12.5% to 73.0% in the main and supplemental studies, respectively. At the end of the first year, individual-tree and family-mean heritability estimates were, respectively, 0.44 ± 0.0935 and 0.85 ± 0.180 for the main study, and 0.57 ± 0.216 and 0.90 ± 0.343 for the supplemental study. The P. cinnamomi isolate from Eastern white pine was more aggressive and there was a large interaction between isolates and pine families. Deploying resistant families will be complicated by this interaction but should, nevertheless, reduce economic losses. Full article

The fungus Neonectria fuckeliana has become an increasing problem on Norway spruce (Picea abies) in the Nordic countries during recent years. Canker wounds caused by the pathogen reduce timber quality and top-dieback is a problem for the Christmas tree industry. In this study, four inoculation trials were conducted to examine the ability of N. fuckeliana to cause disease on young Norway spruce plants and determine how different wound types would affect the occurrence and severity of the disease. Symptom development after 8–11 months was mainly mild and lesion lengths under bark were generally minor. However, N. fuckeliana could still be reisolated and/or molecularly detected. Slow disease development is in line with older studies describing N. fuckeliana as a weak pathogen. However, the results do not explain the serious increased damage by N. fuckeliana registered in Nordic forests and Christmas tree plantations. Potential management implications, such as shearing Christmas trees during periods of low inoculum pressure, cleaning secateurs between trees, and removal and burning of diseased branches and trees to avoid inoculum transfer and to keep disease pressure low, are based on experiments presented here and experiences with related pathogens. Full article

In Iceland and Denmark, there is an interest in planting Abies lasiocarpa for use as Christmas trees. To search for usable genetic material for both countries, 26 provenances of subalpine fir, covering most of its natural range, were planted in eastern Iceland and Jutland, Denmark. Flushing, bud set and survival rates were assessed. Artificial freezing of twigs, from field trials in eastern Iceland and Denmark, was done to rank the provenances for frost tolerance in the spring and autumn. The northernmost provenances showed earliest bud set, highest autumn frost tolerance and a latitudinal cline was delineated. Differences between provenances in flushing and spring frost tolerance were less than that found for bud set and autumn frost tolerance. The southernmost provenances showed earliest flushing and the most spring frost damage on buds. Mortality of single provenances in the field tests could not be attributed to low freezing tolerances in the autumn or spring. The southernmost provenances of Abies lasiocarpa from New Mexico and Arizona showed the highest survival rate in the field trial in East Iceland, while the eastern provenances showed a low survival rate except for two provenances from Utah and Wyoming. The western provenances from Washington state showed the best survival in Denmark, followed by the southernmost provenances. Full article