Brynjar Skúlason

Winter storage of seedlings in freezers reduces the amount of heat sum available for growth in the following growing season compared to seedlings stored outdoors. To test the effects of a reduced growing period on the autumn frost hardiness of the six species most used in Icelandic afforestation, seedlings were stored outdoors or in a freezer during winter. In spring, the seedlings were planted on 24 May, 7 June, 21 June, and 5 July, and the frost hardiness of all treatments was tested on 12 and 26 September. In general, the probability of freezing damage increased with a later planting date, with outdoor-stored seedlings having the lowest probability of damage. The timing of frost events was of great importance; the later the freezing date, the less damage was observed. Growth cessation occurred at different times for each species, and they responded differently to the reduced heat sum. Lodgepole pine and birch accumulated the most frost hardiness in September. Sitka spruce had less autumn frost hardiness than Lutz spruce. Hybrid larch accumulated less frost hardiness than Russian larch and was most sensitive to the reduced heat sum. The results can be used to determine which species should be prioritised in frozen storage with regard to Iceland‘s short growing season.

Scots pine Pinus sylvestris was originally introduced to Iceland in the beginning of the 20th Century. Extensive plantings started in the late 1940s and, in total, 2–3 million Scots pine seedlings were planted, mainly originating from two counties in northern Norway. Part of this plant material was imported as seedlings. Pine woolly aphid Pineus pini was introduced to Iceland before 1940, most likely on imported seedlings in 1937. High mortality of Scots pine, concurrent with high infestation of the pine woolly aphid, was observed in Iceland during the late 1950s and 1960s and planting was discontinued. Provenance trials with Scots pine were established in Iceland in 2004–2006. They consisted of 15 provenances from Norway, four from Finland, four from Scotland, one from Russia, one from the Austrian Alps and three first generation Scots pine provenances from Iceland, collected from survivors of the epidemic in the 1950s and 1960s. In total, there were 28 provenances. The Icelandic provenances had significantly lower P. pini infestation than all the provenances of non‐Icelandic origin, which indicates that natural selection in Scots pine in Iceland has occurred in favour of individuals less susceptible to P. pini .

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.

A field test with 26 provenances of subalpine fir and corkbark fir was established at three sites in Denmark and one site in Iceland in 1999. Survival, growth and Christmas tree quality were assessed after 10 growing seasons in Denmark and 12 in Iceland. Provenances showed significant differences for all measured traits. The western-most subalpine fir provenances from Washington state and British Columbia showed the overall best results in Denmark with the highest survival (after 15 years), fastest growth in height and highest Christmas tree quality and profitability as well as both good postharvest needle retention and high resistance to pests. The corkbark fir provenances also showed high Christmas tree quality and the lowest frequency of fork formation, but corkbark fir provenances also exhibited the poorest postharvest needle retention. In Iceland, the corkbark fir provenances generally showed the highest survival rate and Christmas tree quality. The White River provenance from British Columbia is recommended for use in Denmark. The Mount Taylor provenance from the Cibola National Forest in New Mexico is recommended for use in Iceland.

Summary: Field trials of Abies lasiocarpa were undertaken with the aim of assessing the potential for Christmas tree production in Denmark. Twenty‐six provenances originating from Alaska to New Mexico were tested. Damage by the insect Adelges piceae and the fungus Neonectria neomacrospora was recorded for the first time 8 and 12 years after the initial planting. Damage from N. neomacrospora increased rapidly in the period 12–15 years after planting. Trees from the northern provenances and humid climates exhibited less damage than those from southern ones. Previous attack by A. piceae had a minor effect on N. neomacrospora infection. Greenhouse tests showed that detached shoots from healthy Abies lasiocarpa can be used to rank provenances for resistance to N. neomacrospora, but results varied according to host subspecies.

Spring and autumn frost tolerance was measured using material from a range-wide (50–67° N, 38–158° E) provenance trial of four Russian larch species (Larix sukaczewii Dyl., L. sibirica Ledeb., L. gmelinii Rupr. and L. cajanderi Mayr.) growing in northern Sweden. Shoots were collected in early May and late September and frozen at −8, −12, −16 and −20°C. Cambial damage was assessed visually after development under ideal conditions for 2 weeks. Differences in frost damage among provenances were highly significant in both spring and autumn. Autumn frost damage was significantly correlated with provenance latitude and longitude and spring frost damage was significantly correlated with provenance longitude but not latitude. Frost damage was not correlated with provenance elevation. North-western provenances showed the least damage and far-eastern provenances the greatest damage in both spring and autumn. A possible explanation for less spring frost damage to north-western provenances is adaptation to maritime conditions in proximity to the Barents Sea, which is often ice free in late winter. This would counteract early loss of frost tolerance and bud flushing if warm spells occurred in late winter. North-eastern Siberian provenances did not show similar adaptation and may exhibit increased spring frost damage if global warming eventually results in the Arctic Ocean north of Siberia becoming ice free in late winter.

In three Nordic field trials, 26 provenances of Abies lasiocarpa (Hook.) Nutt. and Abies lasiocarpa var. arizonica (Merriam) Lemmon were evaluated 3 yrs after establishment. The focus was on 16 traits important for adaptation, establishment and production of Christmas trees. For all three sites in Denmark, Norway and Iceland, survival ranged from 89 to 96%. Provenances showed statistically significant differences for all but two traits. For all traits there was significant interaction between provenance and site. Analysis of ecovalens showed that for most traits, only a limited number of provenances contributed substantially to the interaction. Therefore, a large proportion of the provenances showed a fairly consistent ranking across sites. The frequency of potential Christmas trees in the provenances ranged from 10 to 49%. Southern provenances from New Mexico and Arizona seemed to have the best potential for producing high-quality Christmas trees. The southern provenances added substantially to the interaction between provenance and site for budset, lammas growth and colour. When using multivariate statistical methods on all measured traits, the provenance clusters fitted neatly into six distinct geographical regions.