Spring Field Day at JFK Arboretum

TreeSpace was recently in attendance at the Pro Silva Ireland spring field day at the John F. Kennedy Memorial Park & Arboretum. The theme of the day was ‘Tree Species Selection in the Era of Climate Change’. The field day was guided by the Senior Forestry Research Officer with Teagasc who is also the coordinator for The Fit Forests Project. The arboretum where the field day was held is dedicated to the memory of John Fitzgerald Kennedy, the 35th President of the United States of America. The land is close to the Kennedy ancestral home at Dunganstown, Co. Wexford. The plant collection in the arboretum includes approximately five thousand species with woody plants, shrubs, climbing plants, and woody ground cover from all around the world. The soil in the arboretum is acidic with a pH range of 5.4 to 6.7. The arboretum was officially opened in 1968 and today hosts a collection of some of the world’s most endangered conifers. In 2022 four hectares of the arboretum were planted with 1300 trees representing twenty-four endangered conifer species and the single largest planted collection of the International Conifer Conservation Programme (ICCP).

The plant collections at the arboretum are laid out in family groups and there are approximately 190 forestry plots. The field day focused on conifer species that are currently important for commercial forestry in Ireland and other less commonly used conifer species which may become important in the future. Climate change is expected to increase seasonal temperatures and impact weather patterns which will have consequences for ecosystems and plant communities. Average temperatures in Ireland are expected to increase by 1 – 1.6°C by mid-century with the largest temperature increases expected in the east. Summer heatwave events are predicted to become more frequent, precipitation more variable, and an increase in the length of the growing season of between 12 and 16%. The changing environmental conditions have the potential to boost productivity for some tree species, but water deficits are likely to result in stressed trees which makes them more susceptible to pathogens. This scenario is already playing out in mainland Europe where the Spruce Bark Beetle (Ips typographus) has devasted large areas of Norway spruce (Picea abies) forest. Some of the drivers of the beetle outbreak are related to rising temperatures and drought stressed trees.

The forest plots that were visited during the field day include lines of trees from seed sources collected from different geographical locations in a range of latitudes and elevations. One provenance trial plot of Sitka spruce (Picea sitchensis) included seed sources from a 3000 km range from Alaska to Washington. It was very interesting to observe the differences in productivity between the seed sourced from Alaska where the climate is colder with a shorter growing season, and the seed sourced from Washington where the climate is warmer, temperate, and humid similar to the hyper-oceanic and oceanic climate in Ireland. All the trees in the trial plot were planted in the same year and there was a dramatic increase in stem diameter, tree height, and canopy volume as you walked along the forestry plot from Alaska to Washington. One of the highlights of the day was a visit to a stand of Coast Redwood (Sequoia sempervirens) which was planted in the 1970’s. The natural range of Coast redwood is a narrow coastal strip from southern Oregon to southern Monterey County on the west coast of the US. The species is typically found within 50 km of the Pacific Ocean and is highly dependant on the summer fog which condenses on the canopy and drips down to water the roots during the summer months. Coast redwoods are the tallest tree species on Earth and their presence could certainly be felt after entering the stand. The trees were in the very early stages of their life cycle but had reached an impressive height with good crown volume and stem diameter.

The field day focussed on conifer tree species that have relevance to commercial forestry in Ireland. However, the topics discussed during the day have relevance for the cultivation and management of urban trees and woodlands. Species diversity, genetic diversity, provenance, and forest resilience were topics that were repeatedly discussed throughout the day. Selecting trees from local provenance or climate-matched provenance will be essential for productive tree development. In the urban context where the environment is highly modified, we have the luxury of selecting a much broader range of tree species from a broader geographical range. However, the use of cultivars in the urban environment is probably the elephant in the room. Genetic uniformity could potentially be problematic and reduce forest resilience. Diversity stands out as one of the key factors in promoting forest resilience; species diversity, genetic diversity, and structural diversity within stands has the potential to maintain healthy and resilient forest ecosystems.