Analysis: as a species shaped by the last ice age, the Sitka spruce may provide key information about how trees respond to climate change
By Susanne Barth, Tomas Byrne, Niall Farrelly, Teagasc
Teagasc researchers are using genomic and phenotypic data to understand the nature of adaption and evolution of tree species which are used in Irish forestry. This is an important component of climate change adaptation, because as our climate warms, tree species need to adapt to altered growing conditions. That's why scientists are looking at the DNA composition of trees and studying genetic diversity to determine if different species show adaptation to climatic conditions.
To understand the process of evolution and adaptation in trees, the researchers studied the North American Sitka spruce tree, a tree which prospers in a range of climate conditions from Alaska to California. Trees with a lot of genetic diversity may be more successful at prospering and thriving in future altered conditions, while tree species with narrow genetic pools may be more vulnerable to climate change pressures and require increased efforts to conserve.
It's no easy task for researchers based in Ireland to fully understand the genetics of a tree species that spans a narrow strip of 3,000 kilometres in the Pacific North west of America; a 50 hour road trip from Alaska to California. The team used a historic collection, collected from North America and donated to Ireland in the 1960s. These trees were raised in the JFK Arboretum in New Ross, Co. Wexford, a collection that serves as a unique opportunity to study the evolutionary history of trees and levels of genetic diversity present in a specie, crucial for allowing scientists to understanding adaptation to climate change.
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Tree species with a large native distribution range (e.g. Sitka spruce, Oak, etc.) occupy a diverse range of ecological niches and trees from these niches may contain important traits of adaptation. The study therefore takes eighty different locations, separated by 50 km along the entire 3000 km north-south distribution. From each location 15 individual trees are used to study levels of genetic diversity between locations, using DNA sequencing.
Collecting DNA was hampered by the height of the trees (30 m+) and collecting foliage from all the trees would involve a tree climber travelling more than three times the height of mount Everest (>30km). The team therefore developed a method to extract DNA from the cambial layer of the tree: a tiny layer below the bark of the tree, between the wood and the bark. From this tissue we made so-called genomic libraries and then sequenced those to create a huge amount of data for our research.
What did we discover?
The team found that Sitka spruce was less challenged in its evolutionary history, being pushed onto North American islands to the fringes of its range during the Pleistocene ice age 18,000 years ago, when glaciers spread throughout North America. These fringe ranges served as refuge, which, once the ice sheets began to retreat, represented a seed source to recolonise the mainland. Recolonisation started from the islands in Alaska and from Haida Gwaii in British Columbia, Canada.
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The latest recolonisation occurred around 15,000 years ago, when the ice sheet retreated from more southern region and allowed for the recolonisation of coastal Washington and Oregon, which represents the southernmost habitat. The genetics of Sitka spruce has been heavily influenced by glacial history, working to mix the genetic material together throughout the landscape. Southern populations of Sitka spruce share their heritage with sources from Alaska and Haida Gwaii, which are further north in latitude.
To illustrate variability in the natural ecosystem, the 80 locations represent a large range of diversity in habitat: annual precipitation can vary from 620mm to up to 4,000mm (4 metres) with annual temperatures ranging from –7.3°C to 17.4°C. These habitats receive between four-and-a-half to eleven hours of sunlight per day and such diversity in habitats leads to adaption. Trees are likely to adapt to their local environment slowly, a process that can often take millennia to complete.
The team found that the 50th latitude was a key divide in the northern and southern ranges occurring a long the US-Canadian border. At this point north wards trees are more adapted for inclement weather with increased tolerance to colder conditions and snow fall and have lower height growth. South of this point trees are more adapted to positive growing conditions associated with more temperate climates, with adaptations for increased solar radiation, reduced rainfall and trees grow taller.
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Such information is key to provide an understanding of how the genetic diversity in source populations may allow trees respond to climate change. This study reveals how a pathway for increased adaptation may originate from populations from more southern climates, which are adapted to warmer conditions. To maximise the adaptation of tree species southern populations can increase the resilience of tree to the uncertainties of climate change.
This research was supported by the Department of Agriculture, Food and the Marine. The team would like to thank staff at the JFK Arboretum, Co. Wexford for access to their tree collection.
Dr Susanne Barth is a researcher in the Teagasc Crop Science Department, Dr Niall Farrelly is a researcher in the Teagasc Forestry Development Department and Tomas Byrne is a PhD student of Trinity College Dublin based at the Teagasc Crops Research Centre.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
