Juniper (Juniperus communis) is an evergreen, dioecious conifer (Cupressaceae) (meaning it houses male and female flowers on different plants), and is one of only three native conifer species in the UK. It has a long and rich history within folklore and mythology, but it’s probably best known for its dark purple berries used in gin and cooking.
Most juniper populations are found on upland sites in northern England and in Scotland, where it is an important component of the Caledonian forest. In southern England, juniper typically grows on chalk lowlands. It’s commonly seen as a low spreading shrub or a small tree.
In the uplands, individual bushes can live for over 200 years. Its dense foliage provides a habitat for nesting birds and its berries are a food source.
While juniper is found across the UK, populations tend to be small and isolated with low levels of reproduction, also known as natural regeneration. For this reason, it’s listed as a priority species for conservation.
Under threat from Phytophthora austrocedri
In 2010, scientists at Forest Research, led by Principal Pathologist, Dr Sarah Green, detected that juniper was under threat from an invasive pathogen called Phytophthora austrocedri.
The pathogen was found to be the cause of widespread mortality in English and Scottish juniper populations.
Since then, Dr Green and her team have been working to understand more about Phytophthora austrocedri and how it interacts with juniper trees. Their latest study, led by Daisy Crowson, research scientist at Forest Research, suggests we have reasons to be hopeful for this much-loved species.
What did the study involve?
The researchers carried out two inoculation experiments (introducing the pathogen to juniper plants) to determine if resistance to Phytophthora austrocedri was inherited and whether juniper populations have the potential to adapt to the pathogen over time.
The first experiment involved collecting shoots from ten healthy juniper trees taken from a population in the Lammermuir Hills in Scotland. These were then inoculated with six different strains of Phytophthora austrocedri in the lab and the lesions caused by the pathogen measured.
The second experiment saw seeds collected from 54 families across 13 British juniper populations. The sibling trees were grown for six years and infected with a highly virulent strain of Phytophthora austrocedri. The subsequent lesions were measured.
What did the experiments show?
Overall, the study found that lesion length showed moderate to high heritability and lesion development showed very high heritability.
It was also found that different strains of the pathogen caused varying levels of disease in different juniper plants.
Finally, it was found that young juniper grown from populations that had high exposure to the pathogen had a lower predicted probability of developing a lesion, suggesting that natural selection may already be taking place.
What does this mean for the future of juniper?
The scientists at Forest Research discovered that some trees within native British juniper populations carry inherited natural resistance to the invasive tree pathogen, Phytophthora austrocedri.
This suggests that juniper populations could have the potential to develop wider resistance to Phytophthora austrocedri through natural selection.
However, the findings also indicate that juniper trees varied in their ability to resist different strains of Phytophthora austrocedri.
The researchers concluded that conservation efforts should promote natural regeneration and avoid planting which can introduce new variants of Phytophthora austrocedri.
Hearing from the scientists
“Our findings are encouraging as they suggest that resistance to Phytophthora austrocedri has a genetic basis and could become more common in juniper populations through natural selection,” said lead author, Daisy Crowson, research scientist at Forest Research.
“However, what we’re seeing is that resistance, or partial resistance, to one strain of Phytophthora austrocedri doesn’t necessarily mean resistance to another.
“We need to be careful not to introduce new genotypes by, for example, inadvertently transporting variants of Phytophthora austrocedri. Even planting saplings grown in nurseries which are resistant to Phytophthora austrocedri could introduce a new strain of the pathogen that the established population has no resistance to.”
“Juniper plays an important ecological role in UK landscapes,” said Dr Sarah Green, Principal Pathologist at Forest Research. “We must work to encourage natural regeneration within populations to give juniper the best chance of passing on heritable resistance to Phytophthora austrocedri.”
Read the paper: Vulnerable Juniper Populations Show Adaptive Potential in the Face of a Highly Damaging Invasive Tree Pathogen - Forest Research
The research was carried out in collaboration with the University of Edinburgh and the UK Centre for Ecology and Hydrology. It was funded by the Science and Innovation Strategy (SIS), which is jointly funded by the Forestry Commission, Defra, Scottish Forestry and the Welsh Government.
Charis Goodyear is Science Communications Manager at Forest Research
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