Investigation into the causes of Alder mortality in Scotland

The decline of alder (Alnus glutinosa) across the UK has been noted for at least the last 30 years. Steven Hendry from Forest Research and Matt Elliot from the Royal Botanic Garden Edinburgh undertook a study to better understand this phenomenon in Scotland. The work was funded by Scotland’s Plant Health Centre, and the condition surveys were carried out, in part, by Observatree volunteers.

Alder is a key species within riparian habitats, providing important benefits to biodiversity, water quality and temperature, bank stability and flood alleviation. The ecological niche which it fills cannot be easily replaced by any other native species and it is quite frequently the only tree species present in upland riparian habitats. This has become increasingly noticeable since the declines of elm and ash due to disease.

A number of diseases are known to infect alder. Perhaps the most well-known is Phytophthora alni, a pathogen which has had an impact on alder across Europe over the last 30 years. Unusually, P. alni is a hybrid species and its distribution and impact in Scotland and wider UK are poorly understood.

This study was therefore initiated to investigate alder decline to establish if we are experiencing a sudden outbreak of disease, or whether this is a case of a more complex gradual decline. Several sites were identified across Scotland from a review of the literature, observations from Scottish Forestry, and prior knowledge of the researchers. These sites were surveyed by the researchers to look for evidence of specific pathogens, pests, and abiotic causes of poor health.

In addition, Observatree volunteers undertook alder condition surveys to feed further data into the project. Volunteers were initially briefed on alder health in Scotland and the range of symptoms which might be encountered on the species. They were supplied with supporting information to help them carry out surveys. Volunteers chose the sites that they wished to survey and completed reports using an adaptation of the Observatree site survey procedure and returned to the programme managers who collated the data for publication.

The results showed that the current decline of alder at surveyed sites is the result of several interconnected issues rather than the result of the invasion of a single pathogen or pest. The causes recorded include waterlogging and competition from invasive tree species, infection by Phytophthora alni, crown dieback associated with Armillaria colonisation of branches and stems by the weak pathogen Valsa oxystoma, infection by spp., with occasional instances of infection by Heterobasidion annosum being detected. In some instances, predisposition of trees to damage by short-or long- term changes in site conditions were considered probable (e.g., changes in the height of the water table).

It was also noted that there was a marked tendency for alder populations to be even-aged, with few instances of regeneration following their initial establishment. The lack of surviving seedlings may in some cases be the result of grazing, but there may well be other reasons for the lack of regeneration.

It is clear from the study that biosecurity should be a major consideration for riparian planting projects where alder (and other plants) are being introduced. Planting stock needs to be disease free, and machinery, tools and equipment should be clean. In addition, encouraging natural regeneration or employing direct seeding may well be more effective approaches than planting in the long term, although more research is required on the most effective ways to carry out these techniques.

A more detailed report of this work is available from the Scottish Plant Health Centre.

If you find unhealthy-looking alders and wish to report them, you may do so through TreeAlert. Registered TreeAlert users are also able to submit information on healthy alders to provide important information on their distribution across Britain.