Threats

Anthropogenic activities that affect riparian areas can threat alder forests, namely: fragmentation, land use changes, hydromorphologic changes, touristic and recreational activities or waste disposal. Moreover, the introduction of exotic species with invasive behavior in riparian areas, such as Acacia dealbata or Arundo donax, can have negative consequences for alder forest when they spread to riparian areas.

Although a warmer climate could allow its expansion further north, changes in rainfall regime could negatively affect alders in some parts of its current distribution.   Warmer conditions and water scarcity could disrupt water flow in upper catchment sectors with small drainage basins, threatening alder populations.

The oomycete complex Phythophtora xalni is an emerging threat, which was firstly observed in the 90s in the UK, and has widely expanded across Northern European catchments, devastating native European priority forests. The leading-edge of this disease is currently located in Central Portugal. A specific strategy to deal with this emerging threat is urgently needed due to impacts not only alder but also other forest species, and to high risk and increased vulnerability to riparian ecosystems.

In Europe, alders can be infected by several Phytophthora species, including a complex of three species commonly called Phytophthora alni (which comprises Phytophthora uniformis, Phytophthora xmultiformis and Phytophthora xalni, being the latter one the most aggressive and a hybrid between the first two) and Phytophthora plurivora. Alders in Europe could also be potentially infected by Phytophthora siskiyouensis (which already infects Alnus cordata in the UK and A. glutinosa in Australia)

Phytophthora xalni usually infects the collar or adventitious roots and then extends to the trunk, damaging phloem and cambial tissues. Water facilitates the transport of this oomycete, as its zoospores can spread from infected alders to other trees. Therefore, the spread of the disease is usually downstream and upstream areas could potentially serve as refuges from the disease. The life cycle of the pathogen can be consulted here.

The pathology that Phytophthora xalni produces in A. glutinosa is called alder dieback, and it is usually characterized by the following symptoms:

Alders susceptibility to the pathogen can increase when affected by environmental stressors such as prolonged flooding. In addition, global change can influence pathogen emergence and intensity of damages. Warmer winters can increase the survival of Phytophthora and therefore decrease tree recovery. However, tree variability – both phenotypic (observed traits) and genetic – can benefit their resilience to diseases if the required traits are heritable. Therefore, studying the phenotypic and genetic variability of Alnus glutinosa across Europe can be key to find tolerant populations.