Forest Declines and Diebacks

Primary reference(s)

Ciesla, W.M. and M.E. Donaubauer, 1994. Decline and dieback of trees and forests: a global overview. FAO Forestry Paper No. 120. Accessed 13 October 2020.

Additional scientific description

Forest ecosystems are a critical component of the world’s biodiversity as many forests are more biodiverse than other ecosystems. Forests cover 31% of the global land area. Approximately half of the forest area is relatively intact, and more than one-third is primary forest (i.e., naturally regenerated forests of native species, where there are no visible indications of human activities and the ecological processes are not significantly disturbed) (FAO and UNEP, 2020).

Forests provide habitat for the vast majority of the terrestrial plant and animal species known to science. Forests are being rapidly and directly transformed in many areas by the impacts of expanding human populations and economies (Allen, 2009). Forests and the biodiversity they contain continue to be under threat from actions to convert the land to agriculture or unsustainable levels of exploitation, much of it illegal (FAO and UNEP, 2020).

Forest decline is characterised by the presence of symptoms such as reduced growth, shortened internodes, root necrosis, premature fall colouring in temperate forests, yellowing and loss of foliage, dieback of twigs and branches generally beginning in the upper crown, sprouting from adventitious buds and(or) increased prevalence and pathogenicity of root decay fungi (Manion and Lachance, 1992).

Decline has been considered a symptom of disease, a distinct class of disease and as part of forest dynamics. Another widely accepted concept describes decline as a result of interaction of predisposing, inciting and contributing factors (Manion and Lachance, 1992). Predisposing factors are often of long-term duration with slowly changing factors such as soil, site and climate. These factors alter the ability of trees to withstand or respond to injury-inducing agents.

Forests generally produce dieback of small branches. Examples include defoliating insects, late spring frost, drought (Steinkamp and Hickler, 2015) and salt spray. The contributory factors are those which further weaken and ultimately kill the tree. Examples include bark beetles, canker fungi and root decay fungi. These factors are persistent and visible and often wrongly blamed for tree death (Ciesla and Donaubauer, 1994).

Less evident are the pervasive effects of ongoing climatic changes on the condition and status of forests around the world. Recent examples of drought and heat-related forest stress and dieback (defined here as tree mortality noticeably above usual mortality levels) are being documented from all forested continents, making it possible to begin to see global patterns. While climate events can damage forests in many ways ranging from ice storms to tornadoes and hurricanes, the emphasis here is on climatic water stress, driven by drought and warm temperatures (Allen, 2009).

It has been estimated that the world is losing 20,000 hectares of forest a day with 835 hectares of forest disappearing every hour, the equivalent of 1140 football pitches (UNEP, FAO and UNFF, 2009).

Metrics and numeric limits

The Food and Agricultural Organization of the United Nations (FAO) has been monitoring the world’s forests at 5 to 10 year intervals since 1946. The recent Global Forest Resources Assessments have been produced every five years in an attempt to provide a consistent approach to describing the world’s forests and how they are changing.

The guidelines and specifications for the Global Forest Resources Assessments (FAO, 2020) have been adapted both in scope and reporting regularity to better respond to recent developments in the international forest policy arena, such as Agenda 2030 for Sustainable Development (United Nations, 2015), the United Nations Strategic Plan for Forests 2017-2030 (UNDESA, 2017) and the Paris agreement (UNFCCC, 2015). Furthermore, the reporting content has been streamlined and a new online reporting platform has been developed, to make reporting more efficient and to decrease countries’ reporting burden (FAO and UNEP, 2020).

Examples of drivers, outcomes and risk management

The total forest area is 4.06 billion hectares, or approximately 5000 m2 (or 50 x 100 m) per person, but forests are not equally distributed around the globe. More than half of the world’s forests are found in only five countries (Russian Federation, Brazil, Canada, United States of America, China) and two-thirds of forests are found in ten countries (FAO and UNEP, 2020).

Multiple factors such as insect pest and disease outbreaks, and extreme weather events such as drought are considered the main causes for forest decline and diebacks. However, human activities, such as pollution, urbanisation and encroachments on sensitive habitat, can exacerbate what would ordinarily be cyclical occurrences of these factors. Forest decline can be hastened and worsened as the forest ecosystem becomes less resilient and more sensitive to changes, whether they be natural or anthropogenic (Zhu and Liu, 2004; Martínez-Vilalta et al., 2012; DeSantis et al., 2016).

Risk management includes sustainable forest management practises to establish resilient forest ecosystems, establish early warning and early action systems, and increase community participation in forest management.