Major perturbations in the Earth's forest ecosystems. Possible implications for global warming

Forests are among the most important terrestrial ecological systems in terms of the multitude of ecosystem functions and services they provide. These biotic systems are vital not only for ensuring the wellbeing of human society and for preserving global biodiversity, but also for regulating the climate system, decarbonizing the atmosphere via carbon sequestration (in biomass or underlying soil carbon pools) and evaporative cooling processes that mitigate climate warming. However, forest ecosystems are currently being subjected to a wide range of natural and anthropic disturbances that pose a real threat not only to forest health and the various benefits forests provide for human society, but also to the overall functioning of the global system. This paper is a review that aims to analyse, in a brief and holistic manner, the main perturbations Earth forest ecosystems are currently facing, both the obvious (e.g. deforestation) and discrete/silent ones (e.g. defaunation) that have generally not yet been tackled strictly as ecological forest issues in the international scientific literature. At the same time, the paper aims to highlight the possible effects generated by forest perturbations in the global warming process, through carbon fluxes and biogeophysical feedbacks between these terrestrial systems and the atmosphere. Upon analysis of a vast scientific bibliography, it was found there currently are 12 major forest disturbances that can be grouped into three categories based on the prevalence of triggering causes, i.e. climatic (phenological shifts, range shifts, die-off events, insect infestations), anthropic (deforestation, fragmentation, air pollution) and mixed (defaunation, fires, composition shifts, net primary productivity shifts, biogeochemical shifts) perturbations. These ecological issues, which occur frequently, intensely and on large spatial scales, are able to significantly disrupt forest productivity and therefore strongly erode the forests'capacity to stabilize the climate system. All identified disturbances can amplify global warming in various ways, including by means of many positive feedback mechanisms in the case of climatic perturbations. Finally, this review paper proposes five major anthropogenic strategies to fight this multidimensional forest crisis - mitigate, adapt, repair, protect and research actions, which, if implemented rapidly, efficiently and on a large scale via international policies, can successfully stabilize these terrestrial ecosystems and, implicitly, the climate system in the 21st century.