Anthropogenic Disturbances and the Emergence of Native Diseases: a Threat to Forest Health

Abstract

Purpose of Review Human-caused global change is fundamentally altering natural forest ecosystems. More trees are exhibiting a wide range of symptoms indicative of poor vigour, particularly stressed species at the edge of their native ranges and stands growing on marginal sites. This review will focus on complex tree diseases (declines) caused by native pathogens and the key environmental drivers that contribute to this phenomenon. These systems are frequently complex, with multiple drivers at work. Recent Findings Using four cases studies on different continents, we explored the direct and indirect environmental drivers underlying these decline syndromes. Although climate and weather events seem to be usually associated with forest decline, we found that environmental disturbance by either forest management or land-use changes is also a global predisposing factor of decline which deserves more attention. Changes in land use have directly benefited pathogens such as root rots in the Pyrenees (Spain) or indirectly by making the environment more conducive for canker and foliar diseases in Australia and the USA. Focus on land-use changes could improve understanding of current decline problems such as those affecting Araucaria in Chile. Summary The next century will almost certainly see an unprecedented rise in forest pathogen epidemics, requiring a proactive rather than reactive response. Diseases caused by native pathogens with complex aetiologies will become more common, and recognising, characterising and managing these epidemics are difficult because native pathogens are frequently already widespread, and eradication is not feasible. We need to start approaching these issues from a ‘whole ecosystem’ perspective, highlighting the many aspects and entanglements of forest declines and allowing us to respond with management options tailored to each scenario. The approach proposed here provides logical steps based on six questions to untangle the direct and indirect environmental drivers of tree declines.

Anthropogenic land-use legacies underpin climate change-related risks to forest ecosystems

“…On the contrary, climate change-type drought did not result in increased tree mortality in C2 forest structural typology (Fig. I), suggesting that other drivers than drought stress, such as long-term effects of altered stand genetic properties [60] or warmer temperatures in combination with tree antagonist organisms (e.g., pathogens) [72], might explain the larger mortality trends observed in plantations in this structural typology. This study case demonstrates that anthropogenic land-use legacies persisting in plantations can elevate the risk of climate change-related stressors to forests.…”

“…Loss of forest resilience to climatic stressors may also occur if anthropogenic land-use legacies affect interspecific interactions such as mutualisms (Table 1) [48, [62][63][64]. Species replacements may also lead to increased performance of antagonists under changing climatic conditions, including tree heterospecific competitors, invasive species, herbivores, and pathogens (Table 1) [42,54,[65][66][67][68][69][70][71][72][73].…”

“…This alteration may reduce the capacity of trees to cope with drought stress [48, 63,64]. High occurrence of tree stumps or increased stand density and structural homogeneity may alter interspecific interactions and favour, for instance, the spread of pathogen and insect outbreaks [65,70,73] that in interaction with climate-related stress may exacerbate negative impacts on forests [42,72].…”

Population structure and regeneration of Tsuga dumosa and Abies spectabilis across altitudinal gradient in Rasuwa district, central Nepal

“…These gaps might be due to the past anthropogenic disturbances or episodic regeneration events. Anthropogenic disturbances could include human activities such as logging, clearing, or grazing, which can impact the growth and survival of trees (Burgess et al, 2022). Episodic regeneration events could include natural disturbances such as landslides or fires, which can create gaps in the forest canopy (Crausbay & Martin, 2016).…”

Invasion Frameworks: a Forest Pathogen Perspective

“…However, in recent decades there has been a rise in emerging forest diseases caused by native pathogens. This is essentially due to breakdowns in ecosystem resilience due to global change drivers, including climate change, land-use change, anthropogenic disturbance and indeed other biological invasions [28]. Therefore, invasion events involving pathogens are challenging to predict [9].…”