Multiple factors associated with forest decline in the context of control measures for the pinewood nematode

Abstract

Forest decline is a complex phenomenon; however, there is often a tendency to oversimplify by attributing it to a single cause, neglecting the interaction of multiple factors. Ever since the detection of the pinewood nematode (PWN), Bursaphelenchus xylophilus, in Portugal (and in Europe) over two decades ago, pine wilt cases in the country have been primarily attributed to this pathogen, leading authorities to enforce strict containment and control measures. Other contributing factors are generally overlooked, with the system being easily overwhelmed by disturbances such as droughts and fires. In this study, a physiographic approach was used to assess multiple factors potentially affecting tree wilting, including environmental parameters in the landscape, alongside other biological agents, and infestation by the PWN. The study area is a highly managed production forest where control measures have been implemented since the detection of PWN in the 1990´s. Pine decline was found to be aggregated in the landscape, with topographic parameters associated with water availability playing a significant role. Wilting trees were affected by multiple biological agents, even though the density of PWN was positively correlated with the decline. However, B. xylophilus was also found to be present in asymptomatic hosts, suggesting an ability to persist for some time in healthy trees within the forest. This research illustrates the multifaceted nature of pine forest decline and the limitations of managing it as originating from a single cause. A critical overview of current PWN management measures is provided, along with proposals for a more heuristic and holistic approach.