The spatial distribution of trees outside forests in a large open-field region and its potential impact on habitat connectivity for forest insects

Jean-Pierre Rossi, Jérôme Rousselet
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Forest trees are commonly used for ornamental purposes and are thus frequently encountered in non-forest landscapes where they represent an important component of the so-called trees outside forests (TOF). Little is known about the role of these trees in the connectivity between forest patches and their potential impact upon forest organisms’ dispersal. We focused on the tree species belonging to genera Pinus, Cedrus and Pseudotsuga, the potential hosts of the pine processionary moth Thaumetopoea pityocampa, a common defoliator in the western Mediterranean basin. We carried out an exhaustive inventory of trees in a 22 × 22 km sampling window located in the Beauce region (France) an agricultural territory where landscapes consist of intensively managed open-fields (mainly dedicated to cereal crops). The results showed that host trees suitable for the PPM are much more numerous than expected, and form small patches scattered across the study area. They are mostly ornamental trees planted in populated places such as villages. We conclude that hosts suitable for the PPM are available throughout agricultural landscapes. Various forest pests may benefit from TOF coverage that forms consistent ecological continuities linking forest areas. It appears that forest health should be addressed in a large context, encompassing non-forested areas, either agricultural lands or urbanized territories that play an overlooked role in large-scale spatial dynamics of forest pests and diseases.
Keywords: Trees outside forest, Spatial point pattern, Landscape connectivity, Pine processionary moth

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