Four study years on the nest site use of the Great Spotted Woodpecker - the role of two invasive tree species in riparian forests

Abstract

The study has been carried out 2014-2017, in Hungary, in the Central Tisza Landscape Protection Area, in two, approximately 40-50 ha, old, unmanaged softwood willow-poplar gallery forests on the bank of the river Tisza. Two North American invasive tree species that are widespread throughout Central Europe are present in great numbers in the two study areas. One is the green ash (Fraxinus pennsylvanica), the other is the boxelder (Acer negundo). The invasion of these tree species is a significant environmental problem, as the two species hinder the development of native riparian tree species. I studied the nest site characteristics of the generalist Great Spotted Woodpecker (Dendrocopos major). The study areas were differed in terms of the proportion of invasive tree species. Both species are the two most frequent ones at both areas, though these species are present at the second area with much greater proportions. The questions of the study were: Which arboreal characteristics could predict the nest site occupation of the studied woodpecker species? Were the used nest sites differed between the structurally different study areas? What were the preferred characteristics of the nest trees? Nest trees were mapped following the calls of the chicks in four breeding seasons in both areas. Data on arboreal vegetation were collected in 0.05 ha circular plots. Nest site plots were centred at the position of nest trees, habitats were measured with circular plots centred on a 100 by 100 m semi-random grid. Species, diameter at breast height and condition (living, decaying or dead tree) were recorded for each tree that had a diameter at breast height, greater than 3 cm. Continuous variables were extracted from raw data. Principal Component Analysis and Generalized Linear Models were used for the analyses. Comparing to semi-random sites, nest sites had more native and less invasive trees and more willow and less boxelder trees. Both group of nest sites represented more green ash trees than semi-random sites. The differences between nest sites of the two area reflected the main differences between semi-random sites of each study area, as the second area had more native and less boxelder trees. Nesting cavities were mainly made in decaying or dead willow and white poplar trees, their diameter at breast height were between 30 and 90 cm. I feel confident to predict that the further decrease of autochtonous tree species may influence these habitats negatively. As major cavity excavators, the study species could play key role in such transformed communities, these changes would have significant effects on cavity dependent species. As nowadays, these tree species support the essential under- and midstorey layers, the control of their mixed-age stands could even cause harm to the ecosystem processes. Adaptive management strategies should be made, to secure the current processes of such ecosystems, while conserving the native forest communities as much as possible.