Consequences of enriching European beech forests with non-native Douglas fir for generalist arthropod predator diversity, abundance and pest control

As anthropogenic climate induced stress leads to biodiversity loss and an increase in the frequency and severity of pest outbreaks, forest management in Central Europe is looking for ways to balance demands of biodiversity conservation and economic utility. To fulfil these demands Central European forestry is shifting from monoculture plantations to mixtures with phylogenetically distant deciduous and coniferous tree species which have been show to promote biodiversity and top-down control. Furthermore, the utilization of non-native Douglas fir, which combines climate change adaptability and high growth rates, is increasing and Norway spruce dieback is likely to exacerbate this. The ecological consequences of such an increase in non-native Douglas fir utilization are not fully known, but it is likely to lead to diversity and abundance loss of generalist arthropod predators, which in turn could translate to lower top-down control and higher herbivory (Chapter 1). In addition to testing this hypothesis, I will study if non-native Douglas fir effects are tempered in mixtures with phylogenetically distant European beech and to what extent Douglas fir effects are similar to effects of the phylogenetically close Norway spruce. Firstly I studied how Non-native Douglas fir affected the diversity, abundance and biomass of epigeal spiders collected by pitfall traps in mature forest stands (Chapter 2). Contrary to my expectations, epigeal spider abundance, biomass and functional richness was higher in stands with higher Douglas fir proportions, with site conditions playing a crucial role in structuring spider communities. Further analysis on arboreal spiders collected by tree beating in young forest stands (Chapter 3) showed that arboreal spiders have the highest numbers on individual conifer trees, especially Douglas fir. However, at the plot level spider species richness, abundance and biomass decreased with an increasing proportion of both Douglas fir and Norway spruce due to lower species turnover among coniferous trees (Matevski & Schuldt, 2021). These results suggest that it is important to consider matters of scale, stratum and forest age when discussing Douglas fir effects on spider diversity and top-down control potential. To see whether Douglas fir effects on spider top-down control potential translate into effects on predation pressure, we assessed clay caterpillar attack rates on the ground and herb layer in mature forest stands (Chapter 4). The results from this study (Matevski et al., 2021) showed that Douglas fir monocultures harbored significantly higher ground layer arthropod attack rates than all other stands, except for Norway spruce monocultures. Additional analysis on European beech sapling and mature tree leaf damage in monocultures as well as mixtures with Douglas fir and Norway spruce (Chapter 5) showed that stands with higher Douglas fir proportion suffered less sapling and mature European beech leaf damage. The results of these two manuscripts show that Douglas fir effects on spider diversity and community composition translate into effects on arthropod-mediated top-down control. All in all, my thesis suggests that, at least in the context of generalist arthropod predator diversity and ecosystem functioning, Douglas fir effects are mostly congruent with forest management demands by promoting diversity and economic utility (Chapter 6). Nonetheless, these results are dependent on stand age, site conditions, stratum and spatial scale studied, highlighting the importance of considering these factors when studying tree diversity and identity effects on arthropod diversity and ecosystem functioning.

2022-11-04