Abstract
Determining the impacts of invasive pathogens on tree mortality and growth is a difficult task, in particular in the case of species occurring naturally at low frequencies in mixed stands. In this study, we quantify such effects by comparing national forest inventory data collected before and after pathogen invasion. In Norway, Fraxinus excelsior is a minor species representing less than 1% of the trees in the forests and being attacked by the invasive pathogen Hymenoscyphus fraxineus since 2006. By studying deviations between inventories, we estimated a 74% higher-than-expected average ash mortality and a 13% slower-than-expected growth of the surviving ash trees, indicating a lack of compensation by the remaining ash. We could confidently assign mortality and growth losses to ash dieback as no mortality or growth shifts were observed for co-occurring tree species in the same plots. The mortality comparisons also show regional patterns with higher mortality in areas with the longest disease history in Norway. Considering that ash is currently mostly growing in mixed forests and that no signs of compensation were observed by the surviving ash trees, a significant habitat loss and niche replacement could be anticipated in the mid-term.
Highlights
Global warming and globalization put pressure on forests ecosystems and threaten their capacity to deliver goods and services to society[1,2]
Mortality was modelled by including basal area of the plot (BAplot) and tree diameter at breast height (Table 1)
Since tree mortality is an inherent process of forest dynamics, defining a baseline against which mortality caused by a new pathogen can be compared presents a major challenge[30]
Summary
Global warming and globalization put pressure on forests ecosystems and threaten their capacity to deliver goods and services to society[1,2]. Manion and Griffin[14] applied this approach to widespread tree species in eastern US; this model can be difficult to apply in the case of minor species, such as Alnus glutinosa, Fraxinus excelsior or Ulmus minor, affected by exotic pathogens in Europe These species are poorly represented in national forest inventories (NFIs), and often appear in mixed natural stands where baseline mortality may be difficult to estimate. It explicitly models mortality based on biological processes such as inter and intra-specific competition with other trees It allows testing whether mortality shifts are observed on other non-host species, information which can be used to discriminate mortality due to a new pathogen from that caused by unspecific factors such as drought, frost or windstorms. We expect that the approach and the resulting models can be directly applied in forest management and planning
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