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Bosvitaliteitsinventaris 2017. Resultaten uit het bosvitaliteitsmeetnet (Level 1)

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Original languageDutch
PublisherInstituut voor Natuur- en Bosonderzoek
Number of pages80
Publication statusPublished - 2018

Publication series

NameRapporten van het Instituut voor Natuur- en Bosonderzoek
No.41

Abstract

The Level I survey is designed by means of 4x4 km grid. Crown condition assessments were performed on 852 broadleaves and 686 conifers in 71 plots. The most important coniferous species are Pinus sylvestris (n=505) and Pinus nigra subsp. Laricio (n=171). The main broadleaved species are Quercus robur (n=362), Fagus sylvatica (n=116), Quercus rubra (n=91) and Populus sp. (n=48). A subset with ‘other broadleaves’ consists of 13 species with a total of 235 trees, e.g. Alnus glutinosa, Fraxinus excelsior, Betula pendula… There are almost no ‘other conifers’ (n=10). 

The mean defoliation was 24.1% and 21.1% of the trees were in defoliation classes 2‐4. 17.6% showed moderate defoliation. On 2.3% of the trees, more than 60% of the crown was defoliated. The mortality rate was 1.2%. 9.0% of the trees in the survey were considered as healthy and 69.9% were in a warning stage (defoliation class 1). 

The share of trees with more than 25% defoliation was higher than the mean in Pinus nigra (35.1%), Quercus robur (24.8%) and the subset with ‘other broadleaves’ (31.0%). Fagus sylvatica reached the lowest defoliation score with 10.4% of the trees classified as being damaged. Crown condition was also better than the mean in Quercus rubra, Pinus sylvestris and Populus sp. The share of damaged trees for these species amounted to 17.6%, 13.1% and 12.4%. Mortality was high in the subset with broadleaves. Dieback of Quercus robur was noticed in several plots, dieback of Alnus glutinosa in one plot. From 2012 on, dead oaks and alders were reported every year. 

Defoliators caused more than 10% defoliation on 14.6% of the Quercus robur trees, while 8.0% of Q. robur showed severe discolouration caused by Microsphaera alphitoides (>10% of the leaves). Discolouration by fungal infection was also noticed on Pinus nigra. Dothistroma septosporum caused brown discolouration in combination with needle loss. The cause of dead shoots, twigs or branches was often unknown. In one plot crown dieback on Alnus glutinosa was caused by Phytophthora alni. In several plots Hymenoscyphus fraxineus caused dead shoots, twigs and branches on Fraxinus excelsior. 

A slight increase in defoliation and the share of damaged trees was observed. Mean defoliation decreased significantly only for Fagus sylvatica (‐7.4 percentage points). Contrary to 2016 there was almost no seed production on beech. A significant increase in defoliation was registered in Pinus sylvestris (+1.0 percentage points), Quercus robur (+1.8 percentage points), Quercus rubra (+2.2 percentage points) and ‘other broadleaves’ (+2.3 percentage points). Pinus nigra and Populus sp. showed non‐significant changes in defoliation (+0.3 and +0.4 percentage points). 

New plots were selected to monitor the crown condition of Fraxinus excelsior and the impact of Hymenoscyphus fraxineus in Flanders. 252 ash trees in 29 plots have been assessed yearly since 2014. A serious deterioration of the health status was noticed. Mean defoliation increased from 28.8% in 2014 to 43.0% in 2017, while the share of damaged trees almost doubled (from 32.1% to 59.1%). 6.7% of the sample trees died and every year new dead trees were noticed.
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