Since 1984, the IAP investigates the health conditions of several hundred alley and park trees in the city of Basel. Until 2008, the monitoring programme included maple (Acer sp.), horse chestnut (Aesculus sp.), sycamore (Platanus sp.), and linden (Tilia sp.). Since 2009, also European hornbeam (Carpinus betulus), European nettle tree (Celtis australis), and Turkish hazel (Corylus colurna) are included. In addition, since 2012, growth is recorded as stem diameter increments.
In our cities, alley and park trees are severely stressed because they suffer from a limited rooting area, root injuries due to construction work, heavy soil compaction, increased heat absorbance, gas emissions, road salt, inadequate nutrient supply, and parasites. During the last decades, several parasites were introduced like the sycamore lace bug (Corythucha ciliata), leaf blotch of horse chestnut (Guignardia aesculi), horse-chestnut leaf miner (Cameraria ohridella) or Scots pine dieback (Sphaeropsis sapinea). Recently, urban trees are even more stressed by increased drought caused by climate change.
Construction work is a challenge for urban trees (left) and tree health monitoring is a demanding job for IAP staff (right).