A study by Wageningen University says radiation from wi-fi networks may be harmful to trees.   

The researchers note that trees in urban areas in recent years have shown an increasing number of cracks, bumps, discolorations and various forms of tissue necrosis but no cause has been identified, so they sought to examine if it was more than biological factors like pests or disease. 

Wageningen University was commissioned by the town of Alphen aan den Rijn to see if  increasing sources of electromagnetic radiation, such as wi-fi transmitters or cellular towers, could play a role in the deteriorating health of the trees. Alphen aan den Rijn noted ash trees in the city were increasingly suffering from growth disturbances and that correlated to increases in wireless clusters nearby.

Their study exposed 20 ash trees to six radiation sources for a period of three months.    The emitters  had frequencies ranging from 2412 to 2472 MHz and a power of 100 mW EIRP at 50 cm distance.    They found that trees with leaves closest developed "metallic luster appearance",  a discoloration caused by the dying of the outer cell layer of the leaves. 

Although the effects of multiple radiation sources and several trees were found, the researchers want to repeat the test for a longer period and on a larger scale.

A PC World article further notes

In the Netherlands, about 70 percent of all trees in urban areas show the same symptoms, compared with only 10 percent five years ago. Trees in densely forested areas are hardly affected.