Like other native German species, lindens are having increasing problems coping with hot, dry summers and bitterly cold winters, as well as pollution from cars and the large amounts of road-salt that end up in the soil. The trees in the city are under stress. Now scientists at the Humboldt University are looking at species that can withstand it better. Since 2010, they've been testing robust plants from all over the world. Potential candidates are subjected to different kinds of stress test in a nursery on the edge of the German capital.