The city's hidden giants

Barcelona's historic botanical gardens, the Jardí Botànic Històric de Barcelona, were built in the 1930s and are only spotted by the most observant passers-by. They go unnoticed because they are located inside two big hollows that were part of an old quarry, which created a sheltered spot, which is noticeably cooler and shadier than the surrounding area. This means that species that require a wetter, cooler climate than is usually found in Barcelona, can grow there. The temperature in this part of the gardens is 5 degrees lower than at the entrance, and the fact that the plants and trees have been placed inside a deep hollow means they have had to search for sunlight higher up. This is why the gardens are home to the city's tallest trees.

There are three particularly tall trees at the back of the garden: a green ash (Fraxinus pennsylvanica), which is native to North America, a narrow-leaved ash (Fraxinus angustifolia), and a wingnut tree (Pterocarya x rehderiana), which vie with one another to be Barcelona's tallest tree. A few years ago, a tape measure attached to a helium balloon was used to measure the trees. This was the only solution, given the impossibility of seeing their full height because they were planted in a hollow. Two people, one at the base of the tree and another standing 30 metres above in the garden, were able to ascertain the size of these giants, which measure 32, 29 and 26 metres respectively.