Given the task definition and input, reply with output. This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.

Passage: Mexico city primarily rests on what was Lake Texcoco. Seismic activity is frequent here. Lake Texcoco was drained starting from the 17th century. Although none of the lake waters remain, the city rests on the lake bed's heavily saturated clay. This soft base is collapsing due to the over-extraction of groundwater, called groundwater-related subsidence. Since the beginning of the 20th century the city has sunk as much as nine metres (30 feet) in some areas. This sinking is causing problems with runoff and wastewater management, leading to flooding problems, especially during the rainy season. The entire lake bed is now paved over and most of the city's remaining forested areas lie in the southern boroughs of Milpa Alta, Tlalpan and Xochimilco. Question: Where are most of the trees located in Mexico City?
southern boroughs