Además, la contracción poblacional de P. chilensis no se correlaciona con una expansión poblacional de A. caven, por lo cual las dos especies que definen y caracterizan el bosque espinoso de la Región Metropolitana de Chile se encuentran prontas a extinguirse y con ellas el piso vegetacional. Surge, por lo tanto, como necesidad urgente el establecer las medidas de conservación y restauración necesarias para asegurar la persistencia del bosque espinoso en la Región Metropolitana de Chile. PALABRAS CLAVE:Algarrobos, espinos, degradación ambiental, extirpaciones futuras. ABSTRACTAt central Chile grows the spiny forest of Acacia caven and Prosopis chilensis of which less than 40% of its original surface currently persists, but with no representation into the wilderness protected areas. Owing to the action of cattle, has it been previously suggested a population reduction of P. chilensis coupled to a population increase of A. caven which, nonetheless,has not yet verified empirically. At four sites belonging to two micro-basins in the Metropolitan Region at central Chile (Batuco and Huechún), we assessed the incidence of environmental disturbances (cattle, rabbits, weeds, garbage), the phytosanitary stage, and population structure of P. chilensis, as well as the regeneration dynamics of this tree and A. caven by using Markovian transition matrices. Populations of P. chilensis suffer similar degrees of threats, exhibit a similar but depauperate phytosanitary stage and, consequently, they face a senescent stage prone to disappear when the current adult individuals go death. Furthermore, the population decrease of P. chilensis does not correlate with the previously suggested population increase of A. caven; hence, the tree species characterizing and defining the spiny forest of central Chile are prone to be extinct and with them the vegetation belt. It appears, therefore, as an urgent necessity the establishment of conservation and restoration programs in order to assure the persistence of the spiny forest at the Metropolitan Region in central Chile.