THE NATURAL VEGETATION OF THE MEXICAN BAJIO - ARCHIVAL DOCUMENTATION OF A 16TH-CENTURY SAVANNA ENVIRONMENT

Neo-ecologists make assumptions about a 'natural' or potential vegetat ion when they argue whether a particular landscape is in secondary and degraded condition. Similarly, paleoecologists attempt to infer a thr ee-dimensional biotic mosaic from a core taken in a low-lying wetland. Yet with millennia of human disturbance, climatic fluctuation, biotic response to long-term climatic trends or catastrophic 'events', and c oevolution between Holocene vegetation and human land-use,;natural eco systems' have not been in equilibrium. While past vegetation changes c an be traced, efforts to reconstruct potential vegetation are probably unrealistic. This paper assembles 16th century landscape descriptions of the Bajio of Central Mexico from archival repositories, to charact erise the landscape at the time of Spanish intrusion. Attention is foc used on five major landscape elements: (1) Riparian woodlands of mesqu ite, bald cypress and willow, with reed stands; (2) Level, vertisolic plains, with a low-tree savanna (mesquite acacia-grass); (3) Steeper p iedmont plains with stony substrates, probably favoring xeric, thorn-b ush associations; (4) Rough uplands with a mix of mesquite-acacia wood land, scrub oak, and them bush; and (5) Mountains dominated by live an d deciduous oak woodlands. The biotic mosaic of the 16th century appea rs similar to that of the modern spontaneous vegetation in physiognomi c terms, despite changes in structure. Areas of older indigenous settl ement were affected by local vegetation disturbance, with partial defo restation near lakes Cuitzeo and Yuriria. While Spanish-Criollo intrus ion (1540-1640) brought new, potentially destructive landuse methods, there is no evidence of additional landscape degradation in the Bajio until well into the 18th century. Dramatic changes in hydrology and ri parian vegetation are quite recent. Archival documentation provides a complementary methodology to re-examine the interplay of edaphic varia tion, climate and cumulative land-use in understanding contemporary ve getation, and it can assist in converting proxy data into a three-dime nsional landscape. (C) 1997 INQUA/Elsevier Science Ltd.