Long-term forest dynamics in high-altitude mountains of West-Central Mexico

This thesis presents the results of a study to examine long-term forest dynamics in the high-altitude mountains of West-Central Mexico. Vegetation dynamics on temporal scales ranging from 102 to 103 years were reconstructed in order to provide essential information on the temporal variability of ecological patterns and processes in these forests; information that is of direct relevance for their current and future conservation and management strategies. Vegetation and palaeoecological methods undertaken included fossil and modern pollen analysis, vegetation surveys, microfossil charcoal analysis, magnetic susceptibility, inorganic and organic geochemistry, radiocarbon and 210Pb dating. These were used to evaluate the long-term dynamics of three forest types; Pine Forest, Cloud Forest and Transitional Forest on timescales spanning the past 4260, 1340 and 1230 years respectively. The main drivers of change were climate and disturbance events induced by climate fluctuations, for example increased fire frequency. The reconstructed records indicate that the sequences from the Cloud Forest and the Transitional Forest spanned two wet and one dry climatic interval while the Pine Forest sequence spanned two dry and two wet periods. The impact of these climatic fluctuations was significant on all three forest types and resulted in variations in forest diversity, taxonomic turnover and successional change. The climate change episodes observed in these records seem to be the local manifestation of climatic events that were occurring throughout Mexico at these intervals in time. Human influences were evident in the three forests through the appearance of cultural taxa, particularly during the driest period (~ 1200 yr BP). There is little evidence from these records, however, to suggest a widespread clearance of the landscape for agriculture. Results from this study support the current conservation and management recommendations for Cloud Forest to exclude timber extraction, grazing and agricultural activities from this forest type. In the Pine Forest, human interventions such small-scale agriculture, prescribed burning and silvicultural actions are in agreement with the longterm pine ecology and as such, total exclusion of human activities is not necessary. For the Transitional Forest, results from this study suggest that there needs to be the establishment of adequate plans to reduce frequent fires to arrest the development of prone-to-fire taxa.