Modern and fossil pollen assemblages reveal forest taxonomic changes in the Mexican subtropics during the last 1300 years

Abstract

The usual approach to reconstructing long-term vegetation dynamics is through the use of fossil pollen records from sedimentary sequences. This requires, however, an understanding of the relationship between modern pollen and the species composition and distribution on landscapes. Since different taxa produce different amounts of pollen, with different dispersal characteristics, understanding this relationship can be complex. This research aimed to address the following questions: Is it possible to distinguish different high-altitude forests by modern pollen signature? Is modern pollen abundance related to forest structure, forest diversity or to differences in altitude? How well can modern and fossil pollen assemblages reveal changes in the forest composition over the past ~1300years?Data were collected on present-day forest composition and structure and diversity, paired with modern pollen samples in three high-altitude forest types: pine forest, cloud forest and a transitional forest (a mixture of Pinus–Quercus–Carpinus) in west-central Mexico. A sediment core was collected from each vegetation type for reconstructing past vegetation. Constrained and unconstrained ordination methods were used to discern patterns of variation and the correlates behind the datasets. To compare the fossil with the modern pollen assemblages, dissimilarity indexes were computed between each of the fossil pollen sequences and their modern assemblages at different altitudes using the chord distance.Results demonstrated that each of the three forests types was distinctive in composition, and modern and fossil pollen reflected local vegetation at stand level; mean diameter and altitude were the main variables related to differences among modern pollen signatures. When reconstructing high-altitude vegetation in the subtropics, it is important to consider all taxa, even if they are rare. Therefore, in order to reveal responses to climate change and human disturbances at stand level in forests at different successional stages; particularly from 102 to 104m2 and, at temporal scales from 102 to 103years, it is essential to combine modern and fossil pollen together with present-day vegetation records.