North America experienced moderate but often abrupt climate changes since the last glaciation: temperature increases and decreases, precipitation varies, ice retreats, fire regimes shift, etc. Vegetation communities may shift between different biomes across latitudes and elevations or keep stable under these various disturbances. The varying responses of vegetation communities in North America after the last glaciation provide excellent records to study community resilience under climate change. This work uses 19,776 fossil pollen assemblages from 511 pollen Neotoma database records to reconstruct vegetation community stability across North America since the last glaciation (20,500 BP). We used a model-based cluster method (Gaussian finite mixture models fitted via EM algorithm) to align the radiocarbon dates of the fossil pollen assemblages. Log-ratio transformation of abundances – the sum of herbs as the reference – is used in lieu of normalization. Principal coordinate analysis (PCoA) is used to reduce dimensions of pollen taxa. We compare these results to a more established modern analog and biomization method for no-analog communities. We aim to 1) identify stable vegetation communities in North America since the last glaciation, and 2) explore the resilience of each vegetation cluster group.
Results/Conclusions
Two PCO axes represent 99.8% of the information in log-ratio transformed abundance data. All fossil pollen assemblages are classified into 10 groups using mclust package in R, in which the optimal number of 10 clusters is selected using BIC. The average duration time for a long-stable vegetation community, i.e. a community that does not experience a biome shift, is 1,070 years. The upper 5% of the long-stable community duration is 4,720 years. These long-stable sites are sparse and mainly distributed in the Northeast and southern Midwest before Holocene. After a contraction between 13,000 and 12,000 BP, these sites expand rapidly from the southern Midwest to the north-central Midwest and Rocky Mountain regions. After 4,000 BP, though the long-stable sites are still distributed throughout North America, the number of sites declines. The most common vegetation types in long-stable communities are spruce-birch-alder boreal forest and pine-oak-birch cool mixed forest, while the least common vegetation type is mixed temperate forest dominated by various deciduous and coniferous trees. From the perspective of vegetation groups, boreal forests last 1,390 years and cool mixed forests last 1,410 years, on average, while the mixed temperate forests only last 230 years.