Variations in tree cover in North America since the last glacial maximum

Accurate reconstructions of late-Quaternary land-cover change are needed to better understand past interactions of the terrestrial biosphere with other components of the earth system. This paper presents a sequence of reconstructed needleleaved and broadleaved tree-cover densities for North America since the last glacial maximum, generated from fossil-pollen data and present-day tree-cover estimates derived from the Advanced Very High Resolution Radiometer (AVHRR). For this study, a refined form of the modern analog technique was developed, called the hierarchical analog technique, which can constrain paleoenvironmental properties even for fossil-pollen assemblages without close analogs in the modern-pollen record. Pollen taxa from samples that are compositionally unlike any modern-pollen samples are regrouped into plant functional categories based upon phenology, life form, leaf shape, and climatic tolerances, and the analog analysis rerun. Reclassifying individual pollen taxa into broader functional categories enables analogs to be found when no compositional analogs exist, but at a cost of increased uncertainties in the analog estimates. Tests of the hierarchical analog technique shows that it accurately reconstructs present-day tree-cover densities. The median standard deviation for each individual estimate is <10%. Tree-cover densities during the last glacial maximum were low relative to present, and have increased since. Lower-than-present tree-cover densities at the last glacial maximum were likely due to a combination of low temperatures, low precipitation, and low atmospheric CO2 concentrations. By 14 ka, broadleaved tree-cover densities had begun to rise in the southeastern US and needleleaved forests grew in the western US, southeastern US, and as a belt along the southern margin of the Laurentide Ice Sheet. By the mid-Holocene, the northern and western needleleaved forests had joined. Needleleaved and broadleaved tree densities continued to increase until European settlement. Mapping percent tree-cover represents a useful alternative to biome-based classification schemes, enabling a fuller representation of vegetational gradients in space and time, and can be directly compared to the tree distributions simulated by dynamic global vegetation models. In effect, by calibrating the modern-pollen data against the AVHRR-derived estimates of tree-cover, the fossil-pollen data are applied to extrapolate satellite-based observations into the Quaternary, enabling study of vegetation dynamics and land-cover change at timescales beyond the period of direct observation by engineered remote sensors. Analog-based approaches, however, require extensive networks of surface and fossil-pollen samples, making further data collection a priority in sparsely sampled regions of the world.