Abstract. Human impact is a well-known confounder in pollen-based quantitative
climate reconstructions as most terrestrial ecosystems have been artificially
affected to varying degrees. In this paper, we use a "human-induced" pollen
dataset (H-set) and a corresponding "natural" pollen dataset (N-set) to
establish pollen–climate calibration sets for temperate eastern China (TEC).
The two calibration sets, taking a weighted averaging partial least squares
(WA-PLS) approach, are used to reconstruct past climate variables from a
fossil record, which is located at the margin of the East Asian summer monsoon
in north-central China and covers the late glacial Holocene from 14.7 ka BP
(thousands of years before AD 1950). Ordination results suggest that mean
annual precipitation (Pann) is the main explanatory variable of both pollen
composition and percentage distributions in both datasets. The Pann
reconstructions, based on the two calibration sets, demonstrate consistently
similar patterns and general trends, suggesting a relatively strong climate
impact on the regional vegetation and pollen spectra. However, our results
also indicate that the human impact may obscure climate signals derived from
fossil pollen assemblages. In a test with modern climate and pollen data, the
Pann influence on pollen distribution decreases in the H-set, while the human
influence index (HII) rises. Moreover, the relatively strong human impact
reduces woody pollen taxa abundances, particularly in the subhumid forested
areas. Consequently, this shifts their model-inferred Pann optima to the arid
end of the gradient compared to Pann tolerances in the natural dataset and
further produces distinct deviations when the total tree pollen percentages
are high (i.e. about 40 % for the Gonghai area) in the fossil sequence. In
summary, the calibration set with human impact used in our experiment can
produce a reliable general pattern of past climate, but the human impact on
vegetation affects the pollen–climate relationship and biases the pollen-based
climate reconstruction. The extent of human-induced bias may be rather small
for the entire late glacial and early Holocene interval when we use a
reference set called natural. Nevertheless, this potential bias should be kept
in mind when conducting quantitative reconstructions, especially for the
recent 2 or 3 millennia
