Crossdating of disturbances by tree uprooting: Can treethrow microtopography persist for 6000 years?

Establishing disturbance frequencies in different types of forest ecosystems is an area of considerable research. We use several overlapping dating methods - robust repeated tree-censuses, dendrochronology and radiometric techniques (radionuclides C-14, (210)pb, Cs-137, Ra-226) - to date 302 pit-mounds, formed by uprooting, in 3 different forest ecosystems. Our research goal was to better understand the disturbance frequencies in these ecosystems and establish the potential longevity of treethrow mounds. At the same time, we tested the applicability of multiple dating methods in different kinds of stands, including two sites in Central Europe and one in the Great Lakes region, USA. We used tree-census data to establish the ages of the youngest (<37 yr) uprooting events, Pb-210 dating (including Cs-137 and Ra-226) to date treethrow events younger than ca. 200 years, dendrochronological dating for sites less than ca. 400 years old, and radiocarbon dating for assumed older sites. In order to maximize the reliability of these age estimates, we compared the dates by evaluating different methods with maximal overlap - a method we call crossdating. Theoretically, our data provide (i) minimum-lim- iting ages of disturbance events, using dendrochronology, tree-censuses, (210)pb- and/or C-14-dating of samples from the organo-mineral sedimentation funnel within the treethrow pit, (ii) real ages of events, using dendrochronology, and/or (iii) upper-real or maximum-limiting ages of the events, using C-14-dating on remnants of uprooted trunks and charcoal from buried A-horizons in mounds. Responses in the radial growth of trees near by uprooting disturbances were usually detectable for 20 years, and were specific to each region and tree species. Repeated tree-censuses (13,795 trees from 72.84 ha) suggested that the ratio of bole breakages to uprootings was 2-3:1, and that the treethrow rotation period approximated 1250-1380 years for some sites in Central Europe. At our European, beech-dominated sites, the maximal longevity of the treethrow microtopography was ca. 220 years on the fine-textured Haplic Cambisols and exceeded 1700 years on sandy-loam Entic Podzols. Finally, on the sand-textured Albic Podzols of northern Michigan, even greater longevities were achieved, with one mound returning a C-14 age of 5260 +/- 30 BP (median of calibration age 4077 BC) - by far the oldest feature reported in the literature. Sedimentation rates in treethrow pits, calculated using the Pb-210 technique, were 0.05-0.28 cm year(-1) in the beech-dominated European forests, and 0.07-1.4 cm year(-1) in the hardwoods of Michigan. Radiocarbon dating of fungal sclerotia in treethrow pits can also help inform the disturbance history. (C) 2013 Elsevier B.V. All rights reserved.