Crossdating of disturbances by tree uprooting: Can treethrow microtopography persist for 6000 years?
Pavel Samonil, Randall J. Schaetzl, Martin Valtera, V. Golias, Petra Baldrian, I. Vasickova, D. Adam, D. Janik and L. Hort.
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 14C, 210Pb, 137Cs, 226Ra) – 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, 210Pb dating (including 137Cs and 226Ra) 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-limiting’’ ages of disturbance events, using dendrochronology, tree-censuses, 210Pb- and/or 14C-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 14C-dating on remnants of uprooted trunks and charcoal from buried A-horizons in mounds.
Responses in the radial growthof trees near byuprootingdisturbanceswereusually 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-loamEntic Podzols. Finally, on the sand-textured Albic Podzols of northern Michigan, even greater longevities were achieved, with one mound returning a 14C 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 210Pb technique, were 0.05–0.28 cmyear1 in the beech-dominated European forests, and 0.07–1.4 cmyear1 in the hardwoods of Michigan. Radiocarbon dating of fungal sclerotia in treethrow pits can also help inform the disturbance history.