Great Plains vegetation dynamics in response to fire and climatic fluctuations during the Holocene at Fox Lake, Minnesota (USA)

Abstract

Vegetation composition and fire frequency are tightly linked in North American grasslands and have varied considerably throughout the Holocene in response to different drivers. Yet, detailed records of both long-term changes in grassland vegetation composition and diversity, coupled with fire history, are still relatively sparse. In this study, we examine a sediment core from Fox Lake, Minnesota, using pollen, charcoal, magnetic susceptibility, organic carbon (%C), and silica (%Si) records with the aim of understanding grassland structure and function during the Holocene, particularly in the context of vegetation composition and diversity, erosion, and fire activity. Nonarboreal pollen comprises between 37% and 86% of the assemblage throughout the record with the largest percentages occurring during the mid-Holocene (~8000–4000 yr BP). The pollen record also suggests that at 8200 yr BP, there was an abrupt shift from oak-elm woodland to a more open landscape of grassland or savanna, which remained throughout the mid-Holocene. Additionally, the pollen data suggest that vegetation composition exhibited little change in diversity through time despite recurring fire. Charcoal concentrations varied from 30 to nearly 1200 particles cm−3, indicating changes in relative amount of biomass burned, but the morphotypes of charcoal pieces indicate that woody fuels persisted during the mid-Holocene despite the apparent grassland-dominated landscape. Magnetic susceptibility in the sediment ranges from −0.9 to 22.4 (×10−5 SI) throughout the record, with the biggest increase occurring as the vegetation shifted from woodland to grassland entering the mid-Holocene. Organic carbon ranges from 4.6% to 20.0% and exhibits a slow but steady increase after the 8200 yr BP event. Silica decreases slightly but remains generally high between 20.4% and 22.5%.

Regional variation in Holocene climate quantified from pollen in the Great Plains of North America

“…Previous work in this region has described the vegetation transitions into the mid‐Holocene and out of the mid‐Holocene as being asymmetric, with an abrupt shift toward aridity between the early and mid‐Holocene, and a gradual increase in moisture entering the late Holocene (Bartlein et al ., ; Umbanhowar et al ., ; Nelson and Hu, ). In pollen records, this asymmetry has mainly been manifested as an abrupt decrease in woody taxa entering the mid‐Holocene, and a gradual increase in woody taxa entering the late Holocene (Baker et al ., ; Umbanhowar et al ., ; Commerford et al ., ). However, the precipitation and temperature reconstructions presented here indicate that climatically, these transitions were more complicated.…”

“…Fox Lake pollen data were from Commerford et al . (). Moon Lake pollen data (Laird et al ., ) and Kettle Lake pollen data (Grimm et al ., ) were acquired from the Neotoma Paleoecology Database (http://www.neotomadb.org).…”

“…()]. Many pollen records from lacustrine cores in the upper‐Midwest have indicated substantial expansion of grassland vegetation during the Holocene (Webb et al ., ; Baker et al ., ; Wright, ; Laird et al ., ; Umbanhowar et al ., ; Nelson and Hu, ; Williams et al ., ; Grimm et al ., ; Commerford et al ., ). In order to accurately reconstruct the climate from pollen during the grassland periods at these sites, grassland surface samples must be well represented in the datasets used in the calibrations.…”

High dissimilarity within a multiyear annual record of pollen assemblages from a North American tallgrass prairie

“…), including pollen from the Great Plains (Commerford et al. ), but not on annual modern pollen samples. Our ANOVA results demonstrate statistically significant differences in beta‐diversity between some of the years from 2011 to 2014 (Fig.…”

“…Comparatively, Holocene pollen assemblages from Fox Lake in the northern Great Plains had beta‐diversity values that averaged around 0.8 SD units (4 SD units would imply full turnover), suggesting that grassland vegetation at that site exhibited little change in beta‐diversity over much of the Holocene, despite recurring fire and drought (Commerford et al. ). Our beta‐diversity values are similar, suggesting that the amount of change is comparable on an annual time scale.…”