More Than One Way to Kill a Spruce Forest: the Role of Fire and Climate in the Late-Glacial Decline of Spruce Woodlands Across the Southern Great Lakes Region

Abstract

In the North American boreal forest, temperatures are expected to rise by 4 to 11 °C over the next century and fires are predicted to increase in size, severity, and frequency. The combination of warming and intensified fire regimes can cause boreal forests to cross a threshold, in which they transition from coniferous vegetation to deciduous or open vegetation. A model system for studying the interacting effects of changing climate and disturbance regimes on boreal forests is the last deglaciation (19,000 to 8,000 years ago), a period when temperatures rose by 2.5 to 6.5 °C in the southern Great Lakes Region and Picea (spruce) forests and woodlands declined and were replaced by mixed-deciduous or pine-dominated forests. The role of rising temperatures and other climatic factors in driving the regional demise of late-glacial spruce woodlands is widely accepted, but the role of fire is poorly understood. This study presents a synthesis of deglacial vegetation change and fire activity using fossil pollen and charcoal from five lake sediment cores taken across the southern Great Lakes Region: Appleman Lake, Stotzel-Leis, Silver Lake, Bonnet Lake, and Triangle Lake Bog. These records have established radiocarbon chronologies, high-resolution pollen data, and contiguous charcoal counts, enabling a newly-detailed study of the linkages between fire dynamics and vegetation state changes in the southern Great Lakes Region. At the study sites, spruce forests and woodlands were replaced by mixed coniferous-deciduous vegetation between 16,000 and 12,000 years ago. However, the spruce decline followed two different patterns. At Stotzel-Leis and Silver Lake, spruce followed a decline and return pattern, where spruce pollen dropped to low levels by 14,000 years ago, then returned during the Younger Dryas cooling (12,900 to 11,600 years ago), with a final decline after this interval. At these sites, deciduous hardwood taxa like Fraxinus (ash), Ostrya/Carpinus (hop-hornbeam/hornbeam), and Ulmus (elm) made up the majority of pollen after the initial spruce declines, until spruce and pine became abundant in the Younger Dryas. At Appleman Lake, Bonnet Lake, and Triangle Lake Bog, spruce followed a stair-step pattern, where spruce declined over 1,000-2,000 years in a series of abrupt drops. At these sites, deciduous hardwood taxa increased after the first drop between 15,000 and 14,000 years ago, but spruce remained at mid-level abundance until pine reached high abundance during the Younger Dryas. The charcoal records showed changes in fire activity that corresponded to regional climatic trends, including an early rise in fire activity with late-glacial warming, a low point in fire activity in the Younger Dryas, and increasing fire activity through the early Holocene. However, the timing and magnitude of these changes varied significantly across sites. At Stotzel-Leis and Silver Lake, fire activity reached maximum levels leading up to the early spruce declines, and both charcoal accumulation rates (an indicator of landscape fire activity) and fire frequency (an indicator of local fire activity) were significantly and positively correlated with community turnover rates. However, at the other three sites, there was no visual or statistical relationship among fire frequency, spruce declines, and vegetation turnover. Differences in soil drainages might explain why fire was associated with community change at some sites but not others; Stotzel-Leis and Silver Lake have a mixture of loamy soil types and the other sites are surrounded by poorly-drained sediments, which can limit landscape flammability. Hence, frequent fire episodes appeared to accelerate state transitions in parts of the Great Lakes Region by clearing the way for deciduous taxa adapted to warmer climates, while in other parts, rising temperatures appeared to be the primary driver of the demise of spruce forests and woodlands.