The Localized Influence of Fire on Historic Spruce Forest Death

The study of paleobiology provides ecologists the opportunity to see into the past how previous ecosystems in a place were influenced by environmental changes. Not only does paleobiology allow us to see into the past, but also to anticipate how future species and ecosystems will react to the rapidly changing climate today. The relationship between warming climate trends and the transition from boreal species to ones that are better suited for the new climate conditions. In the eastern United States, this shift was characterized by boreal taxa moving primarily northward where climate conditions better suited their environmental needs. However, the impact of fire on this transition is not well studied, so Jensen et al. (2021) set out to investigate the role of both fire and climate in the shift from Picea (spruce) conifer forests to broadleaf deciduous and Pinus (Pine) woodlands from the late Pleistocene to the early Holocene in the southern Great Lakes Region (Fig. 1). 

Vegetation History of the southern Great Lakes Region

The analysis of cores collected from five different kettle lakes across the southern Great Lakes region (Fig. 1) provided pollen and charcoal samples to inform the researchers about the vegetation history of each site. In their research, the first step Jensen et al. (2021) took was to define four different temporal zones based on patterns of species pollen abundance at the sites (Fig. 2). The first zone, Zone A, begins at the record onset, 19-17.5 kyr BP, to 15.8-14 kyr was dominated by Picea (spruce), with some Pinus (pine) and cold tolerant deciduous taxa including Alnus (Alders), Salix (Willows), and Betula (Birches). Transitioning into Zone B (15.8-14 to 13.3–11.6 kyr BP), a marked decline in Picea, resulting in mixed deciduous forests and woodlands, composed of waning populations of Picea and new populations of deciduous hardwood trees. Zone C (13.3–11.6 to 12–10.5 kyr BP) notably occurred during the Younger Dryas, in which all sites saw an increase in Pinus and decrease in deciduous hardwoods. During this time, some sites also saw a second Picea peak where it recovered, but this was not a universal phenomenon at all five sites. Finally, Zone D (12–10.5 to 8.8–7.8 kyr BP) had a sharp decline in Pinus and Picea (where second peaks occurred) as well as a steep  rise of temperate deciduous forests at all of the sites. Quercus (Oak) species showed a significant rise in abundance in this zone, indicating the presence of oak savannas, similar to those restored at Conard Environmental Research Area (CERA) here in Iowa.

So what’s the deal with fire?

Jensen et al. (2021) used their construction of the vegetative history at each site to identify patterns of vegetation change coinciding with signs of fire at that time. The results of combining charcoal analysis and Picea vegetative history divided the sites into two distinct patterns of Picea vegetation change. The first pattern, termed “decline and return”, was observed at Stotzel-Leis and Silver Lake sites. It is characterized by abrupt declines in Picea that coincided with periods of enhanced fire activity (Fig. 3). After that decline, deciduous forest primarily replaced Picea, with some return of Picea and Pinus during Zone C (Fig. 2). In this case, fire significantly accelerated the process of Picea replacement. The second pattern, “stair step” was observed at Appleman Lake, Bonnett Lake, and Triangle Lake Bog sites. It is distinguished by the decline in Picea through several abrupt events, however the relationship between Picea declines and fire activity is less clear (Fig. 3). Picea was primarily replaced by Pinus after the decline, and deciduous forests later on (Fig. 2). These two patterns indicate that the effects of fire vary significantly at the local level. Of the five sites studied, the two that were significantly impacted by fire had different patterns of replacement after significant fire events, pointing to fire significantly affecting what species colonize disturbed areas. 

Why Should We Care?

The rapidly changing climate due to global warming is creating new environmental circumstances that species have little time to adapt to. During the late-glacial period, the warming climate after the Last Glacial Maximum provides scientists an opportunity to see how species reacted to that warming period. Spruce forests and other boreal species habitat zones are farther North now, and may fare similarly as the global temperature continues to rise. Changing patterns of fire frequency are also associated with climate change. Jensen et al. addresses how fire has interacted with climate to produce different ecological outcomes, which can inform how areas with frequent fires will be impacted differently. 

Reference

Jensen, A. M., Fastovich, D., Watson, B. I., Gill, J. L., Jackson, S. T., Russell, J. M., ... & 

Williams, J. W. (2021). More than one way to kill a spruce forest: The role of fire and climate in the late‐glacial termination of spruce woodlands across the southern Great Lakes. Journal of Ecology, 109(1), 459-477.