How can we protect our forests from the effects of climate change? By burning them.

 

For many, the thought of fire as a management tool is something that only applies in prairie environments. The idea of burning a forest in order to protect it seems incredibly counter-intuitive. However, in the northern Midwest and Canada, jack pine Pinus banksiana communities depend on fire to survive. Pinus banksiana cones are serotinous; the seeds are not released until they are exposed to temperatures of 50°C or more. This trait allows Pinus banksiana and associated early-successional species to grow back quickly after fire, while making it difficult for less fire-tolerant species like white pine Pinus strobus to invade. This means that fire will often preserve the species distribution of the forest by maintaining jack pine dominance in the region. In a recent paper, Lynch et al. (2014) investigated whether this stabilizing relationship with fire would give Pinus banksiana forests more resilience to climatic changes. Fitting with the theme of this year’s blogs, their answers came by looking to the past and analyzing how previous changes to the region’s climate affected these communities.

(a)                                                                                         (b)

Fig. 1: (a) Locations of the four study sites within Wisconsin. Adapted from Fig. 1 of Lynch et al., 2014. (b) Picture of red pines Pinus resinosa along the shore of Little Round Lake. Image courtesy of https://goo.gl/maps/hgAvtWymZ6szLjy5A.

 

The authors conducted their study on four lakes in the northwestern Wisconsin sand plain (NWSP) (Fig. 1). The NWSP is a region that consists of very sandy soils left by glacial runoff. The area is highly xeric as a whole, and when the area surrounding each lake was scored on a scale from 1 (somewhat poorly drained soils) to 5 (excessively drained soils), all four study sites scored higher than 4.69. In past research within the NWSP, the authors found sites with scores ranging from 2.09 to 4.99 with a mean of 4.23, so these four are highly xeric sites even for the area. To determine the percentage of the surrounding area that contained fire breaks, the researchers created a distance-weighted measure of the surface area of surrounding lands that were made up of lakes and wetlands. Both Little Round Lake and Ferry Lake scored higher than 23%, while Cheney Lake and Elevenses Lake scored less than 9%, demonstrating the lack of fire breaks near the Northern study sites.

 

To understand how the study sites differed in their species composition and responses to climatic events, the researchers took sediment cores from each lake and analyzed both the pollen counts and the charcoal fragments, while also radiocarbon dating each core. The cores provide data as far back as 2500 BP. The same authors have since conducted a study focusing on Cheney Lake with data going back to 7200 BP (Check out Veronica Henderson’s blog for more) (Calcote et al., 2021). Pollen analysis conducted on the cores was able to distinguish Pinus subgenus Strobus from subgenus Pinus, but could not distinguish within subgenus Strobus between Pinus resinosa and Pinus banksiana. This is important as Pinus strobus are less fire-tolerant than Pinus resinosa, but Pinus banksiana are much more fire-tolerant than both other species.

Fig. 2: Boxplots of pollen percentages of three major taxa at three different time periods. Blue circles show important results of the study. Adapted from Fig. 5 of Lynch et al., 2014.

 

In their charcoal analysis, the authors found that there were significant differences in the magnitude of charcoal peaks, with much larger peaks at the sites without fire breaks. This shows that the fires near those lakes were larger or more severe. Looking at pollen data, they found that the species composition of the sites with few fire breaks were far more stable during periods of climatic change. For example, during the period from 1400-700 BP, which contained many intense droughts that intensified during the Medieval Climate Anomaly (MCA) from 1000-700 BP, the sites without fire breaks dramatically shifted away from Quercus towards Pinus resinosa and Pinus banksiana. During the Little Ice Age (LIA), a period of cooler and moister conditions starting at 700 BP, all four sites show an increase in Pinus strobus percentage, but it is far more extreme in the sites with fire breaks.

 

In this study, Lynch et al. have demonstrated just how powerful the stabilizing relationship between jack pine and fire can be. Even when undergoing dramatic climatic shifts like those that occurred during the MCA and LIA, the sites that had fewer fire breaks (and thus burned more) were much more resilient to community change. This is important as we try to mitigate the effects of anthropogenic climate change in the region. Prescribed burns are widely used in prairie communities to preserve the habitat, but they are not as common in forests (although see this news article about prescribed burns in a National Forest close to the NWSP). Our understanding of the relationship between forests and fire is growing, and it is important that our management techniques keep up.

 

References:

Lynch, E. A., Calcote, R., Hotchkiss, S. C., & Tweiten, M. (2014). Presence of lakes and wetlands decreases resilience of jack pine ecosystems to late-Holocene climatic changes. Canadian Journal of Forest Research, 44(11), 1331-1343.

 

Calcote, R., Nevala-Plagemann, C., Lynch, E. A., & Hotchkiss, S. C. (2021). Late-Holocene climate changes linked to ecosystem shifts in the Northwest Wisconsin Sand Plain, USA. The Holocene, 31(3),