Claire Waldman is a senior at Centre College. This summer, she worked with CCSD scientist Matthew Albrecht to study the legacy of burn pile scars – the ashy leftovers from burning thinned tree trunks – in a restored woodland at Shaw Nature Reserve as part of MBG’s NSF-funded Research Experience for Undergraduates (REU) program.
In December 2016, the staff at Shaw Nature Reserve began a major restoration project focusing on 60 acres of woodland that were heavily invaded by bush honeysuckle (Lonicera maackii), Japanese privet (Ligustrum japonicum), and other invasive shrubs. This work, funded by the Institute of Museum and Library Services, commenced with removal of the invasive shrubs and thinning the canopy through selective tree cutting to promote a more open woodland structure. An open canopy structure allows for future use of prescribed burns in the area to help maintain the open canopy and facilitate diverse understory vegetation.
Canopy thinning left land managers with excess woody debris – that is, there were many large, fallen trees covering the forest floor. Rather than use heavy machinery to remove the logs (which often creates a large amount of soil disruption), the wood was stacked into piles and burned. These slash pile burns create an extremely localized but intensive disturbance. The soil at the burn site becomes sterilized and covered in a layer of ash. These sites are referred to as burn scars. The layer of ash that remains, as well as the absence of vegetation, make burn scars easily identifiable.
Slash pile scars were distributed across the restored site at Shaw Nature Reserve. Previous research has suggested the combustion of biomass and extreme temperatures of the burns can be lethal to the soil seedbank, alter soil structure, moisture, and nutrient availability. The rate of native vegetation recovery on slash pile scars depends on burn intensity, pile area, and properties of the surrounding plant community. The native plant community in burn pile scars, without active seed addition by people, are expected to recover slowly. If the native plant community recovers slowly over time, it raises the concern that slash pile scars could serve as foci for the reestablishment and spread of invasive or undesirable species.
This summer, as part of the NSF-funded Research Experience for Undergraduates program at the Missouri Botanical Garden, I worked under the guidance of Matthew Albrecht to further our understanding of these slash pile burn scars.
First we developed a field experiment focused on native vegetation recovery in slash pile burns. In order to characterize the changes in soil nutrients, compaction, and moisture that occur in these burn piles, we took soil samples from the burn piles as well as adjacent control areas and sent them to the University of Missouri Soil and Plant Testing Laboratory. The results we obtained from this soil analysis were dramatic. Soil pH, P, Ca, Mg, and K were all significantly higher in the burn pile. Soil compaction and soil organic matter were both significantly lower in the burn pile.
While there was an apparent flush of nutrients available in these burn piles, in the first growing season after a burn occurred, we found that burn scars remained essentially bare. We created experimental burn scar plots in which we seeded six native species. The six species sown into the plots were Bromus pubescens (grass), Chasmanthium latifolium (grass), Lespedeza violacea (legume), Senna marilandica (legume), Solidago ulmifolia (composite), and Symphyotrichum drummondii (composite). We also seeded these species in adjacent unburned control plots. We then monitored plant occupancy in these plots over the course of eight weeks.
We found four of the six species established significantly better in the control plots relative to the burn scar plots. These results support that while there is a significant influx of nutrients in the burn scars, there are other factors that are limiting native species establishment in the burn scars.
There are restoration implications of our results. We found, of the six species sowed into the burn plots, native grasses established best. While the microenvironment created by slash pile burns presents a barrier to the restoration of native vegetation in burn pile scars, seed additions of native grasses provide a practical management strategy for promoting native vegetation recovery in burn pile scars.