Skye M. Greenler, Robert K. Swihart, and Michael R. Saunders, 2020
RESEARCH BRIEF #34
Acorns buried in simulated caches had higher emergence rates in burned areas.
Microsite conditions are more favorable to acorn emergence in burned areas.
Red oak acorns had higher emergence rates following the fall fires than spring fires.
In this study, authors assessed the effects of simulated underground caching and burning season on acorn survival and seedling emergence for northern red oak (NRO, Quercus rubra) and white oak (WO, Q. alba) in west-central Indiana, USA. Oak regeneration is a common challenge in many eastern North American forests, and understanding of the relationship between periodic surface fires and historical oak prominence in the region has recently increased. However, very little is known about how fire interacts with individual components of the oak regeneration process, including acorn germination and seedling emergence.
The underground caching of acorns by small mammals is an important factor in oak regeneration dynamics, and can be antagonistic or mutualistic depending on environmental factors (Greenler et al. 2019). Though rodents consume a portion of the cached acorn crop each year, unretrieved acorns exhibit higher germination rates than those left on the surface, due to protection from environmental factors and other seed predators. Cached acorns may experience less fire-caused mortality than those not buried, but this has not been experimentally assessed prior to this study. Because the timing of acorn germination varies by species (white oak group in the fall, red oaks in the spring), the interaction between oak species and season of burn may also impact acorn mortality. Beyond heat-induced acorn mortality, fire may affect acorn germination and seedling emergence due to changes in microsite conditions, changes which may persist from months to years.
The study was conducted in two long-unburned hardwood forests owned and managed by Purdue University. For all treatments (fall and spring fire, control/ no-fire), 40 NRO and 40 WO acorns were placed inside seed predator-proof exclusion cages in the fall preceding fire treatments. Thirty of each species were buried in simulated caches, and 10 were placed on the soil surface beneath the leaf litter. Researchers mimicked the caching practices of the grey squirrel, by placing acorns in a 2cm deep depression, and then backfilling with soil and leaf litter. To assess how fire-caused environmental changes altered microsite conditions for the different caching treatments, moisture measurements were taken before and after fires, as well as periodically throughout the experiment. Temperature sensors were buried in a simulated cache and also located with the surface-placement acorns. Fall prescribed fires were conducted in mid-and-late-November, and spring fires occurred in mid-March.
Statistical analyses assessed how burn treatment (fall burn, spring burn, no-fire/control), species, and the interaction between burn treatment and species influenced spring seedling emergence. Additionally, the effect of burn and caching treatments, and their interaction on microsite temperatures were compared.
Contrary to the popular assumption that fire kills most acorns, acorns buried in simulated caches within prescribed fire units had much higher emergence levels than buried and unburied acorns in unburned treatments. Across all treatments (fall, spring, and no-fire), emergence ranged from 6.0% and 6.7% for buried acorns, compared to only 0.42% and 1.3% of those placed on the soil surface. Therefore, emergence probability was only analyzed for the buried acorns for the different fire treatments. Both WO and NRO acorns had higher emergence in the burned treatments than in controls. NRO acorns had emergence rates about 12 times higher following fall burns and almost 8 times higher following spring burns compared to the no-fire treatments. WO acorns were about 4 times more likely to emerge following fall burns, and almost 3.5 times more likely following spring burns.
The higher rate of emergence for acorns buried within burn units is likely driven by increased protection from fire, as well as fire-induced environmental microsite changes. Though the cause of acorn mortality was not quantified, researchers observed that many of the acorns appeared to have rotted below the leaf litter layer in the unburned control treatments, a barrier to acorn germination well-known by regional managers. Deep litter and duff layers in long unburned forests are known to inhibit radicle growth into the soil; a previous study showed that NRO emergence rates were the highest when buried, but not covered by litter (Garcia et al. 2002).
Emergence of NRO was lower following spring burns than fall burns, but it is unclear if this was due to higher mortality during the spring burn, more favorable winter and spring conditions following the fall burn, or by a combination of both. Emergence of WO acorns, which do not require cold-temperature stratification, was not affected by burn season. Cold-degree-hour accumulations were higher for acorns in the fall burn treatment, which may provide benefits to NRO in areas of their range that do not usually meet their cold-stratification requirements.
FOR FURTHER READING
García, M.J. Bañuelos, G. Houle. Differential effects of acorn burial and litter cover on Quercus rubra recruitment at the limit of its range in Eastern North America. Can. J. Bot., 80 (2002), pp.1115-1120.
The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, landowners and the public about the use, application, and effects of fire in the region. www.oakfirescience.com
This research brief was funded by The Joint Fire Science Program. www.firescience.gov