The Acorn Effect in Fire-Reduced Upland Oak (Quercus L.)

Prescribed fire and canopy reductions are increasingly being used to manage upland oak (Quercus L. spp.) ecosystems, but little is known about their impacts on recently fallen acorns and seedlings in these environments. This study examines how acorn size and nursery fertilization affect seedling growth and survival in field-based experiments following fire and under differing fuel loads.

In December 2018, plot-level (1 m2) prescribed burns were conducted in upland oak forests in northern Mississippi using single, double, and triple fine fuel treatments to replicate the conditions of unburned and burned sites with different basal area and species composition. The acorns were collected directly from the forest floor immediately post-fire and stored in 34 x 24 cm polyethylene bags that were permeable to carbon dioxide, but impermeable to oxygen and moisture. The mean maximum fire temperature detected by pyrometers was recorded in each bag and used to determine the treatment effect.

Acorns were subsequently placed into containers filled with a 3:1 mixture of peat (pH 6.0, screening 0-6) and perlite (pH 5.2, granulate 0-6 mm, Stuewe & Sons, Corvallis, OR) growing medium. One acorn was planted in each container at a depth of 2 cm, and 20 trays were prepared for each acorn class. Acorns were watered as needed when soil surface became visibly dry, approximately two times a week.

Planting acorns directly on the forest floor allows for direct observation of potential legacy effects of fire, including desiccation potential, insect predation rates, and heat damage to acorns (Morales and Hutchinson 1998). Additionally, burning can significantly reduce the amount of leaf litter available for acorns to hide in, increasing their visibility to animal predators (Garcia et al. 2010).

Germination and plant growth patterns of acorns were similar between groups, with the exception of height, which was reduced 9 to 13% in burned acorns of both species (Fig. 2). The lower height of seedlings from burned acorns may be partially due to legacy effects associated with acorn storage on the forest floor (Kormanik et al. 1998).

Although burning negatively affected germination of both acorn classes, there was no interaction between the effect of fuel load and acorn species on germination or on outplanting survival. In addition, acorns of both species were affected by fertilization, with seedlings from small acorns having greater emergence but less root mass and a higher root-to-shoot ratio than seedlings from large acorns.

The results of this study support the hypothesis that acorn size and fertility interact to determine seedling morphology and nutrient status, and outplanting survival. The findings also suggest that acorn sizing may not be a viable strategy for improving outplanting survival in upland oak ecosystems. However, further research is needed to determine whether the nutrient status of acorns is more important than their germination rate and to test the hypothesis that acorn size and fertilization interact to influence seedling emergence in nurseries and outplanting survival. Data were analyzed with a generalized linear model, followed by Duncan’s test for multiple comparisons. All statistical analyses were conducted with SPSS 18.0 for Windows (IBM, Chicago, IL).