Northern Red Oak Acorns Research Articles

Prescribed fire promotes acorn survival and seedling emergence from simulated caches

Prescribed fire increasingly is used to promote oak (Quercus) establishment in many hardwood forests across North America. However, how prescribed fire affects several key components of the oak regeneration process, including acorn survival and germination, is relatively unknown. Acorn mortality during a prescribed fire is likely affected by acorn dormancy, burn season, and previous burial (caching) by scatterhoarding rodents. We assessed the effect of fall and spring prescribed fires on seedling emergence for artificially cached and un-cached northern red oak (Q. rubra) and white oak (Q. alba) acorns in situ in granivore-proof exclosures through eight prescribed fires over two years at two sites in central Indiana. The odds of cached acorn emergence were 3.4–12.1 times higher following fall and spring prescribed fires than the unburned control. Red oak emergence was 1.7 times higher following fall burns than spring burns, whereas white oak emergence was unaffected by burn season. Lower winter temperatures following fall burns may have driven observed differences between burn seasons for northern red oak acorns. Our study links the oak-rodent conditional mutualism hypothesis with the oak-fire hypothesis, by suggesting that prescribed fire may produce environmental conditions that shift interactions between oaks and scatterhoarding rodents toward a more mutualistic relationship.

Invasive oaks escape pre-dispersal insect seed predation and trap enemies in their seeds.

Species introduced to habitats outside their native range often escape control by their natural enemies. Besides competing with native species, an alien species might also affect the native herbivores by introducing a new source of different quality food. Here, we describe the case of northern red oak (Quercus rubra) invasion in Europe. We collected data on insect (moth Cydia spp. and weevil Curculio spp.) seed predation of northern red oak in its native (USA, North America) and invasive (Poland, Europe) range, as well as for sessile oaks (Quercus petrea) in Europe. We also evaluated the quality of acorns as hosts for weevil larvae by collecting infested acorns and measuring weevil developmental success, and quantifying acorn traits such as seed mass, tannins, lipids and protein concentration. We used DNA barcoding to identify insects to the species level. The predation by moths was similar and very low in both species and in both ranges. However, red oaks escape pre-dispersal seed predation by weevils in Europe. Weevil infestation rates of northern red oak acorns in their invasive range were 10 times lower than that of sessile oaks, and also 10 times lower than that of red oaks in North America. Furthermore, even when weevils oviposited into northern red oaks, the larvae failed to develop, suggesting that the exotic host created a trap for the insect. This phenomenon might gradually decrease the local abundance of the seed predator, and further aid the invasion.

The effect of storage temperature and duration on northern red oak acorn viability and vigour

Three separate collections of Ontario sources of northern red oak (Quercus rubra L.) acorns were made to determine the effects of long-term cold storage at +2°C, −1°C, and −2°C on their viability and vigour. We measured acorn moisture content, percent germination during storage, speed of germination and total germination values, root regrowth of seeds that germinated in storage, incidence of fungal contamination, and condition of ungerminated acorns. Viability and vigour peaked six to 12 months after acorns were placed in storage, but decreased with continued storage. After 18 months in storage, ≥60% of the acorns germinated in four of the five seedlots tested and, after 30 months in storage, ≥53% of the acorns germinated in three of the five seedlots tested. Acorn viability was only minimally affected by storage temperature; however, since temperatures above −2°C allowed acorns to germinate during storage, the preferred long-term storage temperature was −2°C. Our results suggest that, assuming proper storage conditions, most red oak seedlots will maintain relatively high germination levels when stored at −2°C for 18 months. Seedlots with particularly high initial germination and vigour may be successfully stored for up to 30 months.

Acorn viability following prescribed fire in upland hardwood forests

Restoration of structure and function of mixed-oak (Quercus spp.) forests is a focal issue of forest land managers in the eastern United States due to widespread regeneration failure and poor overstory recruitment of oaks, particularly on productive sites. Prescribed fire is increasingly used as a tool in oak ecosystem restoration, with the goal of reducing competition, and creating light and seedbed conditions conducive to germination and growth of oak seedlings. Yet, oak seedling establishment is dependent on the presence of viable acorns, which may be vulnerable to prescribed fire. We assessed the effect of prescribed burning and fire temperature on the viability of white oak and northern red oak acorns placed on the leaf litter surface, in the duff, or in the mineral soil during five winter prescribed burns in southern Appalachian upland hardwood forests. Fire temperatures varied among acorn plots, ranging from <79 to <371°C. After the burns, acorns were planted in trays with water-saturated vermiculite, exposed to 14 continuous hours of light daily, and maintained at 27°C on germination beds in a greenhouse. After three weeks we recorded the proportion of acorns germinating and the proportion of germinants with shoots. Our study indicated that patchy, low-intensity dormant season prescribed fire in upland hardwood forests reduced viability of white oak and northern red oak acorns located on the leaf litter surface, but did not generally affect acorns in the duff or mineral soil. Germination rates of both northern red oak and white oak acorns on the leaf litter surface decreased with increasing fire temperature. Shoot production by northern red oak germinants from acorns on the leaf litter surface (and less so in the duff), also decreased with increasing fire temperature. Acorns of both species on the leaf litter surface burned at temperatures ⩾204°C showed high mortality levels, with mortality virtually 100% at temperatures ⩾260°C. Fall burns, especially after a heavy acorn crop, could result in high acorn mortality, potentially impacting oak regeneration from seedlings for many years given erratic acorn production patterns among years and species. Frequent burning that reduces litter and duff depth could compromise availability of ‘safe sites’ where acorns are insulated from high fire temperatures. When oak ecosystem restoration is a goal, land managers should consider the timing and size of acorn crops, as well as the forest floor condition when determining the timing and frequency of prescribed burning.

Responses of northern red oak seedlings to lime and deer exclosure fencing in Pennsylvania

In Pennsylvania, two hypotheses compete to explain the chronic oak ( Quercus spp.) regeneration problem: excessive deer browsing and soil cation depletion. We tested these hypotheses by evaluating the effect of forest liming and deer exclosure fencing on northern red oak ( Quercus rubra L.) seedling growth and nutrition in five oak shelterwood stands in Pennsylvania over 6 years. In each stand, four planting plots were located inside a 2.4 m high woven wire fence and another four were established outside the fence. About 225 northern red oak acorns were planted in each plot in spring 2004. Dolomitic limestone was applied to randomly selected plots at rates of 0, 4.5, 9.0, and 13.5 Mg·ha–1during May 2004. There were no statistically significant (P ≤ 0.05) growth responses to lime applications. The only significant growth responses resulted from the fence versus no-fence treatment. A significant (P &lt; 0.003) fence × year interaction for seedling height and root collar diameter indicates differential impacts of deer browsing. By 2009, seedlings inside fences averaged 32 cm tall, while seedlings outside the fences averaged 17 cm. Similarly, root collar diameter averaged 6.6 mm outside the fences and 9.1 mm inside fences.

Assessing Viability of Northern Red Oak Acorns with X-rays:Application for Seed Managers

Rosa C Goodman, Douglass F Jacobs and Robert P Karrfalt

Assessing Viability of Northern Red Oak Acorns with X-Rays: Application for Seed Managers

We recently completed research that showed X-ray analysis was a better predictor of northern red oak (L. [Fagaceae]) acorn viability and early growth than was moisture content. X-ray image analysis is a rapid and nondestructive test of acorn viability—tissue desiccation can be readily quantified. This article describes how to bring this proposed seed test into practice, with the goal of improving nursery efficiency and quality of oak seedlings.

Evaluating desiccation sensitivity of Quercus rubra acorns using X-ray image analysis

Desiccation of northern red oak (Quercus rubra L.) acorns can impact seed viability. We examined use of X-ray image analysis of cotyledon damage in dried acorns to predict germination capacity and seedling vigor. Acorns collected from five half-sib sources were X-rayed before and after drying to one of four moisture content (MC) levels (30%, 25%, 20%, or 15%) or maintained as nondesiccated controls (35%–38% MC). X-ray images were scored qualitatively according to degree of cotyledon–cotyledon and cotyledon-pericarp separation. Following sowing, acorns were evaluated for number of days to reach each of three developmental stages (emergence of radicle, epicotyl, and first leaf flush) and growth for 80 d. Both MC and family significantly affected all variables. The percentage of acorns to reach each developmental stage, as well as final height and root-collar diameter declined with decreasing MC and dropped most notably between 20% and 15% MC. X-ray separation scores more effectively predicted the percentage of acorns to reach each of the first three developmental stages than MC level (R2 = 0.49–0.63 vs. 0.40–0.59). Our results confirm the recalcitrant nature of northern red oak acorns and demonstrate the potential of X-ray image analysis to provide a rapid and nondestructive means to successful predict acorn viability.

Effect of Overstory and Understory Vegetation Treatments on Removal of Planted Northern Red Oak Acorns by Rodents

Abstract We documented patterns in the removal of planted northern red oak (Quercus rubra) acorns by rodents in red pine (Pinus resinosa) stands during a field study of oak regeneration in northern Lower Michigan. Each stand contained four overstory treatments assigned at random: clearcut, 25% cover, 50% cover, and uncut (control). Each level of canopy cover contained four randomly assigned understory treatments: shrub-layer removal, herb-layer removal, litter removal, and a control. Rodents removed acorns from 59% of the seed spots in the uncut control and 12% of the seed spots in the clearcut treatment. This five-fold difference was significant. Acorns were removed from 48% and 40% of the seed spots in the 75% and 50% canopy cover treatments, respectively. Differences in acorn removal between understory treatments were not significant. These results suggest that overstory vegetation treatments can influence the removal of planted acorns by rodents, which has implications for the use of direct seeding for artificially regenerating northern red oak.

Regeneration of Northern Red Oak in Relation to Ectomycorrhizae in Oak and Pine Stands after Overstory and Understory Manipulations

Abstract Growth of northern red oak (Quercus rubra L.) seedlings in relation to colonization by indigenous ectomycorrhizal (ECM) fungi was studied in oak and pine stands in northern Lower Michigan that were subjected to overstory and understory manipulations. Two stand types (oak and pine), three blocks of each stand type, four canopy cover treatments (clearcut, 25% cover (50% cover in the first year), 75% cover and uncut), and two understory treatments (shrub removal and untreated control) were involved in the experiment. Northern red oak acorns from a common seed source were sown in May 1991 to simulate natural regeneration. Seedling growth and its relation to percent ECM were evaluated for the first two growing seasons. A significantly larger root-collar diameter of northern red oak seedlings was found in pine stands than in oak stands for the first growing season (P &lt; 0.001). However, this difference could not be explained by overall ECM colonization. Seedling growth and ECM colonization were not affected by the shrub removal treatment during the first two growing seasons. In contrast, northern red oak seedling size and weight were strongly influenced by the overstory treatment, with lower levels of canopy cover resulting in larger seedlings. Seedlings had the greatest percent ECM in the partial cover treatment (25-50%) and the lowest percent ECM in the clearcut. After accounting for the effects of canopy cover, the relationship between total biomass of northern red oak seedlings and percent ECM was positively correlated (P = 0.001) during the first growing season and negatively correlated (P = 0.038) during the second growing season. A positive relationship between root/shoot ratio and percent ECM also existed in the first year (P = 0.003) in both oak and pine stands, but only in the oak stands in the second year (P = 0.039). These results indicate that ECM promoted more root development than shoot development, particularly underpartial canopy cover (25%-50%) treatments, where the greatest percent ECM and largest root/shoot ratio were found. Moreover, our results suggest that these partial canopy cover treatments provide a favorable balance between ECM abundance and northern red oak seedling development in both oak and pine stands on intermediate quality sites, and may lead to northern red oak regeneration success on such sites. North. J. Appl. For. 15(4):182-190.

REGENERATION OF NORTHERN RED OAK: POSITIVE AND NEGATIVE EFFECTS OF COMPETITOR REMOVAL

The decreased ability of northern red oak (Quercus rubra) to regenerate throughout its range in the eastern United States has important ecological and economic implications. We studied regeneration of northern red oak in oak and pine stands on moderately productive sites in northern Lower Michigan. Our objectives were (1) to investigate the hypothesis that regeneration of northern red oak is more successful in pine than in oak stands and (2) to test whether removal of potential overstory and understory competitors increases regeneration success on moderately productive sites. Northern red oak acorns and 2-yr-old nursery seedlings were planted in spring 1991 in three natural oak stands and three red pine (Pinus resinosa) plantations on comparable, moderately productive sites. Each stand contained four canopy cover treatments: clearcut, 25% cover (50% the first year), 75% cover, and uncut. Each canopy cover treatment contained four understory treatments: herb-layer removal, shrub-layer removal, litter removal, and control. Seedling survival, performance, and damage due to deer and late spring frosts were quantified along with vegetation characteristics, light, soil moisture, air temperature, soil temperature, and precipitation during the 1991 and 1992 growing seasons. Survival of seedlings was significantly lower in the pine than in the oak stands; the pine stands had a higher incidence of white-tailed deer browsing and lower levels of belowground resources than the oak stands. Canopy cover treatments produced a gradient from high stress (low light and soil moisture) and low disturbance (slight browsing and frost damage) in uncut plots to low stress (high light and soil moisture) and high disturbance (high browsing and frost damage) in clear-cut plots. Understory vegetation development and soil temperature were low in uncut plots, high in clear-cut plots, and intermediate in plots with partial overstory removal. Despite poor growth, the mortality of seedlings planted in uncut plots was low. In contrast, mortality was high and surviving seedlings exhibited good growth in clearcuts. Understory treatment effects were slight compared with overstory treatment effects. Results on our sites (1) do not support the hypothesis that regeneration success of northern red oak may be greater in pine than in oak stands and (2) indicate that the positive effects of removing potential competitors on seedling growth and physiological status may be compromised by simultaneous negative effects of browsing and frost damage.

Regeneration of Northern Red Oak: Positive and Negative Effects of Competitor Removal

The decreased ability of northern red oak (Quercus rubra) to regenerate throughout its range in the eastern United States has important ecological and economic implications. We studied regeneration of northern red oak in oak and pine stands on moderately productive sites in northern Lower Michigan. Our objectives were (1) to investigate the hypothesis that regeneration of northern red oak is more successful in pine than in oak stands and (2) to test whether removal of potential overstory and understory competitors increases regeneration success on moderately productive sites. Northern red oak acorns and 2-yr-old nursery seedlings were planted in spring 1991 in three natural oak stands and three red pine (Pinus resinosa) plantations on comparable, moderately productive sites. Each stand contained four canopy cover treatments: clearcut, 25% cover (50% the first year), 75% cover, and uncut. Each canopy cover treatment contained four understory treatments: herb-layer removal, shrub-layer removal, litter removal, and control. Seedling survival, performance, and damage due to deer and late spring frosts were quantified along with vegetation characteristics, light, soil moisture, air temperature, soil temperature, and precipitation during the 1991 and 1992 growing seasons. Survival of seedlings was significantly lower in the pine than in the oak stands; the pine stands had a higher incidence of white-tailed deer browsing and lower levels of belowground resources than the oak stands. Canopy cover treatments produced a gradient from high stress (low light and soil moisture) and low disturbance (slight browsing and frost damage) in uncut plots to low stress (high light and soil moisture) and high disturbance (high browsing and frost damage) in clear-cut plots. Understory vegetation development and soil temperature were low in uncut plots, high in clear-cut plots, and intermediate in plots with partial overstory removal. Despite poor growth, the mortality of seedlings planted in uncut plots was low. In contrast, mortality was high and surviving seedlings exhibited good growth in clearcuts. Understory treatment effects were slight compared with overstory treatment effects. Results on our sites (1) do not support the hypothesis that regeneration success of northern red oak may be greater in pine than in oak stands and (2) indicate that the positive effects of removing potential competitors on seedling growth and physiological status may be compromised by simultaneous negative effects of browsing and frost damage.

Water Uptake and Germination of Red Oak Acorns

When pericarps were intact, water was absorbed by Nuttall, pin, cherrybark, and northern red oak acorns, mainly through the vascular openings of the cup scar. Uptake through the pericarp was greater for acorns with a thin waxy coating on the pericarp than for acorns with a heavy waxy coating. Splitting or removing the pericarp greatly speeded uptake. Osmotically induced water stresses decreased germination of pin oak acorns, but germination was still 62% at 10 atm.