Fire Ecology of Seeds from Rubus Spp.: a Competitor during Natural Pine Regeneration’

Air-dried blackbeny (Rubus spp.) fruits were placed at three depths in a reconstructed forest floor and subjected to a simulated prescribed summer bum. Within the forest floor, fruits were placed on the L layer, at the upper-F/lower-F interface, and at the lower-F/mineral-soil interface. Wind for a headfire was generated by electric boxf ans . Extracted seed viability was assessed during each of six 30or 60day germination tests that alternated with 30 or 60 days of cold stratification over a period of 18 months. As depth within the forest floor increased, germinative capacity of b l a c k b e r r y s e e d s i n c r e a s e d . F o r b l a c k b e r r i e s placed on t h e L l a y e r a n d a t t h e u p p e r F / l o w e r F i n t e r f a c e d u r i n g presctibed b u r n i n g , c u m u l a t i v e s e e d g e r m i n a t i o n a v e r a g e d o n l y 0 . 0 3 a n d 0 . 3 3 p e r c e n t , r e s p e c t i v e l y . A t t h e l o w e r F / m i n e r a l s o i l interface, mean seed germination did not differ (P= 0.74) from the 23 percent achieved by unburned control seeds. INTRODUCTION Forestry benefits from prescribed burning include: site or seedbed preparation, control of unwanted vegetation, disease control, thinning of dense young pine stands, increased growth and yield of pines, and improvementof wildlife habitat (Crow and Shilling 1980, Davis 1959). Therefore, prescribed burning continues to be widely used in southern pine management. Prescribed burning can have positive and negative effects on wildlife habitat by increasing certain essential nutrients and palatability of forage, initially reducing leafy biomass followed by increases, and initially decreasing fruit yields followed by increases (Landers 1987). Consequently, it is important to determine the effects of prescribed fire on early successional plant species that both hinder natural pine regeneration and contribute wildlife habitat. Blackberry (Rubus spp.) was chosen for this investigation because it occurs throughout the Southeastern U.S., is a predominant herbaceous vegetation component in naturally regenerated pine stands (Cain 1991, Shelton and Murphy 1994) and is an important food source for wildlife (Landers 1987, Matthews and Glasgow 1981). Because blackbeny seeds have hard, impermeable seedcoats, germination has been improved when seeds are scarified with concentrated sulfuric acid for 20 to 60 minutes (Brinkman 1974). Sumorization, or heat pretreatment of seeds before they germinate (Barbour and others 1987), may also enhance germination of blackberry seeds. The purpose of the present investigation was to experimentally determine if germination of seeds from airdried blackberry fruits might be enhanced when subjected to a simulated prescribed summer burn, depending upon vertical stratification of the fruits in a reconstructed forest floor. METHODS The study was located in southeastern Arkansas. The soil, a Sacul loam (clayey, mixed, thermic, Aquic Hapludult), is described as a moderately well drained upland soil with a site index of 80 ft for loblolly pine (Pinus faeda L.) at age 50 years (USDA 1976). Within a pine seed-tree stand, an area was cleared of vegetation down to mineral soil and a 5by 7-ft bum bed was framed with steel railings. Soil was leveled within the bed and allowed to settle 6 months at which time a forest floor was reconstructed on the bed using procedures developed by Shelton (1995). In early July 1996, 7 days before burning. undisturbed forest floor material was obtained from beneath a closed forest canopy 300 ft from the bum site, where pine basal area averaged 90 ft* per acre. The forest floor was typical of similar stand conditions found elsewhere in the South (Switzer and others 1979). To facilitate reconstruction on the bum bed, forest floor material was collected in three layers L, upper F, and lower F using 1.3 f? sampling frames. The L layer refers to the litter layer consisting of unaltered dead remains of plants (Pritchett 1979). The fermentation (F) layer was immediately below the L layer and consisted of fragmented, partly decomposed organic materials that were sufficiently preserved to permit identification as to origin (Ptitchett 1979). For this experiment, the F layer was subdivided into upper and lower zones based on visual evidence of decay. The undisturbed L layer averaged 0.24 in. in thickness: the upper F layer averaged 0.18 in.; and the lower F layer averaged 0.65 in. Each layer was removed separately: then layers were transferred from the undisturbed forest floor in paper bags and reconstructed on the bum bed during the day of removal. Within the bum bed, a 3by 5.2-ft interior plot was subdivided into twelve 1.3 f? cells (replications) for placement of the reconstructed forest floor and blackberry fruits. A reconstructed forest floor ensured uniform fuel conditions for burning (Hunger-ford and others 1994) and uniform litter layers for placement of blackbeny fruits. Fresh blackberry fruits for the study were obtained in late June 1996 from forested pine stands in southeastern Arkansas, north-central Louisiana, and southwest Mississippi. Fruits were collected from a minimum of 25 blackberry canes per geographic location. These fruits were air-dried on wire screen at room temperature from the time of collection until the burn date. For each replicated cell in the burn bed, 12 blackberry fruits were used, with four fruits taken from each of the three geographic areas. To relocate ‘Paper presented at the Tenth Biennial Southern Silvicultural Research Conference, Shreveport, LA, February 16-18, 1999. ‘Research Foresters, USDA Forest Service, Southern Research Station, Monticello, AR 71656. all fruits per treatment cell after burning, the fruits were glued at l-in. intervals onto fiberglass cord with hightemperature silicone. This process was done 24 hours before the scheduled burn to permit the glue to cure. At the time of burning, fruit moisture content averaged 16 percent (oven-dry basis). Just before fire ignition, three fiberglass cords containing four blackberry fruits each were transferred to the center of 12 bum cells at one of three randomly assigned litter depths in the reconstructed forest floor. Fruits were placed on the L layer, at the upper-F and lower-F interface, or at the lower-F and mineral-soil interface (fig. 1). Blackberry Placement Figure l--Before prescribe burning, blackberry fruits were placed at three depths within the forest floor on the L layer. within the F layer, and at the lower-f and mineral-soil interface. Prescribed burning was conducted on July 8, 1996 (table 1). Wind for the simulated fire was provided from two 20-in. electric box-fans positioned side-by-side at ground level. Fan-blade rotation was varied during burning to maintain a constant wind speed at the fire front. Fuel burned with the wind (headfire), and wind speed was determined using an electronic Turbo-Meter@ wind speed indicator. While burning was in progress, flame lengths were ocularly estimated to the nearest 0.5 ft. Fireline intensity was calculated from flame lengths (Byram 1959). To accurately measure temperatures generated by the fire, Tempil@ temperature indicator pellets with known melting points were placed atop the burning litter. The melting temperature for these pellets ranged from 119 to 1490 “F. To determine fuel moisture, a separate 3% subplot containing a reconstructed forest floor was set up at the bum site. Immediately after burning, three unburned litter samples were taken from this subplot within each of the three litter layers in proportion to the weight of each layer. Moisture determination was on an oven-dry basis. After the burn, four 1-V samples of residual litter were taken from within the burn bed to determine the weight of this unburned material on an oven-dry basis. For each treatment (fruit location) and replication, blackberry fruits were removed from the fiberglass cords after burning Table l-Fuel and weather conditions during a simulated prescribed summer bum in southeastern Arkansas Fuel and weather variables Values Date of bum Days since last precipitation Amount of most recent precipitation (inches) Time of burning (hours DST) Dry bulb temperature (“F) Relative humidity (percent) Wind direction Wind speeda (mph) ,Forest floor moisture (percent) L IayeP Upper F Lower F Forest floor weight (tons/acre) L layer Upper F Lower F Mean fire line intensityC (Btu/%see) Rate of spread (ft/min) July 8, 1996 2 1.10 1,020 96 49 From the South 5.5