A Case Study
A degraded longleaf pine sandhill community within Wekiva Springs State Park in Orange County,
Florida, was the location of an experimental application of hexazinone. Aerial photographs from the
1950s showed that the area had once been characterized by an open stand of longleaf pine and
scattered turkey oak. Park records indicated the site had not been burned for 40 years, because fire
control and exclusion was park policy until the 1980s. As a result of these management practices, a
sparse overstory of longleaf pines was scattered over the site. Herbaceous ground cover was spotty
and the growth of isolated wire grass clumps was inhibited by the accumulation of turkey oak leaf litter.
In this study, three different rates of liquid hexazinone (0.42, 0.84 and 1.68 kg/ha) were applied in 1991
to estimate hexazinone effects on plant community restoration. The chemical was applied with a
backpack-mounted Solo spot gun in a 1m-x-1m-square grid pattern on 12 plots in Wekiva Springs
State Park. Plants within permanently marked plots were measured before and one year after
hexazinone application. Trees were marked with paint and their diameters were measured. The basal
area of pineland threeawn was calculated by measuring right-angle diameters of clumps at the ground.
The relationship between basal area and aboveground biomass (in dry weight) was determined by
clipping and weighing the vegetative growth of a portion of the plants measured for basal diameter.
Oak Mortality from Application
Increasing the rate of hexazinone resulted in increased mortality of oaks ( Table 1).
Susceptibility to the herbicide, however, also was related to tree diameter. There was a
trend toward increased mortality with decreasing diameter of oaks. The largest oaks
(14-16 cm in diameter) survived even the highest application rate, 1.68 kg/ha. Survival of
these large oaks is desirable from a management perspective, since some oaks with larger
diameters are a natural part of longleaf pine sandhill communities and are known to
provide both valuable food and nesting sites for many species of wild.
|Table1. Mean Oak (Quercus spp.) mortality one year following hexazinone applications.|| || |
|Rate (kg/ha)||Siameter (cm)||Mortality (percent)|
The response of the understory to hexazinone treatment was favorable, which again
demonstrates the potential applicability of this chemical in longleaf pine sandhill
restoration. Oak seedlings died and grasses became more dominant, following increasing
rates of hexazinone application. Although there was a notable development of
saw-palmetto on the plots treated with hexazinone, the plant was not present on the
untreated plots; therefore, no comparisons could be made. Potential stimulation of
saw-palmetto through hexazinone treatment could raise concerns among forest managers.
Because saw-palmetto is a natural component of scrub and xeric hardwood forests, its
apparent tolerance to hexazinone will not affect sandhill restoration. Understory Response
The response of longleaf pine to hexazinone was a chief point of interest for this study.
Longleaf pine seedlings dramatically increased on all plots, including those with the lowest
treatment rate. On plots with the highest treatment rate, low panicums ( Dicanthelium
spp), foxtail grasses ( Setaria spp.) and Paspalum spp. all increased or invaded the site
following hexazinone treatment. An expansion in early successional stage grasses suggests
that the resources on the site (e.g., minerals and nutrients) were released for use by the
understory plants. Vegetative cover of forbs also spread on all the treatment plots, mainly
because of an increase in several legume species. Results of this case study illustrate that
herbaceous species were not eradicated after hexazinone treatment and that non-oak
woody species persisted.
Response Pineland threeawn was a focal point for this study because of its importance to
the functioning of the sandhill ecosystem's natural fire regimen. Field measurements one
year following herbicide treatment showed pineland threeawn foliar cover had expanded
in proportion to increasing application rate, experiencing up to a fourfold increase at the
1.68 kg/ha rate ( Table 2). Responses in biomass were not as consistent, however, and
actually indicated a loss of biomass at the lowest and highest rates. This was probably the
result of direct contact between the plants and the herbicide, which could have caused
top-kill of the vegetation.
Proportional (percent) change in biomass and foliar cover of pineland threeawn |
Aristida stricta) one year after hexazinone treatment.
| || |
|Hexazinone Rate (kg/ha)||Biomass (percent change)||Foliar Cover
The + symbol represents increase and the - symbol represents decrease.|| || |