Short-term effects of hexazinone applications on woody species diversity in young loblolly pine (Pinus taeda) plantations

Forest Ecology and Management, 24 (1988) 183-189 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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Short-Term Effects of Hexazinone Applications on Woody Species Diversity in Young Loblolly Pine (Pinus taeda) Plantations B R U C E R. Z U T T E R 1 and

SHEPARD M.

ZEDAKER 2

1School of Forestry, Alabama Agricultural Experiment Station, Auburn University, AL 36849 (U.S.A.) eDepartment of Forestry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.) (Accepted 15 October 1987)

ABSTRACT Zutter, B.R. and Zedaker, S.M., 1988. Short-term effects of hexazinone applications on woody species diversity in young loblolly pine (Pinus taeda) plantations. For. Ecol. Manage., 24: 183189. Data from four studies determining the susceptibility of woody plant species to foliar applications of several rates of liquid hexazinone (Velpar L ®) were used to examine the short-term effects of the herbicide on woody species diversity in young loblolly pine plantations. Woody species diversity, as measured by four similarity measures between pretreatment and post-treatment species importance or diversity, decreased significantly with increasing hexazinone rate. Woody species shown to be among the most susceptible (sumacs (Rhus spp.) and oaks (Quercus spp.) ), and least susceptible (loblolly and other pines, and Vaccinium spp.) to hexazinone in previous herbicide screening trials decreased and increased in importance, respectively. Differences in similarity measures between the no-hexazinone treatment and the 1.0 × hexazinone rate (1.2 to 1.5 kg a.i./ha depending on the study location) 2 years after herbicide application were small.

INTRODUCTION H e r b i c i d e s are o f t e n applied to reduce c o m p e t i t i o n for available water, nutrients, a n d light in conifer p l a n t a t i o n s . In t h e s o u t h e a s t e r n U n i t e d States, h e x a z i n o n e is a herbicide o f t e n u s e d to release y o u n g p l a n t e d loblolly p i n e ( P i n u s taeda) f r o m c o m p e t i n g w o o d y v e g e t a t i o n (Gonzalez, 1985). W o o d y p l a n t species have b e e n s h o w n to v a r y greatly in t h e i r susceptibility to hexaz i n o n e ( N e a r y et al., 1981; Gonzalez, 1983; M c L e m o r e , 1983; Griswold et al., 1984; Miller, 1984). T h e g r e a t e r t o l e r a n c e o f loblolly p i n e to h e x a z i n o n e in c o m p a r i s o n to o t h e r w o o d y v e g e t a t i o n allows b r o a d c a s t a p p l i c a t i o n s of t h e herbicide for release (Gonzalez, 1983; Griswold et al., 1984).

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Since woody plant species differ in their susceptibility to hexazinone, species diversity within a stand may be altered as a result of herbicide application. Previous investigators have studied and reported herbicide effects (percent topkill, crown reduction, etc. ) on an individual-species basis and have not considered effects on species diversity. Potential reductions in species diversity contribute to accusations that silvicultural practices that favor the establishment and maintenance of monocultures create 'biological deserts'. This study was initiated to determine whether applications of liquid hexazinone in young loblolly pine plantations leads to an alteration in woody species diversity in the short term. Herbicide rates and changes in individual species were also considered. METHODS AND MATERIALS

Data used were from a study designed to examine the relationship of soil factors and efficacy of hexazinone formulations for release of loblolly pine (Minogue et al., 1988 ). This paper considers data from hexazinone foliar spray treatments (Velpar L ~ ) from four sites having the most diverse woody species compositions. Selected plantations were beginning their third (Prattville), fourth (Monticello, Douglas) or fifth (Pell City) growing season. A hexazinone prescription rate, denoted by 1.0 × , was assigned to each study location based on the results of soil textural analysis and plantation age. The prescription rate varied little among these four sites, ranging from 1.2 to 1.5 kg a.i./ha (loamy sand to loam surface soil textures). Foliar sprays were tested at 0.6 × , 1.0 × , 1.4 × and 2.0 × rates at each site; an untreated check represented the 0.0 × rate. The herbicide was applied using a C02-pressurized backpack sprayer calibrated to deliver 94 L / h a of total spray solution. A single Spraying Systems KLC SS-9 Fieldjet nozzle was mounted at a sufficient height to produce a 9-m spray swath. Applications were made in the spring when hardwood leaves were approximately 75% of full size. A randomized complete-block design was used to test the treatments at each of the four locations. Treatment plots were 9 m × 46 m, with a minimum of 3 m between plots. Evaluation of woody species was conducted in August of the year of herbicide application, and in August of the following year. Planted loblolly pines within a 3-m × 40-m subplot centered within the treatment plot were assessed for survival. Other woody vegetation, including naturally regenerated pines (loblolly, and shortleaf Pinus echinata Mill. ), was evaluated on six 2.0-m-radius circular plots centered on randomly selected planted loblolly pines. Individual rootstocks were classified by species or species group (hereafter referred to collectively as species) and assessed for height class and survival. A rootstock was defined as a hardwood stem or clump of stems which obviously originated from the same root system. For each evaluation date, data from all blocks were used to estimate density

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(number of rootstocks/ha) and dominance (sum of rootstock heights, m/ha) for each species, by treatment and across treatments, on a site. Data collected the August after herbicide application was used to estimate pretreatment woody species composition by including both living and dead rootstocks in the calculation of density and dominance. It is understood some rootstocks may have been omitted since the evaluation took place a few months after herbicide application. Relative density and relative dominance were computed, summed, and divided by 2 to yield a decimal importance value for each species. Simpson's index (sI) and the Shannon-Weiner index (H') were calculated using these importance values as follows (Whittaker, 1975): sI= ~ P~ and H ' = - ~, Pi log Pi t

t

where Pi = relative importance value, in decimal form, for the ith species. These indices, along with the number of species, were used to represent the alpha or within-habitat diversity for each treatment and across each site (Whittaker, 1975). Diversity increases with increases in H' and decreases with increases in sI. Two beta diversity indices, coefficient of community and percent similarity (cc and PS; Whittaker, 1975), were determined for each treatment on a site, based on pretreatment and post-treatment importance values. These indices are given by Whittaker (1975) as: CC "~ 2 S a b / ( S a "]- S b )

and PS-=-I--0.5~/IP.,-Pbi I where Sa is the pretreatment number of species, Sb is the post-treatment number of species, Sab is the number of species common to both pre- and posttreatment, Pai is the pretreatment decimal relative importance value for the ith species and, Pb~ is the post-treatment decimal relative importance value for the ith species. These indices provide a comparison of diversity before and after treatments. Changes in both Simpson's and Shannon-Weiner indices were also determined for each treatment by site. Using the four sites as blocks, Page's distribution-free test for ordered alternatives (Hollander and Wolfe, 1973) was used to test the null hypothesis that hexazinone treatment had no effect on diversity versus the alternative hypothesis that woody species diversity decreased with increasing hexazinone rate. Separate tests were made considering the diversity of all species and of arborescent species only. Changes in individual species were examined by plotting pretreatment against post-treatment importance values.

186 R E S U L T S AND D I S C U S S I O N

Six species accounted for 77-86% of the pretreatment woody species density at each site (Table 1 ). Rootstock density before treatment was in the range of approximately 11-14 000 rootstocks/ha, with average heights of 1.27-1.99 m. The sites were reasonably similar in one or more diversity measures prior to spraying {Table 1). The Monticello site had a greater number of species, 23 versus 17 or 18 on the other sites, but 9 of the least-frequent species on this TABLE1 Pretreatment woody species composition, total rootstock density, mean rootstock height, Simpson's index and Shannon-Weiner index at four locations in a study examining the effects of hexazinone applications on woody species diversity in young loblolly pine plantations in the southeastern United States Species/ species group

Percentage of total rootstocks Douglas, TX

Rhus spp. Pinus spp. Quercus spp. (red) Vaccinium spp. Nyssa sylvatica Marsh Carya spp. Sassa[ras albidum (Nutt.) Liquidambar styraciflua L. Diospyros virginiana L. Quercus spp. (white) Cornus spp. CaUicarpa americana L. Ulmus spp. Crataegus spp. Prunus spp. Myrica ceri[era L. Acer rubrum L. Ilex opaca Ait. Ilex vomitoria L. Fraxinus spp. Oxydendron arboreum (L.) DC Other species Total number of species Rootstocks/ha Mean height (m) Simpson's index Shannon-Weiner index

Monticello, MS

15 15 21 2 2 1 <1

18 17 11 7 11 10 10

7

2 3

4 12 11 <1 <1

1 1 <1 <1 3 1

4 <1

1 3

2

<1

Pell City, AL 35 12 8 15 10 4 2 <1 6 4 2 <1 1 1

Prattville, AL 17 8 21 15 15 9 6 3 1 1

1 1

<1

<1

<1

1 <1

<1 <1

23 13972 1.76 0.12 2.35

17 10934 1.80 0.17 2.10

18 14342 1.27 0.13 2.21

4 1 18 13739 1.99 0.13 2.27

187 site together comprised less t h a n 1% of the total composition. Species dominance (Simpson's index) was greater on the Pell City site due to the high proportion of sumacs (Rhus spp.; Table 1). For all four similarity measures, the null hypothesis t h a t hexazinone treatm e n t has no effect on species diversity was rejected (at P= 0.05 for coefficient of community, P = 0 . 0 1 for other measures) when either all woody species or only arborescent species were considered. Significant decreases in coefficient of community and percent similarity with increasing hexazinone rate indicate a decrease in similarity between p r e t r e a t m e n t and post-treatment species composition {Table 2). Significant increasing positive changes in Simpson's index and increasing negative changes in the S h a n n o n - W e i n e r index as hexazinone rate increased suggests the decrease in similarity following hexazinone application to be due to an increase in species-dominance concentration and a reduction in species equitability. In general, pines (greater t h a n 95% planted loblolly pine on all sites) and Vaccinium spp. increased in importance, while oaks (Quercus spp.), and sumacs decreased in importance following hexazinone applications (Fig. 1 ). This is not unexpected as herbicide susceptibility trials have shown oaks and sumacs to be among the most susceptible species, and pines (loblolly, shortleaf) and vacciniums to be among the least susceptible species to various formulations of hexazinone (Neary eta!., 1981; Nelson et al., 1982; Gonzalez, 1983; McLemore, 1983; Griswold, 1984; Miller, 1984). Species intermediate in susTABLE2 Similarity measures of pretreatment and post-treatment woodyspecies diversity for each of five hexazinone treatments in a study examining the effects of hexazinone applications on woody species diversity in young loblollypine plantations in the southeastern United States Species included

H e x a z i n o n e Similarity index rate

Change relativeto pretreatment diversity

Coefficient of community

Percent Simpson's Shannonsimilarity index Weiner index

All species

0.0 X 0.6 × 1.0 X 1.4 X 2.0 X

0.950 0.954 0.923 0.904 0.856

0.895 0.836 0.781 0.774 0.726

- 0.004 - 0.031 0.001 0.025 0.066

0.031 - 0.003 0.068 - 0.104 - 0.279

Arborescent species only

0.0 X 0.6X 1.0 X 1.4 X 2.0 ×

0.972 0.985 0.955 0.954 0.889

0.927 0.907 0.879 0.812 0.702

- 0.007 0.013 0.035 0.052 0.129

0.027 -0.039 - 0.109 - 0.122 - 0.326

188 3025

P 0.0 x

/

1

:

1

1.0X

p

1:1

2.0x

1:1

20 >i b.. (/)

15

0a.

10

S Q

N

I S

I 10

V

R N

| 15

I 20

I 25

0

i 5

J 10

115

i 20

i 25

I 5

[ 10

R

i 15

i 20

! 25

P R E - IV (%)

Fig. 1. Comparisons of average pretreatment (PRE-IV)and post-treatment (POST-IV)importance values of individual species for 0.0 × , 1.0 × and 2.0 × hexazinone rates across four locations in a study examining the effects of hexazinone applications on the diversity of woody plant species in young loblolly pine plantations in the southeastern United States. (Plotted symbols denote first letter in genus name for a species or species group: C=Carya spp., D--Diospyros virginiana, L = Liquidambar styraciflua, N = Nyssa sylvatica, P = Pinus spp., Q = Quercus spp. (red), R = Rhus spp., S = Sassafras albidum, V = Vaccinium spp.)

ceptibility such as hickories (Carya spp.) and blackgum (Nyssa sylvatica) changed little in importance. In applying hexazinone to release loblolly pine, the goal of the forest manager is to increase the importance of loblolly pine by killing woody competitors. In this study, the importance of loblolly pine increased with increasing hexazinone rate, and was accompanied by a decrease in diversity of woody species. It is difficult to say conclusively if the observed changes in woody species diversity are of a long-term nature, and if they have significant implications for forest management. It is, perhaps, most meaningful to compare results for the 1.0 X hexazinone rate to that for the 0.0 X rate, as a rate close to 1.0 X would be applied in practice. Negative effects on diversity are substantially lower at 1.0 X rate compared to the 2.0 X rate (Table 2). Differences in similarity measures between no hexazinone treatment and the 1.0X rate are small (Table 2). Studies involving more intensive disturbance of vegetation than in this study have demonstrated the ability of woody vegetation to become established rapidly following disturbance. Five growing seasons after cessation of yearly herbicide treatment following harvesting of hardwoods in the Allegheny mountains in the eastern United States, Shannon-Weiner and Simpson's indices for woody species were 2.42 and 0.11, respectively (calculated from Table 4 of Kochenderfer and Wendel, 1983). In slash pine (Pinus elliottii) flatwoods two growing seasons after clearcutting, chopping, bedding, and planting slash pine, Shannon-Weiner and Simpson's indices for woody species were 2.54 and 0.14, respectively, based on percent

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ground cover, and 2.83 and 0.08, respectively, based on frequency (calculated from Table 1 of Conde et al., 1983a). Woody species diversity in a companion study 2 years after more-intensive site preparation (burning, windrowing, discing, bedding) was somewhat lower, with values of Shannon-Weiner and Simpson's indices of 1.61 and 0.27, respectively, based on percent ground cover, and 2.06 and 0.15, respectively, based on frequency (calculated from Table 1 of Conde et al., 1983b). Given the above observations and the small differences in similarity measures observed in the present study, it appears unlikely that significant longterm changes in woody species diversity would result from a single hexazinone application on sites similar to those used in this study. However, carefully designed, long-term studies to examine the effects of hexazinone on woody species diversity are needed to provide a more conclusive answer. Comparisons with other herbicide treatments and manual cutting would also be very useful.

REFERENCES Conde, L.F., Swindel, B.F. and Smith, J.E., 1983a. Plant species cover, frequency, and biomass: early responses to clearcutting, chopping, and bedding in Pinus eUiottii flatwoods. For. Ecol. Manage., 6: 307-317. Conde, L.F., Swindel, B.F. and Smith, J.E., 1983b. Plant species cover, frequency, and biomass: early responses to clearcutting, burning, windrowing, discing, and bedding in Pinus eUiottii flatwoods. For. Ecol. Manage., 6: 319-331. Griswold, H.C., Fitzgerald, C.H., Presnell, R.F. and Gonzalez, F.E., 1984. Pine release with aerially applied liquid hexazinone. Proc. South. Weed Sci. Soc., 37: 230-236. Gonzalez, F.E., 1983. Southern pine release with hexazinone formulations. Proc. South. Weed Sci. Soc., 36: 223-237. Gonzalez, F.E., 1985. Hexazinone and sulfometuron methyl herbicides for pine forestry. Proc. South. Weed Sci. Soc., 38: 146-156. Hollander, M. and Wolfe, D.A., 1973. Nonparametric Statistical Methods. Wiley, New York, 503 pp. Kochenderfer, J.N. and Wendel, G.W., 1983. Plant succession and hydrologic recovery on a deforested and herbicided watershed. For. Sci., 29" 545-558. McLemore, B.F., 1983. Four formulations of hexazinone for controlling hardwoods in pine stands. Proc. South. Weed Sci. Soc., 36: 212-217. Miller, J.H., 1984. Soil active herbicides for single stem and stand hardwood control. Proc. South. Weed Sci. Soc., 37: 173-181. Minogue, P.J., Zutter, B.R. and Gjerstad, D.H., 1988. Soil factors and efficacy of hexazinone formulations for pine release. Weed Sci., (in press). Neary, D.G., Bush, P.B. and Douglas, J.E., 1981.2-, 4- and 14-month efficacy of hexazinone for site preparation. Proc. South. Weed Sci. Soc., 34: 181-191. Nelson, L.R., Gjerstad, D.H., Knowe, S.A. and Glover, G.R., 1982. Auburn Univ. Silvic. Herb. Coop. Annu. Rep., 1981, 110 pp. Whittaker, R.H., 1975. Communities and Ecosystems {2nd edition). MacMillan, New York, 385 pp.