**Vegetative Response to Weed Control in Forest Restoration**

John-Pascal Berrill and Christa M. Dagley *Humboldt State University California, USA* 

#### **1. Introduction**

362 Herbicides – Properties, Synthesis and Control of Weeds

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Longleaf pine (*Pinus palustris* Mill.) stands once occupied an estimated 24 million ha in the southeastern USA (Stout & Marion, 1993). Fire suppression, timber harvest, and land conversion reduced its extent to around one million ha (Outcalt & Sheffield, 1996). In recent times, widespread interest in restoring longleaf pine ecosystems or planting the species for timber production has motivated private landowners, industrial forest owners, and public agencies to establish more longleaf pine forest. Over 33 million longleaf pine seedlings were produced for the 2005-2006 planting season in the southeastern United States (McNabb & Enebak, 2008), and 54 million produced in 2008-2009 (Pohl & Kelly, 2011).

Longleaf pine ecosystems are fire-adapted and support a diverse understory plant community when ground fires are frequent (Peet & Allard, 1993). Longleaf pine seedlings germinate and develop into a grass-like clump, and later transition from this "grass stage" to become woody saplings. Seedlings in the grass stage resist fire, but become vulnerable to fire upon emergence from the grass stage until height growth elevates their terminal bud beyond reach of fire and their bark thickens (Boyer, 1990). Early fire resistance is thought to be an adaptation to frequent fire. During the grass stage, seedlings invest energy in root development in preparation for rapid shoot extension upon emergence. This strategy for reoccupying disturbed sites gives the slower-growing longleaf pine a competitive advantage over less fire-hardy pines and hardwood competitors (Outcalt, 2000). However, in the absence of fire, longleaf pine seedlings are quickly overtopped by competing vegetation. Therefore rapid restoration of longleaf pine forests will necessarily involve some disturbance of competing vegetation. Hardwood regeneration is usually prolific following disturbances such as removal of forest cover. A suite of hardwood species regenerate as stump sprouts and root suckers, developing quickly from established root systems. Grasses and vines also develop quickly after disturbance in the warm humid climate of southeastern USA. Various forms of above- and belowground competition impact on survival and growth of planted longleaf pine seedlings (Harrington et al., 2003; Pecot et al, 2007) and other pine species (e.g., Richardson et al., 1996b; Amishev & Fox, 2006).

Tools available for control of competing vegetation in longleaf pine forest restoration include prescribed fire, mechanical methods, and chemical weed control with herbicides. Prescribed fire most closely mimics the natural disturbance regime in longleaf pine forests,

Vegetative Response to Weed Control in Forest Restoration 365

iv. What is the response of weeds to the various treatments? How quickly did each type of

The restoration experiment was established at four disturbed sites on the 1,900 ha Hitchiti Experimental Forest (N 330 02' W 830 42') in Jones County, Georgia, USA. Southern pine beetles (*Dendroctonus frontalis* Zimmerman) had killed patches of even-aged conifer plantation throughout the forest in 2007. The kill areas totalled 10% of the forest area. Salvage harvesting in 2007 was followed by broadcast burning that consumed most of the scattered woody debris and residual hardwoods. Fire failed to carry through some areas due to lack of fuels. Containerized 1-0 'mountain variety' longleaf pine seedlings were hand planted in late March 2008 at a spacing of approximately 3.65 x 3.65 m (740 stems/ha).

Vegetation naturally regenerating throughout the study sites consisted primarily of hardwood stump sprouts and root suckers, vines, forbs, and various grasses. Natural regeneration of 22 tree species was recorded, including an abundance of dogwood (*Cornus florida* L.), loblolly pine (*P. taeda* L.), persimmon (*Diospyros virginiana* L.), sweetgum (*Liquidambar styraciflua* L.), and water oak (*Quercus nigra* L.). Five shrub species, 49 forb species, and eight vine species were recorded. The most common forb was American burnweed (*Erechtites hieracifolia* (L.) Raf.). Throughout the four sites selected as experimental replicates for the restoration study, muscadine grapevines (*Vitis rotundifolia* Michx.) were

Elevation of the four study sites ranged from 120-150 m above sea level. Soils were classified as a mixture of Davidson and Vance soil series with remnants of loamy surface layer over clay subsoil. The rolling hills were incised by a series of narrow, shallow gullies (Brender 1952). Before the beetle attack in 2007, the four study sites were forested with planted stands of loblolly pine 24-100 years old. Site index ranged from 24.4 m to 27.4 m at base age 25

Climate at the study site is humid and warm in summer months, and cool in winter. Monthly average low temperatures range from -1°C in January to 19°C in July, and monthly highs range from 13°C in January to 32°C in July. Extreme temperatures were the record high of 40°C in July 1986 and the record low of -20°C in January 1985. The average annual rainfall of 1180 mm is distributed throughout the year; March being the wettest month with 140 mm, and October the driest with 70 mm average monthly rainfall (www.weather.com).

One experimental replicate block was established in each of four beetle-killed areas at different locations across the forest. Within each replicate block (study site), four treatment plots were established. The 25 x 25 m square treatment plots were surrounded by 4 m wide buffers. Treatments applied to each plot were either mechanical weed control, chemical weed control (repeated in two plots), and control (i.e., no weed control). In a split-plot arrangement, each chemical weed control treatment measurement plot (considered the experimental unit for main treatments) was divided into approximately 12 replicates of

and tree seedling response in terms of both survival and growth?

weed (grasses, vines, woody vegetation) develop after treatment?

abundant and expanding laterally to occupy the disturbed sites.

years for loblolly pine (Clutter & Lenhart, 1968).

**2.1 Experimental design** 

**2. Study sites** 

More specifically, what is the trade-off between size of treated area (termed 'spot size')

but it may not carry in areas with insufficient quantity or quality of fuels, and it may not be appropriate or acceptable on some ownerships. Mechanical weed control methods include portable saws and machine-mounted mowers or masticators. These methods are more expensive than prescribed fire treatments, but can have similar effects: competing vegetation is disturbed above ground but not always killed; much of it re-sprouts. Herbicides can provide effective and economical control of competing vegetation, but their use may not be appropriate or acceptable in some areas amid concerns over effects on non-target organisms, movement and drift, and persistence in the environment. Fire or broadcast herbicide treatments can eliminate live vegetation cover, exposing soil to erosive forces and temporarily reducing biodiversity. Applying herbicide in spots as opposed to broadcast applications has the advantage of reducing chemical usage while maintaining some continuity of vegetation cover and preserving biodiversity between treated spots (Richardson et al., 1996a).

Research into longleaf pine forest establishment and weed control has focused on the Coastal Plain region of the southeastern USA. Field research on the Coastal Plain indicated that mechanical weed control treatments were inferior to chemical weed control in terms of enhancing longleaf pine seedling survival and growth (Knapp et al., 2006). Chemical weed control with herbicide has proven effective in several longleaf pine restoration studies on the Coastal Plain (Brockway & Outcalt, 2000; Ramsay et al., 2003; Knapp et al., 2006; Haywood, 2007; Freeman & Shibu, 2009; Shibu et al., 2010). Longleaf pine is native to the Coastal Plain, but also occurs naturally in the mountainous regions further inland, and across the Piedmont Region. The Piedmont is a physiographic region extending from the State of New Jersey down to central Alabama, spanning over 200,000 km2 of rolling foothills between the Appalachian Mountains and the Coastal Plain (Anon, 2000). Little has been reported on longleaf pine restoration in the Piedmont, but restoration experiments have been established (Berrill & Dagley, 2009).

Data from a replicated field experiment established on degraded Piedmont forest sites are presented here. To our knowledge no other experiment simultaneously addresses questions of repeat herbicide applications versus single treatments each of varying spot sizes, and compares all these weed control treatments to non-herbicide management options. We established non-contiguous single-tree plots in a randomized complete block design with multiple treatment levels nested in a split-plot arrangement within contiguous fixed-area treatment plots. Our objective was to determine the influence of frequency and extent of chemical weed control on planted trees and competing vegetation using commonly-used, widely-available herbicides, and to compare herbicide treatments with mechanical weed control and a no-treatment control. Specifically, we sought to answer the following four questions:


More specifically, what is the trade-off between size of treated area (termed 'spot size') and tree seedling response in terms of both survival and growth?

iv. What is the response of weeds to the various treatments? How quickly did each type of weed (grasses, vines, woody vegetation) develop after treatment?
