Silvicultural Herbicides Research Articles

Detection and accumulation of environmentally-relevant glyphosate concentrations delivered via pulse- or continuous-delivery on passive samplers

Glyphosate is the most commonly used herbicide globally, which has contributed to its ubiquitous presence in the environment. Glyphosate application rates and delivery to surface water vary with land use. Streams in urban and agricultural catchments can experience continuous delivery of low concentrations of glyphosate and aminomethylphosphonic acid (AMPA), while their presence in forest streams occurs as an episodic pulse following silvicultural application. We assessed whether trace concentrations of glyphosate delivered as a 1-day pulse (mimic silvicultural applications) followed by flushing with deionized water would affect the detection of glyphosate or AMPA on novel passive samplers, Polar Organic Chemical Integrative Sampler with Molecular Imprinted Polymer (POCIS-MIP), compared with continuous delivery (mimic agricultural or urban applications). Within each delivery type, POCIS-MIP were exposed to seven treatment concentrations of Rodeo (equivalent to 0.0 to 1.84 μg glyphosate L−1). Experimental results demonstrate POCIS-MIP can detect differences in relative glyphosate concentrations above 0.115 μg L−1 (pulse-delivery) or 0.23 μg L−1 (continuous-delivery), but were unable to distinguish trace concentrations (i.e., < 0.115 or 0.23 μg L−1). Our results suggest POCIS-MIP may better retain glyphosate when delivered as a pulse than when delivered continuously, but both underestimated actual treatment concentrations by 46 to 56%. There is a need to demonstrate the field applicability of passive sampling methods to improve environmental monitoring of silvicultural herbicides, and our results demonstrate passive samplers were unable to distinguish lower concentrations, suggesting a limited utility for determining trace concentration levels such as those experienced during or immediately after silvicultural application.

Silvicultural herbicides and forest succession influence understory vegetation and nutritional ecology of black-tailed deer in managed forests

The increasing intensity of forestry practices, especially using herbicides to promote early survival and growth of planted conifers, has the potential to influence the abundance of nutritious, shade-intolerant forbs and shrubs on which black-tailed deer (Odocoileus hemionus columbianus) and other forest ungulates depend. Therefore, we compared summer forage resources and nutrition of female black-tailed deer in stands that received herbicide treatments paired with stands that did not in Douglas-fir/western hemlock forests of western Washington. We investigated how these responses changed as forests aged from early seral to canopy closure (2–20 years post-harvest) in both treatments, and also in unmanaged mid-seral stands (30–90 years old). We measured the biomass and nutritional quality of understory vegetation and the diet composition, diet quality and nutrient intake of tractable black-tailed deer within each paired stand. We found that herbicides did not affect the rate of closure of the overstory canopy, but influenced plant species composition of the understory vegetation and reduced total and non-conifer understory biomass, primarily in the first 9 years before canopy closure. Herbicides also reduced the biomass of two of the six plant functional groups, evergreen shrubs and deciduous shrubs and trees, but did not reduce forb, graminoid, or fern biomass. Herbicides reduced bite size, harvest rate, daily dry matter intake and daily digestible energy (DE) intake of deer, but increased dietary digestible protein (DP) content, and these effects occurred primarily in the first 3 years post-treatment. Biomass of plants suitable for black-tailed deer, and the deers’ bite rate, daily feeding time, dietary DE, and daily digestible DP intake did not respond to herbicides. Our yearling and sub-adult, non-lactating deer met requirements for yearling growth and adult maintenance for dietary DE in 78%, and DE intake in 27% of plots, but only met the higher requirements for lactating females in 5% of plots. All plots where daily DE requirements of lactation or growth were met were <10 years old. Deer were equally likely to obtain diets that satisfied requirements in treated and untreated plots, indicating that although herbicide application reduced understory biomass and DE intake, it did not explain whether deer were able to meet requirements. We also found that the biomass of deciduous shrubs and trees, forbs, and suitable forage was ~25 times greater in 4–9 year old stands than closed canopy stands (14–90 years old), indicating the importance of maintaining early seral habitat for sustaining populations of black-tailed deer.

Abiotic Factors Influence Surface Water Herbicide Concentrations Following Silvicultural Aerial Application in Oregon's North Coast Range.

ABSTRACTNontarget impacts of routine aerial silvicultural practices on surface water quality are not well documented. Thus, uncertainty remains regarding herbicide treatment effects on ecological and human health. To investigate factors that influence silvicultural herbicide concentrations in surface water and identify any potential risks, we conducted a 2‐year study that monitored multiple streams for herbicide residues following aerial application of glyphosate, clopyralid, sulfometuron methyl (SMM), and metsulfuron methyl (MSM). The monitored streams drain recently harvested forest lands that also serve as municipal water sources for nearby communities in western Oregon's north coast range. A paired watershed design targeted predicted episodic pulses with water samples collected before, during, and after herbicide application, and during the first posttreatment storm events. We report no relic herbicide detections in control or test streams. Aerial application of glyphosate, clopyralid, SMM, and MSM resulted in no detections in control streams and only trace, episodic concentrations in test streams. Across all test streams from both study years, maximum SMM and MSM detections (≤0.030 μg/L) consistently occurred during the first storm event at sampling locations closest to the treated harvest unit. Results indicate that proximity to the treatment site, time from application, and rainfall influence herbicide presence and concentrations in surface water. Furthermore, detections of trace SMM and MSM concentrations were more than 25 000‐fold and 60 000‐fold below federal human health safety benchmarks for chronic exposure, respectively. We provide empirical context for understanding surface water herbicide presence following aerial silviculture application under modern forestry best management practices and identify potential risk to ecological and human health. Integr Environ Assess Manag 2019;00:1–14. © 2019 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC)

Elk nutritional resources: Herbicides, herbivory and forest succession at Mount St. Helens

Concern exists about the effects that silvicultural herbicides, past herbivory, and forest succession may have on the carrying capacity for ungulates in the vicinity of Mount St. Helens in southwest Washington. We independently evaluated the effects of both operational herbicides and ungulate herbivory on biomass and available digestible energy of forages in a chronosequence of early-succession (ES) forest stands using a retrospective, paired-site design. We distinguished between trends in biomass by classifying species as avoided (used less than available) or accepted (used equal or more than available) forages for elk based on recent research involving foraging trials of hand-raised, tractable elk in western Oregon and Washington. Herbicide application reduced biomass of accepted forages (kg/ha) for only two years, whereas the initial reduction in avoided biomass persisted throughout the 13-year ES period that we evaluated. The reduction in avoided species was not associated with an increase in accepted species, which may be related to herbivory. We observed that forest canopy closure even at 10–13years was similar in stands treated and untreated with herbicides, suggesting herbicides did not shorten the seral window when palatable forages were abundant. Because dry matter digestibility of accepted species was generally higher than avoided species, digestible energy (DE, kcal/ha) available to elk was similar to trends in accepted and avoided biomass. An initial 2-year increase in modelled estimates of dietary DE in herbicide-treated stands in years 1–2 resulted from a rapid recovery of plants with high digestibility. In contrast to herbicide treatments, effects of ungulate herbivory on ES communities increased with stand age with a reduction of deciduous shrub height but not densities, reduced biomass of accepted but not avoided species, and reduced standing DE of accepted species outside exclosures. Despite the influence of herbicide applications, herbivory, or their interactions, nutritional resources for elk were equal or more available in ES stands than in mid- and late-succession stands, highlighting the importance of maintaining ES stands for elk in this region.

Plant diversity enhances moth diversity in an intensive forest management experiment.

Intensive forest management (IFM) promises to help satisfy increasing global demand for wood but may come at the cost of local reductions to forest biodiversity. IFM often reduces early seral plant diversity as a result of efforts to eliminate plant competition with crop trees. If diversity is a function of bottom-up drivers, theory predicts that specialists at lower trophic levels (e.g., insect herbivores) should be particularly sensitive to reductions in plant diversity. We conducted a stand-level experiment to test bottom-up controls on moth community structure, as mediated by degrees of forest management intensity. Using a dataset of 12,003 moths representing 316 moth species, moth richness decreased only slightly, if at all, as herbicide intensity increased (P=0.062); themoderatetreatment, which is most commonly applied in the northwestern USA, was estimated to have 4.72 (±2.14 SE, P=0.039) fewer species than the control. Structural equation modeling revealed strong support for an effect of herbicide on plant abundance, which influenced plant species richness and subsequently moth species richness. Moth species richness was associated with plant species richness and followed a power law function (z=0.42, P=0.006), which is surprisingly consistent with a recent large-scale experiment in agricultural systems, and provides support for bottom-up drivers of moth community structure. Moth abundance was not influenced by the direct effects of silvicultural herbicide treatments. Site-level effects and variation in pre-harvest vegetation communities resulted in residual broadleaf and herbaceous vegetation in even the most intensive treatment. Even at low densities, these residual deciduous and herbaceous plants supported higher than expected moth abundance and richness. We conclude that forest management practices that retain early seral vegetation diversity are the most likely to conserve moth communities.

Runoff of Silvicultural Herbicides Applied Using Best Management Practices

Nine small (2.2-2.9 ha) and four large (70 -135 ha) watersheds in East Texas, USA, were instru- mented to compare herbicide runoff under different silvicultural systems with best management practices (BMPs). Two treatments were evaluated: conventional, with clearcutting, aerial herbicide site preparation, and hand-applied banded herbaceous release; and intensive, in which subsoiling, aerial fertilization, and a 2nd-year aerial herbicide application were added. Herbicides were applied as operational tank mixes. The highest imazapyr concentration found in stream water was 39 gL 1 during the first storm after application (23 days after treatment (DAT)) and in-stream concentrations during runoff events dropped to 1 gL 1 in all streams by 150 DAT. The highest hexazinone concentration was 8 gL 1 for the banded application and 35 gL 1 for the broadcast application the following year and fell to 1 gL 1 in all streams by 140 DAT. The highest sulfometuron methyl concentration found during a runoff event was 4 gL 1 and fell to 1 gL 1 in all streams by 80 DAT. Approximately 1-2% of applied imazapyr and 1% of hexazinone and sulfometuron methyl were measured in storm runoff. Herbicide was found in streams during storm events only (all herbicides were 1 gL 1 in all true baseflow samples), and peak concentrations during runoff events persisted for relatively short times (24 h). These results suggest that silvicultural herbicide applications implemented with contemporary BMPs are unlikely to result in chronic exposure of aquatic biota; therefore, herbicide use under these conditions is unlikely to degrade surface waters. FOR .S CI. 59(2):197-210.

The effects of triclopyr and glyphosate on lichens

Two commonly used silvicultural herbicides (triclopyr and glyphosate) were examined for their effects on lichens in northeastern Ontario. One hundred 1 m 2 plots were randomly established throughout a single, open canopy forest stand with tree cover dominated by Picea mariana and Pinus banksiana and ground cover dominated by lichens. Herbicides were applied to the plots along a gradient of concentrations ranging from 0.71 to 6.72 kg acid equivalent ha −1. The pre- and 1 year post-application abundance (percent cover) of 25 lichen species in 1 m 2 plots were compared. Triclopyr and glyphosate reduced the abundance of 40% and 56% of the lichen species studied, respectively. Tolerance to these herbicides varied among lichen species and a cluster analysis was used to define four tolerance classes. Analysis of variance was used to compare lichen abundance among the tolerance classes and multivariate analysis (canonical correspondence) was used to explore variation in lichen abundance as constrained by species traits post-herbicide application. Except for Trapeliopsis granulosa, the most herbicide sensitive species were richly branched. The species with the highest mortality were Bryoria furcellata, Cladonia uncialis, and T. granulosa. In general, lichen response to herbicide treatments cannot be assessed using ecological units; species must be considered individually or, as proposed here, by tolerance class. Study results provide forest managers with improved understanding of the effects of herbicide applications on non-target organisms.

Toxicity, transport, and fate of forest herbicides

Public opposition to use of herbicides in forests typically centers around concerns over potential toxicity to wildlife. Characterization of the risk of silvicultural herbicides to wildlife requires an understanding of herbicide toxicity and environmental fate and transport. The fate and chemistry of herbicides and adjuvants within environmental media determine how and which organisms may be exposed and duration of those exposures. The nature of the toxicity of herbicides, adjuvants, and their decomposition products, and levels at which those toxic responses may be observed, determine which organisms or life stages may be most susceptible to any toxic effects and which exposure concentrations and durations can be considered safe. In general, herbicides most commonly used for vegetation management in forestry (glyphosate, triclopyr, imazapyr, sulfometuron, metsulfuron methyl, hexazinone) degrade quickly once they enter the environment and thus are neither persistent nor bioaccumulative. Because modern herbicides have been designed to target biochemical processes unique to plants, they exhibit a low level of direct toxicity to animals. When used according to label instructions, modern silvicultural herbicides pose little risk to wildlife.

Implications of silvicultural pesticides on forest soil animals, insects, fungi, and other organisms and ramifications on soil fertility and health

Sufficient information is available to show that pesticides can be properly applied and used safely in forest management. However, much of the information on the effects of pesticides on soil organisms is often based on methodologies involving soil processes (i.e., toxicology studies) rather than on individual groups of organisms. Because more than one group of soil organisms may carry out the same function, the effects of pesticides on a particular group can be missed. On the other hand, while laboratory test may show pesticide effects on one group under an array of controlled conditions, the same organism may respond differently in a field setting. Not much information is available on soil organisms resulting from combining various silvicultural herbicides or pesticides. Information is presented to show that with few exceptions, pesticides can affect soil organism populations, but the effects may be short-term with little noticeable long-term affect on soil health and quality.

The effects of glyphosate and triclopyr on common bryophytes and lichens in northwestern Ontario

The effects of two silvicultural herbicides (Vision®, Release®) on bryophytes and lichens were studied in a harvested boreal mixedwood ecosystem. A completely randomized design with 115 plots of 1 m2 allowed direct comparison between herbicides and their effects on community dynamics. Regression models were used to analyze the relationship between herbicide application rates (0.71-6.72 kg active ingredient/ha) and changes in bryophyte and lichen abundance and species richness for 2 years following herbicide application. Results showed that bryophyte and lichen abundance and species richness decreased after herbicide treatments. In general, herbicide applications reduced the diversity of forest mesophytes and weedy colonizers to an ecosystem with only a few species of colonizers. A combination of clustering techniques and ANOVA were used to divide bryophytes and lichens into three ecologically defined response groups: herbicide-tolerant colonizers, semi tolerant long-term stayers from dry open forest, and sensitive forest mesophytes.

Influence of alternative conifer release treatments on soil nutrient movement

Zero tension soil solution samplers were used to collect soil solution at 75 cm for one and one-half years after the following treatments: 1) helicopter applied Release® silvicultural herbicide (a.e. triclopyr @ 1.9 kg a.e. ha−1), and 2) helicopter applied Vision® silvicultural herbicide (a.e. glyphosate @ 1.5 kg a.e. ha−1); 3) motor-manual cutting (brush saws); and 4) control (no treatment). Results show no substantial treatment related differences in the movement of selected nutrients (total organic N, NH4+, NO3−; K, Ca) among these treatments. Mean nitrate concentration was less than 1.5 μg mL−1 throughout the sampling period and was marginally higher, but not statistically different, following release treatments. Key words: alternative conifer release treatments, brushcutting, calcium, ecosystem research, Fallingsnow Ecosystem Project, glyphosate, herbicides, nutrients, nitrogen, potassium, soil solution, tending, triclopyr, vegetation management

Short-term Performance of Two Sulfonylurea Herbicides: Efficacy, Seedling Survival, and Growth

Abstract Metsulfuron (ALLY or ESCORT), 36 and 72 g ai/ha (~ 0.5 and 1.0 oz ai/ac), and sulfometuron (OUST), 150, 300, and 450 g ai/ha (~ 2.1, 4.2, and 6.3 oz ai/ac), were applied to a northern New Brunswick clearcut (silty clay loam and silty clay soils) to reduce raspberry competition. Treatment, using skidder-mounted herbicide application equipment, occurred in May and August 1986, with planting of 2+2, bareroot, black spruce seedlings in June 1986 and in June 1987. Seedling survival and growth were measured yearly for 5 growing seasons after planting. By August 1991, raspberry cover was less in some treatments than in controls. Survival of seedlings planted 1 month after spring treatments was less than controls, and no significant stem volume increases were observed. Survival of seedlings planted approximately 1 yr after spring treatments was greater than that of control seedlings. Survival of seedlings planted after some summer treatments also was poor, and no significant stem volume increases were noted for seedlings planted after site preparation with sulfometuron. Optimal stem volume increases over control seedlings were observed for seedlings planted 1 yr after spring sulfometuron treatment. These increases occurred sooner than for seedlings planted 1 yr after spring metsulfuron treatment or after summer metsulfuron treatment. Fifth-year stem volume for these seedlings was correlated with raspberry cover (r² = 0.44), decreasing as raspberry cover increased. We conclude that the use of other less expensive silvicultural herbicides may provide equally effective raspberry control and better black spruce seedling survival and growth. North. J. Appl. For. 12(2):80-85.

Nonindustrial Private Forest Owner Attitudes toward the Use of Silvicultural Herbicides

Nonindustrial Private Forest Owner Attitudes toward the Use of Silvicultural Herbicides

Comparison of duration and method of herbaceous weed control on loblolly pine response through midrotation

Herbaceous weed control studies installed by the Auburn University Silvicultural Herbicide Cooperative to examine response to methods and duration of herbaceous weed control in eight loblolly pine (Pinustaeda L.) plantations were analyzed to determine stand response through age 9. Studies were designed to compare weed control treatments with an untreated check, weed control methods (band vs. broadcast), and weed control duration (first year vs. first 2 years). Pine growth was increased by weed control on all sites. Growth was increased by an additional year of weed control (duration) on about one-half of the sites, but did not differ between band and broadcast treatments (method). Age 9 volume response above the check averaged 27.3 m3/ha for first-year weed control and 42.9 m3/ha for the first 2 years of weed control. Individual-tree height growth between ages 7 and 9 did not differ by treatment at most sites, but stand volume growth was higher with weed control at six of the eight sites. Uniformity of individual tree size, as represented by the standard deviation of DBH adjusted for dominant height, was more dependent on survival, hardwood encroachment, and level of fusiform rust stem infection, which varied by treatment and site, than on the result of herbaceous weed control per se. Growth projections made with the least intensive weed control treatment at each site indicated that on average, merchantable volume at age 22 with weed control will equal that of an age 25 stand without weed control. Largest gains were on sites where weed control increased survival.

Silvicultural herbicides in Canada: registration status and research trends

At present, there are five herbicides (2,4-D, glyphosate, hexazinone, simazine, and triclopyr) available to forest managers in Canada. It is unlikely that any new active ingredients will be registered for forestry in the foreseeable future. The only additions likely are extensions of existing labels (e.g., aerial application of Pronone® and Release®) and registration of new formulations (e.g., Velpar® ULW). Research in the short term future will concentrate on improving efficacy, efficiency and versatility, and reducing environmental impact of currently registered products. Some of the current work is described. Key words: silviculture, herbicide, 2,4-D, glyphosate, hexazinone, simazine, triclopyr, research trends, application technology, cost-benefit, vegetation management model, environmental impact

Effects of the herbicides glyphosate, 2,4,5-trichlorophenoxyacetic acid, and 2,4-dichlorophenoxyacetic acid on forest litter decomposition

Laboratory studies with two leaf litter and one forest floor substrate showed that the herbicides 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), a 50:50 mixture of these, and glyphosate all had toxic thresholds at which they reduced decomposition. However, in all cases, the thresholds were &gt; 50 times higher than residue concentrations that occur in the field after silvicultural herbicide treatments. In a field study at one site, no measurable 1-year postspray effects on litter decomposition were found among treatment plots sprayed at 0.0, 3.4 or 6.7 kg 2,4,5-T/ha.