Injection-based approaches for controlling Douglas-fir (Pseudotsuga menziesii) invasion in conservation efforts of the Patagonian forest

Abstract

Invasion by nonnative woody species poses a major threat to the environment, biodiversity, and economies worldwide. Nahuel Huapi National Park in Argentina is a protected area for habitat conservation that harbors several invasive Pinaceae species, where Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] is one of the most aggressive and abundant conifer tree invaders. Management of invasions in protected areas must include efficient, easy to deploy, and cost-effective techniques, while reducing the impact on native ecosystems. Because the region has no control measures applied other than conventional felling, we analyzed the effectiveness of two systemic herbicides (glyphosate and aminopyralid + triclopyr) at two different concentrations, applied with the drill and fill method. We then quantified defoliation of P. menziesii trees 6, 12, and 24 mo after application and performed an economic cost analysis to determine profitability. For the application, the trees were grouped into diameter at breast height classes and randomly assigned to one of the four treatments. Herbicide doses were adjusted according to tree size. We found that glyphosate at high concentrations completely defoliated 33% of the trees after 6 mo and 87% after 12 and 24 mo. Glyphosate at low concentrations defoliated almost 30% of the trees after 24 mo, most of which were smaller trees. The aminopyralid + triclopyr treatment did not produce significant defoliation at any of the tested concentrations. When compared with conventional felling, the drill and fill method was found to reduce removal costs by 98%. We observe that differences in costs are mainly due to dead trees that remain standing, decompose slowly, and do not generate costs associated with their removal and debris management. Drill and fill is a suitable method for treating scattered trees in a native forest community, with reduced environmental consequences compared with other removal techniques currently applied within conservation areas of the Patagonian forest.

Management Implications

Management of tree invasions within conservation areas is particularly challenging, as the strategies to be applied in control programs are restricted by regulations aimed to preserve the environment. In the total protected area of Argentina (4,993,722 ha), about 50 invasive woody species have been identified, 12 of which are conifers. As tree invasions are significant and a growing threat to biodiversity, it is critical to explore cost-effective techniques that can be implemented in protected areas with a high value for conservation, particularly in economically challenged and/or developing countries. This work focuses on herbicide application by the drill and fill method as a potential tool to treat invasive tree species, as this method allows targeted application and calibrated doses at low cost and reduced labor times. The treated trees showed high levels of defoliation, even in the larger trees. As such, this method could be useful on medium to large reproductive individuals (diameter at breast height between 20 and 30 cm), which are an important source of propagules and secondary loci formation, but are difficult to control without causing significant disturbance to the surrounding environment. Applying the drill and fill method significantly reduces removal costs when compared with conventional felling, which is the current removal method used in the area. This economically viable approach expands the possibilities for tackling ecological problems, even in scenarios marked by limited funding for environmental policies. So far, herbicides are not allowed to be used in national parks, because there are very few local studies demonstrating their effectiveness and low environmental risk. In this study, the drill and fill method emerges as a practical, applicable and cost-effective tool for the management of invasive trees in areas of high conservation value.

Introduction

Invasion by nonnative woody species is an important cause of ecological degradation and biodiversity loss (Gioria et al. 2023; Richardson et al. 2014), resulting in significant economic losses (Diagne et al. 2021; Fernandez et al. 2023). Some conifers are considered invasive worldwide, and their establishment has been especially successful in many areas in the Southern Hemisphere, such as South Africa, Australia, New Zealand, and South America (Nuñez et al. 2017; Richardson 1998; Simberloff et al. 2010). Studies in these regions have documented serious ecological impacts from the invasions, including changes to hydrological cycles (Jobbágy et al. 2013; Le Maitre et al. 2000), fire regimes (Brooks et al. 2004; Taylor et al. 2017), and habitat loss (de Abreu and Durigan 2011), as well as economic impacts related to damage and management costs (Duboscq-Carra et al. 2021; Velarde et al. 2017).

The management of woody species invasion typically involves the application of mechanical and/or chemical control methods (Weidlich et al. 2020). For example, in South Africa, mechanical procedures such as tree felling, pruning, or ring-barking are commonly implemented and are particularly efficient in areas with low invasion density (Boast 2021). In New Zealand, mechanical methods are also employed. However, significant progress has been made in recent years in the use of herbicides for the management of Pinus species (Gous et al. 2015a, 2015b; Ledgard 2009; Rolando et al. 2021). Herbicides have the potential to be the most cost-effective approach for the control of large and well-established invasions (Lange et al. 2022; Nuñez et al. 2017), especially in areas where wood extraction is not profitable. The suitability and efficient application of these methods depend on multiple factors ranging from the age of the invasion, the size of the invaded area, characteristics of the terrain (Nuñez et al. 2017), financial limitations (Kettenring and Adams 2011), and regulations of each country regarding control of invasive species (Wagner et al. 2017).

Herbicide application methods most frequently used to control Pinaceae invasion involve aerial boom sprays, aerial bark application with wands (both applied from a helicopter), and ground basal bark applications, including ring-barking, stem injection, and cut stump applications (Briden et al. 2014; Ledgard 2009; Raal 2005). Aerial sprayings are highly effective in controlling large areas of dense invasion within short time periods (Briden et al. 2014). However, their uncalibrated application can generate negative environmental impacts in the surrounding area (Rolando et al. 2023) and/or kill non-target native species (Cornish and Burgin 2005) due to herbicide drift and runoff (Baillie 2016; Richardson et al. 2020). As such, this procedure would not be a viable option in areas of high conservation value, where prioritizing low-impact methods is essential to ensure the regeneration of native plant cover.

A promising method with low impact on the surrounding native plant community is stem injection (Itou et al. 2015; Raal 2005), also known as stem poisoning or the “drill and fill” method. Although labor-intensive in the context of treating invaded stands, it is a precise method that allows highly calibrated doses to be applied directly into the tree, thus avoiding “off-target” effects (Boast 2021). Once applied, the herbicide is rapidly absorbed and transported through the vascular system to the target tissues (Gous and Richardson 2008), causing physiological failure, defoliation, and ultimately death. Compared with other chemical methods, stem injection has several favorable features, such as no spillage of chemicals into the environment or on non-target species, as the herbicide remains inside the tree (Boast 2021). For the same reason, it can easily be carried out in variable weather conditions. Because the trees are killed standing, the surrounding system remains mostly undisturbed (Macalister 2010) and light gaps that may favor reinvasion are not suddenly generated in the canopy, which can occur with mechanical removal (Paul and Ledgard 2009). Dead trees gradually decompose until they eventually collapse after 10 to 15 yr (Boast 2021; Ledgard 2009). Decaying nonnative trees can serve as perches for native plant seed–dispersing birds and provide habitat for native wildlife (Herrera and García 2009; Menvielle et al. 2020). Mechanical methods to kill standing trees, such as ring-barking, are not always effective, as the tree may repair the damage if the method is improperly implemented (Ledgard 2009), or are directly ineffective in resprouting species (Burch and Zedaker 2003).

The most commonly used systemic herbicides for chemical control of woody species (including Pinaceae) are glyphosate, triclopyr, picloram, and metsulfuron (Ledgard 2009). Several studies have tested the effectiveness of mixing these herbicides (Gous et al. 2014, 2015b) at different rates, also including other chemical compounds such as aminopyralid and dicamba (Gous et al. 2015b; Rolando et al. 2020). The efficacy of each herbicide or mixture varies and depends on the species to be controlled, the size of the individual, and the application method used (DiTomaso and Kyser 2007; Ledgard 2009). Therefore, it is critical to the successful application of these herbicides to adjust their implementation strategies according to the tree species, environmental factors, and land use (e.g., conservation area or forest plantation).

Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] is a Pinaceae species native to North America that is invading large areas within the Patagonian forest of Argentina (Moyano et al. 2023; Simberloff et al. 2010). It is a particularly difficult tree species to manage, because it is invading a protected area with a high conservation value. As such, control plans must be carefully designed and implemented to avoid impacting the native ecosystem. The available information about how chemical methods could be applied to tree species invading conservation areas is currently scarce and often limited to anecdotal reports by park staff. To date, conventional felling has been the only method applied for the control of P. menziesii in the region, while the economic benefits of using chemical approaches remaining unknown. As such, the objectives of this study were: (1) to test the efficacy of two systemic herbicides applied by drill and fill method on defoliation of P. menziesii trees and (2) to perform an economic analysis for the removal of invasive trees using the drill and fill method and compare it with conventional felling.

Materials and Methods

Study Site

The study was conducted in Isla Victoria (–40.9530100, –71.5381000), Nahuel Huapi National Park, Patagonia, Argentina. The island has an area of ca. 3,710 ha, its extension from northwest to southeast is ca. 18.2 km, and its maximum elevation point is 1,025 m above sea level (256 m above lake level) (Fig. 1). The climate is temperate with two distinct seasons. Mean summer temperature is 16 C, and mean winter temperature is 4.3 C. The mean annual rainfall in the area is 1,300 mm with 70% falling in the cold season (May to September). Winds are predominantly from the west, northwest, and north (Simberloff et al. 2002). Most of the island’s surface area is covered with forest co-dominated by Nothofagus dombeyi (Mirb.) Blume (Coihue) and Austrocedrus chilensis (D. Don) Pic. Serm. & Bizzarri (Chilean cedar). In 1925, the Argentina government established a forestry program (Koutché 1942), which resulted in the planting of 62 broadleaved and 73 conifer species (Richardson and Rejmánek 2004). At that time, tree plantations occupied a total area of 180 ha, of which 150 ha occurred in the central part of the island. After several introduction events, some of these species have succeeded in establishing themselves within the native plant communities (Simberloff et al. 2002) and continue to expand within the park (Moyano et al. 2023). Nearly 80 yr later, P. menziesii was found to be the most abundant nonnative species outside the original plantations (Simberloff et al. 2002), successfully invading native forest (Nuñez and Paritsis 2018).

Figure 1. Location of the study site within Isla Victoria, in northern Patagonia, Argentina. The treated area is outlined in yellow.

Field Trial

The trees to be treated were selected from a stand densely invaded by P. menziesii (∼5,000 stems ha⁻¹), representing about 30 ha in the central area of Isla Victoria (Fig. 1). We selected healthy-looking trees (i.e., without signs of anomalies or damage) growing along similar topography. For the herbicide application, 50-mm-deep holes were drilled with an 8-mm-diameter drill bit around the tree trunk at ∼1.5 m in height and at an ∼45° downward angle to avoid spillage (Fig. 2A). Two different systemic herbicides were tested (Table 1): (1) glyphosate, a nonselective herbicide (Roundup Full II®; composition: glyphosate [N-(phosphonomethyl)] glycine potassium salt), at 662 g L⁻¹; and (2) aminopyralid + triclopyr, a selective herbicide (Tocón Extra®; composition: aminopyralid [4-amino-3,6-dichloropyridin-2-carboxylic acid] + triclopyr butoexyl ester [2-butoxyethyl 3,5,6-tricholoro-2-pyridyl oxyacetate]), at 40 g L⁻¹ + 166.9 g L⁻¹. For each herbicide, two concentrations were tested and defined as “high” and “low,” resulting in four treatment combinations: glyphosate at high concentration (G_H), glyphosate at low concentration (G_L), aminopyralid + triclopyr at high concentration (AT_H), aminopyralid + triclopyr at low concentration (AT_L) (Table 1). Glyphosate is usually recommended by manufacturers to control broadleaf weeds as well as annual and perennial grasses. However, it has been shown to be effective in eliminating invasive species such as Salix trees, shrubs, and Pinaceae species using the stem injection method (Australian Weed Management 2003; Ledgard 2009; unpublished internal reports of Argentina’s National Park Administration). Aminopyralid and triclopyr are commonly used for dicotyledonous woody species by basal application or cut stump (i.e., not injected), and although the manufacturer does not recommend it specifically for conifers, triclopyr has been successfully used for controlling Pinaceae (Briden et al. 2014; Rolando et al. 2021). In Argentina, neither product is label-recommended for injection; however, Tocón Extra® is occasionally used within the national park for cut stump applications.

Figure 2. Herbicide treatment applied by drill and fill method in invading Pseudotsuga menziesii trees. (A) Drilling holes into tree bark; (B) injection of herbicide into the vascular system with a syringe; (C) P. menziesii individual (32-cm diameter at breast height) completely defoliated after 12 mo of being injected with glyphosate at high concentration (G_H); (D) aerial photo taken from a drone, the left arrows show completely defoliated trees, and the right arrow shows a tree with brown needles. Scale bar: 10 m. Source: Santiago Quiroga.

Table 1. Herbicide specifications per treatment, including treatment code, commercial product names, manufacturer, active ingredient (ai), concentrations and carriers used to control invasive Pseudotsuga menziesii trees by drill and fill

^a Ricardo Gutiérrez 3652, Munro, Argentina. https://cropscience.bayer.com.ar/distribuidores.

^b Avenida Libertador 101, Piso 1, 1638, Buenos Aires, Argentina. https://www.corteva.com.ar/contactanos.html.

The dose per tree was adjusted according to the diameter at breast height (DBH; Table 2). For this purpose, DBH was categorized into three classes: class 1, less than 10 cm, 1 hole; class 2, between 10 and 20 cm, 2 holes; and class 3, between 20 and 30 cm, 3 holes (Table 2). Each P. menziesii tree within a DBH class was randomly assigned to one of the four herbicide treatments. Once assigned, each tree hole was filled with 6 ml of herbicide (maximum hole capacity for the specified size; Fig. 2B; Table 2). Thus, the dosages were as follows: trees with DBH < 10 cm were filled with 6 ml per tree, trees with DBH between 10 and 20 cm were filled with 12 ml per tree, and trees with DBH between 20 and 30 cm were filled with 18 ml per tree. The amount of active ingredient in grams (g ai) applied for each volume varied between herbicide types (glyphosate or aminopyralid + triclopyr) and between concentrations (high or low); see Table 2 for details. We obtained five replicates per herbicide type per concentration per DBH class, which amounted to 60 trees. Herbicide treatments were applied in April 2022 (Austral autumn) and surveyed at 6, 12, and 24 mo after application (October 2022, April 2023, and April 2024, respectively). To calculate the proportion of dead foliage, the crown was divided into four sections and assigned a defoliation value by visual inspection, which was then averaged to obtain the final defoliation ratio. Additionally, the health of 15 neighboring trees (five per DBH class) not treated with herbicides or drilled was monitored to exclude the possibility that defoliation was associated with other external factors such as drought or insect or pathogen attack. The greatest bark thickness recorded for treated trees was 12 mm. In addition to DBH, tree height and crown width were measured and used to estimate tree size.

Table 2. Details of herbicide concentration of active ingredient (ai) applied into Pseudotsuga menziesii trees according to diameter at breast height (DBH) classes and herbicide treatment

^a G_L, glyphosate at low concentration (33.1 g ai L⁻¹); G_H, glyphosate at high concentration (662 g ai L⁻¹); AT_L, aminopyralid + triclopyr at low concentration (0.31 g ai L⁻¹ total for both active ingredients); AT_H, aminopyralid + triclopyr at high concentration (1.55 g ai L⁻¹ total for both active ingredients).

Economic Analysis

To determine the cost-effectiveness of the drill and fill method and conventional felling, an assessment of the costs of treating 10,000 medium to large invasive trees (DBH between 20 and 30 cm) was carried out. To calculate the costs, we first estimated the time it takes a single person to remove 30 trees between 20 and 30 cm DBH with a chainsaw or to inject herbicides. The estimated time was then extrapolated to a total of 10,000 trees. We ensured that the sets of trees were in similar terrain conditions for both estimates (felling/injecting). A list of required items was made and classified into four categories: labor, safety clothing, equipment, and tree residues. Because chemical control requires a competent professional to elaborate the prescription for herbicide purchase and application (Menvielle et al. 2020), this item was included as “professional service”. Those fees were then estimated based on “technical field consultation/advice cost per hour” from the website of the Professional Council of Agricultural Engineering of Rio Negro, Argentina (https://cpiarn.org.ar). As for the operators, the estimated payment was calculated by consulting the amount that Argentina’s National Parks Administration pays its operators per hour of work in this particular region (Internal Document No. 206/2024). The prices of each item within the three remaining categories, safety clothing, equipment, and tree residues, were taken from Mercado Libre (www.mercadolibre.com.ar), which is the main platform dedicated to e-commerce in Latin America. Prices were converted to U.S. dollars considering that US$1 is equivalent to 1,005 pesos ARG according to the Argentinian National Bank (www.bna.com.ar) at time of writing.

Data Analysis

To analyze differences in the proportion of dead foliage between treatments, a nested generalized linear model (Zuur et al. 2013) with a beta error distribution and logit link were used. Because this distribution only allows values between zero and one, the proportion data were transformed according to Smithson and Verkuilen (2006) to exclude extreme values (Equation 1).

([1]) $Y_{\rm a}= (Y_{\rm b}\times (N-1) + 0.5)/N$

where Y _a is the transformed proportion value, Y _b is the raw proportion value (containing zeros and ones) and N is the sample size. Because the tested concentrations are nested within herbicide types, herbicide treatment was used as a four-level explanatory variable (i.e., G_L, G_H, AT_L, AT_H). Statistical models were fit for each postapplication time surveyed (6, 12, and 24 mo). During model selection, the explanatory variables herbicide treatment, herbicide dose (three levels: 6, 12, and 18 ml per tree), tree height (covariate), DBH (covariate), and crown width (covariate) were included. Model selection was based on the Akaike’s information criterion (AIC; Akaike 1998) and the Akaike weights (Burnham and Anderson 2002). The best-fit models for each subset according to AIC only included the variables herbicide treatment and herbicide dose (Supplementary Table S1). Differences between explanatory variables were determined by an analysis of deviance using a chi-square test and pairwise post hoc multiple comparison using Tukey’s Honestly Significant Difference (HSD) test. Because all the untreated trees had no defoliation (i.e., 0%), they were excluded from the statistical analyses. All the statistical analyses were performed using R v. 4.3.1 (R Core Team 2023) and the packages glmmTMB (Brooks et al. 2017), car (Fox and Weisberg 2019), ggplot2 (Wickham 2016), and emmeans (Lenth 2023).

Results and Discussion

Effectiveness of the Drill and Fill Method

The proportion of dead foliage was significantly affected by the interaction between herbicide treatment and dose for 6 and 12 mo after application (Fig. 3; Table 3; Supplementary Table S2). After 24 mo, the herbicide treatment and the dose still showed considerable effects on tree defoliation, yet without a statistically significant interaction (Table 3). Glyphosate at high concentration (G_H) caused the greatest proportion of dead foliage on P. menziesii trees at 6, 12, and 24 mo after application (Figs. 2C and D and 3). After 6 mo, all small trees were 100% defoliated within the G_H treatment (Fig. 3A), while in medium-sized trees, the average defoliation rate was 0.7, and in large trees, 0.5 (Fig. 3B and C). After 12 mo, the average defoliation for medium and large trees increased from 0.7 to 0.98 and from 0.5 to 0.9, respectively (Fig. 3E and F). The proportions remained the same after 24 mo (Fig. 3H and I). The G_L treatment produced a mean defoliation of 0.85 after 12 mo only on the smallest trees (Fig. 3D). However, after 24 mo, medium and large trees within this treatment exhibited a wide range of defoliation, between 0.1 and 1 in medium trees and between 0.15 and 0.6 in large trees (Fig. 3H and I). Compared with glyphosate treatment, AT_H produced a reduced effect in the smallest trees after 24 mo (Fig. 3G); however, this was not statistically significant among the different doses (i.e., among the different DBH classes; Fig. 3G–I). Associations between defoliation and other measures of tree size, such as height and crown width, were visually explored, but no trends were detected (data not shown).

Figure 3. Proportion of dead foliage as a function of herbicide treatment and herbicide dose at 6, 12, and 24 mo after application into Pseudotsuga menziesii trees. Codes: G_L, glyphosate at low concentration (33.1 g ai L ⁻¹); G_H, glyphosate at high concentration (662 g ai L ⁻¹); AT_L, aminopyralid + triclopyr at low concentration (0.31 g ai L⁻¹ total for both active ingredients); AT_H, aminopyralid + triclopyr at high concentration (1.55 g ai L⁻¹ total for both active ingredients). The solid circles represent the mean with 95% confidence intervals. Sizes of circles represent the three diameter at breast height (DBH) classes. Five trees were treated per DBH class (N = 5). Statistically significant effects determined by Tukey’s Honestly Significant Difference (HSD) test are shown with different letters (P < 0.05). The specific concentrations per volume applied per herbicide are presented in Table 2.

Table 3. Analysis of deviance summary table (type II chi-square test) for the proportion of dead foliage of Pseudotsuga menziesii trees as a function of herbicide treatment, dose, and their interaction, applied in each month postapplication surveyed (6, 12, and 24 mo)

Estimated Tree Mortality after Herbicide Applications

After two growing seasons postapplication, the trees treated with G_H that were 100% defoliated had no new growth. This is a strong indicator that the trees were likely dead. Although P. menziesii individuals experience epicormic regrowth from the trunk (Collier and Turnblom 2001), this phenomenon was only observed (at least at our sites) when small trees were damaged or cut with machetes (data not shown). Only 2 of the 15 trees treated with G_H maintained a small proportion of green-yellow needles after 24 mo (Fig. 3G–I). One of the trees had a DBH of 19 cm and had 90% of its crown defoliated, yet no new shoots were observed. The other tree, with a DBH of 28 cm, had 50% of the crown defoliated at 12 mo and 70% at 24 mo, and also had no new shoots.

Because there is strong local opposition to using herbicides in conservation areas, both from citizens and park managers, the local authorities only allowed us to conduct our study on a restricted number of trees, at a limited temporal and spatial scale. Regardless, our results provide evidence that herbicide application by drill and fill is a useful tool to control the invasion by P. menziesii in a forest. The highest concentration of glyphosate (662 g ai L⁻¹, G_H) killed 90% of the trees, and the lowest concentration (33.1 g ai L⁻¹, G_L) killed 30%. That is, a 20-fold lower concentration (G_L) killed one-third of the trees compared with G_H. This could suggest that concentrations lower than 662 g ai L⁻¹ may also be effective in controlling invasive conifers. Further work should explore the dose–response curves to accurately determine the grams of active ingredient per volume needed to kill other invasive trees for both herbicide types using the drill and fill method. Identifying the optimum dose is essential to avoid the overuse of the product as well as the costs associated with the herbicide and its application.

Environmental Effects of Herbicides Used for Controlling Tree Invasions

Glyphosate is a low environmental risk herbicide (Duke 2020; Hagner et al. 2019). Nevertheless, its global application has sparked discussions among different community members (Weidlich et al. 2020), mainly triggered by the large-scale overuse of herbicides in agriculture (Pergl et al. 2020). Glyphosate has a mean half-life in water and soil of 30 d (Blake and Pallett 2018), and its degradation is mainly through microbial action (Borggaard and Gimsing 2008). Once it enters the soil, it binds tightly to organic compounds that transform it into an inactive product (Borggaard and Gimsing 2008), unable to generate phytotoxicity in non-target plants (Duke 2020). Due to its high affinity to soil particles, its movement through surface and ground water is minimal (Saunders and Pezeshki 2015). The main source of glyphosate entry into the soil is via sprayed droplets (Duke 2020); thus, with targeted applications such as the drill and fill method, glyphosate would not cause undesirable effects on ecosystems (Pergl et al. 2020). Aminopyralid has a mean half-life of 76 d in soil and 240 d in water, while triclopyr has a mean half-life of 30 d in soil and 25 d in water (Rolando et al. 2023). Aminopyralid in particular is highly soluble in water and therefore prone to move into surface water by runoff or groundwater by leaching (Kashuba et al. 2005). Both herbicides are mobile in soils (Thompson et al. 2000; Tomco et al. 2016) and can be degraded via photolysis, hydrolysis, and microbial actions (Tu et al. 2001; USDOI 2015). In New Zealand, triclopyr was found to be present on the forest floor, retained in litter of dead needles, 2 yr after an area densely invaded by conifers was sprayed (Rolando et al. 2023). Moreover, it has been observed that aminopyralid and triclopyr can cause damage to non-target plants via root exudation after basal bark application (Graziano et al. 2022). Although the concentrations found do not pose a risk to the environment in some cases (Rolando et al. 2023), these aspects will definitely influence subsequent management decisions (Dickie et al. 2022). The presence of herbicides in the soil may not only affect non-target plants, but will also impact the success of regeneration or restoration of the system. Addressing these aspects is critical, as the management actions chosen to remove invasive individuals will have a substantial effect on the expected long-term outcomes.

Lessons Learned from Applying Herbicides with the Drill and Fill Method

The insignificant effect of the aminopyralid + triclopyr treatments on tree defoliation was probably due to the less than optimal concentrations of the active ingredient. In Argentina, Tocón Extra® is label-recommended for stump application at 1.5% (0.31 g ai L⁻¹) in woody shrub species. Neither this dose, nor a dose five times higher, was sufficient to cause significant damage to the crown of P. menziesii trees. In the United States, triclopyr is indicated for injection as triethyl amine salt (Kochenderfer et al. 2012); however, this product is not authorized in the region of the present study (SENASA 2024) and therefore is not locally sold. Future studies should address the optimal concentration of the aminopyralid + triclopyr formulation applied by the injection method. Season of application can affect herbicide efficacy, but this variation is usually species specific (Graziano et al. 2022). Application during the growing season is usually recommended for most conifers (Ledgard 2009); however, early fall application has resulted in significant defoliation rates with glyphosate at high concentrations. This may be due to active transport to the roots as the plant is preparing to enter dormancy before winter. Comparing the effectiveness of injection-applied herbicides as a function of seasonality in temperate forests remains to be assessed.

Economic Analysis

In terms of labor time, the action of injecting or felling 10,000 trees consumes 333 h and 362 h, respectively. Based on our economic analysis (Table 4), it would cost ∼US$2,900 to eliminate 10,000 medium to large trees (DBH between 20 and 30 cm) using the drill and fill method. In comparison, conventional felling would cost ∼ US$3,500 (Table 4). When comparing costs, no substantial differences between the two methods per se were found. This is partially due to the fact that equipment costs (e.g., a drill) are very high in the local market. In this sense, the drill could be replaced with cheaper tools that fulfill the same function, such as a hand drill. This could reduce the cost from US$2,900 to US$2,400, but might increase the operating time and therefore the operators’ pay. Mechanical removal requires the processing of post-felling residues to avoid the accumulation of dry woody material, as dead wood may hinder future regrowth of native plants (Boast 2021) or increase the risk of forest fires (Holmes et al. 2000). Processing post-felling residues is quite time-consuming and, according to estimations, has a significantly high cost and increases the risk of injury to workers (Dampier et al. 2006). Mechanically removed trees are usually chipped in Nahuel Huapi National Park, so we estimated the cost of this particular task. According to the manufacturers, a large chipper can process about 7 trees per hour, so processing 10,000 trees takes 1,428 hours. The machinery rental as well as the fuel needed for this amount of time adds up to a cost of US$120,000 (Table 4). It is worth mentioning that the chippers process small- and medium-sized branches, so the residues from the main trunks must go through a preprocessing that includes delimbing the tree, cutting the trunk into operable pieces, and transporting and stockpiling the segments. The time and cost associated with all those stages are variable and difficult to calculate, as they depend on the distances and challenges presented by the terrain. In summary, the cost of applying the drill and fill method is approximately US$0.29 per tree, while felling and processing the residue would cost approximately US$12 per tree.

Table 4. Detailed list of costs and inputs required to remove 10,000 invasive Pseudotsuga menziesii trees with diameter at breast height (DBH) between 20 and 30 cm using the standard felling and drill and fill methods

^a US$6.64 per hour of work × 362 h for felling and × 333 h for drill and fill.

^b Calculated for 0.09 L of chain lubricant, which is the consumption of a 2.4-kW chainsaw at its maximum power for 362 h.

^c Calculated for 180 L of Roundup Full II^® herbicide, which is required to inject 10,000 trees (0.018 L per tree). The price per liter of herbicide is US$0.32.

^d Calculated for 5.02 L of 2t oil (two stroke (2T) oil for engines), which is the consumption of a 2.4-kW chainsaw at its maximum power for 362 h.

^e Calculated for 241 L of fuel, which is the consumption of a 2.4-kW chainsaw at maximum power for 362 h.

Unlike the drill and fill method, felling and post-felling action often requires trained workers and heavy and difficult to transport equipment that can be expensive, with its use limited to easily accessible areas (Lange et al. 2022). This is particularly true in remote areas, natural parks, and ecological reserves. Large-scale removal programs using mechanical methods may have impacts on the surroundings, as falling trees can kill nearby native plants or expose mineral soil (Culliney 2005). Moreover, the disturbance associated with felling can promote the growth of other nonnative invasive species (Hierro et al. 2006).

Despite its economic advantages, the injection method may not be suitable for treating large, densely invaded stands, as it could pose a potential risk to natural areas. Large-scale applications would favor a major accumulation of dry woody material in the coming years, which could increase the risk of forest fires (Clifford et al. 2013). Furthermore, the residual biomass of dead wood and needles would result in a large input of carbon and nutrients modifying the abiotic properties of the ecosystem (Dickie et al. 2022). Instead, we propose using the drill and fill method to treat medium to large trees in areas where the target individuals are near water sources, such as wetlands, lakes, or streams, or to stop long-distance dispersal prompted by isolated distant trees. An integrated woody invasive species management program should therefore consider including drill and fill for large isolated specimens while mechanically removing small trees. In protected areas, such as Nahuel Huapi National Park, this approach becomes an excellent option for managing not only P. menziesii invasion, but potentially other Pinaceae species present in the area in moderate to high abundance. Moreover, exploring the effectiveness of injecting other nonnative invasive species of the National Park, including Salix, Acer, Acacia, Sorbus, and Crataegus, would be of considerable interest.

This research underlines the efficacy of drill and fill method in controlling invasive P. menziesii trees. This method achieved a significant defoliation rate among the target individuals 2 yr after the application of glyphosate at high concentration, although the optimal concentration needs to be determined to balance costs and benefits. Drill and fill is a promising tool, both economically and environmentally, for controlling medium to large invasive trees in remote areas with high conservation value. This technique minimizes potential impacts on the ecosystem and eliminates the need for costly waste-removal processes associated with mechanical extraction methods.