Genus Acacia in Mainland Portugal: Knowledge and Experience of Stakeholders in Their Management

Abstract

The global rise in issues related to the invasion of alien tree species is becoming increasingly prominent. Genus Acacia is one of the most pervasive invaders among the non-native flora introduced to Portugal over the centuries. This research aimed to pinpoint the key players engaged in its management, their expertise, and the methodologies applied to control acacias or decrease affected regions. Data were gathered via an online survey and subjected to diverse statistical analyses. Numerous stakeholders have emerged in recent decades, employing various methods to fight acacia invasion. Mechanical, chemical, and combined methods are the most frequently applied. The findings highlight the need for continued funding for long-term acacia control in the same invaded areas, increased training actions and best practices demonstration in the field and increased public awareness and engagement in local communities and the general public.

1. Introduction

In January 2020, the EU Parliament called for the implementation of the EU Biodiversity Strategy 2030, which aims to address the main drivers of biodiversity loss [1], including the means of combating invasive alien species (IASs). The assessment of IAS and their control management carried out by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) identified these species as one of the five major direct drivers of environmental change worldwide [2]. It is estimated that the direct impacts of IAS, their management, and ecosystem restoration cost the EU economy billions of euros annually [3].

Since the beginning of land and sea journeys between continents, society has exported thousands of woody plant species from their natural areas, which later became naturalized or invasive in their new habitats [4]. These woody plant species have been planted far beyond their natural ranges to provide or improve economic and ecosystem services such as wood, fiber and tannin production, soil erosion control, aesthetics, and other benefits [5,6,7,8]. Some plants were introduced by accident, without purposeful intent. Over time, some of these species began to have a strong negative impact on the environment. As a result of their naturalization and development of dense stands, they become invasive, disrupt or transform ecosystems [9], and decrease the number of native plant species [10]. The invasion of woody plants received little attention and was not widely recognized until the second half of the last century [11], but only since 1990 did research on IAS expand significantly, and the interest in the topic continues to grow today [12,13].

Among the many exotic plants introduced over the past two centuries into the Mediterranean basin, some acacia species from Australia (wattle) are among the most aggressive [14,15]. The Acacia genus has more than 1350 species, of which 1021 are Australian, while others are distributed through southern Asia, Africa, and the Americas [16]. The Australian acacia species deserve special attention as they have become invasive in many areas beyond Australia [17,18]. They change biodiversity, community structure, nutrient cycles, ecosystem productivity, mutualistic interactions, fire regimes, and water availability [14,17,19,20].

1.1. Acacias in Portugal

The introduction of acacias for gardening and forestry purposes in Portugal, as in other southwestern European countries, began in the 19th century [21]. Several species were introduced to Portugal, Spain, France, and Italy for their ornamental interest, to restore vegetation cover in degraded areas, to control soil erosion, and for their economic potential as a source of raw materials [22,23]. One of the first pieces of evidence of the invasive behavior of these species (namely of Acacia melanoxylon) was noted and recorded a few years later [22,24]. As a result, the first laws were passed in the 1930s in Portugal restricting the planting of certain species of acacias (Law No. 1951 and Decree-Law No. 28039 of 1937) [25].

Currently, considering the invaded territory and the impact on the respective natural ecosystems, the species that show the most invasive behavior in Portugal are Acacia dealbata, Acacia longifolia, Acacia melanoxylon, Acacia mearnsii, Acacia saligna, Acacia cyclops, Acacia pycnantha, Acacia retinodes (all native from Australia) and more limited, also Acacia karroo (native from South Africa) [26]. Although most of these species, according to the citizen science platform Biodiversity4All (www.biodiversity4all.org, accessed on 3 September 2023), were mainly spotted in coastal areas, Acacia dealbata was spotted throughout mainland Portugal. Acacia spp. in Madeira and the Azores are characterized by very low species diversity, and their invasive character has not yet been proven [27,28].

According to the last Portuguese National Forest Inventory (2015) [29], acacia species occupy 16,600 hectares of mainland Portugal (i.e., less than 1% of the mainland country), of which a quarter are pure stands, another quarter are mixed stands, and the rest are stands where other tree species dominate acacia species. Transitions in land use land cover in areas dominated by Pinus pinaster, and shrubs and pastures, in 1995, gave place to 2300 hectares and 1900 hectares of acacias, respectively, in 2015. The presence of this IAS in nature-protected areas reached 2000 hectares [30]. The consequences of these acacia invasions are changes in the composition and community structure of ecosystems [19,20], physicochemical changes in the soil, changes in microbial function and structure [15,20], and changes in the fire regime in conditions of management abandonment [31]. According to the Decree-Law No. 92 of 2019 [32], developed for the prevention and control of IAS, all species of the Acacia genus are considered invasive in Portugal.

1.2. Acacia Control Process and Its Main Challenges

The management of IAS, especially acacias, is associated with several constraints, namely their high re-invasion capacity, the large invaded areas, and the high intervention cost [33]. Some key issues for successfully tackling these IAS are a good understanding of the causes of the invasion, specific knowledge to choose the correct control method and its accurate application, the monitoring of the acacia recovery and recovered areas [18], and raising awareness among citizens and municipalities for everyone to be involved [15,34].

Three general weed control methods are biological, mechanical, and chemical. An integrated management approach can be used for acacia control, using the previous three methods alone or combining them, and with the inclusion of prescribed fire (

Table 1. Types of Acacia genus control and conditions for their application. Source: www.invasoras.pt, 2020.

Type	Subtype	Application Requirements	References
Mechanical control	Hand pulling	Seedlings and small plants are manually removed	[35,36]
	Cutting	Small germinating plants are cut with a brush cutter or other cutting tool	[36]
	Ringbarking	Continuous rings are cut around the tree trunk with smooth intact bark, and all the bark and vascular cambium are removed until the ground surface	[37,38]
Chemical control	Foliar application of herbicide	Herbicide spraying is sprayed on recent sprouts or in areas with high germination rates	[35,36]
	Stem injection	Herbicide is applied directly to the vascular system of the adult plants	[39]
Biological control		Natural enemies (e.g., insects and fungi) from the regions of origin of the acacia or herbivores are employed to reduce its viability or reproductive potential	[40,41]
Prescribed fire		Prescribed fire is used to reduce the seed supply in the soil and eliminate the germinated plants	[42]
Mechanical and chemical control	Cut stump method	Acacia trunk is cut followed by herbicide application	[39,43]
Mechanical and biological		Plants are cut and herbivores control their regrowth	[44,45]
Mechanical and prescribed fire		Acacia is cut and followed by prescribed fire use	[42]

). All Acacia genus plants have the same basic life cycle, enabling the same control measures to be applied to all species, although the effectiveness of application may vary [26].

Despite the available variety of control methods and some knowledge of their advantages and disadvantages, in Portugal, few projects are devoted to controlling these IASs, as seen in the low number of applications to the Rural Development Program (RDP2020) [46]. In addition, only two of the mechanical treatments to control acacia (cut and cut followed by herbicide) are supported by the Portuguese Recovery and Resilience Plan (RRP) [47]. The land ownership distribution in Portugal emphasizes the struggle, as taking measures for acacia control in non-public lands is challenging. Private forest areas are predominant (more than 80%) in Portugal, followed by communal lands (15% of forested areas) with different types of tenure rights, management models, and managers [48].

The need to understand stakeholders’ and landowners’ knowledge of acacia control processes has been highlighted in relevant initiatives on non-native and invasive species, namely, in the EU Regulation 1143 of 2014 [49] on the prevention of IASs, and in the Sixth Plenary of the Intergovernmental Platform on Biodiversity and Ecosystem Services [50].

We aim to assess the knowledge and experiences of different stakeholders in Portugal regarding the Acacia genus management and identify the most known and/or applied control methods. More specifically, we seek answers to the following four research questions: (1) Who manages and where is the Acacia genus control in Portugal implemented? (2) Which control methods are most commonly selected in Portugal? (3) Which of the control methods applied is more sustainable? (4) How can acacia control in Portugal be improved based on the data obtained?

With this study, we intend to help identify gaps in stakeholders’ knowledge of the acacia control process and define how the process can be improved.

2. Materials and Methods

We designed an online survey to collect information from stakeholders (e.g., technicians from Municipal Forestry Technical Offices, Forest and Nature Conservation Services, Forest Associations, and Academic researchers) actively controlling Acacia spp. in Portugal to understand their knowledge and the control methods applied. First, according to official online reports of research and financial assistance projects about acacia control in Portugal over the past ten years, we created a database registering their managers and participants. The contact details of these individuals were identified through online research and the snowball method, through which existing contacts identified further potential contacts [51]. As a result, the survey was sent to forest owners, public and private organizations related to forest management at the state and local levels, and research institutions (Academia). Before launching the final version of the survey, a pre-test was conducted with six technicians and forest owners. A few adjustments to the language were made, and these answers were not included in the survey results.

The survey included 38 questions (Appendix A) and was divided into three sections: the first section collected demographic information, including sex, age, and job occupation; the second section focused on the location of the acacia plots controlled by the respondents, their size, and the applied control techniques (methods and frequency), and the sustainability (environmental, economic and social components) of different techniques; the third section explored the sources from which respondents built their knowledge on the different acacia species and techniques to control their invasive behavior, as well as suggestions for improving their sources of knowledge. Various categories of Google Forms questions were used in the survey design, such as multiple-choice, checkboxes, multiple-choice grids, tick-box grids, and paragraphs. Seven questions (18% of the total) were answered with “yes” or “no”. The estimated time to complete the survey was from 10 to 15 minutes.

In August 2020, we sent each potential participant an email with a brief description of the project and an online survey designed in Google Forms. After two weeks, a new email with a gentle reminder was sent to the potential participants.

The answers were analyzed using frequency and descriptive statistics (measures of central tendency as means, variances, standard deviation, percentage values, frequency analyses, ANOVA, and chi-square analysis) using IBM SPSS Statistics 28.0. Prior to the survey questions, there was agreement from the respondents to participate in this study, where it was stated that all the data were anonymous and each question was analyzed in a group and not individually, following the GDPR rules (EU data protection).

3. Results

The survey was sent to 420 people via email, among which 228 belong to Municipal Forestry Technical Offices and 58 to Academia (project managers studying acacia and acacia control processes). The remaining 134 people belong to agroforestry management associations and enterprises whose contacts were found in online acacia control advertisements. In total, 124 people, i.e., 30% of the potential participants, answered the survey (

Table 2. Response rate by type of institution surveyed.

Institution	Sent Emails	Responses Received	% of Responses Received
Municipal Forestry Technical Offices	228	56	25%
Academia	58	68	35%
Associations and Enterprises	134
Total	420	124	30%

).

Although the response rate obtained for the online survey falls below the 44.1% considered by Wu et al. [52] in their meta-analysis as a good average response rate for online surveys, several factors that influence a lower rate are to be considered. These factors are similar to our study’s, i.e., a survey targeting a non-student audience over 30 years old and a niche research topic. Moreover, according to Wu et al. [52], with a sample size below 500, a 20–25% response rate provides a confident estimate. Therefore, we consider that a response rate of 30% is reasonable for our study’s online survey.

3.1. Respondents Profile and Their Experience in Acacia Control in Mainland Portugal

Respondent profiling showed a slight predominance of males (57%) aged between 30 and 50 years (82%) (

Table 3. Demographic characteristics and profile classes (PC) of respondents.

Characteristics	Categories	Nr.	%
Sex	Male	70	57
	Female	50	40
	Prefer not to answer	4	3
Age	<30 years old	6	5
	30–50 years old	101	82
	51–65 years old	14	11
	>65 years old	1	1
Profile class	PC1—Forest owners, community forest managers	17	14
	PC2—Association forestry technicians	19	15
	PC3—Forestry enterprises technicians and/or managers	16	13
	PC4—Forest and Nature Conservation Services technicians	8	7
	PC5—Municipal Forestry Office Technicians	56	45
	PC6—Scientists/Researchers	8	7

).

The group with the largest respondents (45%) was technicians from Municipal Forestry Technical Offices. Scientists/researchers and technicians from Forest and Nature Conservation Services represented the profile class (PC) with the lowest number of respondents (7%).

Most respondents (97%) considered acacia as an IAS. Among the respondents, 92% also recognized that acacia creates a serious or catastrophic ecosystem problem. Although there is a general acknowledgment of the problems created by the different species of acacia, only half (51%) of the respondents were confident in their ability to distinguish between the several species present in mainland Portugal, particularly the younger and older respondents compared to the middle-aged group. As for the differences between PCs, forest owners and community forest managers (PC1) were the most confident in their knowledge, whereas, among the group of scientists/researchers (PC6), only half of the respondents answered that they could distinguish between acacia species. The difference between the responses of the analyzed PCs was not statistically significant.

Although 95% of the participants in the study dealt with acacias in their working areas, only 65% implemented some periodic control methods. The work developed by the respondents to control acacia usually occurred in only one municipality (58%), while six respondents had experience in all regions of mainland Portugal (Figure 1B). We plotted the acacia presence throughout Portugal according to the citizen science data available online (Biodiversity4All) and the official forest data from the most up-to-date National Forest Inventory, i.e., from 2015 [30]. Points were then extrapolated to municipalities, considering that data confirmed by two sources reinforces the presence of acacia (Figure 1A). It is interesting to note that at least six respondents have referred to developing work in municipalities with no acacia presence and that there are municipalities, e.g., in Northern Portugal, with the presence of acacias, according to our two sources but with only a few entities working there (Figure 1B).

Acacia control measures were applied to different types of forests and protected areas/natural parks (Figure 2). As expected, each PC worked on their land or where they were responsible for the management. PC3 (Forestry enterprise technicians and/or managers) intervenes in all forest tenures with a smaller presence in public forests. PC6 (scientists/researchers) has more work developed in public forests, likely because of the respective research activities.

The size of the areas where respondent-controlled acacias present different degrees of invasion (from moderate up to 10% of tree coverage, to very high, with more than 30% of tree coverage), and in general, do not exceed five hectares (62%). The analysis of the responses showed that the size of these controlled areas depended on the PC (p-value < 0.05; F = 2.51). In most cases, PC4 (Forest and Nature Conservation Services technicians) controls areas over five hectares. In 66% of the answers, the respondents controlled between one and three different species of acacia (Figure 3). The difference between the six PCs was not statistically significant.

3.2. Acacia Control Methods Most Known and/or Applied in Mainland Portugal and Their Sustainability

Regarding the methods known or applied by the respondents to control these invaders, mechanical cutting seems to be the most chosen, followed very closely by other mechanical methods (Figure 4). Among the respondents, 96% selected one mechanical method, and 72% selected the combination of one mechanical and one chemical method. Among the respondents, 23% and 13% selected prescribed fire and biological controls, respectively.

Although there is a wide variety of control methods (Table 1) that can be specific and more efficient for a given species [26], the survey results showed that the most selected methods are those without any specificity, with financial eligibility from the Rural Development Funds, and without requiring specific professional training on IAS from the operational teams. In a small number of cases (23%), the respondent himself chose the control methods. Among the answers, 37% indicated that the respondents’ team made the decision and 36% involved experts on the topic. Independent decisions were made by PC1 (Forest owners and community forest managers), whereas team decisions with expert support were, in most cases, made by PC5 (Municipal Forestry Office technicians). A comparison between the PCs at this analysis stage showed that the difference was statistically significant (p-value < 0.05; F = 2.6). A decision made with the support of a team or a group of experts to choose the method to be applied likely hides some individual unawareness. To a certain extent, this unawareness can be explained by the lack of responses in the different components of sustainability for each control method (Figure 5). Given the scarcity of responses in assessing the sustainability of different control methods, we attempted to identify the limits of respondents’ knowledge by analyzing the no-answer data (“Do not know/Do not answer”). Whereas biological control, prescribed fire, and their combination with mechanical methods received the highest percentage of no-answers, the three most selected methods (mechanical, chemical, and combination) had the lowest percentage of no-answers. However, among the three components of sustainability (environmental, social, and economic) in these methods, the social component showed more unawareness. On the other hand, when analyzing other methods, the economic component received the lowest responses from the respondents, showing the respondents’ lack of market price knowledge.

Giving the same weight to each sustainability component (environmental, social, and economic) and calculating the average, the most sustainable method is the biological control, followed by mechanical control and the combination of mechanical and biological control (Figure 6). In most cases, when the percentages are high for the environmental and social components, the economic component receives lower values. The opposite trend was also observed. This result must be carefully analyzed since, although the most sustainable method seems to be biological control, it was also the one with the most lack of answers and the least known and/or applied by the respondents (Figure 4). During this sustainability assessment, the greatest divergence in opinions regarding the method’s sustainability was observed in the case of chemical control (p-value < 0.05; F = 3.9). This statistical difference is mainly due to the position of the two groups: PC1 (forest owners, community forest managers), which, in most cases, considers chemical control to be sustainable, and PC6 (scientists/researchers), which assesses it with low scores due to its cost and environmental impact.

3.2.1. Control Methods Applied to Nine Acacia Species Occurring in Mainland Portugal

To better understand which method is more or less effective in controlling a certain species of acacia in Portugal, we included questions in the survey regarding each method’s efficiency. This part of the survey was not mandatory. More than 75% of the respondents did not answer the questions about six of the nine acacia species considered. Results showed that, regardless of the acacia species, the combination of mechanical and chemical, and exclusively mechanical control methods are applied for eradication in most cases. The biological control and prescribed fire methods were the least applied (

Table 4. Control methods known and/or applied in different species of acacia. Light grey highlights the three most applied methods by mainland Portugal’s three most representative acacia species.

Acacia Species	Control Methods
	Mechanical	Chemical	Biological	Prescribed Fire	Mechanical and Chemical	Mechanical and Biological	Mechanical and Prescribed Fire	Do Not Know
Acacia dealbata	35%	29%	10%	10%	44%	16%	18%	22%
Acacia melanoxylon	29%	24%	11%	10%	35%	15%	15%	34%
Acacia longifolia	26%	15%	19%	7%	23%	19%	10%	48%
Acacia mearnsii	11%	9%	4%	3%	13%	6%	5%	75%
Acacia saligna	10%	9%	5%	2%	12%	6%	5%	77%
Acacia cyclops	9%	6%	3%	2%	9%	6%	4%	79%
Acacia karroo	8%	6%	3%	2%	6%	6%	2%	83%
Acacia pycnantha	11%	10%	4%	4%	7%	6%	3%	78%
Acacia retinodes	9%	6%	4%	4%	5%	6%	3%	81%

).

Besides all the efforts to control the Acacia genus in mainland Portugal, the high dispersion of the plants is actively spreading. The respondents were faced with this tendency and asked in the survey how the dispersion of acacias could be limited in their intervention areas. Most of the suggestions can be divided into three categories: (i) detection of invaded areas, (ii) definition of the action plan and treatment, and (iii) long-term monitoring, when possible. In the case of the detection and treatment of invaded areas, special attention was given to improving knowledge and dissemination to foster an increase in the number of participants in the process. Such improvement can be carried out through, e.g., the development of awareness campaigns and the actions of acacia control and eradication. Several respondents underlined the importance of controlling the seed bank of acacia species, managing areas invaded by small plants using different control methods, and the importance of the subsequent monitoring of these sites. In addition, given the easy colonization of recently burned areas with acacia, the restoration of burned forest areas with native species has been proposed.

3.2.2. Information Sources on Acacia

Most respondents’ knowledge about these IASs and their different control techniques was obtained through webpages and publications, such as technical journals and scientific papers (Figure 7). Respondents under 30 used more information sources than other age groups.

3.2.3. Suggestions for Improving Acacia Control

Controlling IASs is a task in which complementarity is required between specific actors and society. Focusing on the Forest and Nature Conservation Services, Academia, and the general public, we assessed how their complementarity may improve acacia control processes (Figure 8). According to respondents, more than 60% considered that the Forest and Nature Conservation Services should increase technical support from their experts and provide more information and available funding for acacia control measures in the coming years. More than 70% of the respondents considered that Academia should increase the number of workshops and training events, promote the demonstrations of best practices, and disseminate the information created during their research in a more accessible language. Finally, more than 60% of respondents considered it important to increase society’s knowledge of the acacia control problem and their will to contribute to controlling invaded areas in the coming years, given the high percentage of private forest areas in Portugal and the high costs involved.

A large proportion of respondents (80%) were aware of the policy measures of Decree-Law No. 92 of 2019 [32] on the detection and control of exotic species, although a similar share (82%) considered that the funding and support to acacia control available by the state were insufficient to comply with the required measures.

3.2.4. Funding for Acacia Control in Mainland Portugal

Our survey included questions to determine how the effectiveness of the allocated funds could be improved. Results showed that most respondents (98%) considered ongoing financial support to the invaded areas extremely important. However, opinions on redistributing these funds diverge both geographically and in time. Controlling the IAS of the genus Acacia requires several years and is hardly carried out without access to funding. The length of the current financial support from the state is, in most cases, up to two years without the possibility of applying more than once for treating the same area. Thus, many respondents believe that success in tackling this IAS implies funding projects with a timespan of more than three years. Respondents believe discrimination should be avoided based on the type of institutions requiring support (regarding land ownership and protection type). In addition, it was suggested that acacia control measures projects be integrated into regional and national territorial planning. Another issue the respondents raised was the need to train and create specialized groups to manage IASs. Finally, minority opinions suggested, e.g., that we no longer consider acacias as IASs due to the ineffectiveness of its control and starting to manage invaded areas as regular forest areas using acacia species as a raw material.

4. Discussion

4.1. Increased Knowledge of Acacia Control

The survey results confirm an increase in the acknowledgment of acacias as IAS compared with previous studies [53]. There is an increased understanding of the strong negative impacts of acacia spreading on Portuguese ecosystems, as they are amongst the most hostile invasive species [18,20]. Some survey respondents’ inability to distinguish among acacia species can constrain the choice of a more suitable control method, hampering the fighting process efforts [54]. The easiest to distinguish is the Acacia dealbata, followed by the Acacia melanoxilon, and the Acacia longifolia. Some acacias are very similar to other species, making it difficult to distinguish them. Some control methods can also be hard for respondents to evaluate due to their infrequent use or usage limitations (e.g., prescribed fire and biological means).

Our results showed that not all stakeholders dealing with acacia on their land in mainland Portugal take measures to control it. Nevertheless, the ones that control acacia apply mechanical controls, followed by chemical treatments and combinations of both. Silva [55] obtained similar results after surveying forestry operators working on controlling IASs in Portugal. These are general, non-specific methods that do not require specific professional training on IASs from the operational teams and have the option to receive funding from the RDP or RRP. These methods have a low cost-effectiveness ratio (they are expensive and require manual labor) and do not allow long-term control. Due to the abundant acacia seeds in the soil, the emergence of new shoots requires repeated control treatments for these plants and those that survived the previous control [43]. Respondents also recognize this challenge when assessing the sustainability of the different methods [35,36,37,38,39,40,41,42,43,44,45] and when referring to the need to increase the timespan for the funded projects.

A less general and more specific control method, such as biological control using an insect, was introduced in Portugal in 2015 after successfully being used in South Africa. This introduction followed consultation and permission from the national phytosanitary authorities and the European Commission. This specific biological method uses an insect (Trichilogaster acaciaelongifoliae) that is a natural enemy of Acacia longifolia, and has been demonstrated to be quite effective and inexpensive, though its use is limited [41]. However, the knowledge about this method has not yet become widespread among our respondents, but the ones who know it consider it an effective technique to fight Acacia longifolia [53].

It is fair to say that each method has pros and cons. From a physical effort and financial point of view, large-scale mechanical control is difficult to undertake [56] but is one of the safest approaches from an environmental perspective. Chemical control over large areas takes less time but is environmentally risky, while biological control is limited by the species present [41].

4.2. Shifts in the Process of Acacia Control

In recent years, there has been an increase in the diversity of actors controlling IASs. Such an increase occurred through volunteer actions for acacia control and other events promoted for wider audiences, e.g., seminars, workshops, citizen science activities, and collaborations with local communities [26,36,53,57,58]. The results of our study show that many survey respondents’ efforts simply focused on eliminating the tree, with little attention paid to the consequences of the invasion and the long-term results of the control efforts. Nevertheless, the respondents assume that more training and knowledge must be improved to increase their experience of the acacia control process. It will create the right conditions for longer and better adaptive management of acacia [59].

In Portugal, regardless of the acacia species, there are three phases recommended in the control process: first intervention (drastic reduction in the populations of IAS, which implies a high cost); monitoring (effective control of episodic outbreaks of the IAS in the medium and long term); and re-control, applied a few months after the first intervention (control of regenerated acacias by, e.g., root or seed germination) [60]. The implementation of these three stages, although guaranteeing high efficiency in dealing with acacias, is highly costly and directly affects managers’ decisions on planning control periods. The respondents are aware of the need to follow the three steps and the costs associated since the majority believe that current government financial support for the fight against acacias is insufficient and does not consider the need to repeat control in already treated areas.

Public awareness of IAS has steadily increased since 2005, but more work is still required [61]. Methodologies for tackling public awareness include theory, practice, or interactive activities involving participants over a long-term period. Actions such as identifying local knowledge and practices and developing participatory management programs at local levels must be considered [62]. These methodologies increase attractiveness and effectiveness in raising awareness and social learning [63]. As a lack of public awareness is recognized as one of the reasons for the ineffective management of IAS [53], it is important to pay special attention in the coming years to the knowledge and ability of the general public to identify acacia species, as well as their ability to apply and combine methods of dealing with them. Over the last four years, it has been promoted as an annual IAS week at the national and Iberian levels to raise public awareness of the IAS spread and control struggle. More than 836 activities, such as volunteer control actions, species identification (e.g., Bioblitz), and environmental education activities, were carried out during this period by 679 entities (e.g., schools, R&D centers, municipalities and NGOs) [58]. Several entities promote environmental education activities on IAS all year round. Schools develop activities through formal education (e.g., School of Silvares in Portugal [57,64], and Environmental Education Equipment through non-formal education (e.g., Centre for Applied Ecology ‘Prof. Baeta Neves’ of the Instituto Superior de Agronomia, and the Portuguese Association of Environmental Education, ASPEA).

Following our study, a communication effort should be carried out to deliver the existing information adapted to several target audiences. Thus, the communication channels identified as the most used (online publications and scientific events) should be reinforced and improved with more accessible and clear language. In addition, communication channels such as social networks should be further explored to understand how to improve their efficiency [63,65]. According to respondents, visits to acacia-controlled areas to demonstrate best control practices and other hands-on activities can be used in social networks to increase trust in science and learning with peers on acacia control.

In recent years, Portugal has spent many resources on acacia control. In recent years, EU and national funds have been permanently allocated to IAS prevention, control, and eradication. Nevertheless, it seems this problem is far from being solved [66]. The results of our study are enough to state that public awareness and environmental education should be one of the major targets of the funding to come. Building knowledge skills and training our target audience in the early detection of acacia species, especially in protected areas, will contribute to a more proactive rather than reactive management [59]. Given the struggle to control acacia invasions and the little attention paid by respondents to monitoring the treated areas and planning new control actions, adaptive management (including monitoring) seems to be one of the priority approaches to be fostered among them.

The National Forest Inventory 2015 estimates 16,600 hectares of the forest area covered by Acacia spp. [30]. Other studies consider that its occupation is larger and is still increasing [67]. In addition, projected climate change scenarios in the coming decades in the Mediterranean region [68] may increase the area where invasive Acacia spp. are present [69,70]. Acacia produces millions of seeds per hectare per year, creating huge reserves in the soil, and fire greatly stimulates their germination [42], which is challenging for Portugal considering the generally high wildfire risk in areas invaded by acacia. The time required to eradicate the acacia is similar to its seed viability, i.e., decades [8,12,14]. In Portugal, funding for acacia control activities is usually short-term and without an extension option for the same treated area [46]. In this sense, ensuring funding to control these IASs in the medium and long term is the way to mitigate their impacts. Otherwise, most previous management efforts would be compromised. One possible option to obtain extra funding would be introducing payments for ecosystem services through biodiversity restoration actions. It will likely increase landowners’ interest in managing their invaded lands [59]. From another point of view, there are examples worldwide that exploit some acacia derivatives, such as biomass pellets and high-value bioactive extracts, which could open the possibility of reducing the costs associated with its control [71,72]. Scientists have differing opinions on this controversial topic [73]. Similar conflicts of interest often arise when multiple stakeholders participate in IAS control. Many stakeholders are often unaware of the complex impacts of IASs and the potential benefits of their management, resulting in a lack of cooperation and support for management [59].

5. Conclusions

Stakeholders in mainland Portugal have different knowledge and experience regarding the invasion of the Acacia genus and their respective control methods. We developed and applied a 38-question survey to six profile classes of land managers. Results showed that acacia spread to almost all of mainland Portugal, and land managers recognized their presence. However, not all control measures were applied, and there was no significant difference between profile classes concerning the control.

Managers mainly applied mechanical and chemical control methods or a combination of both. Biological control is, above all, applied to Acacia longifolia, since a specific insect was introduced to control this particular species. According to the respondents, the more sustainable control method applied is the biological. This method is neither very well known nor commonly applied. The same occurs with prescribed fire.

The three following steps can help to improve the acacia control: (1) continued funding for long-term acacia control in the same invaded areas; (2) increased training actions and best practices demonstration in the field; and (3) increased public awareness and engagement in local communities and the general public into early detection and permanent control actions. Cooperation between all actors, increasing social learning, and becoming part of the solution are key factors in solving the acacia problem.