Arnaud Monty, Alexis Jorion, Carline Pitz, Charly Géron & Grégory Mahy

Alien invasive plants in Belgian limestone quarries

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Résumé

Plantes exotiques envahissantes dans les carrières calcaires de Belgique

Description du sujet. Une végétation de haute valeur conservatoire peut s’établir dans les carrières, pendant ou après l’exploitation. Les plantes exotiques peuvent empêcher ce processus et représenter des couts supplémentaires lors de la réhabilitation. La situation concernant les invasions végétales en carrières est largement méconnue.

Objectifs. L’objectif était de quantifier les invasions végétales en carrières calcaires (en activité ou non) et d’identifier les secteurs des carrières les plus envahis.

Méthode. Nous avons échantillonné 6 692 placettes dans 31 carrières en Belgique et noté l’occurrence, la densité et le recouvrement des 65 espèces exotiques envahissantes listées au niveau belge.

Résultats. Quatorze espèces ont été observées et 25 carrières contenaient au moins une espèce. Les deux espèces les plus fréquentes, Buddleja davidii Franch. et Senecio inaequidens DC., étaient plus répandues dans les carrières en activité. Tous les secteurs étaient concernés par les invasions.

Conclusions. La problématique des plantes exotiques envahissantes dans les carrières calcaires est très variable, mais significative. Vu le potentiel écologique de ces sites, celle-ci devrait être mieux prise en compte.

Abstract

Description of the subject. Vegetation of high conservation value can establish in quarries, during or after exploitation. Alien plants could hamper this process and cause additional rehabilitation costs. However, the situation of plant invasion in quarries is largely unknown.

Objectives. We aimed to assess alien plant invasion in active and abandoned quarries, and to identify the most invaded quarry sectors.

Method. We surveyed 6,692 plots in 31 quarries in Belgium and recorded occurrence, density and cover of the 65 listed alien invasive plants in Belgium.

Results. Fourteen species were recorded and 25 quarries contained at least one species. The two most occurring species, Buddleja davidii Franch. and Senecio inaequidens DC., were more widespread in quarries in activity. All sectors of the quarries were affected by alien plant invasion.

Conclusions. Alien plant invasion in limestone quarries is highly variable, but significant. Considering the ecological potential of quarry sites, this issue should be better tackled.

Tabla de contenidos

1. Introduction

1Herbaceous plant assemblages of high conservation value, e.g. calcareous grasslands, can spontaneously develop or be implemented in limestone quarries (Prach et al., 2013; Pitz et al., 2018). However, invasion by exotic plants may represent a challenge to biodiversity conservation and ecological restoration in quarries (Hobbs & Huenneke, 1992; Pitz et al., 2019), since it can hamper the installation of native plants or alter ecosystem functioning (Vilà et al., 2011). It can also increase rehabilitation costs and alter the corporate image of mining companies through negative publicity on nature conservation issues.

2Human activity and disturbances have long been recognized as drivers of plant invasions (e.g. Alston & Richardson, 2006; Monty et al., 2013). Current operational quarries are generally larger; host more diverse machinery and more abundant transporter traffic than ancient (abandoned) quarries. As such, they might be more easily invaded, notably by colonizers, i.e. early successional species (Rejmanek, 1989).

3Limestone quarries exhibit a variety of habitat types, notably due to the exploitation that requires the delimitation of sectors. The topsoil is removed and piled on embankments that can serve as physical barriers reducing noise and dust pollution. The rock is blasted in the extraction pit and transported to the factory where it is processed and stocked. The rest of the excavated material is disposed on site in the form of spoil heaps. Several water bodies are also present. In some cases, parts of the quarry have been rehabilitated into forest or agricultural areas. As not all sectors have the same ecological conditions, they do not have the same ecological potential.

4If previous studies have reported the occurrence of particular invasive plants in limestone quarries (e.g. Prach et al., 2013; Gilardelli et al., 2015), no studies to date have evaluated the general situation of plant invasion. The present study aims at:

5– quantifying the occurrence, density and cover of alien invasive plants;

6– comparing the situation of alien plant invasion between active and abandoned quarries;

7– identifying the most invaded quarry sectors.

2. Materials and methods

2.1. Field survey

8The study was performed in 31 limestone quarries in Southern Belgium (Figure 1a), with a minimum area of 5 ha. All sites were limestone quarries, i.e. sites where calcareous rocks (limestone sensu stricto, dolostone, tuffeau stone or chalk) or limestone gravel were being extracted or have been extracted in the past. Fifteen quarries were still operational, whereas the 16 others were abandoned for at least five years. The quarries and sectors within quarries were digitalized using ArcMap 10.3 (ESRI©). The sectors were delimited as: facilities (buildings, roads and parking lots); extraction pit; spoil heaps; embankments; stocks; agricultural lands; forests; and water bodies. The latter was not surveyed for security reasons.

9The surveyed species were the 65 plant species of the official list of alien invasive species in Belgium (Branquart, 2016). Plots of 2 m x 10 m were surveyed across sites using a random walk (Figure 1b): after the first plot had been randomly located and surveyed, the operator walked 10 m in a random direction then located another plot to survey, and so on, until the operator had surveyed 1% of the site area. In the six largest sites (more than 80 ha) however, the number of plots was limited to 800. Each plot was assigned to a sector. If a plot covered two different sectors, it was assigned to the sector containing the largest proportion of the plot. A total of 6,692 plots (4,619 in active and 2,073 in abandoned quarries) were surveyed.

10For each plot, the presence, the number of individuals and the cover of all alien invasive plant species were recorded. For clonal species, ramets were considered as individuals. The cover was estimated according to the modified Braun-Blanquet scale (Barkman et al., 1964).

2.2. Data analyses

11The species occurrence (%) was calculated as the proportion of quarries where the species was present. The density of each species was calculated as the number of individuals per unit area and expressed in individuals·ha^-1. A Kruskal-Wallis test with the quarry activity as fixed factor (active vs abandoned) was performed using the number of occurring species in each site. A Kruskal-Wallis test with the quarry activity as fixed factor, using the average density and average cover at the site level, was performed for each species and for all species together. The average cover of each species was calculated based on the median value of each Braun-Blanquet class. The proportion of invaded plots was calculated for each sector i present in each quarry j as P_ij = N_ij/T_ij, where N is the number of plots where at least one invasive species was found and T the total number of plots. For active and abandoned quarries separately, a Kruskal-Wallis test was performed on the proportion of invaded plots with the sector as fixed factor.

3. Results

12In total, 14 alien invasive plant species were recorded. Their occurrence, average density and average cover in active and abandoned quarries are synthesized in table 1. Alien invasive species were recorded in 25 out of the 31 quarries. The three most occurring species were Buddleja davidii Franch. (19 quarries, 61%), Senecio inaequidens DC. (17 quarries, 55%) and Robinia pseudoacacia L. (14 quarries, 45%). The number of species ranged from 0 to 8 in active quarries and from 0 to 6 in abandoned ones, with medians of respectively 3 and 1 species. The Kruskal-Wallis test showed a difference close to significance in the number of occurring species according to quarry activity (H = 3.39; DF = 1; p = 0.066). The density and cover of all species considered together did not significantly differ between active and abandoned quarries (respectively H = 2.84; DF = 1; p = 0.092 and H = 1.19; DF = 1; p = 0.275).

13It is worth noting that three species of woody alien invasive plants (Rosa rugosa Thunb.; Prunus serotina Ehrh.; and Quercus rubra L.) were only recorded in active quarries, whereas four other species (Fallopia sachalinensis [F.Schmidt] Ronse Decr.; Ailanthus altissima [Mill.] Swingle; Rhus typhina L. and Mahonia aquifolium [Pursh] Nutt.) were only found in abandoned quarries.

14When considering each species separately, significant effects of the quarry activity were found for B. davidii density (H = 5.48; DF = 1; p = 0.019) and for S. inaequidens density (H = 7.38; DF = 1; p = 0.007) and cover (H = 6.94; DF = 1; p = 0.008). These two species were more widespread in active quarries.

15The proportion of invaded plots significantly varied among sectors in active quarries (H = 11.26; DF = 4; p = 0.024), being highest for spoil heaps (Figure 2). In abandoned quarries, no significant difference was found (H = 7.23; DF = 6; p = 0.300). It has to be noted that not all sectors were present or sampled in all quarries.

4. Discussion

16Our results indicate a significant level of alien plant invasion in limestone quarries, but with a high heterogeneity. Two highly occurring and widespread species, B. davidii and S. inaequidens, are pioneer wind-dispersed species typical of dry and open habitats (Hill et al., 2002; Monty et al., 2008). In active quarries these two species are most abundant and the number of occurring species is slightly higher. This can hamper future ecological restoration in those sites. However the overall density and cover of alien invasive plants did not significantly differ based on quarry activity. This indicates that disturbances are not the sole driver of plant invasion and highlights the continuation of the invasion process after the cease of the quarrying activity. Our survey notably revealed the importance of intentionally planted trees and shrubs, such as R. pseudoacacia, Cotoneaster horizontalis Decne., R. rugosa and P. serotina. Considering the ecological potential of quarry sites, managing plant invasions appears to be a necessity. It is important, from a cost/benefit point of view, to address the problem early in the invasion process. Preventing the introduction of the species recorded in this study in non-invaded quarries is a key aspect in the management of the problem. In parallel, running regular vegetation surveys should make early detection and eradication of emergent species feasible at minimal costs.

5. Conclusions

17Our survey revealed the importance of alien invasive plants in quarries and emphasizes the need for managing the issue. Woody species have often been intentionally planted for rehabilitation, in abandoned sites but also in active quarries. This should be highly discouraged.

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Acerca de: Alexis Jorion

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Acerca de: Charly Géron

Acerca de: Grégory Mahy