Research articleConstructed pine log piles facilitate plant establishment in mining drylands
Introduction
Ecological facilitation between plants is one of the main processes that structure biodiversity and assemble communities in abiotically stressful ecosystems (Brooker et al., 2008). Nurse plant species colonize barren substrates and, in a period of a few years, significantly modify the microclimate and soil conditions in their influence area (Navarro-Cano et al., 2015). This process triggers plant nucleation through expanding the realized niche of other less stress-tolerant species (Bruno et al., 2003), thus shaping multispecific plant patches that work as isles of resources in barren areas (Brooker et al., 2008). Both the local alleviation of warm and dry conditions and the co-existence of evolutionarily distant species within the patch neighbourhood can eventually promote functional diversity (Navarro-Cano et al., 2019a). Nurse-based restoration practices have been implemented in research afforestation projects in drylands (Castro et al., 2002; Rey et al., 2009; Tongway and Ludwig, 2011; Kimiti et al., 2017). Wild nurse plants can also provide suitable microsites for the establishment of target species in mine tailings (Navarro-Cano et al., 2018). However, their usual low density and plant cover in these extremely stressful ecosystems can lead to a fragmentary and incomplete restoration.
Mining activities are among the most environmentally harmful industries worldwide. Mine pollutants can persist for decades and be exported through soil erosion and runoff to neighbouring areas. Many drylands across the world are major areas where metal-mining industry operates, and thousands of hectares of mine tailings from the metal concentration process still wait for restoration (Mendez and Maier, 2008). These tailings create barren and polluted micro-deserts where high metal and metalloid concentration, as well as salinity, coincide with warm and dry climatic conditions. All of these factors, together with mass movement risks, pose a big challenge for the use of conventional mine-restoration methodologies based on physical or chemical stabilization such as on-site containment by dam building, sealing with a geological layer or with organic adhesives (Tordoff et al., 2000; Gómez-Ros et al., 2013; Mendez and Maier, 2008). These expensive methods do not guarantee soil stabilization due to crack formation from wetting-drying cycles or earthquakes (Mendez and Maier, 2008). Phytoextraction through hyperaccumulator plants is usually a long-term and limited-efficiency method only applicable to low or moderate levels of pollution (Ali et al., 2013). For mine tailings enclosed within natural areas, phytostabilization poses a cheaper and environmentally friendly method, as it is based on the use of local metal-tolerant species and can be executed with a limited use of heavy machinery (Mendez and Maier, 2008; Conesa and Schulin, 2010; Navarro-Cano et al., 2018). The main limitations of this method are: i) its low effectivity in the short term due to the lack of sufficient plant cover, which needs time to grow, and ii) its partial effectivity for soil remediation, since this technology reduces soil erosion but can contribute to exporting bioaccumulated pollutants out of the system through herbivory or runoff of released leaf litter (Gómez-Ros et al., 2013; Parraga-Aguado et al., 2014). Recent advances in phytostabilization combine physical and organic amendments with plantation of local tolerants to accelerate the growth of a plant cover by extensive soil preparation methods (Zornoza et al., 2012; Gil-Loaiza et al., 2016).
Log piles made by packing bundles of woody material from pruning, logging or burnt wood are also named fascines and have been used as ecological engineering solutions for slope stabilization and erosion control (Coppin and Stiles, 1995). They are used as erosion barriers on slopes up to 45° (Howell et al., 1991). Densely packed bundles of fresh wood have also been used to vegetate riverbanks by rooting of live cuttings (see e.g. Evette et al., 2009). In less steep slopes in drylands, cut branches have been grouped (Padilla and Pugnaire, 2009) or piled (Ludwig and Tongway, 1996; Castillo-Escrivà et al., 2019Castillo‐Escrivà et al., 2019) to assist plant establishment, protect planted saplings or serve as bird perches. All of these techniques are absent in mine tailing restoration programmes (but see the pioneer experiences in Australia reviewed by Tongway and Ludwig, 2011), as their extreme edaphic stress has been traditionally considered an overwhelming filter for plant colonization. However, our recent experiments indicate that metal-tolerant pioneer plants that naturally recruit in mine tailings can significantly reduce the climatic stress beneath their canopy and subsequently allow the emergence of functionally-diverse plant communities (Navarro-Cano et al., 2019a). A diverse understory within these facilitation-driven patches also has a medium-to long-term effect on edaphic stress, mainly promoting soil fertility and microbial productivity both through biodiversity and biomass effects (Navarro-Cano et al., 2019b).
The Cartagena – La Union district (SE Spain) has a 2500 year mining tradition up to its closure in 1991. Currently, around 5000 ha are affected by mining structures, including up to 218 ha of highly polluted mine tailings. After three decades of abandonment, only 15 out of 89 tailings have been removed or capped (Conesa and Schulin, 2010). For most of the remaining tailings, their locations are relatively far from urban areas that has delayed their costly rehabilitation. Most of these tailings are enclosed by a valuable natural landscape, a feature that makes the use of phytostabilization a likey softer and cheaper restoration technology. Within the mining district, the Calblanque Natural Park holds one of the most prominent semi-arid Western Mediterranean forests. The necessity for better management of the fire-prone Pinus halepensis and Tetraclinis articulata mixed forest in the Park suggests reusing local burnt wood and woody debris from firebreaks for in situ restoration programmes. Based on previous field observations, we hypothesized that, besides a role as erosion barrier, pine log piles (fascines) can mimic the nurse plant canopies in barren tailings, thus facilitating the establishment of a plant community through the amelioration of abiotic stress conditions. We further expected, based on ecological theory, that the attenuation of temperature and radiation through fascine construction might increase the diversity of the facilitated community. We tested these hypotheses in a pilot restoration programme based on the pine fascine construction on an abandoned mine tailing. Through recording natural and sowed seedling recruitment underneath fascines and adjacent open areas we specifically tested whether fascines i) ameliorate the microclimatic conditions, ii) facilitate the establishment of taxonomically and functionally diverse plant communities and iii) improve the nutrient status of facilitated seedlings.
Section snippets
Study site and soil properties
We studied the effect of pine fascine construction on plant establishment in a mine tailing from Cartagena-La Unión Mining District (SE Spain; 30 S 689151 E, 4164433 N). The area is a coastal mountain range shaped by metamorphic nappes, limestones and igneous rocks where different heavy metals (iron, copper, lead, zinc, manganese, etc.) have been exploited since ancient times. The climate is semi-arid Mediterranean with 17.9 °C mean annual temperature, 316 mm precipitation and 762 mm
Fascines ameliorated microclimatic conditions
Fascines significantly decreased surface soil temperature (5 °C on average) and light intensity (56%), while increasing humidity (10%) compared to adjacent gaps (Fig. 2). Moreover, light intensity was significantly lower on the N side of the fascines (N Fascine). Overall, fascines shaped an ameliorated microclimate for seedling establishment.
Fascines facilitated natural plant establishment
Fifteen months after the construction of fascines, we found seedlings from 13 species. Most of them belonged to Pinus halepensis (60% of counts), followed
Discussion
Within the framework of a pilot restoration project of dry mining areas, we show for the first time that arranged log piles promote plant colonization processes by alleviating the abiotic stress similarly to the effect of nurse plants.
Conclusions
We constructed log piles on an abandoned mine tailing based on natural wastes from local forestry operations, thus avoiding transportation costs.
We show that logged wood can be arranged to create microsites that alleviate abiotic stressors, thus promoting plant establishment, functional diversity and a more balanced nutritional status of plants.
Turning log piles from silvicultural or agricultural management into a resource not only for erosion control but also for plant nucleation can help to
Author contribution
J.A.N.-C. designed and conducted the research. J.A.N.-C., B.O. and M.G. collected data. J.A.N.-C. wrote a first manuscript and all authors contributed to the final version.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The restoration project was funded by the LIFE Tetraclinis European Project (LIFE13 NAT/ES/000436). We acknowledge trust and support of Fundación Sierra Minera for the project implementation and the contribution of ANSE. Thanks to J.M. Escarabajal, G. Gutiérrez, D. López, D.A. Rodríguez, B. López, A.H. Montero and M. Bindang for helping with the field work. Five anonymous referees kindly reviewed this article.
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