Elsevier

Ecological Engineering

Volume 159, 15 January 2021, 106097
Ecological Engineering

Survival and early growth of 51 tropical tree species in areas degraded by artisanal gold mining in the Peruvian Amazon

https://doi.org/10.1016/j.ecoleng.2020.106097Get rights and content

Abstract

Artisanal gold mining in Amazon forests and rivers has been reported in all Amazonian countries. Amazon mining has a wide range of negative effects and severe environmental and social consequences. Given that the activity in the region is mostly illegal, there are few studies published in the scientific literature on recovery of areas degraded by gold mining. This study conducts an experimental reforestation project aimed to evaluate soil degradation and explore the seedling survivorship and early growth of 51 tropical tree species in gold mined areas at 5 study sites distributed across the Madre de Dios region, in the Peruvian Amazon. The study evaluates the effect of biochar amendments on the survivorship and growth of 51 tropical species. The study also analyzes the influence of species wood density on seedling performance one year after planting. In order to inform further restoration plantation strategies, species were chosen with the end goals of timber production, biodiversity enhancement, and soil restoration. Site degradation, soil properties and mercury levels were analyzed in degraded areas and paired reference forest patches. Soils after gold mining are found to be highly degraded, with soil C being nearly absent, cation content greatly decreased, and loss of fine sediment. Soil mercury levels were found below national and international environmental quality standards. A positive correlation and a statistically significant relationship were found between survivorship and wood density. This reveals that the higher the wood density of the species, the higher the survival percentage. Growth and overall performance of mid, and especially low wood density species were significantly increased by biochar additions, while no effect was recorded on high wood density species growth. The study provides guidance on the post-ASGM restoration potential for 51 common and useful tree species and gives practitioners recommendations for combinations of species and fertilization treatments to optimize restoration designs.

Introduction

The Amazon has great potential for mineral production, particularly copper, tin, nickel, bauxite, manganese, iron ore, and gold (Sonter et al., 2017). As a result, governments are providing incentives in order to boost mineral resource development. As extractive technologies improve and the prices of commodities remain high due to uncertainty in global markets, it is likely that the scale of Amazon mining will increase, particularly illegal and poorly regulated artisanal and small-scale gold mining (ASGM) (Alvarez-Berríos and Aide, 2015). While illegal mining has always been present in the region, current levels are unprecedented. A recent analysis reported 2312 illegal gold mining sites with more than 500,000 informal gold miners active across six South American countries – Venezuela, Brazil, Bolivia, Colombia, Ecuador and Peru (RAISG, 2018).

Artisanal and small-scale gold mining is responsible for the largest fraction of forest loss and disturbance in the Amazonian region of Madre de Dios, in Peru (Caballero et al., 2018). ASGM is unique compared to other drivers of deforestation because of the severity of its impacts, leaving a highly altered landscape (Peterson and Heemskerk, 2001; Asner and Tupayachi, 2017) and threatening local communities, including indigenous people, by affecting the quality of the food supply (Diringer et al., 2015). Mining causes the destruction of natural ecosystems through removal of soil and vegetation, and contamination with heavy metals beneath waste disposal sites (Cooke and Johnson, 2002).

Previous studies on abandoned mining sites in the Neotropics suggests that forest recovery following ASGM is slow and qualitatively inferior compared to regeneration following other land uses (Parrota & Knowles, 2001; Peterson and Heemskerk, 2001; Rodrigues et al., 2009). It has the lowest residual forest carbon of any other land use, and leads to loss of ecosystem services, removal of fine sediments, defaunation, severely impaired water quality, and mercury contamination of soil, water, and air (Román-Dañobeytia et al., 2015; Wyatt et al., 2017). Compared to normal soils, mining substrates can present extreme challenges to plant colonization and the formation of any kind of self-sustaining ecosystem (Bradshaw, 1997; Ilunga et al., 2015; Buch et al., 2017).

Most reforestation projects centered on mining recovery in the tropics fail due to inappropriate knowledge on species choice, their growth and survival rates under different site conditions, as well as their fertilization needs in areas degraded by gold mining (Cooke and Johnson, 2002). Species selection based on functional traits is also of vital importance for the success of restoration or rehabilitation plantations (Baraloto et al., 2010). Wood density has been shown to be a key functional trait in tropical forest communities (Chave et al., 2009) and is being used to predict performance of tree species in degraded areas (Charles et al., 2017). Despite the evidence of a survival–growth trade-off in tropical trees (Wright et al., 2010), early successional species with low wood densities are often selected for restoration projects under the assumption that restoration plantings follow the same recovery trajectory as naturally regenerating forests (Berenguer et al., 2018). However, there is growing evidence that planting high wood dense species with slower growth rates may also help to maximize mid-term restoration success (Cardinale et al., 2007; Ostertag et al., 2015).

Successful restoration following mining usually depends on transplanting seedlings and applying soil amendments such as fertilizer, lime, and organic manure, on severely degraded sites as a means of catalyzing forest succession (Ni et al., 2015). Biochar, or charcoal intended for use as a soil amendment, has received great attention in recent years because of its recalcitrance (at least 100–200 years and up to 1000s) and potential for enhancing carbon sequestration and soil properties (Thomas and Gale, 2015; Purakayastha et al., 2019). Also, some studies have documented reduced bioavailability of heavy metals and sulfates in the presence of biochar incorporated in contaminated soils (Beesley et al., 2010; Borchard et al., 2012). Though promising, biochar's potential for land remediation and for improving plant growth in areas degraded by gold mining has not been explored so far (Wang et al., 2012; Lefebvre et al., 2019).

In Madre de Dios, government and community views on post-mining land use have resulted in regulations and programs emphasizing the reestablishment of native ecosystems inside protected areas that were illegally mined (SERNANP, 2017), as well as the establishment of tree plantations and improved agroforestry systems near indigenous or mestizo communities. In both of the cases, the species most suitable for restoration goals in soils altered by ASGM, soil amendments, and position on the landscape that offer greatest success, are still unknown (https://www.nature.com/articles/d41586-020-00119-z).

Here we report the results of a large-scale restoration experiment distributed across the Peruvian department of Madre de Dios to develop strategies for the recovery of ASGM mined lands. The main objectives of the study were to (1) test 51 species with potential for ecological restoration and/or forest rehabilitation; (2) assess the potential for biochar amendments for use in reforestation efforts; and (3) explore species performance regarding their contrasting wood density traits. To enhance our understanding of the functional response of planted species, we used wood density, a key plant functional trait, as a predictor of species seedling performance. Due to the fact that fast growing early successional species starts the regeneration process and later successional species move in to replace them (Whitmore, 1989), we predicted a negative relationship between seedling performance and species wood density. We also hypothesized that biochar and fertilizer amendments will improve seedling performance, especially of lower wood density species (Huante et al., 1995; Lefebvre et al., 2019).

Section snippets

Study area

The study was conducted in five gold mining sites totaling 7.6 ha in the southeastern Peruvian Amazonian region of Madre de Dios, province of Tambopata, districts of Laberinto and Inambari (see Table 1 for specific locations and site characteristics). The study sites were located between 220 and 360 m and had a seasonal tropical climate (MAT 25 °C) with a mean annual rainfall of 2200–2400 mm concentrated in a ~ 8 month wet season (Malhi et al., 2002). The forest types of the region are

Survivorship and growth among species

One year after plantation establishment, mean seedling survival percentage across sites and treatments ranged from 41% to 66%. Survivorship was >75% only for 4 of 51 species tested, two of those being high wood density species (Syzigium and Flemingia) and the other two mid wood density species (Copaifera and Tithonia). Survivorship was 50–75% for 22 species, including 5 high wood density species (i.e. Dipteryx, Calliandra, Hymenaea), 13 mid wood density species (i.e. Inga, Acacia, Amburana),

Discussion

In our study, at least 27 of 51 species (53%) recorded acceptable performance standards in ASGM areas one year after planting. Species from the leguminous family, both low, mid and high wood density species (i.e. Erythrina, Inga, and Dipteryx, respectively) were ranked with the highest diameter and height growth rates, revealing the great potential of this family for soil rehabilitation in areas degraded by gold mining. The inclusion of different N2-fixing tree species with different life

Conclusions

Species survivorship and growth in ASGM substrates were strongly influenced by species wood density, and in many cases greatly modified by soils amendments with pure and enriched biochar. High wood density species showed higher survival rates than lower wood density species. Growth and overall performance of mid, and especially low wood density species was significantly increased by biochar additions. Therefore, planning of restoration plantings in areas degraded by gold mining, would include

CRediT authorship contribution statement

Francisco Román-Dañobeytia: Conceptualization, Data curation, Formal analysis, Investigation. France Cabanillas: Investigation, Methodology, Supervision. David Lefebvre: Methodology. Jhon Farfan: Methodology. Jesús Alferez: Methodology. Fredy Polo-Villanueva: Methodology. Juana Llacsahuanga: Methodology. Claudia M. Vega: Methodology. Manuel Velasquez: Methodology. Ronald Corvera: Supervision. Edith Condori: Supervision. Cesar Ascorra: Project administration, Supervision. Luis E. Fernandez:

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.

Acknowledgements

We thank the team of field assistants that helped carry out this study, in particular Massiel Cueto, Martín Pillaca, Jorge Caballero, Vivian Ruiz, Aldo Rivas, Shamir Delgado, Midori Chinen, Miguel Macedo, Yulissa Barrios, Alfredo Rivera, Judith Soethe, Alex Boyd, Julinho Quispe, Yhonatan Calla, José Carlos Valadez, Raul Nava, Daniel Sotelo and Miguel Torres. We thank landowners and concessionaires Pedro Ynfantes, Jose Flores, and Policarpo Monzon who gave permission to work in their areas. We

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