Limited biomass recovery from gold mining in Amazonian forests

Gold mining has rapidly increased across the Amazon Basin in recent years, especially in the Guiana shield, where it is responsible for >90% of total deforestation. However, the ability of forests to recover from gold mining activities remains largely unquantified. 
Forest inventory plots were installed on recently abandoned mines in two major mining regions in Guyana, and re-censused 18 months later, to provide the first ground-based quantification of gold mining impacts on Amazon forest biomass recovery. 
We found that woody biomass recovery rates on abandoned mining pits and tailing ponds are amongst the lowest ever recorded for tropical forests, with close to no woody biomass recovery after 3-4 years. 
On the overburden sites (i.e. areas not mined but where excavated soil is deposited), however, aboveground biomass recovery rates (0.4 - 3.5 Mg ha-1 yr-1) were within the range of those recorded in other secondary forests across the Neotropics following abandonment of pastures and agricultural lands. 
Our results suggest that forest recovery is more strongly limited by severe mining-induced depletion of soil nutrients, especially nitrogen, than by mercury contamination, due to slowing of growth in nutrient-stripped soils. 
We estimate that the slow recovery rates in mining pits and ponds currently reduce carbon sequestration across Amazonian secondary forests by ~21,000 t C yr-1, compared to the carbon that would have accumulated following more traditional land uses such as agriculture or pasture. 

Synthesis and applications. To achieve large-scale restoration targets, Guyana and other Amazonian countries will be challenged to remediate previously mined lands. The recovery process is highly dependent on nitrogen availability rather than mercury contamination, affecting woody biomass regrowth. The significant recovery in overburden zones indicates that one potential active remediation strategy to promote biomass recovery may be to back-fill mining pits and ponds with excavated soil. 

Methods
Plots were installed in two central gold mining areas in Guyana, namely Mahdia (5°16'0.01"N 59°08'60.00"W) and Puruni (6°00'0.00"N 59°11'60.00"W), from January to March, 2016, and re-censused approximately 18 months (June to August, 2017) later. Both sites are currently mined by artisanal and small-scale miners. At each study site, measurement plots were established on previously mined vegetation patches, ranging in age from 0.6 to 3 years since abandonment of mining activity at the time of establishment.

Nine 0.25 ha (50 m x 50 m) plots and a control old-growth forest plot (100 m x 100 m) were established at each location. However, approximately half of all plots (five in Mahdia and four in Puruni) were re-mined before the 2017 re-census and thus were excluded in this study. Each plot was positioned to include all three mining zones, i.e. the mining pit, the mine tailings and the overburden. Within each plot and within each mining zone, three nested subplots of 3m x 3m and 1m x 1m were established. All trees >2 cm DBH (diameter at reference height of 1.3m) were measured and tagged with a unique number in the larger 0.25 ha plot and identified to species level where possible. All tree saplings 25-200 cm tall and all tree seedlings 5-25 cm tall were identified and counted in the 3m x 3m and 1m x 1m subplots respectively. Nested subplots were also established in the 1-ha control plot, where all trees >10 cm DBH were measured and recorded and standard RAINFOR protocols applied to map, measure and identify trees (Phillips, Baker, Feldpausch, & Brienen, 2015). Tree species in all plots were identified by a local botanist.

Heights of larger trees were measured using a laser rangefinder, whereas a tape measure was used to measure the height of saplings and seedlings. In the re-census in 2017, the diameter of all individuals still standing from the first census was re-measured, along with all new recruits >2 cm DBH in each 0.25 ha plot and >25 cm and >5 cm tall in our nested 3m x 3m and 1m x 1m subplots respectively. Trees not present in the second census but present in the first census were assumed to have died. Vouchers were only collected for non-woody species which were unknown and logged at the Centre for Biodiversity, University of Guyana.