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Establishment of native vegetation on a closed coal mine without soil cover

Rehabilitation of disturbed land is an important aspect of mine closure, and revgetation of mine wastes is a major step in this rehabilitation process. One of the greatest problems encountered during mine site revegetation is that the main growing medium (usually crushed rock) has not supported plant life previously, and is typically poor in nutrients. Some mine waste may also be toxic to some plants. Hence, definition of suitable species and surface preparation is necessary for each mine site, to ensure that appropriate planting can be done from the outset.

This study involved establishment of nursery-raised seedlings at an opencast coal mine site near Kaitangata, south-east Otago. The mine was developed in quartz conglomerate interlayered with siltstones and multiple coal seams. The coal-bearing strata were overlain by a thin (1-4 m) veneer of wind-blown loess silt derived from schist bedrock.

The mine site has been undergoing final rehabilitation since 2001 by Solid Energy N.Z. Ltd, and has involved the planting of over 100,000 seedlings into a range of substrates. Planting at the mine site began in autumn 2003, and dead seedlings were replaced on an annual basis as part of a maintenance programme. This study examines only those seedlings planted during the original phase of planting at the site, which was completed at the end of 2003. The survival and growth of seven indigenous species were examined in three different types of mine waste: quartz-rich, silt (loess)-rich, and coal-rich waste rock. Seedlings were planted with individual peat-rich root blocks.

sample selection of 463 individual seedlings was included in this study, and these plants were initially tagged. The first assessment of seedling status was made in September 2003, approximately three months after planting. Repeat measurements were made occasionally until September 2006. Plant growth in the different substrates was compared using species with an adequate number of survivors, i.e. kohuhu, koromiko, and manuka. The height of individual seedlings at the time of first measurement in 2003 was compared to height attained in 2006 to assess relative growth during the study period

Native shrubs planted on loess substrate in 2003, and photographed here in early 2007. Weeds between shrub plantings have been suppressed with herbicide
Shrub planted in coal-rich substrate in 2003 and photographed here in early 2007. No herbicide was needed to suppress weeds between plantings on this substrate
Survival of the selected seedlings three years after planting was low on all substrates, but seedling deaths were particularly acute on coal-rich waste rock
Plant height growth was generally low on both quartz-rich and coal-rich waste rock. Height growth after three years on loess was twice that on any type of waste rock type. Poor survival on coal-rich waste rock is probably related to low moisture levels, substrate acidity, and boron toxicity

It is notable that natural colonization by manuka (Leptospermum scoparium) from nearby stands has occurred on all substrates. This colonization has been most effective on coal-rich substrate.

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