Individual study: Survival and growth of Banksia seedlings on a sand mine rehabilitation site, South Eneabba Flora Reserve, Western Australia
Enright N.J. & Lamont B.B. (1992) Survival, growth and water relations of Banksia seedlings on a sand mine rehabilitation site and adjacent scrub-heath sites. Journal of Applied Ecology, 29, 663-671
An area within a flora reserve 270 km north of Perth, Western Australia is mined for heavy minerals. Legislation requires that the mined areas are revegetated, specifically returning at least 80% of the native flora to restoration sites. In order to provide guidance to the restoration process, survival, growth and water relations of first-year seedlings of the shrub species Banksia attenuata, B.leptophylla, and B.hookeriana, were compared in scrub-heath and the sand-mine rehabilitation sites; first summer drought is known to be critical in natural recruitment of Banksia seedlings.
Study areas: Three representative sites were chosen in or near the South Eneabba Flora Reserve (29º55'S, 115 º17'E), Western Australia. These comprised: i) reconstituted soils in the 1989 mining rehabilitation block; a sandy dune crest where all three banksias occur naturally; and iii) a flat inter-dune area 500m from the dune crest dominated by low heath but lacking banksia.
Experimental design: On the dune crest and inter-dune sites, a flat area of 4 x 8 m was cleared of vegetation and major roots to 20 cm depth were removed. All three sites were fenced to exclude kangaroos (Macropodidae) and emus Dromiceius navaehollandie that might otherwise graze or trample seedlings. Locally collected banksia seeds of all three species were sown in autumn (May) 1989 (the normal time for rehabilitation coinciding with the natural pattern of germination with the onset of winter rains). The three species were sown together at differing densities (six seedlings placed equidistantly in rings of 1, 5, 20, and 40 cm radius) with 12 replicates at each site. More seeds were sown initially to ensure the required densities. Plants were thinned in July 1989.
Six replicate plots were left intact through the study to record plant stature, survival and stomatal conductance (five occasions at about 20month intervals fromSeptember1989-May 1990. At the end of the experiment Root systems were excavated (five seedlings of each species at each site) to record dry root mass, and tap root and major lateral root lengths. The remaining plots were used for destructive sampling to record xylem pressure potential (up to five times between spring 1989-autumn 1990).
Soil analysis: Soils were analysed for: nutrient concentrations; available phosphorous, ammonium nitrogen, nitrate nitrogen; basic cations; extractable iron; organic carbon; % gravel, sand, silt, clay; and soil impedance.
Weather: The pattern of temperature and rainfall was compared with the long term average for Eneabba. Total rainfall for the year May 1989 to April 1990 was slightly lower than the long-term median (489 mm vs. 526 mm). However, the summer was less dry (70 mm vs. 9 mm median rainfall) and less warm (32.5ºC vs. 34.9ºC mean daily maximum temperature) than usual.
Survival: Survival and growth of the broad-leaved B.hookeriana and B.attenuata were lower in the rehabilitation site (40 and 35% respectively) than either on the dune crest (55 and 67%) or the interdune site (both 67%), but the needle-leaved species, B.leptophylla, survived and grew equally well in all three sites (78% rehabilitation site; 77% dune crest; 70% interdune).
Root system: Root system development was poorer in the rehabilitation site as here the tap-roots stopped at the topsoil/subsoil boundary at 0.1 m, while they reached 2 m depth at the two natural sites.
Water relations: Seedlings in the rehabilitation site had much lower pre-dawn xylem pressure potentials (XPP) and stomatal conductance.
Soil water potentials (psi) in the top 0.5 m of the soil profile were generally lowest at the natural sites, and almost always lower than XPP. At these sites, seedlings must have been obtaining water over summer-autumn from deeper in the soil via their well-developed tap-roots. Topsoil psi in the rehabilitation site was more similar to XPP than the much higher values in the subsoils, which rarely contained roots.
Conclusions: Higher initial germination success in the rehabilitation areas was probably due to higher moisture levels in the newly deposited soils at the time of sowing, and also reduced insect herbivory compared to the two natural areas. Impedance of the rehabilitation soils was significantly greater than that at the natural sites and appeared to explain the poor root development and dependence of seedlings on soil water stored near the surface; and the consequent lower xylem potentials, stomatal conductances and growth rates. This ultimately led to higher mortality levels.
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