COMPARISON OF ROOT AND MYCORRHIZAL CHARACTERISTICS IN PRIMARY AND SECONDARY RAIN-FOREST ON A METAMORPHIC SOIL IN NORTH QUEENSLAND, AUSTRALIA

Root biomass, root lengths, and mycorrhizal associations were compared in a series of primary and Acacia-dominated secondary rainforest stan ds on nutrient-poor, red podzolic soils developed from low grade Palae ozoic metasediments. Five soil cores to 200 mm depth were collected at random locations from each of 20 sites. Ten of these sites were in 20 -25 m high closed secondary forest (30-40 y old) dominated by Acacia a ulacocarpa and ten sites were located in primary, selectively-logged, rainforest (28-32 m tall). Arbuscular mycorrhizas were the only form o f association found in the primary forest sites. Ectomycorrhizas domin ated the secondary forest sites although arbuscular mycorrhizas were a lso present. The primary forest sites had significantly higher root bi omass (34.4 +/- 17.8 t ha(-1)) and root length (33,400 +/- 3,200 km ha (-1)) than the secondary forests (11.6 +/- 4.6 t ha(-1) and 25,200 +/- 4,800 km ha(-1) respectively), and this was interpreted as a reflecti on of the greater allocation of biomass to roots necessary to support the greater above ground biomass. The specific root length in the seco ndary forest (340 +/- 119 cm g(-1)) was twice that of the primary fore st (154 +/- 65 cm g(-1)) indicating that the trees in the secondary fo rests achieved a degree of soil exploration which was comparable to th at in the primary forest with less than half the biomass allocation to roots. The dominance of ectomycorrhizas in the secondary forest was a ssociated with the prevalence of Acacia aulacocarpa, and the results c annot be extended to other secondary forests in the region. The implic ations that the dominant ectomycorrhizal associations have for the pat terns of successional development and the patterns of species coloniza tion in these Acacia-dominated secondary forests are discussed.