Jarrah Forest Restoration in Western Australia: Canopy and Topographic Effects



The restoration of the northern jarrah (Eucalyptus marginata) forest after bauxite mining is a major objective of Alcoa of Australia Limited. The typically variable and sometimes low emergence of broadcast seed of jarrah-forest plant species may relate to microclimatic changes associated with mining disturbance. This study examined the effect of the presence and absence of a canopy and topographic position in the post-mining landscape on the emergence of four canopy species (E. marginata, E. calophylla, E. patens, and E. diversicolor) and related these patterns to detailed measures of surface soil temperature and moisture. The absence of a canopy in the restoration appeared to result in adverse microclimatic conditions for the successful early establishment of E. marginata and E. calophylla from seed, particularly in the low topographic regions of the restoration. Emergence beneath a canopy compared to that in the open was 17% and 6%, respectively, for E. marginata and 23% and 2%, respectively, for E. calophylla. For both species, emergence was also greater at upland than at lowland open restoration sites (9% and 3%, respectively, for E. marginata; 4% and 0.3%, respectively, for E. calophylla). In contrast, canopy removal and position on the topographic landscape did not reduce the early establishment success of E. patens and E. diversicolor. Field measurements revealed that soils were drier and that diurnal temperature fluctuations were wider in the open restoration sites than beneath a canopy. Furthermore, cold conditions were more frequent at lowland than at upland restoration sites, suggesting the occurrence of cold-air drainage to Jew-lying areas. It is therefore possible that the field emergence patterns reflected the lower tolerance of E. marginata and E. calophylla than both E. diversicolor and E. patens to cold and dry surface-soil conditions. The ecological significance and practical implications of the results are discussed.