The benefits of fertiliser application on tree growth are transient in restored jarrah forest

Abstract

The application of fertiliser, to both replace nutrients lost during mining and facilitate rapid vegetation re-establishment, is viewed as a key step in the restoration of post-mining landscapes. However, few studies have examined the long-term effects of a single initial fertiliser application on tree growth in restored sites. We report on a large-scale, fully replicated study that investigated the effect of an initial N and P fertiliser application (0, 80 and 120 kg ha−1 elemental N and P) on sites restored after bauxite mining. Growth of the two main jarrah forest tree species (jarrah - Eucalyptus marginata and marri - Corymbia calophylla) was monitored 9 and 20 years after the completion of restoration. After 20 years, soil NO3− and NH4+ were unaffected by N-application, although soil Colwell-P concentrations remained elevated following P-application. N-application had no effect on marri growth at either time interval, but increased jarrah diameter at breast height over bark (DBHOB), height and stand basal area at 9 years and DBHOB at 20 years. Applied-P increased height and DBHOB of jarrah after 9 years, but these effects did not continue. In contrast, applied-P benefitted marri growth (DBHOB and stand basal area) at both 9 and 20 years. Tree growth rates in the fertilised treatments declined more between the two-time intervals (0 – 9 years and 9 – 20 years) than the unfertilised plots, particularly for jarrah, suggesting that resource limits were reached more rapidly in the fertilised treatments. Further, for both N and P there was no additional benefit from application rates above 80 kg ha−1. These results demonstrate that while fertiliser addition may benefit initial growth in restored jarrah forest, the effects reduce with restoration age and may have limited practical benefit after 20 years.