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

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.