4.1 Site effects on growth and survival
We used three common garden sites to contrast tree performance across two major environmental axes commonly encountered by restoration practitioners: nutrient availability (DRO vs Cow Bay) and climate (DRO vs Thiaki). We found a substantial impact of site on both survival and growth of tropical trees planted under common garden conditions, with trees at the DRO gaining on average over twice the height increment seen at the other two sites. This resulted in the DRO planting site reaching effective site capture and partial canopy closure after 1.5 years of growth with a planting density of just 1100 trees ha-1, far lower than the planting density commonly employed in the Australian Wet Tropics (Goosem & Tucker 2013). In contrast, poor overall growth at the Cow Bay site necessitated continued site maintenance. The differences between the DRO and Cow Bay highlight the significant impact that edaphic context can have on tree growth and restoration success. The DRO and Thiaki sites were not limited in nutrient availability, and differed primarily in climate conditions, with the DRO site experiencing on average higher temperatures, vapour pressure deficit, and rainfall than Thiaki. The higher growth rates observed at the DRO compared to Thiaki are consistent with elevational patterns of growth observed in natural populations across the Australian Wet Tropics (Bradford et al. 2014, Bauman et al. 2022). This has been linked to warmer temperatures stimulating growth and biomass accumulation in tropical tree species that can be masked or limited by the associated increase in vapour pressure deficit (Smithet al. 2020, Middleby et al. 2024a). While the contrasting temperatures of the DRO and Thiaki sites likely contributed to the growth differences across these sites, it is also important to note the site differences in precipitation and therefore soil moisture. Although both sites receive relatively high levels of mean annual precipitation, the cumulative rainfall at the DRO was over 2x higher during the study period compared to Thiaki. We therefore cannot rule out the effect that soil moisture may have had on plant performance during this trial.