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.