Ecosystem restoration targets are increasingly citing higher resilience and adaptive capacity as desired project outcomes in response to the challenges posed by rapid climate change. One strategy to increase climate resilience in restored communities is to selectively harvest and transplant seed from regions that are historically similar in climate space to the projected future climate of the restoration site (assuming adaptation to local conditions). Here, we investigate the potential for climate-adaptive seed sourcing in the Warren Catchment of south-west Western Australia using a full reciprocal transplant experiment of the riparian tree, Eucalyptus rudis, to identify mechanisms underpinning observed trait differentiation in natural populations across a 1200-550 mm per annum (mmpa) rainfall gradient. We reared 1,880 seedlings under greenhouse conditions using seed sourced from 31 maternal lineages across the rainfall gradient, before transplanting them to six sites where survival, growth and leaf traits were measured at 6, 18 and 30 months post-transplant.
We show that E. rudis responses are highly plastic when transplanted to drier climates: seedlings sourced from high rainfall sites were indistinguishable in responses traits from low-rainfall sourced seedlings. Under wetter conditions, however, we identified conserved growth traits in maternal lineages sourced from low-rainfall sites. This effect was only detected in individuals transplanted 400 mmpa greater than their source, a shift in climatic space which exceeds current projections to 2090 for our restoration sites. We demonstrate that while a dry-adapted provenance exists with the potential to enhance adaptive capacity in our restored sites, naturally high plasticity in early establishment traits confers substantial resilience to future climatic changes without intervention in this keystone riparian species.