Look to seedling heights, rather than functional traits, to explain survival during extreme heat stress in the early stages of subtropical rainforest restoration

Ecological restoration is urgently needed to arrest and reverse land degradation, reinstate ecosystem services and recover threatened species. Tree planting is a commonly used strategy to restore forests; however, planted seedlings often experience low survival and slow growth during the establishment period. Critical evaluation of factors that lead to poor establishment is vital to improve practice but is often constrained by a lack of monitoring and reporting. We took advantage of a large monitoring dataset (~7,000 native trees from 23 species) to interrogate an extensive suite of environmental conditions and plant characteristics that potentially contributed to poor survival and growth of seedlings in a restoration planting in southeast Queensland, Australia. The initial height of planted seedlings strongly influenced survival and growth. Intermediate heights generally performed best and the optimal heights for different species were significantly but weakly modulated by functional traits including SLA, lamina area and leaf structure (compound vs. simple). Survival was lowest when seedlings were planted in high bulk density soils on hot days, and growth was fastest in plots with higher average elevation. Synthesis and applications. In this large restoration experiment in Australia's subtropics, initial seedling height was a much stronger predictor of rainforest seedling survival than plant functional traits. We recommend planting rainforest seedlings between 25 and 35 cm tall, especially in restoration projects facing a high likelihood of hot weather during planting and establishment.