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

Abstract

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.

Climate and species stress resistance modulate the higher survival of large seedlings in forest restorations worldwide

“…We argue that the contradictory results on the relationship between seedling size at planting and outplanting survival may be due to inherent differences in species' functional characteristics, which could modulate the effect of seedling size on outplanting survival (Gardiner et al 2019), and this effect could be contingent upon environmental conditions at planting sites (Charles et al 2018). Increasing seedling size may positively affect outplanting survival in stress-resistant species because they may more effectively offset the negative ecophysiological effects linked to large size, whereas the opposite may occur in less stress-resistant plants.…”

“…We argue that the contradictory results on the relationship between seedling size at planting and outplanting survival may be due to inherent differences in species’ functional characteristics, which could modulate the effect of seedling size on outplanting survival (Gardiner et al. 2019), and this effect could be contingent upon environmental conditions at planting sites (Charles et al. 2018).…”

Better left alone: Trying to control pasture grasses in untended rainforest plantings incurs multiple costs and delivers few benefits

“…Funded by the Australian Government's ‘20 Million Trees’ programme, the project incorporated an experiment investigating two distinct species mixes (‘ Fast ’ and ‘ Hardy ’). Tree species were selected using a trait‐based target approach (Gardiner et al., 2019; Laughlin, 2014); the ‘ Fast ’ mix contained species with traits reflecting high resource acquisition, whereas the ‘ Hardy ’ mix included species with traits reflecting more conservative strategies. Twenty‐four species were selected (Table S1 in the Supporting Information), with ‘ Fast ’ and ‘ Hardy ’ mixes each possessing eight unique species and sharing eight species with intermediate trait profiles.…”

“…Functional trait data were sourced for all 24 study species from earlier research at the study site (Gardiner et al., 2019). Traits included specific leaf area (SLA), leaf dry matter content, lamina area, leaf type (simple or compound), wood density, root tissue density and specific root length.…”

“…The previous investigation at this site (Gardiner et al., 2019) focused on survival and growth of planted seedlings during the establishment period (i.e. seedlings were monitored in February 2017, 3.5 months after planting).…”