Rapid ontogenetic niche expansions in invasive Chinese tallow tree permit establishment in unfavourable but variable environments and can be exploited to streamline restoration.
Abstract
Reinvasion pressure is the rate of new exotic recruitment following mature exotic removal and it can vary broadly among similarly invaded habitats. Reinvasion pressure drives restoration outcomes and costs but is difficult to predict and poorly understood. Chinese tallow tree Triadica sebifera is a major invader demonstrating broad variation in average reinvasion pressure. We hypothesize this variation arises from differences in moisture regimes among habitats, which drive differences in Triadica recruitment but are masked because Triadica exhibits ontogenetic niche expansions (increases in niche breadth during development) that enable dominance in habitats where average recruitment success is low. We began testing this hypothesis by quantifying Triadica's capacity for ontogenetic moisture niche expansions. We performed greenhouse experiments examining how soil moisture affects germination likelihood and timing and how soil moisture and seedling age influenced seedling survival and performance, plus a field experiment quantifying survival and performance of differently aged seedlings within a restoration site that has exhibited low average reinvasion pressure. Moisture requirements for germination were the most constrained of any life stage. No germination occurred in saturated or flooded soils despite high seedling survival in comparable conditions. Germination timing varied among suitable moisture treatments. Seedling survival increased rapidly with age across moisture treatments and in the field. Above-ground biomass increased with age; tissue-specific performance metrics also increased but reflected seed provisioning, age-specific carbon allocation preferences, or tissue shedding for survival. Crucial moisture*age interactions that signify ontogenetic niche expansions were significant for all survival metrics and all but one performance metric. Survival and performance were generally highest in intermediate moistures, modestly reduced in saturated conditions and lowest in water-limited conditions. Synthesis and applications. Our results show Triadica exhibits rapid ontogenetic moisture niche expansions, which could decouple mature Triadica density and average reinvasion pressure. Therefore, density is an unreliable predictor of reinvasion but is commonly used to guide Triadica management, and cryptic opportunities exist for inexpensive and straightforward restorations. Reliable moisture niche-based estimates of Triadica's average reinvasion pressure are feasible and may improve restoration efficacy and efficiency by informing site selection and optimal management strategies.