How ontogenetic niche shifts in the moisture tolerances of an exotic tree influence its invasion dynamics

Background/Question/Methods:  Water is crucial to transitions between biomes because species have finite moisture tolerances and moisture availability shapes competitive hierarchies. Many invasive species exhibit both broad abiotic tolerances and high competitive ability, which promote successful colonization, establishment and spread, and can be influenced by individual age and/or size. When abiotic tolerances change during development of an individual, such changes are termed ontogenetic niche shifts (ONS). If a plant’s abiotic niche broadens as individuals grow, populations could persist within and even dominate habitats with abiotic conditions typically unsuitable for recruitment but sufficiently variable to temporarily permit germination and growth to a more tolerant stage. Thus ONS can decouple average recruitment success from mature plant density in variable environments. Triadica sebifera (Chinese tallow tree) is a woody invader with broad abiotic tolerances and high competitive ability. Experimental restorations of Triadica-invaded ecosystems demonstrate broad variability in average Triadica recruitment, suggesting aforementioned decouplings may occur in this system and could have profound implications for restoration and management. We asked: (1) Does Triadica exhibit ontogenetic niche shifts in its moisture tolerances? (2) How do soil moisture, propagule availability and competition influence Triadica recruitment? To address (1) we performed greenhouse and field experiments quantifying germination, survival and performance of Triadica seeds and differently-aged seedlings along a range of moisture treatments. To address (2) we conducted eleven replicated experimental restorations of Triadica-invaded habitats along a geographic moisture gradient and established mesocosms at each site manipulating moisture and native and exotic seed abundance in a full-factorial design.

Results/Conclusions:  (1) Moisture requirements for Triadica germination are more restrictive than for seedling survival, and moisture tolerances of seedlings broaden rapidly after germination. Seedlings aged ≥14 days tolerated permanent flooding, and we observed increased drought tolerance after 28 and 36 days. (2) Preliminary results suggest site identity influenced native plant community composition and abundance, and all metrics of Triadica survival and performance. Moisture treatments impacted Triadica germination and native plant community composition and abundance, and Triadica abundance correlated with average soil moisture. Triadica germination, performance and abundance were strongly affected by Triadica seed addition. Native seed addition influenced native community structure and abundance and Triadica performance but not survival. The native seed*moisture interaction influenced total Triadica abundance. To identify predictors of Triadica recruitment we tested for correlations between initial site characteristics and recruitment success. Triadica seedling or sapling abundances are far better predictors of recruitment than mature Triadica density.