Early life conditions and precipitation influence the performance of widespread understorey herbs in variable light environments

1. The understorey of tropical forests is heterogeneous across time, and plants that inhabit this layer may exhibit adaptations (e.g. trait plasticity) that enable them to exploit this variability to their advantage. We tested the hypothesis that two widespread understorey herbs would perform equally well in a variable as in a constant environment, using a 2-year shade-house experiment. 2. We measured demographic traits (growth and survival), a physiological trait (maximum photosynthetic capacity), and life-history traits (leaf life span and biomass allocation) of Heliconia tortuosa and Calathea crotalifera. We investigated how these traits were affected by light availability at the seedling stage, precipitation, and whether individuals experienced a constant or variable light environment. 3. Whether or not a variable environment was favourable for plants depended upon precipitation and the environment in which individuals started life. At low precipitation, plants in a variable light environment grew more than those in a constant environment, but only when individuals had lived as seedlings in low light. At high precipitation, plants in a constant environment grew more than those in a variable environment, regardless of early conditions. Survival was lower in a variable environment at low precipitation, and more so at high precipitation. Photosynthetic capacity was lower for individuals in a variable environment than in a constant environment when they had lived in high light as seedlings. 4. Calathea grew faster and survived more poorly than Heliconia, independently of the treatments. Calathea grew more at high than low precipitation while Heliconia grew more at low than high precipitation. Leaf life span and biomass allocation did not differ among treatments, although Calathea had a significantly greater proportion of its biomass above-ground vs. that of Heliconia. 5. Synthesis. Environmental variability had a neutral or positive effect on biomass allocation, photosynthetic capacity, and leaf life span for these species. Survival was the only trait that was always lower in a variable environment. The effect of environmental variability was dependent on early life conditions as well as precipitation, suggesting that generalist species may experience high fitness as forest environments become more variable by maintaining high growth at the expense of survival.