Abstract
For understory plants in tropical forests, light strongly influences rates of growth, survival, and reproduction, i.e., vital rates. To better understand how light availability influences the vital rates of two co-occurring understory herbs, Calathea crotalifera and Heliconia tortuosa, I monitored their growth, survival, and reproduction in forest plots. Plant size influenced the effect of light on vital rates, and increasing light did not always increase vital rates. Both species grew at small sizes but shrank at larger sizes, and larger individuals were more sensitive to changes in light than small individuals. I also found evidence of tradeoffs among vital rates, which were influenced by the interaction between plant size and light. These results support the hypothesis that life stage (ontogeny) influences the ability to capture and utilize light, and reveal that high light may negatively influence the demographic performance of plants that are adapted to deep shade. To better understand how the ability to capture and utilize light influences growth, I estimated photosynthetic light responses for individuals in the forest plots and used model averaging to determine the importance of size, light, and photosynthetic responses for estimating future size. I found minor differences between the species in their photosynthetic traits, but found significant differences in the importance of size, light, and physiology on growth. Calathea that diminished in size had one of two combinations of photosynthetic efficiency and respiratory costs, either higher respiratory costs coupled with lower photosynthetic efficiency, or, higher efficiency coupled with maximum photosynthetic capacity compared to individuals that increased in size. Heliconia that diminished in size also had different combinations, lower respiratory costs coupled with photosynthetic capacity or lower efficiency, coupled with lower respiration, and lower photosynthetic capacity than individuals that increased in size. These results do not support the hypothesis that that shade tolerant species should have high efficiency but low respiration and low photosynthetic capacity and therefore they indicate differences in mechanisms and degrees of shade tolerance among species. I used a shadehouse experiment to determine whether demographic traits and functional traits were positively influenced by variability in light, light availability during the seedling stage, and soil moisture. I measured growth, survival, leaf lifespan, photosynthetic capacity, and biomass allocation of Heliconia and Calathea over two years. Plants in a variable light environment had greater growth than those in a constant light environment when moisture was low. At low moisture, a variable light environment increased growth when individuals started in low light and had no influence on growth when individuals started in high light. At high moisture, a constant light environment increased growth whether individuals started in low or high light. Survival decreased with increasing environmental variability but more so at high moisture. Photosynthetic capacity decreased for individuals in a variable light environment, when they had lived in high light as seedlings, but was unaffected by environmental variability when they had lived in low light as seedlings. Calathea had a significantly greater proportion of its total biomass aboveground than Heliconia. Leaf lifespan was unaffected by the treatments. Thus, although these species inhabit highly heterogeneous and variable light environments, these results do not support the hypothesis that environmental variability positively influences demographic and functional traits. Instead they reveal that environmental variability may be stressful even for plants found in intrinsically heterogeneous environments. They may have low plasticity, i.e., a low capacity to acclimate. To determine the effects of static and dynamic light environments on population growth rates, I used Integral Projection Models. Growth was modeled as a function of plant size, maximum photosynthetic capacity (Amax), and light, and all other vital rates were modeled as functions of plant size and light. I estimated the population growth rates for both species over a range of light levels and Amax values. Finally, I evaluated three types of elasticity (proportional sensitivity) of the population growth rate for three levels of Amax: perturbations in the mean and variance of vital rates (ES), increased variance of vital rates (ESσ), environment-specific perturbations of vital rates (ESβ). The latter are especially of interest as it addresses the relative impact on overall fitness of events that occur in different light environments, in other words the potential strength of selection of events that occur in high light vs shady environments. Adaptation to shady environments means a higher impact of events that occur in the shade on fitness whereas adaptation to high light environments means a higher impact of events that occur in high light on fitness. As light availability increased, the population growth rate (λ) increased for Calathea but shrank for Heliconia, and increasing Amax had no effect on λ for Calathea but increased λ for Heliconia in low light. As Amax increased, the population growth rate in a dynamic light environment (λS) increased for Heliconia, but not Calathea. These results suggest that Calathea is more strongly adapted to shade than Heliconia and indicates that increasing the ability to use light has a direct positive influence on population growth, and therefore fitness. Photosynthetic capacity (Amax ) had an impact on how sensitive the population growth rate was to changes in life history rates for Heliconia, but not Calathea. Calathea λS was most sensitive to perturbations in intermediate-sized individuals under high light, and changing Amax had little effect on this relationship. When light availability was low, elasticities were more widely distributed among the size classes, but λS was much more sensitive to seeds and seedlings. In contrast, Heliconia λS was sensitive to intermediate- and large-sized individuals when light availability was low, and became much more sensitive to seeds and seedlings as light availability increased. Changing Amax had much more of an effect on elasticity of Heliconia when light was abundant than when light was scarce. These results demonstrate that photosynthetic physiology can have large consequences for the population dynamics of plants in both static and dynamic light environments, and that the effect of light on population dynamics is influenced by photosynthetic rates. In conclusion, I found that increasing light and increasing the capacity to use light does not always improve demographic performance for plants adapted to living in the shade. Plant size interacts with light availability to influence rates of growth, survival, and reproduction. Growth is in turn influenced by different combinations of physiological responses for my study species. I found that the effect of light variability is influenced by soil moisture and early life conditions. Finally, population growth rates, an indicator of fitness, are significantly influenced by photosynthetic capacity for one species but not the other, and reflect differences in the ability to use light. The broader impact of this study is that physiological responses can be used to predict the fates of species in temporally variable environments.