Many ecosystems experience frequent cloud cover and rainfall that alter the rate that plants fix carbon. Clouds scatter incoming radiation and result in apparent radiation at the plant canopy that is predominantly diffuse rather direct light. In addition, these clouds often bring environmental conditions (rainfall, cool temperatures, fog) that result in wet canopies. Despite this, our understanding of how the interactive effects of light quality and leaf wetting on photosynthesis remain largely unexplored and have significant implications for ecosystem productivity. We studied how photosynthetic rates respond to direct versus diffuse light and canopy wetting among canopy tree species in a tropical montane forest ecosystem.
Results/Conclusions
We demonstrate that there is significant variation in species-level response to light quality independent of light quantity. Three species demonstrated up to 100% higher rates of photosynthesis in direct versus diffuse light conditions, whereas two other species demonstrated 10 to 20% higher rates under direct light. While leaf wetting more consistently lowered rates of photosynthesis, variation among species still ranged from a 20% increase to a 60% decrease. The response to leaf wetting was explained by the stomatal density and leaf water storage. The variation in the response to diffuse light and response to wetting had similar tradeoffs, leading to species that maximize photosynthesis during direct light and dry periods and those that maximize photosynthesis during diffuse light and wet conditions. Our results have implications for the ecosystem productivity given projected changes in cloud cover and precipitation, particularly in wet and cloudy tropical systems that store much of the Earth’s carbon.