| The interrelation between canopy structural and functional traits and light-use efficiency was examined across a variety of plant species (herbaceous crops, evergreen woody species, and deciduous woody species). Among structural and functional traits, canopy nitrogen content, adjusted for the fraction of photosynthetically active radiation intercepted by the canopy, and leaf mass per unit area were combined to create a canopy efficiency index (CEI) that was a significant predictor of light-use efficiency across a wide array of plant species, functional groups and growth environments. While numerous factors could have contributed to this finding, an underlying mechanism may be that CEI accounts for the dependence of light utilization on the quantity of photosynthetic machinery (canopy nitrogen content) and its inherent efficiency, which is negatively related to leaf mass per unit area.; Although, CEI provided a solid foundation for determining variation in light-use efficiency, I found that the canopy average for leaf conductance to water vapor (Gc) was strongly and positively related with residuals from the CEI versus light-use efficiency regression in monoculture stands of four poplar hybrids. It appeared that CEI failed to fully capture the implications of differing patterns of intra-canopy light distribution, as Gc and canopy light extinction were negatively related. Therefore, at a given CEI, a greater G c was associated with a higher light-use efficiency. As a result, a model containing both CEI and the canopy average for leaf conductance to water vapor was a better predictor of light-use efficiency than either variable alone.; Finally, I found that a fungal leaf spot adversely affected photosynthesis in two susceptible poplar hybrids and that this negative relationship was dependent upon the density of incident light. Declines in leaf photosynthesis seemed to result from a disruption of the photosynthetic apparatus. Consequently, the fungal leaf spot affected clone productivity and light-use efficiency, but the consequences of the pathogen for light-use efficiency appeared to be reflected by CEI and Gc. |
