Canopy architecture of clonal hybrid Populus: Implications for light reflectance, interception, and physiology

Various-sized monoculture plantation blocks of P. deltoides x P. nigra (DN) and P. trichocarpa x P. deltoides (TD) hybrid poplars were distinguishable in an AVIRIS image of a fiber farm in eastern Washington State. Presuming an operational plantation would use clones of equal productivity, three hypotheses were posited and tested: (1) the two Populus hybrids would have equal above-ground productivities; (2) clonal differences in canopy architecture would be linked to differences in canopy light dynamics; and (3) for equal clone productivity, differences in clonal hybrid leaf morphology and physiology must exist to compensate for differences in canopy architecture and light interception. Destructive harvests and canopy architectural and leaf physiological measurements were made from 1996 to 1998. Mean whole-tree aboveground biomasses and leaf areas were similar between the two clones. Significant clonal canopy architectural differences were: the DN clone had more branches, narrower branching angles, a deeper live crown, a more vertical upper canopy leaf angle distribution, and a lesser light extinction coefficient (k); the TD clone had fewer, more open branches, a shorter live crown, a more horizontal upper canopy leaf angle distribution, and a greater k. For DN and TD, leaf mass per unit area, chlorophyll, leaf nitrogen, leaf thickness, and cell size were influenced by relative canopy position but did not differ inter-clonally. Similarly, both clones shared similar leaf photosynthetic responses to controlled manipulations of light or CO2 conditions within a relative canopy position. A multi-layered canopy model with equal leaf photosynthetic parameters but different canopy architectural and light interception parameters was used to simulate whole-tree maximum potential net photosynthesis (MPNP) for DN and TD. Equally illuminated leaf areas, and consequently, similar MPNP for the two clones resulted when the model was parameterized with clone-specific means for vertical leaf area distribution (VLAD), k, and leaf angle distributions. Clonal differences in leaf angle most strongly influenced MPNP by altering whole-canopy leaf area illumination. Demonstrating the interactions between canopy architecture, light interception and leaf physiological processes provided a contemporary example of two plants operating within the same morphospace (sensu Niklas, 1999), demonstrating the flexibility of plants in achieving similar productivities.