Seasonal patterns of phenology, photosynthesis, and biomass production in 10 hybrid poplar clones

Background/Question/Methods

Recent and projected warming trends may significantly influence the timing of bud break, leaf senescence, and the duration of the growing season for deciduous trees. Furthermore, leaf photosynthetic capacity is not constant throughout the growing season, and changes in leaf area duration and leaf physiology driven by warming may alter annual carbon accumulation in trees. We investigate the seasonal patterns of leaf development, photosynthesis, and the resultant biomass of hybrid poplar trees in an irrigated common garden experiment at the Biosphere 2 research facility in Oracle, AZ. We used hybrid poplar lines typically grown at a range of latitudes to investigate the linkage between phenology and carbon gain in closely related genotypes. To quantify bud break and leaf senescence, we used USA National Phenology Network protocols to make observations of leaf phenophases. We used infrared gas analysis to monitor photosynthetic capacity (Vc_max and J_max) over the growing season, as well as the diurnal pattern of photosynthesis in each genetic line. To quantify total aboveground biomass, we coppiced the trees at the end of the growing season.

Results/Conclusions

We found that bud break occurred earliest and senescence began latest in lines suited to higher latitudes. Vc_max and J_max were not constant throughout the season, and peak values were observed in the middle of the growing season. Low values of Vc_max and J_max early in the season may have reduced the potential impact of longer leaf area duration on seasonal biomass accumulation, but differences between lines were evident. Phenology is a key control of potential carbon gain in plants, and in systems were water is not limiting, longer growing seasons may lead to greater annual carbon uptake.