Determining growth properties dictated strictly by genetics is difficult to distinguish from environmental variation among short rotation woody crops (SRWCs) particularly when comparing studies across years and site conditions. Growth and yield are highly impacted by varying environmental site conditions (i.e. precipitation, soil type, relief, temperature), meaning that generalized recommendations for genetically superior planting stocks cannot be easily made. In addition, many sites are limited in available nutrients, and management inputs, such as fertilizer application, are costly and increase carbon emissions from biofuel products. To evaluate indicators of growth across variable site and environmental conditions, physiological gas exchange measurements were gathered from three hybrid poplar (two Populus trichocarpa x P. deltoidesand one P. deltoides x P. maximowiczii), and three eastern cottonwood (P. deltoides) varietals on a monthly basis from two sites, one alluvial and one upland, located in the southeastern United States. Leaf area and dry mass were measured to obtain leaf mass per area (LMA) and calculate mass-based photosynthetic rates. At the end of the first growing season, height and diameters were measured at the upland site to assess relationships between physiology and growth.
Results/Conclusions
Averaged across the growing season, mass-based photosynthetic rates did not differ significantly between hybrid poplar and eastern cottonwood varietals across sites, averaging 0.22 μmolg-1 s-1. However, a genotype by environment interaction was significant for photosynthetic rates with hybrid poplars exhibiting higher rates at the upland site and eastern cottonwood exhibiting higher rates at the alluvial site. Transpiration rates and stomatal conductances were similar across sites and between hybrid poplars and eastern cottonwood and averaged 8.0 mmol m-2s-1and 0.57 mol m-2s-1 for transpiration and stomatal conductance respectively. However, water use efficiency (WUE) was significantly greater for eastern cottonwoods compared with hybrid poplars (2.83 vs. 2.58 mmol mmol-1 respectively) and WUE was significantly higher at the alluvial site compared with the upland site. At the upland site, hybrid poplars were significantly taller than eastern cottonwoods (4.83 vs. 4.08 m respectively) but had similar diameters (3.2 vs. 2.94 cm respectively). Across taxa, transpiration and WUE parameters were the only physiological parameters that were significantly correlated with overall height growth. Therefore, water availability is more important for growth than photosynthetic capacity and WUE across poplar genotypes at an upland site may be used to predict future growth.