Net Photosynthesis and Growth of Three Novel Woody Species Under Water Stress: Calycanthus occidentalis, Fraxinus anomala, and Pinckneya pubens

Frequent episodes of water stress in managed landscapes have led the nursery industry to look for attractive woody species that perform well under extreme conditions of drought and flooding. We chose to evaluate three taxa with highly localized natural distributions in the United States, Calycanthus occidentalis (north-central California), Fraxinus anomala (northeastern Utah), and Pinckneya pubens (northeastern Florida), each of which may merit further use under cultivated conditions beyond their respective ranges. Although widespread cultivation of each taxon may not be possible as a result of limitations related to cold hardiness, we hypothesized that each species can tolerate extremes in soil moisture availability more so than their native habitats imply. Our objective was to characterize, under greenhouse conditions, how the quantity of soil water affects gas exchange of potted plants of each species. Plants were divided into five groups, each exposed to treatment conditions ranging from complete submersion to severe drought. Complete submersion killed plants of C. occidentalis and F. anomala, although in drought or severe drought conditions, C. occidentalis plants had lower net photosynthesis and less leaf area and plant dry weight than control plants. Net photosynthesis, leaf area, and plant dry weight of partially flooded plants, however, were not found to be significantly less than that of the control plants. Mean net photosynthetic levels and plant dry weights of severe drought, drought, and control F. anomala did not differ. While severe drought plants of P. pubens exhibited much lower levels of net photosynthesis, but not plant dry weights or leaf area, than the control plants, those exposed to drought, partial flood, and complete submersion were not found to differ in net photosynthesis levels from the control plants. Due to the sustained tolerance of F. anomala and P. pubens to a range of extreme soil moisture conditions, as exhibited by net photosynthetic responses, carbon accumulation, and survival, we conclude that use of these species in landscapes is warranted if invasiveness and other potential problems are not identified. Calycanthus occidentalis, however, appears unsuitable for cultivation in areas with organic soils greater than approximately equal to 66% and lower than approximately equal to 30% soil moisture content as a result of its high mortality in flooded conditions and poor physiological responses under dry conditions.