Massive tree planting initiatives are underway in communities across the world. A major hurdle in tree planting projects is transplant shock, which occurs when newly planted trees grow slowly and exhibit signs of stress. This usually results from a tree having limited access to water either because part of its root system was removed during the transplant process and/or because of lack of irrigation. Research is lacking on the interaction of species and stock size on transplant shock and the associated tradeoffs between planting trees from large or small stock sizes, determined by container size or the size a tree was when dug from a field. We used leaf-level gas exchange to track changes in tree physiological function over time to assess the variability of acclimation time for four stock sizes of six different species (n=10; total of 240 trees) following transplanting next to a highway in soil amended with biosolids. Stock sizes included #3, #7, and #15 containers and 5 cm ball & burlap (B&B). Species were selected that varied in their growth rates and tolerance to urban conditions. Gas exchange measurements were made monthly from June to August 2019, 8 months after initial planting.
Results/Conclusions
We hypothesized net photosynthesis (Anet) and stomatal conductance (gs) would increase during the growing season as tree roots grow out into the site soil. This increase was only observed in B&B Quercus bicolor and Tilia americana. Some species-size combinations showed no change from June to August while gas exchange rates of other groups decreased in July 2019 following a period of low precipitation and hot weather. These responses were consistent within, but not between, genera. Acer xfreemanii ‘Autumn Blaze’ and A. saccharum ‘Green Mountain’ #7 and #15 Anet and gs were significantly lower in July. Acer B&B rates were similar between all months. Anet and gs of Q. bicolor and Q. rubra #3 and #7 did not vary between months. Anet for all sizes of T. americana did not change between months but gs decreased in July for #3, #7, and #15. These initial findings suggest that stock size effects may vary by species, possibly because of differences in water use strategies or patterns of root growth. Understanding the interplay of stock size and species characteristics is critical for providing urban foresters with recommendations to maximize the success of and ultimately the benefits provided by tree planting programs.