City Trees

In a third study, Bassuk and team transplanted small (1.5 inch/38 mm caliper) and large (4 inch/102 mm caliper) bare root trees of six species in Ithaca in both spring and fall. They left a control group of the same trees at Schichtel's Nursery, in place/undug. The six species were Q. bicolor and Q. macrocarpa, ironwood (Carpinus car- oliniana), Kentucky coffeetree (Gymnocladus dioicus), tupelo (Nyssa sylvatica), and scarlet oak (Q. coccinea). The measurements taken beginning the growing season after transplanting were hydraulic conduc- tance, shoot extension, and rate of photosynthesis. For the smaller caliper (1.5 inch/38 mm), spring-trans- planted trees, hydraulic conductance was significantly better for the smaller-caliper trees than for the larger caliper (4 inch/102 mm) trees; this was true across species. For fall-transplanted trees, the results were more mixed: smaller caliper trees still had better hydraulic conductance than larger caliper trees, but not to the same degree as with the spring-trans- planted trees. Further, fall-planted trees showed more variation by species in hydraulic conductance. Shoot extension was measured starting in the year after transplanting. Spring-transplanted, smaller caliper trees had greater shoot extension across species than larger caliper trees planted in spring. For fall-transplanted trees, the shoot extension results were more mixed, varying by species. Spring-transplanted smaller caliper trees showed better rates of photosynthesis than spring-trans- planted larger caliper trees. However, for fall-planted trees, the rate of photosynthesis was roughly equal between smaller and larger caliper trees. In a related study, Bassuk and team transplanted three species (Q. bicolor, Q. coccinea, and Q. macro- carpa), balled-and-burlapped (B&B), at three different caliper sizes (1.5 inch/38 mm, 2.5 inch/64 mm, and 4 inch/102 mm). They harvested and replanted the B&B trees at Schichtel's Nursery to minimize the effects of soil conditions. Trees of the three caliper sizes were dug, balled and burlapped, and then replanted in the same nursery plot. This replant-on-site approach was to provide maximum control for site conditions, with the knowledge that the nursery soil and the nursery's maintenance practices were ideal for tree growth. The study used 54 trees altogether, six for each permutation of species × caliper. Some plants estab- lished and thrived, while others lived but showed significant dieback. The larger the tree, the more dieback was observed, and all six of the 4 inch/102 mm-caliper Q. macrocarpa died. All six died, despite impeccable planting and maintenance at Schichtel's Nursery with its ideal nursery soil. The failure of all six 4 inch/102 mm-caliper bur oak trees is no surprise; it fits with Bassuk's decades of field observation. "Larger is not better," Bassuk says. "If the tree's too big, problems arise with water uptake—and even if the tree survives, you get dieback … not the thriving tree you want," she says. Bassuk emphasizes that regardless of production method, transplanting aftercare remains essential, espe- cially in year one. "So even if you planted in spring and watered throughout the summer, we really recommend watering in the fall as well, because that's when you get the most intimate contact between soil and roots," she says. "Watering is the most important component of aftercare. How much water is needed varies on climate and year; in central New York where I live, we water newly planted trees at least twice a week when it's dry." Graduate students harvest fine oots from the root balls of containerized swamp white oak (Quercus bicolor) used to measure the hydraulic conductivity of the xylem. Courtesy UHI 30 CityTREES

• Cover