Rapid Growth of Southern California Black Walnut (Juglans californica) through Stump Salvage and Transplantation

Richard B. Lewis, III, Psomas

Kai T. Palenscar, PhD, San Bernardino Valley Municipal Water District

California’s native walnut taxa (Juglans spp.) are rare and declining across their ranges, and effective methods of regeneration are needed as part of a strategy to retain and restore these species and the plant communities where they dominate (California walnut groves). California walnut species exhibit slow growth from plantings of seed or standard nursery container stock, delaying the establishment of high-functioning woodland habitat characteristics—e.g., broad arboreal canopy, nesting opportunities (including cavities), and substantial fruit production—on habitat restoration sites. Alternatively, salvage of whole trees via excavation, root pruning, and establishment in large nursery boxes, is a labor-intensive process.

The development of the Three Oaks tract in the City of Walnut included 61 acres of oak-walnut woodland habitat restoration to mitigate impacts to this habitat type caused by development activities. As part of restoration implementation in 2006, the developer approved the experimental salvage and transplantation of several Southern California black walnut (Juglans californica) (walnut) trees, along with coast live oak (Quercus agrifolia var. agrifolia) (oak) and blue elderberry (Sambucus nigra ssp. caerulea). Psomas developed a methodology for the strategic salvaging of individual trees, including (a) pruning of suitable specimens to retain a limited portion of stem/trunk, (b) excavation and removal (via heavy equipment) of the trunks with a substantial amount of crown/root tissue attached; (c) transplantation into prepared pits and replacement of backfill soil; and (d) initial maintenance including irrigation and weed control. Some of the transplants were installed within the habitat mitigation site, while others were placed in nearby natural areas in undeveloped portions of the property. The plantings outside the mitigation site did not receive long-term maintenance. The purpose of this experiment was to determine if stump transplantation could be a cost-effective alternative to whole-tree salvage, while yielding superior growth results versus plantings from seed or standard nursery container stock.

The current landowner (Lennar Corporation) provided access to the Three Oaks property in July 2021 for Psomas to evaluate the performance of the salvaged individuals approximately 15 years following transplantation. A total of 12 transplanted walnuts and 2 oak trees were evaluated for canopy diameter/height, trunk diameter at breast height, phenology/fruiting, and other characteristics. The walnut tree growth was exceptional, with canopies measuring up to 34 feet diameter and trees reaching a height of up to 28 feet. Abundant fruit was present on multiple walnut individuals. Notably, the transplanted oak trees had attained a canopy diameter of up to 23 feet and a height of up to 25 feet and fruit production was observed.

This experimental approach demonstrates that stump transplantation can be an effective method to achieve rapid growth of these native hardwood species with the additional benefits of conserved local genetics and enhanced habitat complexity (e.g., provision of nesting cavities and inclusion of coarse woody debris).