Recovery of California black walnut ( Juglans californica ) trees from drought, fire, and biotic stress: Is resprouting an elixir?

PREMISE: Resilience to the interaction of multiple stress factors of drought, wildfire, and canker disease has been little studied. During the 2012 to 2016 megadrought in California, black walnut trees were observed to resprout from the base due to water stress-induced dieback. After monitoring plants for a year, wildfire burned all the plants at two of our study sites, which was followed by vegetative resprouting. Plants also expressed symptoms of canker disease. We thus had an opportunity to assess the impacts of water stress, wildfire, growth form, and canker disease on plant performance. METHODS: Water potentials and photosynthetic rates of drought-induced resprouts were monitored and compared to nearby adults that had not experienced drought-induced dieback, both inland (San Dimas, California) and at coastal exposures (Malibu, CA). We monitored the recovery of adults and drought-induced resprouts from 2017 to 2019, including fire-induced resprouts at coastal exposures following a 2018 wildfire. In addition, stem canker number and canker length were compared among irrigated and non-irrigated growth forms, including drought-induced resprouts, coppice-induced resprouts, adults, and resprouting adults. RESULTS: Drought-induced resprouts displayed higher water potentials and photosynthetic rates than adults, especially during dry periods, but had lower (more negative) values than fire-induced resprouts. Non-irrigated walnut trees displayed greater canker initiation and canker elongation rates than irrigated trees, with irrigated coppice-induced resprouts showing the least canker development. CONCLUSIONS: Resprouting can temporarily relieve water stress, but xylem water stress and canker disease agents feed off one another. Repeat stresses of drought, fire and disease compound to reduce the resilience of individuals and populations of walnut.