Opportunistic growth phenology and water use of semi-arid tree species of northern Arizona

In forests of the southwestern USA, the seasonality—or phenology—of tree growth is affected by a combination of limiting temperatures and water availability. However, in this topographically diverse area, temperature and precipitation vary by elevation and therefore may have differing effects on tree phenology across the landscape. Our over-arching research question was: how does variation in temperature and water availability drive differences in the timing of tree growth and water use across species and sites in northern Arizona? We analyze 3 years of high-frequency measurements of stem growth and sap flow velocity collected across five sites from 1400 to 2600 meters elevation. We supplemented these data with lower-frequency measurements of xylogenesis, budburst, and potential photosynthetic quantum efficiency. Our study species—Pinus ponderosa, Pinus edulis, Juniperus osteosperma, Juniperus scopulorum and Quercus gambelii—represent the dominant tree species across northern Arizona. We found that all metrics of tree phenology tended to be limited by water availability at lower elevations. Lower elevations had bimodal patterns of growth and sap flow velocity which reflected precipitation inputs. Higher elevations had more consistent unimodal patterns which aligned with daylength, and growth was rarely limited by water availability. Microcore data supported growth rates from dendrometers, but microcores were able to capture xylogenesis when dendrometers were unable to—surprisingly, even when stem diameter was shrinking due to water limitation. A machine learning model showed soil volumetric water content was the best predictor of radial growth and sap flow velocity at lower elevations but was only marginally better than daylength and temperature at higher elevations. Thus, tree activity in trailing-edge, low elevation forests was more impacted by moisture, and less affected by temperature, compared with forests at higher elevations. Our results, which show that arid woodlands and forests in the North American Southwest are adapted to grow opportunistically when neither water availability nor air temperature is limiting, are synthesized in a new conceptual model.