UC Santa Cruz

Increased climatic variability can impact tree physiological processes beyond what is predicted from changes in mean conditions. We assessed the sensitivity of conifer saplings to spatial and temporal variability in meteorological conditions, taking advantage of the end of California's historic drought and the exceedingly wet winter of 2017. We sought to understand how very dry and very wet conditions constrain photosynthesis and growth in four regionally dominant conifers and whether sensitivity in these processes changes across a 500 m gradient in elevation. All species demonstrated phenotypic plasticity in response to temporal differences in precipitation on both inter-annual and seasonal timescales. Net photosynthesis in Pinus contorta decreased from an early season 2016 average of 12.4 to 6.89 μmol CO₂ m^-2 s^-1 later in the summer, but increased 14.1% between seasons in the wet year. By contrast, elevation had almost no effect on instantaneous photosynthetic gas exchange, CO₂ response curve parameters, or stem water potential in any of the years for any of the species. Effects of the heavy snow year (2017) on needle growth differed between elevations. Pinus contorta showed a 38.9% increase in average needle length at the lower two elevations but a 31.6% decrease at the highest site compared to the height of the drought. Despite these differences, biological variation was dampened compared to the physical variation between years, suggesting these trees can effectively withstand substantial meteorological variability. Our results show that these species demonstrated considerable ability to tolerate and recover from an extreme drought event.