Experimental drought and heat can delay phenological development and reduce growth in semiarid trees
Higher temperatures associated with climate change are anticipated to trigger an earlier start of the growing season, which could increase the terrestrial C sink strength. However, greater variability and altered timing of precipitation are also expected with climate change, bringing increased drought stress to many ecosystems. We tested the effects of higher temperature and drought on the foliar phenology and growth of mature trees of two semi-arid conifer species. We exposed field-grown trees to a ~45% reduction in precipitation with a rain-out shelter, a 4.8°C temperature increase with open-top chambers, and a combination of both simultaneously (heat+drought).
Over the 2013 growing season, heat, drought, and heat+drought treatments reduced shoot and needle growth in piñon pine (Pinus edulis) by 39% or more, while juniper (Juniperus monosperma) had low growth and little response to these treatments. Needle emergence on primary axis branches of piñon pine was delayed in heat, drought, and heat+drought treatments by 19 to 57 days, while secondary axis branches were less likely to produce needles in the heat treatment, and produced no needles at all in the heat+drought treatment. Growth of shoots and needles, and the timing of needle emergence were significantly correlated with xylem water tension and non-structural carbohydrates. Our findings demonstrate the potential for delayed phenological development and reduced growth with higher temperatures in species that are vulnerable to increased xylem tension and reduced non-structural carbohydrates with drought stress. Climate change projections of an earlier growing season with higher temperatures, and consequent increases in terrestrial C sink strength, may be incorrect for regions where plants will face increased drought stress with climate change.