Slow-Growing Pines in a Rapidly Changing World: Physiological Impacts of Severe Drought on Three High-Elevation Pine Species

Background/Question/Methods

High-elevation five-needle pines are ecologically important, long-lived tree species that currently face grave threats due to global change. Warming and drying climates increase the physiological stress, susceptibility to pest and pathogens, and the already widespread mortality of these species. It is therefore vital that we understand and identify physiological traits associated with drought resistance and recovery. To investigate this, we exposed greenhouse-grown 5-year-old whitebark pine (Pinus albicaulis), limber pine (Pinus flexilis) and Great Basin bristlecone pine (Pinus longaeva) populations from climatically distinct sources to an eight-week experimental drought by completely withholding water, followed by a four-week recovery period, in which soil moisture levels were equivalent to the control group. We measured physiological traits such as gas exchange (net assimilation, stomatal conductance) and leaf water potential throughout the experiment. Needle samples were collected at weekly intervals for analysis of non-structural carbohydrate (NSC) content. At the end of the drought and recovery periods, individuals were destructively harvested to assess biomass of needle, stem, and root tissues, NSC content, leaf carbon isotope ratios (d13C) and N content.

Results/Conclusions

During the experimental drought, severe drought was achieved when stomatal conductance reached an average value of 0 mol H20 m-2s-1. P. longaeva reached this severe drought stage after four weeks of treatment, while P. albicaulis and P. flexilis reached this stage after six weeks of treatment. P. longaeva exhibited significantly lower predawn and midday leaf water potentials, and net assimilation values than the other species by the end of the drought treatment period. After the recovery period, P.flexilis exhibited the greatest increases in stomatal conductance, returning to 12% of pretreatment values. In contrast, P.albicaulis stomatal conductance returned to 4% of pretreatment values, and P. longaeva stomatal conductance did not increase during recovery. Sample and data analysis are well underway to further elucidate physiological mechanisms underlying these observations. For example, preliminary analyses of root biomass to shoot biomass ratios suggest that P. albicaulis and P. flexilis invest more heavily in belowground biomass than P. longaeva seedlings, allowing greater access to water and reduced hydraulic impairment during severe drought. Additional analyses will examine non-structural carbohydrate reserves in needles, stems, and roots, leaf N, and leaf d13C values to further reveal physiological mechanisms of drought resistance and recovery.