Differences of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) populations in height growth, water use efficiency, and carbon partitioning responses to changes in climate and water availability

This work deals with understanding differences of Douglas-fir populations in height growth, water use efficiency, and carbon partitioning responses to changes in climate and water availability. Chapter 1 highlights the importance of understanding these differences in the context of climate change. Chapter 2 addresses population differences in three-year height growth in response to changes in climate of interior Douglas-fir populations. Climate change is measured by changes in mean temperature of the coldest month. The data comprised 236 interior Douglas-fir populations from Idaho, Montana, and Eastern Washington, USA, grown in a common garden. The modeling approach used combines species-level and population-specific responses, which was key to detect population-level differences. Results indicate that population maximum height and height growth responses to changes in climate were dependent on their seed source climate. Chapter 3 explores whether Douglas-fir populations presented genetic differences in the response of water use efficiency (WUE) to changes in water availability and whether the different responses are associated with climate characteristics at each population's place of origin. The data comprised thirteen populations and three trees per population growing in a field test in British Columbia during a period of eleven years. WUE was estimated from tree ring delta 13C composition. Tree ring delta13C response to plant transpiration deficits was evaluated with a linear mixed-effects modeling framework. Results were: (1) tree ring delta13C (and WUE) increased linearly with increases in plant transpiration deficits for all populations, (2) populations differed in their mean tree ring delta 13C, (3) these population differences were positively correlated with 30-year tree height, and (4) a population's tree ring delta 13C was positively related to the mean vapor pressure deficit during the frost-free period at its seed source. Chapter 4 assesses population differences in partitioning of photosynthate (A) to aboveground biomass increment (ABI/A), whether differences in ABI/A correlate with the climate at which populations originated, and whether ABI/A is affected by current biomass or the plant transpiration deficit in a given year. The data comprised nine populations, with three trees per population, growing in a common garden in British Columbia during a period of eleven years. Annual photosynthesis was estimated from WUE (from delta13C), vapor pressure deficit, and annual transpiration estimates. Annual aboveground biomass increment was estimated from diameter at breast height measurements and allometric equations. Results are: (1) populations differed in their mean ABI/I, (2) populations from warmer and wetter climates partitioned more of their photosynthate to aboveground biomass, (3) for a given population, ABI/A increased with annual plant transpiration deficit, which we interpreted as a consequence of the occurrence of the deficits late in the growing season at the test site location.