Douglas-fir Seedlings in the Pacific Northwest: The Genetics of Drought Adaptation Public Deposited


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  • Douglas-fir (Pseudotsuga menziesii) is a widely distributed, ecologically important, and commercially valuable tree species in North America. However, climate change is expected to adversely impact Douglas-fir trees, and assisted migration may become necessary to lessen the effects of climate change. Because drought stress is one of the projected effects of climate change in the western U.S., it is increasingly important to include drought adaptation traits in breeding programs and in reforestation decisions.This study assesses genetic variation in drought adaptation traits in Douglas-fir as part of the Drought Hardiness Study that was initiated by the Bureau of Land Management (BLM). Currently, it is being managed as collaboration among the BLM, Pacific Northwest Tree Improvement Research Cooperative (PNWTIRC), Northwest Tree Improvement Cooperative (NWTIC), Weyerhaeuser, Silver Butte Timber Company, and Washington Department of Natural Resources.In this study, I addressed the following objectives: (1) obtain baseline measurements and climate data to help in the analysis and interpretation of future measurements in theDrought Hardiness Study; (2) characterize the quantitative genetics of drought adaptation traits; and (3) determine whether drought adaptation traits are associated with the climatic origin of Douglas-fir seedlings.To achieve these objectives, data were collected from about 10,000 Douglas-fir seedlings from 429 families from western Oregon and Washington that were planted at two sites (Sprague and Lost Creek) in southern Oregon. Measured variables, which I refer to as drought adaptation traits, included height, second flushing, spring bud flush, damage (foliage, stems, and leaders), and survival.Each drought adaptation trait was subjected to an analysis of variance (ANOVA) to obtain variance components. Then, these components were used to estimate quantitative genetic parameters, including genetic variances, heritabilities, family-level breeding values (BLUPs), and genetic correlations. Climate variables (1961-1990 normals) from the female parent source locations were estimated using the ClimateNA software program. Simple correlations and lasso regressions were calculated between drought adaptation traits (family BLUPs) and climate variables.Based on ClimateNA models and weather station data collected in the year of the study (2015-2016), the Sprague site is typically hotter and drier than Lost Creek. Results also indicate that the trees at the Sprague site grew less, were more damaged, and had greater mortality than the trees at Lost Creek. Therefore, differences in climate and seedling growth between the two sites indicate that this experiment should be effective forscreening families for drought adaptation. In later analyses of the Drought Hardiness Study, early height measurements will be helpful for the analysis and interpretation of later measurements. For instance, either height in the greenhouse or height in the field can be used as an “initial height” for comparison with later height measurements to remove the confounding effects of family height variation resulting from early seedling growth in the greenhouse.In the first growing season, heritabilities and genetic variances differed widely among traits. I also found that estimated genetic gains were large for drought adaptation traits, primarily because of the large number of families tested (i.e., high selection differentials). For example, large potential genetic gains were observed for flushing (Flush), second flushing (SFlush), and height increment (Htinc). Although genetic correlations were found among drought adaptation traits, low correlations were found between growth in the greenhouse and other drought adaptation traits, flushing versus height growth, and flushing versus mortality. Additionally, genotype-by-environment interactions at the family level are reported.Drought adaptation traits were significantly correlated with some parental climate variables. Large and significant correlations were found between growth in the greenhouse and parent source climates. However, I did not find any correlations with growth in the field. I found moderate correlations for spring bud flush, and low correlations between other drought adaptation traits and climate. For instance, I foundearly bud flush was associated with warmer and drier climates, suggesting that early bud flush is a drought avoidance strategy.Selection of climate variables associated with drought adaptation traits was investigated using genecological-modeling techniques. I found that the end of the frost-free period (eFFP) was the most relevant variable, based on the data from the Sprague site. However, eFFP only explained a low amount of variation in second flushing (SFlush). The same procedure identified growing degree-days below 18°C (DD_18) as the most relevant variable based on the Lost Creek data.My results help increase the understanding about the importance of climatic-driven genetic differences for drought adaptation traits in Douglas-fir. The results of this study and later analyses of the Drought Hardiness Study will provide useful information for understanding drought, enhancing breeding programs, and potentially adjusting forest management to climate change impacts.
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