- Approximately 3-4 month-old containerized Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings (seed zone 262 and 271) were subjected to 6 moisture stress treatments (65, 53, 41, 29, 17 and 7% soil water content by volume of dry soil) starting July 4 to September 22, 1991 at Forest Research Laboratory's greenhouse at Oregon State University, Corvallis, OR. Seedlings were tested for various phenological, physiological, and morphological parameters. All the parameters were significantly affected by water stress treatments. Moderate soil water content resulted in increased and earlier terminal bud initiation and budset in seedlings whereas, too much and too little water content caused decreased and delayed bud initiation, budset, and bud development. The
driest and wettest treatments apparently stressed these seedlings so severely and
kept them growing respectively that they took longer to set bud. A small percentage of seedlings (13.1% of total 480 seedlings), after they had initiated bud formation, resumed their growth with increasing soil water content. Decreasing soil water content resulted in reduced total needle and root
nutrient concentration and content (N, P, K, Ca, and Mg) except N concentration which was significantly higher at the lowest soil water content (7%). On the other hand, shoot nutrient concentration and content, measured at day 0 and 43, remained unaffected. Moisture stress treatments had also a profound effect on starch reserves in needles and roots. A significant decrease was found in starch concentration in roots (measured at day 0, 43, and 81) and in needles and roots (measured at day 81) with decreasing soil water content. Further analysis of needle:root starch concentration ratios showed a higher concentration in needles than that of roots, indicating that less starch was translocated from needles to
roots due to severe moisture stress. Seedlings treated with the lowest soil water content were most stressed and experienced the greatest plant moisture stress (22.34 and 23.95 bars pre-dawn and mid-day PMS, respectively). There was an approximately 398 and 211% increase in pre-dawn and mid-day PMS respectively from the wettest to driest treatments. Similarly, water stress treatments had the greatest effect on various morphological attributes. Shoot height, caliper, fresh and dry weights of shoots and roots (measured at day 0, 43, and 81), total shoot height and caliper, shoot and caliper growth, and total fresh and dry weights of needles, stems, branches and roots
(measured at day 81) decreased significantly with decreasing soil water content. Most drastic effect was found at the driest treatment (7% soil water content), where, seedlings decreased approximately by 66, 44, 25, 25, and 69% in shoot height and caliper growth, total fresh weight of needles, stems and roots from the wettest to driest treatments. This effect was also prominent in terminal bud development. Both terminal bud length and diameter were reduced approximately by 35 and 29% at the lowest soil water content. Severe moisture stress resulted in complete cessation of growth and breakdown of metabolic system of a few
seedlings, thus, causing their mortality.