Much research has characterized drought effects on tree regeneration, but less effort has looked at the interactive effects of drought and Fusarium, a soil-borne pathogen that causes growth declines in tree nurseries after drought. We used seedlings of different provenances of Douglas-fir and loblolly pine to investigate the interactive effects of a two-week period of water deficiency (“drought”) and Fusarium. From the most to least xeric, the provenances were from the Coast Range, Cascade foothills, and Cascades (Douglas-fir) and Alabama, North Carolina, and Texas (loblolly pine). We grew three-month-old germinants in single-plant pots for eleven weeks in a greenhouse. We inoculated all pots with either sterile plugs or plugs infected with Fusarium and then subjected them to one of four treatments: well-watered (C), well-watered and inoculated (I), droughted (D), and droughted and inoculated (ID). H1: We expected Fusarium inoculation to cause decreased growth in drought but to cause no change in growth in well-watered conditions (e.g., ID smaller than D, but I similar in size to C). H2: We expected that the most xeric provenance of each species would be less affected by a given drought, and therefore, be less susceptible and less affected by Fusarium.
Results/Conclusions
H1: The effects of Fusariumwere much lower on the droughted plants and greater on the well-watered plants than expected. In Douglas-fir, ID plants had larger diameter and root/shoot than D, but similar height and dry weight (total, and most compartments). I plants were taller and had larger diameter than C, but lower root/shoot and dry weight (total, and most compartments). In loblolly pine, ID plants had similar height, diameter, and root/shoot than D, but higher dry weight (total and most compartments). I plants were shorter and had lower diameter and dry weight (total and most compartments) than C, but higher root dry weight and root/shoot.
H2: Plants from the xeric provenances behaved more similarly to those from other provenances when challenged with Fusarium than expected. The most xeric provenances had no significant changes in total dry weight (both species) and root/shoot (Douglas-fir) in D vs. ID (droughted without vs. with inoculation), whereas the other provenances had changes. This research shows that even the patterns commonly described anecdotally were not reproduced, suggesting more research is needed to better understand the effects of these dual stresses on seedling growth and establishment.