Ponderosa pine (Pinus ponderosa) is an ecologically and economically important tree species with a broad distribution in the western United States. P. ponderosa has experienced elevated mortality in many parts of California due to the recent severe drought. Multiple traits can affect the drought responses of conifer trees, including root: shoot ratio, roots biomass and length, specific leaf area (SLA, the ratio of leaf area to dry mass), stomatal conductance, and water use efficiency. However, most drought tolerance research has focused on aboveground responses. In 2017, I conducted a pilot greenhouse experiment to examine the interactive effects of soil type and water on ponderosa pine seedlings. Seedlings were grown in deep cylindrical pots in a greenhouse for 6 months, in soils with three different proportions of sand, organic material, and vermiculite, including high sand, medium sand, and low sand soil. For the last 4 months, half the seedlings were watered every four days, the other half every other day. We measured both aboveground and below-ground physiological traits, including growth, SLA, root to shoot ratio (dry mass), and root length, and compared across treatments.
Results/Conclusions
The seedlings grew roots of over 70 cm length in only six-month of growth, compared to their average height of 11 cm. In the summer-drought conditions of the Sierra Nevada, the rapid root growth makes it possible for the P. ponderosa seedlings to get the water from the relatively deeper weathered bedrock zones instead of the top dry soil. Although the pots were found to retain moisture better than expected, making drought conditions less severe than planned, some water effects were evident. As predicted, SLA of seedlings in wet conditions was higher than in dry conditions. Thicker needles with smaller SLA usually have a higher density of chlorophyll and proteins, hence, have a greater photosynthetic capacity per unit area than thinner. Unexpectedly, the root: shoot ratio and root length of seedlings in wet conditions was higher than in dry conditions for medium and low sand conditions, but similar in high sand soil. Under drought, seedlings in medium sand soil had the highest shoot growth, while those in low sand soil have a lower root: shoot than seedlings in other two types of soil.