Response of shortleaf x loblolly pine hybrid seedlings to water stress

Background/Question/Methods

Increased hybridization between two species can be a result of human-induced global change. One example exists in the southern pine forest, where hybrids of shortleaf pine (Pinus echinata) and loblolly pine (P. taeda) are rapidly increasing in frequency. While the abundance of the hybrid pine is evident, its life history is more opaque. For example, shortleaf pine is considered drought tolerant, while loblolly pine is less able to survive prolonged drought. The hybrid pine's drought tolerance is unknown. We compared the response to water stress of pot-grown, one-year-old hybrid F1 seedlings to shortleaf and loblolly pine seedlings. We hypothesized that drought response of the hybrid pines more closely resembles that of shortleaf pine which may be allowing the hybrids to increase in abundance on drier sites. Towards the end of their first growing season, we imposed drought stress for four months and measured gravimetric water use and leaf gas exchange, At the conclusion of the water stress period, seedlings were harvested and the biomass of each component measured.

Results/Conclusions

The less drought tolerant loblolly pine partitioned a greater amount of biomass to stem than did shortleaf pine; hybrid pine was intermediate (p = 0.01). The water stress treatment reduced partitioning to stem (p= 0.07) and foliage (p=0.08) compared to the well watered treatment. Water use per unit of foliage biomass measured under well watered conditions was greater in shortleaf pine compared to loblolly and hybrid pine (p=0.02) and was reduced by previous water stress (p < 0.0001). During a water stress cycle, whole plant water use of all three genotypes began to decrease at approximately the same gravimetric water content. Our results indicate that there are differences in biomass partitioning and water use between shortleaf and loblolly pine, but that the hybrid's response was either intermediate between the two species or more similar to loblolly pines. Therefore, the hybrid seedlings do not appear to fully inherit the greater drought tolerance of shortleaf pine.