Jessie Cone, right, and Michael Blazier measure photosynthesis, respiration and water use of sampled branches. Photo by Michael Blazier.
A view of the study site from above. White structures are blocking rainfall on some trees to measure their resilience to drought. Photo by Michael Blazier.
By Jesse Cone
The Southeast region of the United States is known as the “Wood Basket” and produces more timber than any other region. During 2011, Louisiana experienced one of the worst droughts ever recorded in the state. Widespread water deficits caused millions of dollars in agricultural losses. Climate scientists predict that harsh droughts will become more common in the coming decades. Pines planted today will experience increased climate variability over their 30-year rotation cycle. Producing drought-tolerant, highly productive plantations is essential to maintain economic growth in the timber industry.
A cooperative study between the LSU AgCenter Hill Farm Research Station and Louisiana Tech University was established to understand how to prepare plantations for future severe droughts. Planting drought-tolerant seedlings is one method to increase plantation drought resilience in newly established plantations. In established plantations, intensive thinning has been suggested as one way to allocate more nutrients and water to robust trees during drought. The current study investigates whether the negative effects of drought can be reduced by thinning and/or using drought-tolerant genotypes.
A pine plantation was established in 2005 with open-pollinated Louisiana seedlings (OP-LA), open-pollinated Carolina coastal plain seedlings (OP-756) and a clonal variety based on a robust OP-756 individual (C93). All plants, including loblolly pines, use photosynthesis to transform atmospheric carbon dioxide into carbohydrates that plants use for growth and development. We can detect whether a plant is experiencing drought stress by measuring its photosynthesis rate. Our study compares the net photosynthesis rate of each genotype to determine which can best maintain high carbohydrate production despite water deficits. We are creating artificial drought conditions to best understand how these genotypes respond to extreme drought events. To create the drought conditions, we excavated trenches to 6 feet deep around every study plot and installed a plastic liner in the soil to block the lateral flow of soil water. We then built shelters over the plots in early summer 2020 to block all rainfall within the study plots.
Prior to the simulated drought, both the OP-756 and C93 trees outperformed the OP-LA seed source in height growth. During a moderate drought in 2010, the C93 genotype displayed the most sensitivity to water deficits through a lower net photosynthesis rate. Despite the reported drought sensitivity, the C93 maintained its superior height growth. As of 2016, the C93 was the tallest tree, while the OP-LA was the shortest.
The superior growth rate of the C93 is evident. Future evaluations are needed to determine if it will continue to maintain high growth rates as severe drought becomes more common. The more conservative growth rates of the OP-LA trees are thought to be an adaptive behavior to reduce drought mortality. Following the study’s extreme simulated drought in early June, the C93 trees again displayed the greatest drought sensitivity and lowest net photosynthesis rates. The OP-756 and OP-LA trees had similar photosynthesis rates and equal drought tolerance. It has not yet been determined if the more conservative strategy of the OP-LA will allow for greater survival and growth during extreme droughts. As the study continues through the remainder of the year, it is expected that the C93 genotype will be the most vulnerable to water deficits. If this results in higher mortality for the C93 trees, landowners may need to rethink prioritizing fast-growing clonal varieties over more drought-tolerant seed sources. The benefits of intensive thinning have not yet been determined, but it is anticipated that these effects will manifest as water becomes more limiting.
— Jessie Cone is a graduate student at Louisiana Tech University working with . Joshua Adams at Louisiana Tech University and Michael Blazier at LSU AgCenter Hill Farm Research Station.