How Coastal Forests Are Managed Can Impact Water Cycle

Younger trees take up and release less water than mature trees 10 years or older, researchers from North Carolina State University found in a new study that tracked how water moves through wetland pine forests near the North Carolina coast.

Their findings, published in Agricultural and Forest Meteorology, suggest managers should time timber harvests to leave older trees alongside new growth to mitigate runoff.

“The water balance, especially in coastal sites, is very important,” said the study’s lead author Maricar Aguilos, postdoctoral research associate in forestry and environmental resources at NC State. “We have so much water there. We wanted to understand how land-use changes impact water use and drainage in the forests, as well as how they affect the growth of the trees.”

The findings come from a long-term research project designed to understand how wetland forests in eastern North Carolina – including pine forests managed for timber and a natural hardwood forest at the Alligator River National Wildlife Refuge in Dare County – are responding to changing climate conditions.

Using meteorological sensors perched on towers above the forest canopy, the researchers are able to track water flow to and from the site, including during a severe drought in 2007-2008. They’ve also used the sensors to track carbon sequestration – an important marker for the forests’ ability to mitigate or contribute to climate change. They have gathered data on forest carbon and water cycling spanning 14 years.

“In order to study the response of coastal ecosystems to climate change and sea-level rise, we need long-term observations,” said study co-author John King, professor of forestry and environmental resources at NC State. “The longer we can let those studies run, the better our data will be, and the more effectively we can help inform policy.”

The latest study evaluated how much water the trees use and release as vapor, compared to how much is lost as drainage.

The researchers found that younger pine plantations had increasingly higher “evapotranspiration,” which is the amount of water released in combination from two sources: through evaporation of water from the soil, and the process in which trees consume water and release it from their leaves as vapor, which is known as “transpiration.” Mature plantations had the highest ratio of evapotranspiration to rainfall, and drained less water than younger pine forests.

“We found that the trees use more water as they mature,” said study co-author Ge Sun, a research hydrologist and project leader at the U.S. Department of Agriculture Forest Service and adjunct professor in forestry and environmental resources at NC State. “Water use stabilized by about year 10 in the pine forests.”

That finding suggests clear-cutting a site and replanting it could lead to increased drainage and flooding off the site initially, but the impacts would diminish as the trees grow.

“The mature plantations help to mitigate effects of forest harvesting on drainage at a landscape scale,” Aguilos said. “If you harvest to leave trees of different ages, they can help each other.”

The study, “Effects of land-use change and drought on decadal evapotranspiration and water balance of natural and managed forested wetlands along the southeastern U.S. lower coastal plain,” was published online in Agricultural and Forest Meteorology on March 6, 2021. In addition to Aguilos, King and Sun, other authors included Asko Noormets, Jean-Christophe Domec, Steve McNulty, Michael Gavazzi, Kevan Minick, Bhaskar Mitra, Prajaya Prajapati and Yun Yang. It was supported by the USDA A.5 Carbon Cycle Science Program award No. 2014-67003-22068, the U.S. Department of Energy NICR award 08-SC-NICCR-1072, the USDA Forest Service award 13-JV-11330110-081, and DOE LBNL award DE-AC02-05CH11231.

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Note to authors: The abstract follows.

“Effects of land-use change and drought on decadal evapotranspiration and water balance of natural and managed forested wetlands along the southeastern U.S. lower coastal plain”

Authors: Maricar Aguilos, Ge Sun, Asko Noormets, Jean-Christophe Domec, Steve McNulty, Michael Gavazzi, Kevan Minick, Bhaskar Mitra, Prajaya Prajapati, Yun Yang and John King.

Published online in Agricultural and Forest Meteorology on March 6, 2021.

DOI: 10.1016/j.agrformet.2021.108381

Abstract: Forested wetlands are important in regulating regional hydrology and climate. However, long-term studies on the hydrologic impacts of converting natural forested wetlands to pine plantations are rare for the southern US. From 2005-2018, we quantified water cycling in two post-harvest and newly-planted loblolly pine (Pinus taeda) plantations (YP_2–7, 2–7 yrs old; YP_2–8, 2–8 yrs old), a rotation-age loblolly pine plantation (MP, 15–28 yrs old), and a natural bottomland hardwood forest (BHF, > 100 yrs old) along the lower coastal plain of North Carolina. We quantified the differences in inter-annual and seasonal water balance and trends of evapotranspiration (ET) using eddy covariance over 37 site-years and assessed key climatic and biological drivers of ET. We found that the rotation-age plantation (MP) had higher annual ET (933 ± 63 mm) than the younger plantations (776 ± 74 mm for YP_2–7 and 638 ± 190 mm for YP_2–8), and the BHF (743 ± 172 mm), owing to differences in stand age, canopy cover, and micrometeorology. Chronosequence analysis of the pine sites showed that ET increased with stand age up to 10 years, then gradually stabilized for the remainder of the rotation of 28 – 30 years. YP_2–8 was sensitive to water availability, decreasing ET by 30 – 43 % during the extreme 2007 – 2008 drought, but reductions in ET at MP were only 8 – 11 %. Comparing to BHF, ditching with management enhanced drainage at YP_2–7 and YP_2–8, while drainage was lower at the mature pine site. This study provides insight into land use-hydrology-climate interactions that have important implications for forested wetland management in a time of rapidly changing environmental conditions of the LCP of the southern US.