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Environmental Science
The secret behind tree growth during drought is hidden underground, according to 911吃瓜 study
911吃瓜 scientists are learning more about the impact that water stored underground has on tree growth across the western United States in a new study.
While trees rely on water to grow, it鈥檚 often thought that more rain means more growth and periods of less precipitation lead to less growth.
A led by Mantek Aulakh, an undergraduate student in 911吃瓜鈥檚 School of Environmental Science and Natural Sciences and Engineering Research Council of Canada Undergraduate Student Research Award recipient, suggests that this isn鈥檛 always the case.
Instead, findings from the study demonstrate that in areas where winter precipitation consistently exceeds what the ground can store, the subsurface 鈥渂ucket鈥� fills up completely even in drier years, providing trees with a reliable water supply that buffers them against droughts.
Aulakh explains that the amount of water that can be stored under Earth鈥檚 surface in soil and bedrock 鈥� or subsurface storage capacity 鈥� is playing a significant role in how the relationship between tree growth and precipitation plays out.
The study, carried out in collaboration with W. Jesse Hahm, an assistant professor in 911吃瓜鈥檚 Department of Geography, and David Dralle, a US Forest Service researcher, looked at the correlation between tree ring growth and winter precipitation throughout the western U.S. to understand how growth patterns differ across areas with varying subsurface storage capacities.
鈥淲e wanted to understand whether the amount of water stored underground controls how strongly tree growth depends on precipitation. In other words, do trees always grow more when it rains more, or does that relationship break down when the ground can store enough water to buffer dry periods?鈥�
The study found that when winter precipitation does not fill underground water storage capacities, tree growth is heavily tied to rainfall. However, in areas where typical winter precipitation consistently exceeds what the ground can store, the subsurface storage is fully replenished every year, making tree growth largely insensitive to year-to-year swings in rainfall totals.
鈥淥ne of the most interesting takeaways from this study is that what happens below the ground can be just as important as what happens above it,鈥� he says. 鈥淭he relationship between precipitation and tree growth depends on how much water the subsurface can store.鈥�
While having access to underground water can help trees withstand periods of lower precipitation, Aulakh explains that this phenomenon can hide early warning signs of stress in forests and calls in to question the accuracy of tree rings for measuring precipitation trends and reconstructing past climates.
鈥淭ree rings are one of our most important tools for understanding past climates, especially precipitation. But if tree growth isn鈥檛 always directly linked to rainfall, then our interpretations can be misleading,鈥� he says.
Better understanding this relationship and knowing where trees can access subsurface water supplies can help determine which forests could be more vulnerable to droughts, and which may be more resilient to changing climates.
鈥淲e often focus on rainfall when thinking about ecosystems, but how that water is stored and accessed can completely change how plants respond. Understanding that hidden part of the system is key to predicting how forests will cope with climate change,鈥� he says.