Saturated soils could impact survival of young trees planted to address climate change

The saturated soil conditions predicted to result from increased rainfall in the UK’s upland regions could have a knock-on effect on the ambition to create more woodland in the fight against climate change, a new study has found.

Researchers from the University of Plymouth have spent a number of years exploring how temperate rainforests could be an effective nature-based solution to some of the planet’s greatest challenges.

They have also shown that the UK’s uplands could in future see significantly more annual rainfall than is currently being predicted in national climate models.

In new research, they found that higher soil water levels within areas such as Dartmoor, the Lake District and the Scottish Highlands could have a significant impact on the survival rates of both acorns and juvenile oak saplings.

Published in the journal Forest Ecology and Management, it is the first study to highlight the importance of factoring in soil conditions when looking at where and how to create the temperate rainforests of the future.

Dr Thomas Murphy, Lecturer in Environmental Sciences at the University of Plymouth, is the study’s lead author. He said: “In recent years, there have been increasing calls to plant more trees as part of the global effort to combat climate change. Restoration and expansion of temperate rainforests, which are a globally rare ecosystem, is seen as one of the potential solutions. But with our previous work also predicting an increase in future rainfall we wanted to know if the woodlands we create will support naturally colonising trees in future. Our results show that higher water levels within soils directly contribute to reduced survival of both acorns and young oak trees. We believe it provides landowners, land managers and policy makers with important information as to which species might work in particular locations to support more resilient future rainforests.”

For the study, researchers planted acorns from English oaks (Quercus robur) in containers with four soil states, from completely flooded to low saturation where the water level was 220mm below the acorn.

The acorns did not survive in the flooded soils, but survival rates improved gradually – 43% at high saturation, 77% at medium saturation, and 83% at low saturation – as the water level dropped.

The surviving seedlings also exhibited reduced root:shoot ratio, leaf photosynthesis, and a lower likelihood of late season shoot growth in soils of higher saturation.

In a concurrent field experiment, juvenile English oak and Sessile oak (Quercus petraea) saplings were planted in a region of Dartmoor that is seasonally waterlogged and frequented by grazing livestock.

In these tests, the English oaks exhibited greater shoot growth and leaf photosynthesis than its close relation in areas where the soil was more saturated.

The researchers, including environmental scientists and ecologists, say the results highlight the need for better understanding of soil influence on tree development.

Dr Murphy added: “There has been extensive talk about how larger trees respond to the effects of climate change. But these results show we need to factor in the response of young trees as well, especially if they are being envisioned as an integral part of the solution. By examining their response to conditions now, while also thinking about what these locations are going to be like in 50 years’ time, we can better understand the right trees for the right locations, and hopefully make these woodlands more resilient in the long-term.”