The South Downs National Park is set to plant more than 20,000 trees in the coming months.
The new trees will be added to a woodland area in Sussex and Hampshire, which is the size of five football pitches.
The park aims to plant a total of 100,000 trees by its 15th anniversary next year.
This planting is part of the park's "major nature recovery drive" this winter.
The park has been working to restore English elms, planting 400 disease-resistant trees to replace those lost to disease.
These elms are important for supporting insect and butterfly species.
So far, nearly 75,000 trees have been planted under the Trees for the Downs initiative.
The goal is to plant 100,000 trees by the middle of the decade.
The trees being planted are a mix of disease-resistant elms and native species such as oak and black poplar.
They are being planted in various locations, including schools, farms, recreation grounds, and historic parks.
The park has reported early signs of success, with green shoots appearing where trees have been planted over the last five years.
National park countryside and policy manager Nick Heasman said: "Trees provide clean air for us to breathe, enrich our soils, provide a vital habitat for wildlife and, crucially, are amazing carbon capturers.
"However, the amazing treescape that makes up almost a quarter of the South Downs National Park and is a vital resource for the entire South East is under unprecedented threat from pests and diseases, as well as climate change.
"Increasing the number and diversity of our native trees, and carefully introducing new species into the landscape, is going to be a big help in responding to all these threats and help to reverse biodiversity loss."
A new corridor of elms has also been created near Wilmington, East Sussex, providing refuges for wildlife.
The South Downs National Park Trust is encouraging landowners and managers to help reach the 100,000 target by expressing an interest in tree planting for next winter.
The Trees for the Downs project is part of the park's ReNature campaign, which aims to transform 13,000 hectares of land into wildlife habitat by 2030.
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