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Secondary forests are more vulnerable to drought than primary forests

Northern boreal forest ecosystems, such as those in Sweden, are predicted to face more frequent summer droughts caused by climate change. In Sweden, only a small part of forests have experienced limited direct human contact – known as “primary forests” – while the majority of forests are human-managed commercial forests with little diversity in structure and species (secondary forests). 

A team of researchers led by Lund University has recently investigated how the massive drought Sweden experienced in 2018 impacted these two forest types, and discovered that secondary forests were the ones most affected.

“We used a unique map of primary forests in Sweden and investigated whether there might be any difference in how these forests and neighboring secondary forests were affected by the drought in 2018. The results showed that secondary forests in Sweden were more affected by the drought than primary forests were,” said study senior author Anders Ahlström, an expert in Physical Geography at Lund.

How the study was conducted

To better understand how 2018’s dry summer impacted Swedish forests, the scientists used satellite images and a map detailing over 300 primary forests. 

By analyzing a high-resolution vegetation index over these forests and the buffer zones surrounding them – which represent secondary forests – the researchers managed to create a picture of how the massive drought affected the two different forest types.

What the researchers learned 

“That primary forests were less affected by drought leads to a number of interesting follow-up questions about what causes this difference. It could be the trees’ generally greater age and size that perhaps leads to deeper roots and more structural diversity,” explained lead author Julika Wolf, a researcher in Cartography and Spatial Analysis at Lund.

“Or it could be that there is more water in these ecosystems, perhaps because they haven’t been ditched. Right now though, we have no idea what makes primary forests more stable during drought.” 

Implications of the study 

Currently, primary forests are quite rare in Sweden and Europe, representing the most pristine forests that we have left. Since examining these forests can inform us how nature looks and functions without major direct human impacts, they are particularly important in understanding how environmental changes and human land use affect ecosystems and their processes.

“This study is a good example of how we can use these primary forests in our research. In this case, we see that they are more stable during drought when compared with secondary forests that have been exposed to change such as felling, planting, ground preparation, and ditching,” Ahlström concluded.

The study is published in the journal Environmental Research Letters.

More about secondary forests 

Secondary forests are forests that regrow after the primary forests have been significantly altered or removed due to events such as fires, logging, or agricultural activities. 

These forests often have different species compositions and structures compared to the primary forests. The time it takes for a secondary forest to mature can vary greatly, often taking decades or even centuries. 

Secondary forests bring several benefits:


Secondary forests can provide new habitats for a wide range of species, contributing to biodiversity. Some species are adapted to thrive in secondary, rather than primary, forests.

Carbon sequestration

As they grow, secondary forests absorb carbon dioxide from the atmosphere, playing an important role in mitigating climate change.

Soil rehabilitation

Secondary forests can help restore degraded lands. The trees’ roots stabilize the soil, preventing erosion and promoting soil health by cycling nutrients.

Water regulation

Forests play a key role in regulating water cycles. They can enhance local rainfall and improve water quality by filtering pollutants.

Livelihoods and resources

Secondary forests can provide resources such as wood, non-timber forest products, and fodder. They can also provide opportunities for recreation and tourism.

Buffer zones

These forests can act as buffers between human settlements and pristine natural areas, protecting the latter from human intrusion.

More about summer drought

Summer drought refers to a prolonged period of dry weather during the summer months. This can happen due to various reasons, including shifts in weather patterns, reduced rainfall, and higher temperatures that cause increased evaporation.

A summer drought can have significant effects on agriculture, leading to reduced crop yields and increased stress on livestock. It can also affect water supplies, energy production, and lead to wildfires in some regions.

Additionally, drought conditions can impact natural ecosystems, stressing plants and wildlife, and leading to long-term ecological changes. In extreme cases, prolonged drought can contribute to desertification, turning previously fertile land into barren landscapes. 

Efforts to manage and mitigate drought include water conservation, irrigation management, and drought-tolerant crop development.


By Andrei Ionescu, Staff Writer

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