By Hannah Corsini, The Kingfisher writer

Published August 10th, 2023

The University of Freiburg, Germany, is developing the Conservation of Forest Biodiversity Project focused on the reforestation of the Black Forest. A 2023 study announced that forest elements like canopy cover, structural complexity and herb cover structure are essential for bee and wasp communities.

In Germany, bees have been facing a big problem. A study published in 2017 estimated that over 75% of the country’s insect biomass had been lost over the last few decades, prompting national concern.

On a global scale, wild bees have largely decreased their range, with several species reducing in numbers and some even going extinct. Due to a reduction in flower abundance and diversity globally, along with being exposed to parasites, pathogens and pesticides spread by humans.

Even worse, despite all this, global demand for pollinator services is at an all-time high, with pollination needed to contribute to crop productivity and nutritional security.

‘On a global scale, wild bees have largely decreased their range, with several species reducing in numbers and some even going extinct.’

Human land use as it currently stands encroaches on the bees’ needs. As the agricultural industry expands, flower-rich habitats are replaced with crop monocultures, which provide insufficient nectar resources for thirsty bees for most of the yearㅡserving as a food supply when harvest season rolls around.

Even tiny variations in air temperature, wind speed and precipitation can affect a bee’s chances of survival, so having a variety of microclimatic conditions available for different speciesㅡwhich all have specific preferencesㅡto forage and nest in, is essential.

But we can learn from conservation successes, like those in Germany. Since the report’s publication, the German government has pledged $25 million a year for research and monitoring of insect populations and implemented controversial decisions such as phasing out herbicides and insecticides in protected areas and prohibiting some chemicals nationwide.

‘Human land use as it currently stands encroaches on the bees’ needs.’

In the German Black Forest of the region of Baden-Würtemberg, specific plots were established by the nationwide Conservation of Forest Biodiversity Project in 2016. These plots are managed under the model known as ‘retention forestry,’ where even after harvesting, important structural elements of a forest are left at the site. Although this varies in practice, these structural elements commonly involve leaving behind deadwood and green trees of many shapes and sizes.

This forestry method emerged in the late 20th century and was refined by forest ecologist Jerry Franklin, who referred to it as a ‘kinder and gentler’ form of forestry. By leaving ‘biological legacies’ in harvested areas, habitats are maintained for the insects and fungi which often underpin forest ecosystems.

Bees have different seasonal needs, so landscapes which exhibit diversity in floral species, amount and composition are essential. Woods are often integral to bees in the early spring seasonsㅡproviding floral resources and nesting habitatsㅡespecially to highly specialised bees. However, woods which have been exploited and degraded often fail to meet this function.

Woodland edge can provide increased light, temperature and floral availability and so can create favourable conditions for some bee species. Although the practice of retaining deadwood has existed for decades, evidence supporting its effect on pollinator populations has previously been sparse.

Scientists in the Black Forest created ‘trap-nests,’ which are artificial cavities where mason bees and wasps (bees and wasps which form their nests in holes in wood, walls and hollow plant stems) can form nests, with the developing offspring then ‘trapped’ in the artificial cavity so that it can be reared to adulthood and identified.

‘Bees have different seasonal needs, so landscapes which exhibit diversity in floral species, amount and composition are essential.’

Their results showed that species richness increased when standing deadwood was retained locally. Species richness is particularly significant for ‘forest specialist species’一those which exclusively inhabit forest areas. Specialist insect species generally decline at far steeper rates than generalist species, and so are often priority targets for conservation.

However, the scientists also found that forest specialist species abundance decreased with an increase in lying deadwood. An increase of deadwood on the forest floor means less space for the plants, providing vital resources for bees and wasps.

In the late stages of decay, lying deadwood can increase the abundance and richness of forest floor plants. Still, the areas examined by the study included only early deadwood, which may also have formed a barrier to bee and wasp flight paths. This is important to note for future forest management decisions.

Although forest specialist pollinators such as wasps and bees prefer shaded conditions, the study found that increased canopy cover correlated with a decrease in species abundance and richness.

‘Scientists also found that forest specialist species abundance decreased with an increase in lying deadwood.’

Increased light exposure leads to greater plant growth in the forest understory, which provides more resources for pollinators. However, forest specialists were more tolerant of shaded conditions than non-forest specialists. Canopy cover may help to create ‘patches’ of land where forest specialists have less competition from generalist species.

‘Increased light exposure leads to greater plant growth in the forest understory, which provides more resources for the pollinators.’

The most important conclusion from the paper was that habitat heterogeneityㅡbe it in the form of standing deadwood, understory plant growth or light-exposed areasㅡis key for pollinators to thrive in our forests.

These findings put into perspective the need for land stewardship and sustainable management. Practices like controlled burning and coppicing can help reduce canopy cover and increase the diversity of forests to ensure that they meet different survival needs, whilst retaining deadwood provides additional habitats for pollinator species to nest in, supports insects which are reliant on it to survive, and also helps to contribute to a varied landscape.

Climate change is adversely affecting bee populations which is set to heighten in the future. Bees, unlike most insect species, prefer temperate conditions rather than warm ones and extreme climate events like flooding and droughts will affect their ability to nest and feed.

If we want to protect our pollinators, taking care of the land over which they roam is essential, and we need to move away from exclusively profit-seeking activities and towards more harmonious ways of living on the land.

Featured Image: Philipp Pilz | Unsplash

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