Why deciduous trees protect our drinking water

Plant deciduous trees and promote ground­water

When we plant deciduous trees, we gain more than just living landscapes – we promote ground­water recharge and, with it, our drinking water. But how exactly are these connected?

 

Where does our drinking water come from?

In Germany, 70 % of drinking water comes from ground­water. Over 40 % of the area of all German water protec­tion zones is forested. The ground­water formed in the forest is of good quality and, at almost all sites, falls below the appli­cable nitrate limit values.

 

More water and life in mixed deciduous woodland

• Deciduous trees lose their leaves in winter. Their shady canopy reduces grass and herb growth. This means that, especi­ally in the rainy winter season, more rain reaches the ground directly. Ground­water recharge increases.
• Per hectare per year, a deciduous forest can generate between 500,000 and 1.2 million litres more ground­water than a pure conife­rous forest.
• Deciduous trees reflect more sunlight, evapo­rate more water through their leaves and thus promote a humid forest interior climate under closed canopies. This lowers the tempe­ra­ture even on hot days and keeps the soil moist.
• The moister soils in mixed deciduous forests can absorb more rainwater during extreme heavy rainfall.

 

Mixed deciduous woodlands are more resilient than monos­truc­tured conife­rous forests

• Deciduous trees reflect more sunlight and evapo­rate more water, creating a moist forest interior climate under closed leafy canopies and more effec­tively lowering tempe­ra­tures during hot spells.
• The moist, humus-rich soils of mixed deciduous forests can absorb more rainwater during extreme rainfall.
• Struc­tu­rally diverse mixed deciduous forests with site-appro­priate species promote biodi­ver­sity: oak alone is home to up to 1,000 animal and plant species, inclu­ding around 700 insect species.
• Mixed deciduous forests reduce the spread of pests and damage events, as diffe­rent species are vulnerable in diffe­rent ways.
• More species improve the balance between pests and benefi­cial organisms.
• Deciduous trees make forests more storm-resistant.
• Deciduous trees such as beech, oak or maple form a humus-rich soil layer with far more soil organisms than under conife­rous forests.

 

Why conife­rous forests are proble­matic

• Spruce naturally occur in cool, moist mountai­nous regions and pines on sandy soils, but due to their rapid timber yield they were also planted in other regions of Germany outside their natural range, with lower resili­ence to the incre­asing droughts and storms.
• Conife­rous forest monocul­tures let less rain through their year-round canopy – a great deal of rain remains on the needles and evapo­rates there. Ground­water recharge is lower than in deciduous forests.
• Their needle litter is rich in lignin and resin, but contains few readily available nutri­ents that can be broken down into humus. The soil becomes acidic, nutrient-poor and offers less habitat for soil organisms.
• Monos­truc­tured conife­rous forests outside their natural range harbour less biodi­ver­sity, and the risk from pests is not spread across several species. Pests such as the bark beetle can spread quickly.

 

Conclu­sion: Mixed deciduous woodlands are climate-tolerant and fit for the future.

Anyone who plants mixed deciduous woodlands protects the forest, drinking water and biodi­ver­sity, also for future genera­tions.