Forest and the water cycle in the landscape

In a small village in southeastern Poland, in a place where perhaps some time ago a Carpathian primeval forest grew, the State Forests – presumably in pursuit of its educational function, which, right next to its protective and economic function, is the mission of its operations – erected an information board: „How does a forest grow?” In the photos illustrating the question, the men are planting trees at equal intervals. And yet they don’t. In our latitude and climatic conditions that have prevailed for several thousand years, the forest is a natural form of land cover. And this means that it will grow on its own, if only allowed to do so.

Currently, forests cover about 30% of the country. Almost all of them, however, are no longer natural forests, but transformed, with the degree of transformation varying – some are commercial forests, planted by the hand of man and dedicated to the production function, while in others human interference is less or more distant in time. Although foresters boast that trees are increasing in Poland, a forest is not equal to a forest, and not every forest planting will perform the same ecosystem services as an appropriate for the morphology of the terrain and weather conditions, a mixed forest consisting of multiple storeys and trees of different ages. In addition, forests in the temperate climate zone, and even more so, like Poland’s forests – growing on poor soils and experiencing shrinking water resources (lowering groundwater levels) – are vulnerable to climate change. Already spruce trees, for which it is too hot, are dying, and it is also getting too dry for pine trees, which grow on the weakest soils. According to dendrologists, if not her, no other trees of our climate zone will grow there. An additional factor that increasingly threatens forests is violent weather events. The hurricane winds that hit the Tuchola Forest and Kashubia region in 2017 wrought unprecedented damage to 120,000. hectares of forests. Foresters already have solutions ready: they are prepared to start planting trees from the more southern zones – they have identified species, seed material and nurseries. How, dear reader, will you feel when a pine-oak forest full of birches, lindens or firs is replaced by plantings of Douglas fir, bird cherry and robinia? Will you encounter woodpeckers, boletes and berries in it?

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Healthy – large, diverse and old enough for natural processes to take place – forest ecosystems provide nature, and us, with a range of services that underpin biological life on the planet. Some of the most important are those related to water – its circulation in nature, renewal of resources, as well as purification. These are ecosystem services provided for free by healthy forests. Their value can be valued, if this were to become a key argument for their preservation.

Before we move on to scientific explanations, I encourage you to exercise your imagination or memory: what does it feel like on a sunburned July day to walk from a rapeseed stubble into a two-hundred-year-old Carpathian beech forest crisscrossed by ravines, lined with moss and full of streams? Or: how is it that the water from a forest stream or river is crisp and crystal clear, and just a few kilometers away, in the midst of the fields, it turns murky, warm, blooming? Which would you, attentive reader, drink?

Forests protect our – increasingly scarce – water resources in at least two ways: first, they slow down rainwater runoff. Diverse, multi-storied and adapted to the morphology of the landscape, the vegetation cover mechanically stops water from escaping by making it possible for more rain to penetrate deep into the soil profile. This effect will be better the more humus in the forest soil, that is, the more dead and fallen trees could decay and join the cycle of matter. Trees capture most of the precipitation, so water losses associated with surface runoff, are fourteen times less than in an area without forest. Drainage is also impeded by all natural depressions of land where water can collect in forested areas – marshes, swamps and moors. (The idea of turning riparian forests into pine plantations through land reclamation, thereby increasing forest productivity, turned out to be a mistake and was abandoned.)

Second, forests are a key component of the water cycle in the landscape. The vegetation covering the land not only slows down water runoff and is a vector for water infiltration, but also stores water in its biomass. An area with dense vegetation will therefore have a much higher evapotranspiration rate – the sum of evaporation from soil and plants – than a field in black fallow or sealed parts of a catchment area. Although it may sound paradoxical at first glance, this effect is beneficial to the water balance of the landscape: water extruded by the trees remains in circulation – it circulates between the atmosphere, plants and soil, increases atmospheric humidity and returns in the form of fog, dew or local storms. Activities aimed at regulating the water cycle in the landscape by means of appropriate shaping of the vegetation cover are called phytomelioration. In this way, the effect of reducing wind power, regulating water relations or retaining snow can also be achieved. It is estimated that trees need about 20 liters of water per week, plus 20 liters for every 2.5 cm of tree breast height. With this demand and deep root penetration, which also reaches groundwater, tree transpiration is the primary factor in controlling the amount of water runoff from the ecosystem.

In Poland, where most of the waters of the two main drainage basins – the Vistula and Oder – flow from south to north, forests in the southern part of the country are particularly important for countering drought and limiting its extent and consequences. Also because precipitation in the mountains in the south is almost twice as high as the average for the whole country, and it is from precipitation that our renewable water resources – those in the landscape in the form of lakes and rivers that benefit the ecosystem, as well as agriculture, industry and us (so-called municipal needs) – come. The role of mountain forests is therefore special – they should capture the maximum amount of precipitation while protecting us from storm surges and the threat of flooding – which is all the more important in a destabilizing climate, when precipitation is rapid and increasingly unpredictable – and then slowly return the water, mitigating the effects of flooding. Water conserved in forests, especially in the mountains, guarantees the stabilization of river flows – reducing the dangers of high and low river levels.

However, this is not the case. The density of logging roads in Poland’s mountainous areas averages 12.5 kilometers per square kilometer. Only Borneo, where there are few forests left, boasts a better record. The roads, often driven across level ground and torn up by heavy equipment, turn into rushing streams with every rain. The creeks have been concreted over. Rather than being retained in the ecosystem, water accelerates at the start and poses a threat to downstream areas. This is how flash floods begin. The exposed soil of skid trails and clear-cutting areas is systematically washed away. Although foresters insist that Poland’s forests are too old, and that young plantings will be even better at retaining water and sequestering atmospheric carbon (“because young trees grow faster”), naturalists are skeptical. If the climate continues to change, forests like the ones we are cutting now may never grow again.

Exactly in the same way, though perhaps a little less spectacularly, the forests in the lowlands work: they provide high rainwater retention, stabilizing the water level in the ground, which guarantees recharge during periods of rainfall shortage. In addition, they positively influence the water cycle in the landscape by generating fog, dew and thunderstorms. Agriculture, which uses 60% of the country’s land area, is beginning to realize the role of trees and canopies in the landscape. In keeping with the principles of agroforestry, progressive farmers are making plantings along pastures, baulks and roads – thus improving water relations in neighboring fields, providing shade for animals or counteracting wind erosion.

The old, rich forest acts as an air conditioner. Thanks to intensive transpiration, it moderates climatic extremes, especially daily temperature amplitudes. It filters and purifies water. Was a common good, is becoming a luxury good. He could be a dream companion for times of climate destabilization.

Author: Joanna Perzyna

The text is based on the Expert Report “Water in Agriculture”.

The text was written as part of the project „Hydrozagadka – jak wygrać z suszą?”