Long-term changes in groundwater levels of the Bialowieza Forest

In the Bialowieza Forest, almost half of which is made up of swamp and wetland habitats, changes in groundwater levels have a direct impact on the persistence and character of local vegetation. A study published on July 5 by scientists at the Forest Research Institute in Sekocin Stary shows how climate and precipitation conditions have shaped the waters over nearly 40 years.

The Bialowieza Forest Reserve is one of the last and largest fragments of the original lowland primeval forest in Europe. Most of its area consists of protected forest stands and valuable swamp habitats, for which groundwater is the most important source of water supply.

Declines in groundwater levels before 2005.

The analysis, prepared by a team led by Prof. Andrzej Boczon, used data collected at 21 monitoring points over two periods: 1985-2005 and 2022-2023.

Between 1985 and 2005, there was a systematic decline in groundwater levels in all habitat types:

• in the swamp by an average of 0.17 meters per year;
• In wet ones by 0.25 m;
• and in fresh ones – by as much as 0.44 m.

The average rate of decline in different habitat types was: 0.08 m/10 years (marsh), 0.11 m/10 years (wet) and 0.21 m/10 years (fresh). The greatest decline was in the summer half-year, when high temperatures and vegetation development increased evaporation and water consumption.

Climate change – increase in temperature and precipitation, less snow

From 1985 to 2023, the average annual air temperature in Bialowieza increased by 1.87°C, with 0.98°C in the 1985-2005 period. At the same time, average annual precipitation increased from 604 mm (1985-2005) to 664 mm (2006-2023). A larger increase was recorded in the winter semester – almost twice as much as in the summer.

In the last decade, the number of days with snow in the Bialowieza Forest area has been noticeably decreasing. Since 2014, it has not exceeded 100 days per year, and in 2020 it lasted for only 10 days.

Water balances for years with similar precipitation (e.g., 2022 vs. 1986, 2023 vs. 1999) showed that despite similar precipitation totals, actual evapotranspiration (ETa) differed significantly. In 2022, it was 312.5 mm, while in rainfall-comparable years it ranged from 240.9 mm (2004) to 336.3 mm (2002).

Slowdown in water level decline over the past two decades

Despite a warming climate, a deceleration in the trend of declining groundwater levels has been observed. The reason was increased precipitation after 2005. At many measurement points, water levels in 2022 and 2023 were comparable to those in the late 1980s. In 2022. – hydrologically similar to 1986. – water levels were similar to or even higher than in 2002 and 2004.

Overall, however, groundwater levels in 2023 were significantly lower than in the comparable year of 1999, despite similar precipitation – the change was as much as 0.36 m. The largest decrease was in fresh forests (0.52 m), followed by wet forest habitats (0.46 m). The smallest changes were in wetlands.

Particularly sensitive watershed areas

The largest and most systematic lowering of water levels – by as much as 1 m – was found at two monitoring points located in the area of the Narew and Narewka river watershed. The study’s authors emphasize that such sites are particularly vulnerable to groundwater loss, as they do not receive recharge from higher ground, but only from precipitation. This can lead to interruption of flow in the upper sections of watercourses, which poses a serious threat to the sustainability of riparian forests and spring habitats.

Snow cover reduction and the consequences for the ecosystems of the Bialowieza Forest

A reduction in the number of days with snow, as well as earlier snowmelt and increased winter precipitation in the form of rain, are causing changes in the timing of groundwater recharge. Earlier occurrence of this phenomenon may shorten the period of water availability in spring, which is crucial for tree growth, especially in April-July, when earlywood is formed. The projected groundwater level in the winter of 2023 was 0.46 meters lower than actually measured – indicating that increased winter precipitation has partially offset the negative effects of climate change.

Although the decline in groundwater levels in wetland habitats has slowed (by 0.19 meters on average; 0.33 meters was predicted), it still poses a threat. If precipitation decreases or evaporation increases, the risk of a rapid decline in water levels will increase again. The disappearance of surface water during periods of drought can lead to a gradual decrease in the area of wetlands and significant changes in the species composition of their vegetation.

Conclusions: what lies ahead for Bialowieza?

Between 1985 and 2023, the Bialowieza Forest experienced a general trend of lowering groundwater levels, but this was less than predicted based on 1985-2005 data. Increased precipitation after 2005, especially in winter, slowed this process.

However, the problem of falling groundwater levels in the Bialowieza Forest is a significant one, and can be exacerbated in years with low rainfall totals. Reduced water availability for the Bialowieza Forest’s vegetation is particularly dangerous at the beginning of the growing season. The greatest danger in this regard is in the watershed areas. The authors recommend continuing groundwater monitoring to better predict and respond to future changes in the Bialowieza Forest’s ecosystems.

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Source:

Boczoń, A.; Wróbel, M.; Kowalska, A. Long-Term Changes in Groundwater Levels in the Białowieża Forest, Poland, Under Climate Change. Water 2025, 17, 2027. https://doi.org/10.3390/w17132027