Water in the soil is essential for the survival of pine forests

In the current time of climate change, characterized by irregular precipitation patterns and rising average air temperatures, water is becoming increasingly vital in landscape ecosystems. While plants can absorb water through their leaves, for terrestrial plants, soil moisture is the essential factor for survival. Water serves as a source of nutrients and helps cool the organism during hot days. This not only affects individual plants but also influences the climate of the broader landscape, particularly in forested areas.

Until recently, pine forests were thought to be stable and healthy, even in regions and during times of low rainfall. It was assumed that even on poor sandy soils, significant problems would not arise, so these forests were largely overlooked. However, the dry years of 2015 and 2018 have highlighted the limitations of Scots pine’s resilience.

A better understanding of the water regime in pine stands in these areas can help us assess their health status in relation to their characteristics, weather patterns, and climate changes. This knowledge could inform the selection of effective silviculture and restoration methods for these ecosystems.

The topic discussed in this research comes from scientists at the Opočno Research Station, VÚLHM, v. v. i. Their recent findings were published in the article titled “Variation of soil moisture in different stand conditions of pine stand on nutrient-poor sandy soils during vegetation period” (Změny půdní vlhkosti v různých variantách borového porostu na chudých písčitých půdách během vegetační doby), which appeared in the journal Reports of Forestry Research, Volume 3, 2024. This work was part of the project QK24010054 on the “Impacts of Climate Change on Small Forest Catchments and Possibilities for Mitigation through Forestry Management and Water Management Measures“ (Dopady klimatické změny na malá lesní povodí a možnosti jejich zmírnění prostřednictvím lesnického managementu a vodohospodářských opatření).

The scientists aimed to describe and compare the development of soil moisture in various Scots pine stands on poor sandy soils in the Elbe region from 2018 to 2020. Based on their findings, they sought to establish fundamental principles governing these relationships. The research was conducted at the “Suchý chobot” experimental facility, which is part of the Municipal Forests of Hradec Králové, a.s. The deep sandy soils in this area are formed from dunes, and the groundwater level is situated 6 to 8 meters deep, beyond the effective reach of the roots.

In the long term, the years 2015, 2016, and 2018 experienced a significant decrease in precipitation during the growing season, falling more than 40% below the average levels recorded from 2001 to 2011. This notably affected the health of pine stands in the monitored area. Furthermore, in 2018, this dry weather coincided with an average temperature that was 1°C higher than usual.

Since January 2018, the given area has experienced low rainfall, receiving a total of 454 mm of precipitation in open areas and 367 mm beneath pine stands. In comparison, the average precipitation from 2001 to 2011 was 558 mm.

In 2019, precipitation levels were more balanced, totalling 633 mm for the year. The temperature conditions were also generally favourable, except for June, which saw an average high of nearly 22 °C. In contrast, the following year was characterized by colder temperatures and higher precipitation, particularly during the growing season, with the exception of April.

Sandy soils generally have a low capacity for retaining water. However, this retention can be improved by the presence of fine particles, particularly organic matter such as humus. Vegetation plays a key role in this process, as humus is primarily concentrated at the soil surface, within the upper layers, and in the top mineral horizon. As a result, these areas tend to be wetter than the deeper layers of soil.

There was noticeably higher humidity at and just below the soil surface, particularly in the clearing. This was due to the low vegetation coverage—with ground vegetation covering only about 10%, dominated by blueberries—which limited the influence of evapotranspiration. As a result, water entering the soil was reduced, leading to lower moisture levels.

In contrast, the effects of woody plants and ground vegetation on restricting water entry into the soil through interception and on extracting water from the soil via evapotranspiration were noticeable under the mature pine stand for an extended period, particularly until the beginning of August 2019.

The humidity data from this period indicates that the higher precipitation levels in August and September 2019 positively affected soil water supply. As a result, the soil has now reached a level that exceeds what is typically drained by vegetation. This means that the period of increased risk of water deficit in the forest ecosystem has come to an end.

The slight variations in humidity observed under the pine stand until early August 2019 may be attributed to the lingering effects of the exceptionally dry year of 2018.

Tree species likely reduced their water content during the dry period of 2018, and re-equilibration did not occur during the winter of 2018-19. Although soil water availability improved, climatic conditions may have hindered sufficient physiological activity in the trees. In spring, there is a significant demand for water to support germination and leaf development, so the water content in the trees’ tissues could balance out by the peak of summer.

Additionally, the decrease in soil moisture with depth suggests that the trees in this location cannot depend on groundwater. Given the sandy nature of the soil, seepage from the surface at depths greater than 6 meters is likely beyond the reach of pine roots. The low nutrient content in the soil, combined with decreasing moisture, poses a natural barrier to active root growth.

The observed increase in soil moisture in the clearing is due to the lack of a functional forest ecosystem, which affects the distribution of runoff from the catchment area.

However, the water used by the forest should not be viewed as simply a loss to the local ecosystem. The forest utilizes most of the solar energy for physiological processes related to water, leading to (evapotranspiration.

By releasing latent heat, the forest directly cools the environment. Additionally, the short water cycle contributes to increased cloudiness and higher air humidity, including fog, which in turn reduces the amount of solar energy that reaches the ground.

The environmental impacts from the current manifestations of climate change represent a significant gain for our ecosystem.

Based on an evaluation of soil moisture levels in various locations within a pine stand situated on a sandy soil that is poor in nutrients and has groundwater beyond the reach of the tree roots, scientists have reached the following conclusions:

• In the clearing, there was a confirmed year-round increase in soil moisture compared to the adult pine stand, particularly in the upper soil horizons.
• Variations in soil moisture levels between the upper mineral horizon under the mound and in the furrow of the cleared area were influenced by the time elapsed since the last rainfall and the amount of rain received. During extended dry periods without rainfall, moisture levels in the furrow decreased.
• The presence of the young pine stand affected the moisture levels in the upper soil layer in a manner similar to the mature stand, meaning that the increased moisture observed in the clearing was no longer apparent.
• After the removal of the spruce understory in the pine stand, there was only a partial increase in soil moisture; however, this increase was still crucial for the development and survival of the pine trees.
• In the clearing, soil moisture levels, even during dry periods, decreased with soil depth. Under the stand, this pattern was only observed when there was sufficient water in the ecosystem, such as after replenishing the precipitation deficit.
• The impact of the growing season on the long-term trends of soil moisture horizons was not clearly evident.

The findings are crucial for understanding soil moisture development in pine forests with similar conditions.

Paper “Variation of soil moisture in different stand conditions of pine stand on nutrient-poor sandy soils during vegetation period” (Změny půdní vlhkosti v různých variantách borového porostu na chudých písčitých půdách během vegetační doby) can be downloaded here.

Author: Ondřej Špulák, VÚLHM, v. v. i., Research Station Opočno, e-mail: spulak@vulhmop.cz

Prepared from the original by Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz

Illustrative photo: Pine forests on poor sandy soils in the Elbe (Labe) region, author Jan Řezáč