Some of the factors for root rot to emerge and develop are the soil diversity and water regime peculiarities. Due to the fact that the cause-and-effect mechanism of the development of the disease has not been thoroughly studied, there are no radical measures developed to control it.
The results of the study of the genetic features of soils affected by the root rot caused by Heterobasidion annosum Fr. (Bref.) indicate that decline of trees occurs in areas where buried soils or dense layers of different mechanical compositions are close to the surface. Thus, a deep development of the root systems gets delayed. In this regard, it is important to find out how the correlation of different soil fractions at different depths and moisture content affects the condition of pine stands and their water regime.
The aim of the research was to study how soil grading at different depths affects condition of stands and their water regime due to the fact that dieback centers have emerged.
Materials and Methods
We studied IV-age-class pine stand affected by annosum root rot. We made 8 boreholes to the depth of 1.5 m inside the dieback focus and outside it. Soil samples were taken from each 10-centimeter depth of these holes to analyze the soil for various sand and clay fractions content. We marked the circular test areas with the radius of 5.7 m around the boreholes and analyzed health condition and trunk diameter of the trees growing there. To assess condition of the stand, we divided the trees into the health condition categories according to the “Sanitary Forest Regulations in Ukraine”. The moisture, physical sand (> 0.01 mm) and physical clay (< 0.01 mm) contents were estimated in the collected soil samples. To define the relative moisture of the soil layers, we applied the weight method according to the standard practice. We determined how the fractional composition of soil and its moisture influence the trees’ health by means of the correlation analysis of the relations between these indices and the cross-sectional area of living and dead trees.
The total moisture content in the soil to the depth of 140 cm was significantly higher in the dieback focus comparing to the area outside it (by 21%). The soil moisture in the dieback focus at the depth of 30–40 cm was more than twice higher than that in the outside it. A higher moisture content in the dieback focus was observed in the soil to the depth of 120–130 cm. The moisture content in deeper soil layers was somewhat higher in the area around dieback focus than in the focus itself. We analyzed the changes in the water-physical parameters of the soil and the stand condition in the affected (the dieback focus) and the intact sites (the area outside the dieback focus) and found the strong correlation between the sum of cross-sectional areas of living trees outside the dieback centers and the soil horizons moisture at the depth of 30–50 and 50–70 cm (r = -0.82 and r = -0.73, respectively).
The stand condition was found to depend on the moisture content in the upper soil layer (30–70 cm deep). The higher the moisture content in this layer was, the smaller the cross-sectional area of the living trees. That means that the less the number of living trees is, the more moisture remains in the soil. The results of the analysis indicate that the moisture content in the soil layers at different depths depends on their granulometric composition. Therefore, the higher the content of physical clay at the depth of 130–150 cm, the larger a moisture reserve in this layer (r0.05 = 0.82). An increase in the proportion of physical sand, on the contrary, reduces the moisture content (r0.05 = -0.82).
A strong negative correlation between the physical sand content in the soil and the sum of the cross-sectional areas of relatively healthy trees growing outside the dieback focus was marked at the depth of 110–130 cm (r0.05 = -0.72). That indicates that the less physical sand at the depth of 110–130 cm is, the better the stand condition will be. It happens due to the fact that sandy soil fractions, in contrast to physical clay fractions, are bad at keeping moisture in the soil. Thus, the strong negative correlation between the physical sand content and the sum of the cross-sectional areas of dead trees at the depth of 10–30 cm (r0.05 = -0.86) indicates that the higher physical sand content at the depth of 10–30 cm is, the smaller the cross section area of dead trees and the better the stand condition. And inversely, with the increase in the physical clay content in the soil layer at this depth, the stand condition get deteriorated, namely the cross-sectional area of dead trees decreases (r0.05 = 0.86).
With the increase of the physical sand content in the soil layer of 10-30 cm, the stand condition tends to get better as moisture is not retained in the layer but goes to the physiologically active roots that grow in the deeper soil layers. And vice-versa, the increase in the physical clay content accounts for the moisture accumulation in this layer and formation of lateral root system. In the soil layer of 130–150 cm, on the contrary, the increase of the physical sand content brings about moisture infiltration to the deeper soil layers and the stand condition get deteriorated. At the same time, the increase of physical clay in this layer, on the contrary, enhances its moisture-retention capacity, which has a positive impact upon the stand condition.
The amount of moisture needed to enable physiological functions of trees will increase with age, which, especially in arid periods, will result in its deficit. It, in turn, will bring about the group-type decline of part of a stand. In turn, because of the gaps in the dieback focus, more moisture gets into the stand, which, in one or another way reduces its deficit.
In Kharkiv Region, the annual precipitation reaches about 500 mm on the average. During the growing season it is about 290 mm (Babichenko et al. 1984). In the stand unaffected by root rot, a part of precipitation is retained by crowns and trunks of trees, and only about 70% of it gets into the soil (Molchanov 1953), that is, 203 mm of precipitation become available for root systems of trees. In the gaps of the dieback focus about 95% of the precipitation gets into the soil, i. e. 275 mm.
We calculated that gaps, covering 10% of the stand area, replenish the underground water by about 7 mm, or 70m3 per ha, which provides about 13 medium-sized trees with moisture, or helps preserve about 6.1m3 per ha of the forest. The larger the area of gaps in the stand, the more moisture gets into the soil. Therefore, the gaps in the dieback foci can replenish the water reserve of the stand by 5-25 mm, which, to some extent, will reduce the amount of pathological abnormalities.
The higher the physical clay content in the upper 10–30 cm of the soil, the worse the Scots pine stand condition is, and vice versa, the increase of the physical clay content in the soil layers at the depth of 110-130 cm improves it. With the increase of the physical clay content in the upper layers of the soil, moisture gets concentrated in them and brings about the development of the lateral root systems in trees. If there are radical changes in the water regime (prolonged droughts), the condition of trees worsens. As for the soil layers of 130-150 cm, on the contrary, a higher amount of the physical clay in these layers helps to keep moisture, which has a positive impact upon the stand condition. Taking into account a fragmentary nature of the sandy soils granulometric composition on the sandy terraces, where the majority of such forests have been planted, the changes in the stand conditions during the periods of changes in the water regime will also be fragmentary. Dieback foci, depending on their area, can increase the water reserve in the stand by 5–25 mm. In stands where dieback gaps cover 10% of their area, the water reserve gets 7 mm increased. It, to an extent, compensates losses caused by the pathological dieback by means additional growth of trees outside the dieback centers.
2 Figs., 1 Table, 16 Refs.