During the 50-60s of the last century, about 80 thousand hectares of sands were afforested mainly by pure Scots pine (Pinus sylvestris L.) and Crimean pine (Pinus nigra ssp. Pallasiana Lamb.) stands. Forest pathology processes, given the climatological characteristics of the region, accompanied the development of these plantations. Periodic drying of pine plantations was observed in 1957, 1971–72, and 1975–76. Since the late 80s of the last century, attention has been paid to the group drying of trees, which was observed mainly in pine plantations predominantly of 3rd–5th age classes. Against the background of a body of adverse influences (fires, droughts, and pests), the soil conditions and water regime of the territory remain the main factors on which the health of plantations depends. In this regard, it is important to clarify how soil conditions affect the health of pine plantations in such an environment.
Materials and Methods
We studied the dependence of the stands’ health on characteristics of the soils, on which they were established, on two soil profiles: in the center of drying and in a healthy forest, in plantations created on sands with deep (15.0–20.0 m in Vinogradove tract) and shallow (2.0–3.0 m in Blyzniy Karabay tract) groundwater. The profiles were a series of circular plots with a radius of 5.0 m with soil section of 1.7 m depth in the middle. Soil sections were placed along the sight line at regular intervals (10–20 m) between them. Trends in the influence of soil characteristics on the condition of the plantations were determined by correlation analysis between soil characteristics (depth of genetic horizons and fractional composition of soil layers to a depth of 170 cm) and indicators of the stand health (number of living and dead trees, basal area for living and dead trees).
The analysis shows that the pine plantations decline appear usually on soils containing ancient soils at a shallow depth, or clay-sandy or sandy deposits. Almost all the soil varieties in which the decline centers have been marked differ by a limited layer of soil available for root growth. All these differences concern, first of all, the texture of the soils and the peculiarities of their morphological structure. In order to determine the effect of the soil texture at different layers on the process of drying, a correlation analysis was conducted between the health indices of forest stand and the soil texture at different depths.
The relationships of the content of various soil fractions at depths of 0–170 cm with the basal area of healthy and dead trees indicate that the higher the content of silt and clay fractions in the soil, especially at a depths of 0–10 and 130–170 cm, the greater the basal area of dead trees. However, the increase of these fractions in 30–50-cm layer of soil, on the contrary, is accompanied by a decrease in the basal area of such trees.
The content of coarse sand fractions also had a significant impact on the trees that had died: with increasing content of coarse-grained sand in the upper 0–10 cm and 80–130 cm layers of soil, the basal area of dead trees decreased. The dependence of the health of the pine trees on the soil texture in the conditions of the Nizhnedniprovsky sands was primarily due to the influence of different soil fractions on moisture infiltration. Large fractions of sand freely pass moisture into deeper layers of soil, and silt and clay factions entrap it. Due to the peculiarities of the formation of sandy areas in Lower Dnieper, the concentration of different soil factions at different depths affects the development of root systems of trees, the physiologically active part of which is concentrated in those layers of soil where moisture lingers.
The content of various fractions of the soil at different depths under the current conditions also affected the health of plantations. In our case, the influence was manifested in the shares of healthy trees and their basal area in the number and cross sections of all trees. The results of the correlation analysis showed that the number of healthy trees increased with the increase of coarse-grained sand (1.0–0.25 mm) proportion at a depth of 70–110 cm (r0.05 = 0.61…0.67) and decreased in the case of an increase of fine sand (r0.05 = -0.64…0.69). Reducing the number of healthy trees is observed in the case of an increase in the proportion of coarse silt (0,05–0,01 mm) in the upper 10–50 cm soil layers (r0.05 = -0,63…0,66) and in the lower layers, especially at a depth of 130–150cm
(r0.01 = -0.83). Better health of plantations is also observed in case of an increase in the content of physical clay fractions in the soil layer at a depth of 30–50 cm (r0.01 = 0.80) and 90–110 cm
(r0.05 = 0.64). In general, the fractional composition of different soil layers as a whole reflects the occurrence of genetic horizons. The description of soils gives grounds for their classification as soddy underdeveloped gley sandy ones, which are underlain at different depths by mature soddy sandy and sandy-loam soils, sometimes black meadow soils. During the study, the groundwater in Blyzniy Karabay area was at a depth of 2.0–3.5 m, and in Vinogradove tract, at a depth of 15–20 m.
The results of the analysis show that the indices of the health condition of the plantations are most closely related to the depth of the horizon of gray fine-grained sand. The deeper the horizon is, the greater the number of living trees (r0.01 = 0.87) and their basal area (r0.01 = 0.73) and, accordingly, the smaller the number of dead trees and their basal area (r0.05 = -0.74). The health of the plantations also depends on the depth of the groundwater level: the deeper the groundwater table, the more the number of living trees and their basal area (r0.05 = 0.66; r0.05 = 0.789) as well as the smaller the number and basal area of trees, which dried up (r0.05 = -0.69; r0.05 = -0.70). The nature of these relations shows that the groundwater table is higher in the sites with dead trees.
In the deep-water sands (Vinogradove tract), the number of living trees and their basal area depend on the slightly humic fine-grained silt sand h-hp horizon’s power (r0.01 = 0.948; r0.01 = 0.870), on the depth of the horizon hP1 – rock with humus inclusions (r0.01 = 0.937; r0.01 = 0.888), and on its power (only the number of living trees – r0.01 = 0.830), as well as on the depth of the horizon Hfos -hfosP2 – clay-sandy humic (r0.01 = 0.999; r0.05 = 0.811), and also with depth of parent rock (Р2) – compressed clay sand (r0.01 = 0.937; r0.05 = 0.888).
The health of a plantation as a whole depends on the thickness of the soil layer above horizon hfos P2, which includes the buried humus horizon: the deeper is the brown loamy sand, the greater is the number of healthy trees and their basal area. There was a weak inverse relationship between the number and the basal area of dead trees and the depth and the thickness of the soil genetic horizons. That is, with the increase in the depth of the humic horizons of modern soils and the thickness of the humic horizons of buried soils, there is a tendency to decrease the drying of trees.
At present conditions, the health of pine stands in the Lower Dnieper zone depends on the peculiarities of the soils on which they were established, their mechanical composition at different depths and power of the layer of soil available for root growth. Healthy trees develop a branched root system with a concentration of physiologically active roots in the soil layers containing physical clay fractions at a depth of 90–110 cm and 130–150 cm. On the contrary, the trees that dry up develop lateral root systems in the upper 0–30 cm layers of soil with the higher content of coarse-grained silt and physical clay or in the 130–170 cm soil layers with a high content of these fractions.
The health condition of plantings is improved with the growth of the proportion of coarse and medium-scale fractions of sand in the soil layers of 70–110 cm and the increase in physical clay content in the soil layers at a depth of 30–50 cm and 90–110 cm. On sands with shallow groundwater table, the condition of plantations depends on the level of groundwater, the depth of parent rock, in particular, on the thickness of the layer of soil above it. The thicker the layer of soil over the parent rock, the larger the number of living trees and the less of the dead ones. In deep-water sands, the state of the plantation as a whole depends on the thickness of the soil layer above the humic compressed horizons of buried soils.
5 Tables, 10 Refs.