Modern manifestations of climate change, such as increasing intensity and frequency of droughts and floods, are the main environmental stress, which negatively affects the growth of trees, complicates their survival, reduces resistance to pests and diseases, and generally causes forest degradation. In Ukraine, the area of forests affected by pests and diseases increased by 40–50% over a period 2000–2015, which led to their weakening and dying (Yavorovskyi 2015). Seven thousand hectares of forests were lost during 1990–2000, while 19,000 ha died during 2006–2010. The most intensive forest health deterioration and decline were manifested in 2017. In that year, the total area of dying pine, spruce and oak stands exceeded 330,000 ha. The area of dying pine stands alone exceeded 142,000 ha, with wood losses reaching about 7.2 million m3 (Shvidenko et al. 2014, 2018).
Conservation and enrichment of biodiversity in the Ukrainian forests can be ensured by introducing resistant tree species, including species of the genus
The genus
Small-leaved lime is of considerable economic importance. It is actively used in landscaping and field shelterbelts planting as a soil-shading-associated species for ravine stands in the forest-steppe and steppe zones in Ukraine (Sovakova et al. 2012; Tsvetkova and Saranenko 2018; Sultanova et al. 2019). Lime stands are not only a source for meeting the ever-increasing demand for wood, bark and non-wood products but also an important bee forage. The melliferous capacity of lime is, according to various data, from 450 to 800 kg per 1 ha of continuous stands (Polishchuk and Bilous 1972; Vasylchenko 2010; Razanov et al. 2019). Wood is widely used in making musical instruments, household items, furniture, containers, barrels, bee houses, etc. (Praciak et al. 2013).
Due to the adaptability of
The aim of this study was to assess the current state of small-leaved lime stands in the context of climate change, as well as to identify optimal conditions for the occurrence and further growth of natural lime regeneration under the stand canopy to develop effective interventions to grow productive and sustainable mixed stands.
The study examined stands with a predominance of small-leaved lime in the canopy layer. The stands were of different origin, age and stocking and covered the area of 13,610 ha. Also, we studied oak stands of different origin, age and relative density of stocking, which had 10–60% of small-leaved lime in the composition. The area of oak stands was 209,780 ha within six administrative regions of Ukraine, including Cherkasy Region, Chernihiv Region, Kharkiv Region, Kyiv Region, Poltava Region and Sumy Region. The study covered the forest fund of more than 60 state enterprises managed by the State Forest Resources Agency of Ukraine.
The data of the last forest survey (as of 2016) were the basis for the relevant calculations. Within the study area, 5,313 subcompartments were analysed where small-leaved lime predominated in the canopy layer and 48,062 subcompartments where it was found as a co-dominant species in oak stands.
Lime natural regeneration was studied under the canopy of oak stands of different ages and relative density of stocking within the specified administrative regions. The lime proportion in the oak stands ranged from 0% to 30% by the stock.
The natural regeneration was estimated on circular accounting sites of 10 m2 each (
where:
In total, 600 accounting sites were established in six administrative regions under the canopy of 20 stands with an area of 5–18 ha, that is, 30 sites under the canopy of each of the 20 stands. For example, there were 60 sites under the canopy of two stands in the Kyiv Region (sample plots number 6 and 12 in Ivankivske Forest Enterprise), 90 sites under the canopy of three stands in the Cherkasy and Chernihiv Regions (sample plots number 2, 11 and 15 in Zolotoniske Forest Enterprise and number 1, 7 and 14 in Borznyanske Forest Enterprise) and 120 sites under the canopy of four stands in Kharkiv, Poltava and Sumy Regions (sample plots number 5, 10, 13 and 20 in Vovchanske Forest Enterprise; sample plots number 3, 9, 16 and 18 in Gadyatske Forest Enterprise; sample plots number 4, 8, 17 and 19 in Trostyanetske Forest Enterprise; Fig. 2).
Depending on the age of natural regeneration, 1-year-old, 2-year-old, …, and 15-year-old individuals were classified. The older regeneration was not found under the stand canopy. Age was determined by cutting the trunk and counting the number of annual rings near the root collar of at least 10 individuals.
In Ukraine, lime stands cover an area of 20,430 ha, which is only 0.3% of the total forest area. Their largest areas are concentrated in six administrative regions of Ukraine, which are located in the central and north-eastern parts of the country, namely Cherkasy Region – 2,660 ha, Chernihiv Region – 1,680 ha, Kharkiv Region – 1,660 ha, Kyiv Region – 1,150 ha, Poltava Region – 2,220 ha and Sumy Region – 4,240 ha. These regions were the objects of the study. Significant areas of lime stands were also registered in Ivano-Frankivsk Region – 1,830 ha and Lviv Region – 1,280 ha, which are in the western part of Ukraine. In general, 81.8% (16,720 ha) of all lime stands in the country grow in these regions (Tab. 1).
Current state of small-leaved lime (
Administrative regions | Total forest area (ha) | Including small-leaved lime stands | |
---|---|---|---|
ha | Percent of the total area | ||
Autonomous of Crimea Republic | 227,700 | 150 | 0.1 |
Cherkasy Region | 255,400 | 2,660 | 1.0 |
Chernihiv Region | 355,500 | 1,680 | 0.5 |
Chernivtsi Region | 157,300 | 350 | 0.2 |
Dnipropetrovsk Region | 65,700 | 50 | 0.1 |
Donetsk Region | 92,500 | 110 | 0.1 |
Ivano-Frankivsk Region | 425,800 | 1,830 | 0.4 |
Kharkiv Region | 282,300 | 1,660 | 0.6 |
Kherson Region | 77,300 | – | <0.1 |
Khmelnytskyi Region | 167,400 | 260 | 0.2 |
Kyiv Region | 355,100 | 1,150 | 0.3 |
Kirovohrad Region | 103,500 | 740 | 0.7 |
Luhansk Region | 228,200 | 150 | 0.1 |
Lviv Region | 428,200 | 1,280 | 0.3 |
Mykolaiv Region | 37,400 | – | <0.1 |
Odesa Region | 90,200 | 80 | 0.1 |
Poltava Region | 157,300 | 2,220 | 1.4 |
Rivne Region | 584,200 | 200 | <0.1 |
Sumy Region | 255,600 | 4,240 | 1.7 |
Ternopil Region | 143,600 | 460 | 0.3 |
Transcarpathian Region | 461,200 | 20 | <0.1 |
Vinnytsia Region | 199,700 | 580 | 0.3 |
Volyn Region | 436,800 | 100 | <0.1 |
Zaporizhzhia Region | 34,000 | 10 | <0.1 |
Zhytomyr Region | 660,300 | 450 | 0.1 |
Total Ukraine | 6,282,200 | 20,430 | 0.3 |
Within the study area (Cherkasy Region, Chernihiv Region, Kharkiv Region, Kyiv Region, Poltava Region and Sumy Region), stands of vegetative origin make up the largest part of the total area of lime stands, amounting to 66.5% (9,044 ha from 13,610 ha total). Seeded stands occupy 24.0% of the area (3,279 ha), and natural seed origin stands account for 9.5% (1,287 ha). By administrative regions, the proportion of stands of vegetative origin ranges within 42.4–83.6%, while stands of natural seed origin are 1.3–18.3%, and stands established by seeding account for 8.3–47.6% (Tab. 2).
Distribution of lime stands by origin within the study area
Administrative regions | Origin of small-leaved lime ( |
Total small-leaved lime | ||||||
---|---|---|---|---|---|---|---|---|
vegetative | natural seed | planted seed | ( |
|||||
ha | % | ha | % | ha | % | ha | % | |
Cherkasy Region | 1,129 | 42.4 | 265 | 10.0 | 1,266 | 47.6 | 2,660 | 100 |
Chernihiv Region | 1,405 | 83.6 | 136 | 8.1 | 139 | 8.3 | 1,680 | 100 |
Kharkiv Region | 1,305 | 78.6 | 38 | 2.3 | 317 | 19.1 | 1,660 | 100 |
Kyiv Region | 574 | 49.9 | 210 | 18.3 | 366 | 31.8 | 1,150 | 100 |
Poltava Region | 1,784 | 80.4 | 29 | 1.3 | 407 | 18.3 | 2,220 | 100 |
Sumy Region | 2,847 | 67.1 | 609 | 14.4 | 784 | 18.5 | 4,240 | 100 |
Total | 9,044 | 66.5 | 1,287 | 9.5 | 3,279 | 24.0 | 13,610 | 100 |
The study showed that stands of vegetative and natural seed origin with 50–70% of lime in their composition were the most productive. For example, their stock at the age of 61–70 reached 253–266 m3·ha-1 for vegetative stands and 262–268 m3·ha-1 for the stands of natural seed origin. The stands’ stock volume decreased with the increase or decrease in the lime proportion in the composition.
Among stands grown from seeds, the highest productivity (190 m3·ha-1) was recorded for those with 60–70% of lime in their composition. The significantly smaller growing stock volume of planted stands is associated with their younger age (51–60 years), as well as slower growth compared to stands of vegetative origin. Another reason is the reduced resistance to adverse environmental factors compared to stands of natural seed origin, particularly at a young age, when planted stands have not yet formed a fully functional forest environment. If the lime proportion in the planted stands decreased or increased, their stock also decreased (Fig. 3).
Small-leaved lime, as already mentioned, serves as an associated species in oak stands in the Ukrainian forests; it has quite high growth rates in rich forest site conditions (fresh and moist fertile sites).
Our results show that small-leaved lime grows as part of oak stands of various origins on an area of 209,780 ha. By regions, the area of oak-lime stands varied from 11,740 ha in Chernihiv Region to 59,860 ha in Cherkasy Region (Tab. 3). The part of the area of oak-lime stands in the total area of oak forests in these regions ranges within 22–51% (38.9% on average).
Percentage of oak-lime stands in the total area of oak forests
Administrative regions | Area of oak stands [ha] | Area of oak- lime stands [ha] | Percentage of oak-lime stands |
---|---|---|---|
Cherkasy Region | 117,690 | 59,860 | 50.9 |
Chernihiv Region | 54,560 | 11,740 | 21.5 |
Kharkiv Region | 150,800 | 52,310 | 34.7 |
Kyiv Region | 64,380 | 14,440 | 22.4 |
Poltava Region | 55,780 | 24,910 | 44.7 |
Sumy Region | 95,500 | 46,520 | 48.7 |
Total | 538,710 | 209,780 | 38.9 |
In these regions, mixed oak stands with 10–60% of lime in their composition are formed. The largest part of the area is occupied by stands, in which lime is 10–20%. For example, the areas of such stands were 82.8% in Cherkasy Region, 75.0% in Chernihiv Region, 85.6% in Kharkiv Region, 86.2% in Kyiv Region, 78.7% in Poltava Region and 76.7% in Sumy Region (Tab. 4).
Distribution of the area of oak stands with different proportions of lime in their composition (numerator – ha, denominator – % of the area)
Origin of small-leafed lime ( |
Unit | Lime proportion in oak stands | ||||||
---|---|---|---|---|---|---|---|---|
10% | 20% | 30% | 40% | 50% | 60% | total | ||
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
Cherkasy Region | ||||||||
Vegetative | ha | 14,894 | 11,734 | 3,285 | 731 | 155 | 60 | 30,860 |
% | 48.3 | 38.0 | 10.6 | 2.4 | 0.5 | 0.2 | 100 | |
Natural seed | ha | 8,503 | 6,205 | 1,848 | 506 | 101 | 10 | 17,172 |
% | 49.5 | 36.1 | 10.8 | 2.9 | 0.6 | 0.1 | 100 | |
Planted seed | ha | 4,078 | 4,126 | 1,959 | 963 | 488 | 214 | 11,828 |
% | 34.5 | 34.9 | 16.6 | 8.1 | 4.1 | 1.8 | 100 | |
Total | ha | 27,474 | 22,065 | 7,092 | 2,201 | 744 | 284 | 59,860 |
% | 45.9 | 36.9 | 11.8 | 3.7 | 1.2 | 0.5 | 100 |
Chernihiv Region | ||||||||
---|---|---|---|---|---|---|---|---|
Vegetative | ha | 3,330 | 2,177 | 922 | 337 | 174 | 28 | 6,969 |
% | 47.8 | 31.2 | 13.2 | 4.8 | 2.5 | 0.4 | 100 | |
Natural seed | ha | 1,208 | 1,018 | 661 | 380 | 48 | 8 | 3,324 |
% | 36.3 | 30.6 | 19.9 | 11.4 | 1.5 | 0.2 | 100 | |
Planted seed | ha | 589 | 484 | 232 | 91 | 33 | 24 | 1,452 |
% | 40.5 | 33.3 | 16.0 | 6.2 | 2.3 | 1.7 | 100 | |
Total | ha | 5,126 | 3,679 | 1,813 | 808 | 254 | 60 | 11,740 |
% | 43.7 | 31.3 | 15.4 | 6.9 | 2.2 | 0.5 | 100 |
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
---|---|---|---|---|---|---|---|---|
Kharkiv Region | ||||||||
Vegetative | ha | 22,774 | 11,231 | 3,730 | 1,053 | 196 | 42 | 39,025 |
% | 58.4 | 28.8 | 9.6 | 2.7 | 0.5 | 0.1 | 100 | |
Natural seed | ha | 4,541 | 2,823 | 723 | 276 | 155 | 26 | 8,543 |
% | 53.2 | 33.0 | 8.5 | 3.2 | 1.8 | 0.3 | 100 | |
Planted seed | ha | 1,917 | 1,467 | 872 | 222 | 166 | 99 | 4,742 |
% | 40.4 | 30.9 | 18.4 | 4.7 | 3.5 | 2.1 | 100 | |
Total | ha | 29,232 | 15,521 | 5,324 | 1,551 | 517 | 167 | 52,310 |
% | 55.9 | 29.7 | 10.2 | 3.0 | 1.0 | 0.3 | 100 |
Kyiv Region | ||||||||
---|---|---|---|---|---|---|---|---|
Vegetative | ha | 4,481 | 1,173 | 422 | 187 | 11 | 29 | 6,303 |
% | 71.1 | 18.6 | 6.7 | 3.0 | 0.2 | 0.5 | 100 | |
Natural seed | ha | 2,850 | 1,116 | 489 | 185 | 83 | 18 | 4,741 |
% | 60.1 | 23.5 | 10.3 | 3.9 | 1.8 | 0.4 | 100 | |
Planted seed | ha | 2,020 | 802 | 329 | 131 | 82 | 31 | 3,396 |
% | 59.5 | 23.6 | 9.7 | 3.9 | 2.4 | 0.9 | 100 | |
Total | ha | 9,351 | 3,091 | 1,240 | 503 | 177 | 78 | 14,440 |
% | 64.8 | 21.4 | 8.6 | 3.5 | 1.2 | 0.5 | 100 |
Poltava Region | ||||||||
---|---|---|---|---|---|---|---|---|
Vegetative | ha | 8,823 | 6,398 | 2,772 | 760 | 208 | 28 | 18,988 |
% | 46.5 | 33.7 | 14.6 | 4.0 | 1.1 | 0.1 | 100 | |
Natural seed | ha | 1,333 | 971 | 423 | 165 | 35 | 13 | 2,940 |
% | 45.3 | 33.0 | 14.4 | 5.6 | 1.2 | 0.5 | 100 | |
Planted seed | ha | 1,191 | 890 | 479 | 254 | 106 | 61 | 2,982 |
% | 39.9 | 29.9 | 16.1 | 8.5 | 3.6 | 2.1 | 100 | |
Total | ha | 11,346 | 8,259 | 3,674 | 1,179 | 349 | 103 | 24,910 |
% | 45.5 | 33.2 | 14.7 | 4.7 | 1.4 | 0.4 | 100 |
Sumy Region | ||||||||
---|---|---|---|---|---|---|---|---|
Vegetative | ha | 12,140 | 9,267 | 4,309 | 1,593 | 462 | 155 | 27,925 |
% | 43.5 | 33.2 | 15.4 | 5.7 | 1.7 | 0.6 | 100 | |
Natural seed | ha | 5,884 | 4,893 | 2,137 | 572 | 151 | 38 | 13,674 |
% | 43.0 | 35.8 | 15.6 | 4.2 | 1.1 | 0.3 | 100 | |
Planted seed | ha | 1,890 | 1,595 | 811 | 418 | 168 | 41 | 4,921 |
% | 38.4 | 32.4 | 16.5 | 8.5 | 3.4 | 0.8 | 100 | |
Total | ha | 19,913 | 15,754 | 7,256 | 2,582 | 781 | 233 | 46,520 |
% | 42.8 | 33.9 | 15.6 | 5.6 | 1.7 | 0.5 | 100 |
The highest productivity was recorded for mixed oak-lime stands with 20–40% of lime in their composition. For example, the stock volume of the mixed stands of vegetative origin was 254–256 m3·ha-1 at the age of 71–75, while that of the stands of natural seed origin amounted to 275–279 m3·ha-1 at the age of 75–80. At that, the stock volume of the stands grown from seeds was 185–202 m3·ha-1 a t the age of 5 0–55. With a decrease or increase in the lime proportion in oak stands, their stock gradually decreases (Fig. 4).
The number of lime natural regeneration aged under 15 was insignificant under the canopy of oak stands and ranged within 100–2,100 trees per ha. Its proportion in the total natural regeneration was 0.8–25.6%. It should be noted that the largest number of naturally regenerated lime seedlings, from 1,000 to 2,100 trees per ha (1,100 ± 210 on average), was recorded under the canopy of oak stands with 10–30% of lime in their composition. These stands were 101–130 years old and had relative density of stocking of 0.65–0.80. In older stands (131–200 years old), the number of lime seedlings was much smaller (100–600 trees per ha, 310 ± 73 on average). In stands aged from 50 to 100, the lime regeneration varied within 200–500 trees per ha (340±53 on average; Tab. 5). Mann–Whitney
Natural regeneration aged below 15 years under the canopy of oak stands of different ages, compositions and relative densities of stocking
Sample plot number | Characteristics of oak stands | The number of naturally regenerated seedlings | ||||
---|---|---|---|---|---|---|
total [trees ha-1] | including small-leaved lime | |||||
stand composition | [years] age | density relative of stocking | trees [ha-1] | % | ||
Oak stands aged 50–100 years | ||||||
1 | Oak 50%, ash 30%, N-maple 10%, lime 10% | 50 | 0.85 | 4,300 | 200 | 4.7 |
2 | Oak 40%, ash 30%, N-maple 20%, lime 10% | 60 | 0.65 | 8,300 | 200 | 2.4 |
3 | Oak 50%, ash 30%, lime 10%, N-maple 10% | 70 | 0.60 | 14,600 | 400 | 2.7 |
4 | Oak 80%, ash 10%, lime 10% | 70 | 0.60 | 14,800 | 400 | 2.7 |
5 | Oak 70%, lime 30% | 79 | 0.80 | 6,500 | 500 | 7.7 |
6 | Oak 90%, lime 10% | 92 | 0.62 | 12,900 | 200 | 1.6 |
7 | Oak 60%, lime 20%, ash 10%, N-maple 10% | 99 | 0.83 | 11,600 | 500 | 4.3 |
Mean | 74 | 0.71 | 10,400 | 300 | 3.7 |
Oak stands aged 101–130 years | ||||||
---|---|---|---|---|---|---|
8 | Oak 60%, lime 30%, N-maple 10% | 104 | 0.76 | 5,900 | 1,200 | 20.3 |
9 | Oak 70%, lime 20%, N-maple 10% | 104 | 0.75 | 22,100 | 1,100 | 5.0 |
10 | Oak 70%, lime 20%, N-maple 10% | 105 | 0.75 | 3,900 | 1,000 | 25.6 |
11 | Oak 70%, ash 10%, lime 10%, N-maple 10% | 108 | 0.70 | 16,700 | 300 | 1.8 |
12 | Oak 70%, lime 20%, ash 10% | 110 | 0.65 | 12,200 | 800 | 6.6 |
13 | Oak 60%, ash 20%, lime 20% | 115 | 0.66 | 8,600 | 2,100 | 24.4 |
14 | Oak 80%, lime 10%, N-maple 10% | 124 | 0.80 | 9,400 | 1,400 | 14.9 |
Mean | 110 | 0.70 | 11,300 | 1,100 | 14.1 |
Oak stands aged 131–200 years | ||||||
---|---|---|---|---|---|---|
15 | Oak 50%, ash 20%, lime 20%, N-maple 10% | 144 | 0.80 | 8,100 | 400 | 4.9 |
16 | Oak 50%, ash 20%, N-maple 20%, lime 10% | 157 | 0.74 | 10,200 | 200 | 2.0 |
17 | Oak 100% | 183 | 0.61 | 12,900 | 100 | 0.8 |
18 | Oak 90%, lime 10% | 188 | 0.66 | 25,000 | 600 | 2.4 |
19 | Oak 100% | 193 | 0.69 | 26,800 | 300 | 1.1 |
20 | Oak 50%, lime 30%, ash 10%, N-maple 10% | 198 | 0.59 | 13,000 | 200 | 1.5 |
Mean | 177 | 0.70 | 16,000 | 300 | 2.1 |
Note: Oak – English oak (
Under the canopy of the investigated stands, the natural regeneration of other tree species such as English oak, common ash, Norway maple (
The data of some studies (Soshenskyi et al. 2015) indicate a decrease in the area of small-leaved lime stands in Ukraine. For example, as of 2014, the area of stands with small-leaved lime predominance was 22,213 ha, with a total stock volume of about 5 million m3. Among them, 77% of the area was occupied by lime stands of natural origin, in particular, 58% vegetative and 19% seed. Planted lime stands occupied 23% of the area. Middle-aged (45%) and mature (27%) stands dominated in the age structure. Approaching maturity and overmature stands occupied an area of 14% and 12%, respectively, while young stands covered less than 2%.
In the forest-steppe zone of Ukraine, small-leaved lime stands occupy 1.1% of the total area of forests. The distribution of their area by origin is as follows: vegetative origin – 66%, natural seed origin – 12%, and stands established by seeding – 22% (Soshenskyi et al. 2018). Our study also revealed a significant predominance of stands of vegetative origin (66.5% of the total area of lime stands) and a small proportion of the most valuable and productive stands of natural seed origin (9.5%). The remaining area (24%) is occupied by artificially seeded stands.
In the forests of Lithuania, only
In the past, in European forests and, in particular, in Ukraine, small-leaved lime occupied much larger areas as the main species or as an admixture in other deciduous stands. However, due to the much lower economic value of its wood at the present-day market, it gave way to more valuable tree species, including English oak and common ash in fertile forest site conditions, as well as Scots pine (
Also, small-leaved lime, black alder (
Small-leaved lime along with Norway maple and European hornbeam (
In 66-year-old mixed oak and lime stands with 60% of English oak and 40% of small-leaved lime in the composition, planted within Kirovohrad Region in central part of Ukraine, the average height of oak was 21.2 m, and the average diameter was 24.5 cm. In 66-year-old planted mixed oak and maple stands with 70% of English oak and 30% of Norway maple, the average height of oak was found to be 20.9 m, and average diameter was 23.0 cm. In 66-year-old planted mixed oak and hornbeam stands with 80% of English oak and 20% of European hornbeam, the average height of oak was 18.1 m and average diameter was 20.8 cm. In 68-year-old planted pure oak stands (100% of English oak), the average height of oak was 19.4 m while the average diameter was 19.2 cm. Thus, the best values of average height and diameter were recorded in oak-lime stands. They exceeded pure oak stands by 8% in average height and by 22% in average diameter (Hordienko and Karpenko 1996).
In oak-lime stands, the crown closure in a row occurs at the age of 5 years while in pure oak stands at the age of 6–7 years, due to the annual decomposition of organic litter by 70% in the mixed stands, while in pure oak stands this value is only 14% (Karpenko 2013; Shpak et al. 2017). Lime leaf litter reduces soil acidity during decomposition and enriches the soil with humus, thus improving the fertility of acidic soils more actively than English oak, European larch (
Natural seed regeneration of small-leaved lime under the stand canopy and on cut-over sites is generally poor (Pigott 1975; Murakhtanov 1981; Hordienko and Karpenko 1996; Radoglou et al. 2009; Shayakhmetov and Sejdafarov 2016; Zaika and Kalenyuk 2018, 2019; Kalenyuk and Zaika 2019, Sultanova et al. 2019, 2020). This is due to unfavourable conditions for the germination of lime seeds and further development of advance growth such as high air temperature, overgrowth of grass vegetation and lack of moisture in the soil.
Small-leaved lime is one of the most shade-tolerant tree species (Murakhtanov 1981). The shade tolerance of
Zaika and Kalenyuk (2019) showed that in hornbeam-oak forests of the Western Podillya in Ukraine, the viability of lime seeds remains high (55–94%) in different years. However, the natural seed regeneration of lime is mostly poor. This is due to unfavourable conditions for the seed germination under the stand canopy and on felling sites (Hordienko and Karpenko 1996; Bondar 2004). Examining the natural regeneration of small-leaved lime in Podillya in Ukraine, Bondar (2004) found the advance regeneration of this species in 44% of felling sites of different ages, where its number ranged from 4 to 1,320 trees per ha. The results of our research also indicate the poor natural regeneration of lime under the canopy of oak and oak-lime stands.
Lime is adapted to different soil types; however, it grows best on chernozems. Fresh and moderately moist, enriched with forest humus, loose and well-drained sandy-loam and light loamy soils are optimal (Murakhtanov 1981; Matusyak 2017; Tsvetkova and Saranenko 2018; Kalenyuk and Zaika 2019). It is intolerant to bogging (De Jaegere 2016) but can tolerate high soil acidity and grow in the pH range from 4 to 8. However, neutral soils are optimal, where pH does not exceed 5–6 (Radoglou et al. 2009).
In Ukrainian forests, the total area of stands with a predominance of small-leaved lime in the canopy layer is almost 20.5 thousand hectares (only 0.3% of the total forest area). More than 80% – about 16.7 thousand hectares – of their total area is concentrated in eight administrative regions of the country, which are territorially assigned to the central, north-eastern and western parts of Ukraine: Cherkasy Region, Chernihiv Region, Kharkiv Region, Kyiv Region, Poltava Region, Sumy Region, Ivano-Frankivsk Region and Lviv Region.
On a large area, almost 210,000 ha, or 38.9% of the total area of oak forests in the above regions, small-leaved lime occurs as part of oak stands of various origins and quite intensively grows in fertile forest conditions (fresh and moist fertile oak sites). The oak-lime stands with 20–40% of lime in the composition have the highest productivity.
Lime natural regeneration is mostly poor under climate change. The largest number of lime natural regeneration, from 1,000 to 2,100 trees per ha, was recorded under the canopy of 101- to 130-year-old oak stands with 10–30% of lime in their composition and relative density of stocking of 0.65–0.80. In older (131–200 years) and younger (50–100 years) stands, the number of lime regeneration was much smaller (up to 600 trees per ha).
The development of highly productive mixed oak-lime forests should be ensured by carrying out appropriate forestry practices in oak forests with a focus on their natural regeneration, in particular, partial main-use felling and further forest tending in young stands on clear-cuts. At the same time, it is necessary to preserve lime advance growth as much as possible. Tending felling should ensure 20–40% of lime in the composition of mixed stands. When focusing on the artificial regeneration of oak stands, it is necessary to introduce 20–30% of lime in the forest crop composition.
The generalization of the research results complements the knowledge system on the current state of lime forests, as well as small-leaved lime regeneration and development specificities. Also, it can be used as a theoretical basis for developing a system of forestry practices to promote natural regeneration.