3.1. Spatio-Temporal Variability Assessment of Pinus Mugo Using Landsat Imagines
The monitoring of forested areas from Romania, and especially of the changes occurring due to human intervention on mountain territory (tree cutting, pests, forestry-cultural works), has been performed indirectly by using Landsat OLI8 satellite images [
49,
50]. Two main categories of interest have hence been identified: forest gain and forest loss. However, the research was limited to areas of approximately five hectares where disturbances occurred.
On larger territories, this process requires considerable effort (a large number of specialists, numerous resources, reduced precision), which imposes the use of indirect monitoring methods of the areas with tree vegetation, by using geospatial techniques, offering an overall view and highlighting, at the same time, the vulnerable areas.
The spatial analysis from the present study was performed based on the derivation of the NDVI index for two years which were considered representative, 1987 and 2018. High-quality LANDSAT 5, 7 and 8 images were identified for these years, without cloud coverage in the analysed area.
The NDVI index derived for 1987 highlights the large extension of the areas covered with vegetation, approximately 84% of the whole territory, while almost the same percentage (86%) was calculated for 2018. The homogeneity of the alpine area due to the associations of species makes the differentiation between grass and wood species a difficult task, a problem which also affects the identification of the studied species.
For the study area, the spectral response of the bands being integrated in the equation of deriving the NDVI index is represented by numerical values between 0.98 and 0.60 for vegetation. For the year 1987, the value is 0.92, while for 2018, the value is 0.51, a result which highlights the reduction of the compact vegetation surface for the studied habitat (
Figure 4). The analysis of the spatial distribution of the vegetation in the study area indicates a very heterogenous distribution of vegetation classes for both years, due to the intercalation of areas without vegetation (bare lands) among those with grass vegetation and with dwarf mountain pines (
Pinus mugo shrub) from the upper forest tree line, section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation as well as the experimental conclusions that can be drawn.
The analysis of the wooden component, with the purpose of highlighting the dwarf mountain pine associations, faces great difficulties due to the fact that most of the study area is covered with different coniferous associations (spruce, juniper, Swiss pine) which have almost the same spectral response as the dwarf mountain pine.
In order to highlight only the areas classified as being covered with Pinus mugo, one used the technique of reclassifying the raster databases to integer values (value 1 was attributed to all the areas with Pinus mugo and value 2 for the rest of the areas where other types of vegetation were identified), which were used in the analysis process of the temporal changes affecting the studied species in this territory.
The analysis of the final results indicates the spread of dwarf mountain pines on 74% of the study area in 1987 and on 63% in 2018, the large surface being motivated mainly by the topography, the favourable altitude as well as by the favourable climatic and pedologic conditions.
A reduction in the areas covered with Pinus mugo is also visible in the time interval under analysis, which is motivated by some studies through massive cutting of dwarf mountain pines for the expansion of alpine pastures and the destructuring of the Pinus mugo habitats by extensive harvesting of its buds for alternative medicine.
The analysis of the changes in vegetation types indicates (
Figure 5 and
Figure 6) firstly the areas without modifications in the structure of the analysed vegetation (approximately 57% of the entire study area), a fact which is explained by the large percentage of areas without vegetation and short grass vegetation, which is characteristic to Carpathian areas above 2000 m. The spatial extension of
Pinus mugo is reduced (approximately 15% of the total area of Piule-Iorgovanu Massif), which is explained by the migration of the dwarf mountain pines towards the areas with loose vegetation and their growth on the debris close to rocks, as well as in the areas with windthrow where the dwarf mountain pine settled as an invasive species. The ratio of expansion-reduction in surface covered by
Pinus mugo is dominated by reductions (with approximately 27%), which is very well correlated with the results of the spatial analysis based on the supervised classification, which validates the fact that there has been a reduction in the extension of the Carpathian dwarf mountain pines between the two representative years.
3.2. Identification of Favourability for Pinus Mugo in The Romanian Carpathian Area
Among the environmental factors, the climate is highly important through its influence on forest vegetation. The most important climatic factors are light, heat, humidity and wind [
51]. The dependency between the environmental factors and the requirements of
Pinus mugo was highlighted by the resulting cartographic database which classifies the territory in favourability classes according to 14 main ecological factors under analysis (
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11,
Figure 12,
Figure 13,
Figure 14,
Figure 15,
Figure 16,
Figure 17,
Figure 18,
Figure 19 and
Figure 20).
The variations in air temperature in Romania between the warm and cold seasons influence the vegetation period of tree species. Thus, the vegetation periods alternate with resting periods during winter, the variations in temperature being influenced by latitude, altitude, slope exposure, etc.
The average annual temperature in the study area ranges between −2.3 and 10.4 °C and manifests its influence by inducing a very low favourability for
Pinus mugo in 50% of the Oriental Carpathian territory, 26.3% of the Occidental Carpathians and, due to their higher altitude, 17.1% of the Meridional Carpathians (
Figure 7). The most extended territories where this influence is present occupy 2870 km
2 in Făgăraș, Parâng and Retezat-Godeanu mountain divisions from the Meridional Carpathians, 1267 km
2 in the Oriental Carpathians at high altitude and smaller areas, of 221 km
2, in the Apuseni Mountains from the Occidental Carpathians. Thus,
Pinus mugo (oligotherm species), must adapt its rhythm of biological activity to the variations in thermal regime from the Carpathian area, as it is necessary to resist to frost in the cold season and to massive snow fall.
The influence of humidity on dwarf mountain pine development manifests itself on two directions, with favourable, but also restrictive effects. From the category of negative effects induced by a large amount of precipitation, one notices the high soil erosion on already skeletal soils and the influence on the pollinating period (which is short). As a positive aspect, however, high humidity allows this species to form pure vegetation associations on large surfaces, creating areas with a specific microclimate.
In the study area there is a variation of the average annual precipitation amount between 557 and 1620 mm/year. In the precipitation range of 901–1400 mm/year, the favourability for
Pinus mugo is low and in the areas where the precipitation amount is below 900 mm/year and above 1400 mm/year, the favourability is very low (
Table 2).
The Oriental Carpathians, through their specific average annual precipitation, offer acceptable growth conditions for
Pinus mugo on 30% of their territory (19,939 km
2), being followed by the Meridional Carpathians, on 13.3% of the analysed area (8810 km
2) (
Figure 9). The least represented division of the Carpathians in what concerns the favourability for
Pinus mugo induced by the average annual precipitation amount is the Occidental Carpathians. Here, the lower altitude and the effect of orographic barriers in the way of the western air masses that bring precipitation determine conditions for low favourability on 20% of the territory (13,261 km
2) and restrictive conditions on 6.7% (4450 km
2) (
Table 3).
The length of the bioactive time period (
Figure 8) is determined by the number of days with temperature above 10 °C. This factor varies in the study area between 0.6 and 6 months, the longest duration being specific to the area of low mountains and depressions inside the mountain range. The favourability for
Pinus mugo is positively influenced on 23.1% of the analysed territory where the bioactive time period is between 1 and 4 months/year, which helps the development of the dwarf mountain pine, while on the remaining 76.9% of the territory (51,035 km
2) this indicator limits the spatial and temporal development of the species (
Table 3).
The relative atmospheric humidity in July (%) is important mainly because, during this month, there is generally the lowest amount of precipitation. This indicator varies in the Romanian Carpathians between 66% and 91% (
Figure 10), the most favourable territories for
Pinus mugo being characterised by values above 75%, mostly found in the Oriental Carpathians (on 77.4% of their territory) and on 23.9% of the Meridional Carpathians (
Table 3). For this indicator, 66.8% of the mapped territory is included in the medium favourability class, being the only one out of the 14 analysed factors which reaches this level of favourability.
In the Carpathian mountain range from Romania, the wind regime ranges between 3.9–36.7 km/h (
Table 2). The negative influence of the wind regime on dwarf mountain pines is felt on 66,071 km
2 (which represents 99.5% of the whole study area), according to the analysis performed (
Figure 11). Thus, according to the Beauford classification, the wind speed in the range of 3.9–5 km/h is correspondent to a light breath of wind, 5.1–11 km/h corresponds to a light breeze, 11.1–19 km/h corresponds to little wind, 19.1–28 km/h corresponds to moderate wind, while wind speeds between 28.1 and 37 km/h correspond to strong winds. Although the wind speed is a limitative factor in the development of the dwarf mountain pine, this species’ resilience to wind is due to its growth pattern (high elasticity of the stems, crawling and ascending shrub aspect) and its plastic rooting system.
Altitude is the determining ecologic factor in the development of the dwarf mountain pine due to its effect on the previously presented climate factors (temperature, average annual precipitation, wind speed, atmospheric humidity) as well as on the soil characteristics which work together in supplying the necessary factors for the development of
Pinus mugo. Specialised literature mentions the altitude of 1350–1450 m as a minimum for the growth of the dwarf mountain pine in the Western Carpathians and a maximum of 1850 m in the Tatra Mountains, and even above 2100 m in the southern Romanian Carpathians (Retezat, Parâng, Bucegi) [
16,
52].
The altitude in the study area is in the range of 56–2544 m (
Figure 12). The mountain areas with lower altitudes than 1300 m offer a very low favourability for the growth of
Pinus mugo, while those above 1300 m offer better conditions (
Table 2). Hence, the Meridional Carpathian range ensures good development conditions on 4439 km
2 (6.7% of the whole Carpathian range), while the most restrictive Carpathian unit (30,939 km
2) is represented by the Occidental Carpathians, characterised by the lowest altitude in the whole Carpathian range (its maximum being only of 1849 m) (
Table 3).
The slope aspect represents a morphometric parameter which influences indirectly the favourability and the restrictiveness for dwarf mountain pines. The mountain surfaces with slopes exposed towards the south or the southeast receive a higher caloric energy than the slopes facing north and northwest. The northern and northwestern slopes are also more exposed to the air masses which are rich in precipitation, followed by the southern and southeastern slopes.
From this point of view, 92.1% (61,160 km2) of the Carpathian territory offers good conditions for the development of Pinus mugo on quasi-horizontal surfaces, which receive a higher quantity of caloric energy and determine for 7.9% (5220 km2) of the analysed area a medium favourability.
In what concerns the influence of soil on the development of forest vegetation, it is known that trees and bushes set their roots in the soil and absorb through them the mineral salts needed for their development. Thus, the thicker soils offer the possibility of growing deeper roots. In the case of shallow soils which are specific to the mountain area above the forest boundary, it is important to know the physical and chemical characteristics of soils as the trees need to adapt their roots and are more vulnerable to windthrow and massive snowfall.
The saturation of the soil with bases in the Romanian Carpathians varies between 0.002 and 82%, the areas with larger values than 45% (624 km
2 of the analysed territory) offer a very low favourability, while the areas with values below 45% offer a better favourability (
Figure 14,
Table 3).
Soil acidity, determined as the value of water pH, varies in the study area in the range of 4.3–8.2, the most favourable areas for the growth and development of dwarf mountain pines are those characterised by values between 5.4 and 3.6, which can be found on 16.3% of the territory (10,801 km
2). The Oriental Carpathians are the best represented mountain range from this point of view, on 8.2% of their territory (5895 km
2) (
Figure 15,
Table 3).
The azote from soil was determined procedurally using the Kjeldahl procedure and the punctually value was interpolated using GIS technologies. In the Romanian Carpathians ranges between the values of 1.06 and 7.11 (
Figure 15). As in this case we are interested in the influence of this indicator on
Pinus mugo, the focus is laid on the territories with values above 4, which represent 97% of the study area, mostly extended in the Oriental Carpathians (on 32,632 km
2) (
Table 3). A reduced quantity of the azote from soil will lead to the yellowing of the needles and a decrease in the plant’s growth. That is why an analysis of the azote trophicity is also required as wooden plants cannot take in atmospheric azote directly, thus, a thick layer of humus is required.
Edaphic volume (Ve, fractions of unity), particle size distribution, as well as waterstable macroaggregates (structural macrohydrostability) have been determined for all monitoring plots at national level and this values was interolated using GIS technologies.
The edaphic volume (m
3/m
2) determines a very low favourability for
Pinus mugo when the values are higher than 0.45. In the study area, this indicator ranges between 0.17–1.67 (
Table 1). The best values of the edaphic volume for the development of dwarf mountain pines are found on 5956 km
2 from the Oriental Carpathians (
Figure 17,
Table 4).
The usable water from soil (%) ranges between 1.8 and 20.9% for the study area, but the best conditions for
Pinus mugo development are found in the range of 10.1–20% (for 90.9% of the study area, the largest territory included in this category being located in the Oriental Carpathians −31,233 km
2) (
Figure 18).
Soil compactness represents an important pedologic factor for the development of trees and bushes, thus, loose soils which are rich in humus and have a low acidity are favourable to plant growth, while on compact soils, the favourability is very low (this being the case of the dwarf mountain pine as well). According to pedologic studies, in the study area, surfaces with a limitative compacteness degree were identified on only 618 km
2 (
Figure 19), these territories being located in the Banat Mountains and in the Retezat Mountain unit from the Meridional Carpathians.
Soil texture conditions the regime of soil humidity, soil permeability and consistency, as well as the value of the edaphic volume. As a consequence, it has an important role in the growth and development of the trees and bushes from the study area. The soils from the Romanian Carpathians offer favourable conditions for the development of the dwarf mountain pines on 90.3% of the whole study area, where there are soils with sandy, sandy-loamy, loamy-sandy and loamy texture. The rest of the territory is characterised by loamy-clayey, clayey-loamy and clayey texture on 6354 km
2 which imposes restrictive conditions for the development of
Pinus mugo (the most extended surfaces with these types of textures being found in the Oriental and Occidental Carpathians (
Figure 19).
In order to identify the favourability classes for the development of
Pinus mugo in the Romanian Carpathians and the vegetal associations in which it is included, a digital database was used, including ecological, climatic, pedologic and morphometric factors which determine through their cumulated influence various degrees of favourability or restrictivity for the analysed species. The final map of favourability for
Pinus mugo was created using the techniques of GIS spatial analysis and the Raster Calculator extension, using the formula:
where: PM Fav—Favourabily for
Pinus mugo, Tma—average annual temperature (°C); PPav—average annual precipitation (mm/an); Q
W—quantitative index for wind; Q
H—quantitative index for relative atmospheric humidity in July; Q
T10—quantitative index for temperature above 10 °C; Q
A—quantitative index for Aspect; Q
B—quantitative index for degree of soil saturation with bases; Q
V—quantitative index for edaphic volume; Q
AC—quantitative index for soil acidity; Q
U—quantitative index for usable soil water; Q
C-quantitative index for soil compactness; Q
N—quantitative index for azote grid; Q
S—quantitative index for soil texture.
By analysing the final result, classified according to the favourability classes for
Pinus mugo, one identifies three classes in the Romanian Carpathians (low, medium and high) (
Figure 21).
In the Oriental Carpathians there is a high favourability for Pinus mugo on 266.1 km2 (0.77% of the mountains’ surface) and medium favourability on 949.5 km2 (2.76%), while in the Meridional Carpathians the high favourability class is found on 525.5 km2 (3.71%) and the medium one on 1957.3 km2 (13.81% of their surface). The most reduced values which offer good development conditions for Pinus mugo are found in the Occidental Carpathians (only 40 km2 with high favourability and only 180 km2 with medium favourability).