1. Introduction
The most useful source of botanical information of a particular area is its floristic checklists [
1,
2]. Since floristic composition is a decent floristic marker, any changing floristic composition in various endogenous milieus highlights the presence of various environmental variables, prompting entomb and intra-particular variations [
3]. The floristic structure of a specific territory provides fundamental. However, information about the plant dissemination and motherland show diversity in landscapes, topographies, and biological/ecological zones, bolstering the distinctive floristic composition [
4]. To contemplate the vegetation of a territory, it is essential to oversee the plant’s life-form, which is an indicator for both micro- and macroclimate [
5]. As indicated by Raunkiaer [
6], the biological spectrum is divided into five major classes (i.e., phanerophytes, hemicryptophytes, cryptophytes, chamaephytes, and therophytes).
The plant species are also categorized based on leaf size classes which are also extremely helpful for association mapping of vegetation; it may also be helpful for comprehension of the plant communities and their physiological processes [
7,
8]. Leaf sizes and shapes (physiognomy) connect unequivocally with moisture and temperature from worldwide to neighborhood scales [
9]. Leaf characteristics assume an especially imperative part in carbon absorption, water relations, and vitality adjust (energy balance) [
9]. Furthermore, a life-form and leaf size spectra provide a clear picture of an area. Therefore, climatic conditions of a certain region may accurately be referred from such observations [
10]. These physiognomic characters have been widely used in vegetation investigations in anthropogenic and naturally vulnerable areas [
5,
10].
The understory vegetation plays a major role in sustaining the structure and function of forest ecosystems [
11,
12], promoting energy transfer and nitrogen cycles, and shaping the canopy succession of forest ecosystems [
13,
14]. The understory makes comparatively little contribution to the overall biomass of forest plants [
13,
15]; it promotes the greatest proportion in floristic diversity [
16]. In addition, diversified understory vegetation boosts the structural complexity of forests and provides other biotic groups with habitats and food, increasing their diversity [
17]. For forest regeneration, understory vegetation is extremely crucial, as it can influence the germination, survival, and development of tree seedlings by competing for light, water, and nutrients [
11,
15,
18]. Therefore, increasing attention is being paid to forest understory vegetation [
16]. It is essential to know about the most influential variables affecting their distribution for ecological protection, conservation, and forest management [
12,
16,
19].
Numerous studies have shown the effect on the species composition and diversity of understory flora. These include flora canopy species and structure [
20], stand management [
21], soil disturbances [
22], light resources [
23], litter properties [
20,
24], and soil nutrients and pH [
23,
25]. Topography can dramatically change microclimates and the availability of energy under the tree canopy [
23,
26] and, in turn, affect the composition and diversity of understory species [
12,
27]. In coniferous forests, hardwood forests, and mixed-wood forests, understory vegetation has been well studied [
25,
28]. The latter two types of forests are generally agreed to create more favorable conditions for their biodiversity conservation and restoration than coniferous forests [
21]. In general, broad-leaved evergreen forest and oak forest have been ignored in understory vegetation and its composition. Beg and Mirza [
29] studied oak-dominated forests in the Hindukush-mountain region. In the study, four oak species have been reported from the region, i.e.,
Quercus lanata Sm.,
Quercus robur L.,
Quercus baloot Griff., and
Quercus semecarpifolia Sm. The research work also reported thirty-six understory species in the oak-dominated forest. So far, no comprehensive study has investigated the details of phytosociological attributes and environmental variables in association with those understories, aiming to understand its diversity. Forest ecosystems’ floristic composition, geographic distribution, and ecological conditions are essential for a rational management plan. In this study, the understory species composition is considered to differ significantly among the major groups concerning species richness and diversity. Additionally, attempts were made to determine the extent to which physiographic and edaphic variables could explain variation in the understory species composition and major distinct groups of oak-dominated forests.
4. Discussion
Vegetation is a group of plants growing together in a particular locality [
43], and it may also be defined as a unit that retains its unique structure and physiognomic characteristics in an adequately great and sufficient way to permit their distinction from other units [
46]. The understory plant species in the oak-dominated forests consist of 58 species belonging to 32 families at 30 different locations in the Hindukush ranges of Swat. The physiognomy of the studied area was dominated by herbaceous growth form with 47 plant species (81%), followed by shrubs with 10 plant species (17%), and trees with individual plant species (2%). Herbaceous dominancy and less tree percentage might be due to altitudinal and geographical variations, which indicates that the climatic factors favor the herbaceous flora. Our findings are associated with researchers in allied, neighboring, and national regions in which the species reported were mostly herbaceous [
47,
48]. Among all families, Fabaceae was recorded as the leading family with eight plant species, followed by Asteraceae and Lamiaceae with seven species each. For instance, several researchers [
5,
32,
33,
34,
38,
46], for instance [
31], have documented Asteraceae and Fabaceae as the two most important families. While other researchers [
38] cited Lamiaceae and Rosaceae, and [
48] cited Lamiaceae, Moraceae, and Asteraceae, as dominant plant families from Darra Adam Khel, KP, Pakistan.
Plant life-forms and vegetation are indicators of the climate [
6]. According to Meher-Homji [
49], life-forms are reflective of the bioclimates of an area. Raunkiaer [
6] designed three major phytoclimates based on life-form spectra on the earth. It includes phanerophytic climate for the tropics, therophytic for the xeric environments, and hemicryptophytic for the cold temperate region. The present study identified seven different life-form classes in the study area. The most dominant life-form was hemicryptophytes with 19 species (33%), followed by nanophanerophytes with 15 species (26%) and therophytes with 13 species (22%). Hemicryptophytes usually prevail in open physiognomies, while phanerophytes are mainly in closed ones [
50,
51]. Biological spectra are important in comparing geography and habitats and might change due to biotic influences, viz., grazing, human activities, and climatic changes [
52,
53,
54]. Hemicryptophytes lose their aboveground parts during most summer and winter months while therophytes remain seeded to avoid summer, drought, and cold winter stresses.
Furthermore, the flora of the study area is under anthropogenic pressure in the form of overgrazing and deforestation by nomadic and native people. Nasir and Sultan [
55] reported a similar finding from District Chakwal, where therophytes were the dominant biological spectra. A community that therophytes dominate can be a characteristic feature of a highly disturbed area under anthropogenic pressure [
48,
54]. In addition, [
56] reported therophytes as the dominant plant species of the rangeland, district Tank, Pakistan, and [
57] assessed that therophytes is the leading life-form of Lahor, District Swabi, Pakistan, showing that the results of the current study are in compliance with the findings of several other studies.
Results of leaf size spectra trait revealed that species with microphyllous leaves dominated the area (with 24 species, 41%), followed by nanophylls (17 species, 29%), leptophylls (11 species, 19%), and mesophyll (5 species, 9%). Nonetheless, 2% with megaphyllous leaf size was also recorded. Microphyllous leaf size class is normally the characteristic feature of meadow plant species, while leptophylls and nanophylls are illustrative of hot deserts [
7,
54]. Comparable outcomes, where microphylls and nanophylls overwhelmed the vegetation and were reported in the work of researchers [
7] and [
45], where the authors linked these two leaf size classes with the physiognomies of mild ranges. A high rate of microphylls may be identified with cool atmospheres of the sub-elevated and snowcapped areas. Here, as well, the top layer was less settled, containing a slim sheet that is not conclusive about the entrance of roots. Microphyllous and nanophyllous leaves species were inexhaustible because of the biological variety for this dry condition. The present results concur with [
58], who detailed that microphylls and nanophylls predominance in the dry, mild atmosphere of District Quetta. Our inferences additionally uncovered that the high extent in leaf size class changes with increasing altitude, and the rate of microphylls was emphatically connected with this as well [
59,
60].
Among the 58 species documented, 43 plant species were found in association with group III clustered by applying Ward’s agglomerative clustering, which indicates wide sociability of the species in the studied oak-dominated forests. Moreover, the analysis reveals that
B. lycium (7.82)
, C. arvensis (6.87), and
A. trichomanes (6.6) were the most abundant species in group III. Group II was less diverse in terms of understory species numbering in total of 25 plant species. In this group,
Calamintha vulgaris (L.) H. Karst.,
Dryopteris stewartii Fraser-Jenk. and
Plantago lanceolata L. were dominant (11.5, 9.34, and 9.86 respectively). Plant species in the understory with varying degrees of resistance to these factors prefer a specific forest type. Since certain plant species are limited to a specific forest type, the extinction of that forest could result in the extinction of certain understory plants [
27,
61].
According to a recent analysis, the richness and diversity of understory species in oak forests differed greatly depending on canopy dominants. Herbaceous species abundance and α-diversity were highest in the oaks forest, but there was less heterogeneity (β-diversity) across sites and major groups. These findings highlight the importance of oak forests in protecting habitat and plant composition in the understory vegetation. An herb layer with higher species abundance and diversity in oak forests may be due to higher solar radiation on south-facing slopes, which is better for herbaceous species [
62,
63,
64].
Environmental variables such as geographic parameters and physiochemical soil characteristics are important in determining the community structure and composition [
65]. In particular, latitude and altitude were found to vary significantly in the understory groups. Clay particles in soil texture and bulk density, saturation point, and electrical conductivity varied significantly in the understory vegetation. Similarly, in the same region [
66], the soil parameters in understory vegetation of
Pinus wallichiana A.B. Jacks. dominated forest have been studied, revealing that elevation, slope, and phosphorus are vital factors in determining the understory vegetation. An oak species,
Q. rubur L., was also reported as a major associated species. Furthermore, many other researchers from other parts of the world have reported that edaphic factors can significantly affect the species composition and species diversity in plant communities [
10,
66,
67,
68].
Forward selection of the RDA ordination revealed that latitude, elevation, clay, wilting point, bulk density, saturation, and electric conductivity were significantly influential in determining the understory vegetation of oak-dominated forests. These results follow the findings of many other previous studies [
12,
59,
69,
70]. Ali and Begum [
62] also investigated the vegetation of Swat and determined congruent results by stating a strong relationship between vegetation and edaphic factors. Many other researchers worldwide have recorded that soil characteristics matter more than canopy organisms in deciding understory vegetation [
62,
63,
64,
70]. The study of species composition analysis in the oak-dominated forest revealed a general pattern of variation in the community diversity and species composition. A similar pattern of species composition was also reported by [
65] in
P. wallachiana understory from the same region. The species diversity was found to increase with time as Beg and Mirza [
29] reported only 36 understory species due to the increase in anthropogenic activities.
5. Conclusions
The current study reveals that Swat oak forests have rich floristic diversity with dominance of the therophytic life-form and microphyllous leaf size class, indicating sub-tropical and moist temperate type climates. The species diversity was observed to increase during the spring and summer seasons and decreased later in the autumn. As the winter season approaches, the decline in diversity was observed, associated with dry environmental conditions, slow growth rate, and other climatic factors. Nonetheless, critical impacts of seasonal variation on life-forms and overall species diversity were evident. Among 58 species, 43 plant species were associated with group III clustered by applying Ward’s agglomerative clustering, indicating the wide sociability of the species in the studied oak-dominated forests. The understory vegetation of these forests plays an important role in the forest ecology of the region.
Moreover, the environmental and soil variables, i.e., latitude, elevation, clay percentage, wilting point, electrical conductivity, and potassium (mg/Kg), affect the understory vegetation. In addition, important overstory variables like Quercus IVI and Total IVI of overstory were also found to affect the understory vegetation in oak-dominated forests. Further, it is concluded that the area is vulnerable due to the pressure from the local inhabitants in the form of overgrazing and deforestation, which may significantly affect the understory vegetation. This information is particularly important for the success of efforts intended to prevent the loss of genetic diversity of species within these forests by destroying their natural habitats. Moreover, it is imperative to conserve the area’s biodiversity and provide alternative means of livelihood for the local communities that may allow sustainable utilization and conservation of the invaluable biodiversity of this area for future generations.