1. Introduction
The Arasbaran biosphere reserve is one of the most important biodiversity hotspots in Northwest Iran. This area has about 1334 plant species from 493 genera and 97 families [
1]. Species diversity plays a key role in the ecological aspects of forest ecosystems, influencing succession, resilience, and nutrient cycling [
2]. In the last international conventions, especially after the Rio Convention on Biological Diversity, loss of biodiversity caused by human activities became a major source of concern for forest ecologists [
3]. Human interventions, such as tree cutting, road construction, and non-timber forest product collection, can alter plant communities in terms of density and composition [
4]. Animal grazing, fuel wood collection, recreation, and other human uses can cause destruction and alter species diversity and woody structure of forests. For instance, Rasquinha and Mishra (2020) showed that tree density, basal area, and tree diversity were lower at sites with the small-scale fuel harvested than sites without harvest because harvesters targeted smaller size classes and particular species in harvested sites [
5]. A study of traditional use in Iranian forests indicates human use impacts affect both the over and understory of woody vegetation [
6].
Averting reduction and degradation of forest cover would require maintaining the remaining forests in their current status as well as the restoration of deforested and degraded areas. Success in these activities requires a proper understanding of the diversity and natural structure of woody species’ establishment, such as causes, mechanisms, and factors that influence the process of regeneration, population change, and replacement through time [
7]. In addition, knowledge about the population structure and regeneration of a given forest is important for developing management and conservation strategies of biodiversity [
8]. Information on the population structure of a tree species indicates the history of the past disturbance to that species and, hence, used to better manage the future population trend of the particular species [
9]. Furthermore, the overall pattern or structure of population dynamics of seedlings, saplings, and adults of a plant species can exhibit the regeneration profile, which is used to determine their regeneration status [
10].
Some researchers have documented the population structure, species diversity indices, and regeneration status in understudied forest ecosystems in Iran and Southwest Asia. In Iran, these include exploring the regeneration and conservation of yew (
Taxus baccata L.) and
Quercus macranthera in Arasbaran forests [
11], assessing impact of human factors on diversity of woody species in the Zagros forests [
12,
13], and investigating the effects of grazing on the natural regeneration in Hyrcanian forests [
14]. In India, Singh et al. (2016) explored tree species richness, diversity, and regeneration status in different oak (
Quercus spp.) dominated forests of Garhwal Himalaya [
15]. In China, Atsbha et al. (2019) studied human factors on the regeneration status [
16] and Chen et al. (2020) explored species diversity of primary and secondary forests in the Wanglang Nature Reserve [
17].
Based on our knowledge, information about the population structure and regeneration status of woody species in zones subjected to a different degree of human interventions at the Arasbaran biosphere reserve in Iran is rare. Local human populations depend on forests to meet livelihood needs through livestock grazing, tourism, and other activities [
18]. The impact of these activities on species diversity is the main management challenge for sustainable forest management. Hence, the main objective of this study was to fill the existing information gap in woody species structure, composition, and regeneration in this region of Iran to efficiently protect and manage this forest ecosystem and its associated services. The degree of anthropogenic modification of these forests could reduce ecosystem integrity and diminishes many of the benefits that these forest ecosystems provide. The specific objectives of the study were to: (i) analyze the population structure in terms of species richness, density, basal area, DBH (diameter at breast height) distribution, height class distribution, frequency, and importance value index, (ii) to assess the regeneration status of woody species, and (iii) to document the impact of human interventions on the population structure and the values of species diversity indices. Our hypothesis is that human interventions significantly impact the population structure and regeneration status of woody species in the Arasbaran biosphere reserve, Iran.
4. Discussion
Today, the biodiversity-oriented forest management has gained much attention in temperate forests of the world [
26]. Maintaining species diversity in forest ecosystems is one of the most important goals in forest management plans. Tree species diversity in forests differs significantly from location to location, and can be related to human impacts [
27].
The high
IVI of some species such as
Q. macranthera,
C. meyeri, and
C. avellana in the
HD sites and
P. communis and
M. germanica in the
LD sites indicates that they were dominant species in their respective environment. The number of light demanding, deciduous species, such as
Q. macranthera,
C. meyeri,
P. domestica, and
C. avellana, were higher in the unprotected
HD area because they took advantage of the canopy openings. The
IVI indicates the structural significance of species in a given ecosystem [
28]. Thus, species with high
IVI values are deemed to be more important than those species with low
IVI.
Q. macranthera has been reported to be a high value
IVI in Arasbaran forests [
29]. Therefore, its abundance in these forests is considered to be the establishment sign of early succession for the ecologists and forest managers. In terms of conservation, species with low
IVI require high conservation approaches and, thus, should be prioritized for conservation. In this study, some of the species that should be prioritized for conservation due to their low
IVI include wild pear (
P. communis L.),
M. germanica L.,
P. domestica L., and
Euonymus sp.
Species diversity is an important characteristic of the forest community [
17]. Density of trees differed between these three destruction classes. The density recorded in this research varied between 102 and 211 tree ha
−1 at different destruction classes. Similarly, as reported by Reference [
4], the stem density was 222 trees ha
−1 and 181 trees ha
−1 at
LD and
HD sites, respectively, from different destruction classes of Himalaya. In a research study about the effects of human use and livestock on woody species composition in the northern beech forests of Iran, revealed that tree density, the mean of tree DBH, and total basal area were significantly higher in the protected than in the unprotected area, which are proxies for
LD and
HD in our study area [
30]. This also emphasized that species diversity indices have decreased in areas subject to intensive human exploitation and livestock grazing [
31]. Grazing reduced both evenness and species diversity by removing and reducing the coverage of some sensitive species [
30]. Therefore, species diversity was at maximum when the disturbance index was low, as reported for other ecosystems [
31,
32]. Many other studies have shown that species diversity is reduced by similar human intervention factors such that many sensitive species have been eradicated [
4,
14,
17,
31,
33]. Contrary to some reports, our study did not show differences. This may be due to the type and regime of anthropogenic disturbances in this region. The primary driver for change in composition is tree removal that is rare in the study area. Livestock grazing as a frequent human-based disturbance in this area highly affects the herbal story in comparison with the trees and shrubs that are less frequently disturbed by grazing. The villages near the Arasbaran forests are not as impactful as other subsistence communities [
14].
Higher tree density of all species in the small diameter classes (
Figure 2) indicates that these forests have tree regeneration, establishing and recruiting into large diameter classes or canopy positions. Tree density in
LD tended to be higher than
HD sites. Lower tree density in large DBH classes as compared to other DBH classes could be the result of tree senescence in the
LD sites and by the cutting of some of these trees by local people in
HD sites [
30]. A J-shape distribution of diameter classes indicated a likely continuous and good regeneration and/or a stable population of woody species [
16]. In contrast, bell-shaped and interrupted reversed J-shape distribution of woody species in communal grazing lands indicated a hampered regeneration status due to several disturbance factors, including deforestation by local people in addition to frequent browsing and trimming by livestock [
16], which were not observed at the stand scale at our sites.
5. Conclusions
Woody plant species are key components of the forest ecosystem and influence the overall composition and structure of forest communities. Documenting the structure of tree diversity and human intervention impacts provides a good database that is useful for management measures in these forests. A comprehensive approach to forest management is needed for the conservation of dominant tree species that are necessary for the canopy formation as well as maintaining the socio-ecological balance of the forests to reduce potential losses in ecosystem services and avoid the impact in the ecosystem processes and properties of this complex forest. The preservation of these forests is crucial not only for conservation of their rich biodiversity, but also for meeting the basic needs of the local population. Due to the impacts of human interventions on the forest structure, composition, and diversity, the conservation programs are recommended to implement in collaboration with local communities to employ management aimed at providing services for local people while restoring these forests. Basic ecological studies, such as this study, are the foundation to begin developing policies and management that meet multiple ecological and social goals. There are ecological studies of Iranian forests.
Tree species density, distribution, and population structure analyzed in this study are example empirical metrics to compare with for restoration or management of forests experiencing human impacts similar to these. This research tries to evaluate the impact of human interventions on the population structure and the values of species diversity indices. According to our results, high destruction (human intervention intensity) impacts tend to be negative on diversity, richness, and the evenness index. For this reason, understanding the impact is essential for evaluating forest management and the ecosystem supply. It could also inform policy and practice for meeting societal demand. In this research, conservation does not mean do nothing. We simply present the human impact data on three scenarios, and this information could help make future decisions about the appropriate level of management to preserve ecosystem services and ecosystem integrity. Decision-makers must gather and consider data before making a choice. Clearly, doing nothing could also be considered a management criterion, but this is not the objective of this work. In future research, we will try to determine the potential capacity to provide ecosystem services and calibrate the appropriate levels of human activity on these ecosystems. While human impacts were isolated to select characteristics (e.g., tree height) in our study area, humans are likely having greater influence than our study captures. Understanding the impact of human interventions on the stability of stands can inform policy and practice for meeting societal demand, preserving biodiversity. Human interventions can regulate regeneration processes, benefiting light-demanding species with high capacity to adapt to disturbances, as noted in this study. In future research, a greater number of sites and adding a reference area would facilitate this area of research.