Changing Dynamic of Tree Species Composition and Diversity: A Case Study of Secondary Forests in Northern China in Response to Climate Change

Extensive experiments by many scientists have long demonstrated the issue of global climate change. There is a consistent trend of rising temperatures with climate change from the current trend and the predictions of climate change. The global surface temperature in the first two decades of the 21st century (2001–2020) was 0.99 °C higher than in 1850–1900 [1]. While the unprecedented changes in global temperatures due to increased greenhouse gases affect the terrestrial ecosystem, which is highly sensitive to temperature changes [2,3], humans are increasingly altering the diversity of life on Earth through activities that drive climate change, e.g., the overexploitation of resources, severe pollution, and habitat fragmentation [4,5,6]. However, various arguments have been made regarding the decrease or increase in species diversity and richness caused by climate warming [7,8,9,10,11]. This indicates that the response of plants to climate warming varies depending on the characteristics of the site conditions and functional groups of plants, which profoundly influence their growth, distribution, and reproduction. Kazakis et al. [12] and Pickering et al. [13] found that the distribution of shrubs adapted to warm temperatures, and invasive plants gradually migrated towards high-altitude areas. Klanderud et al. [14] investigated the dominant shrubs in the mountainous areas of southern Norway and showed that they were gradually replaced by suitable herbs owing to climate change. Rafferty et al. [15] noted that the phenological period of flowering plants is affected by climate change, which leads to the extinction of other related species. Wehn et al. [16] studied the impact of climate change on plant diversity. They reported that species richness increased with increasing temperature; however, it did not correlate with changes in precipitation gradients. Extensive studies have shown that one of the critical driving forces of changes in global plant diversity in forest ecosystems is climate change, which will replace different forest ecosystems, especially in the coniferous forests of frigid zones, shrubbery areas, and northern forests [17]. Therefore, understanding the interaction between climate change and plant diversity is of great theoretical and practical significance for protecting biodiversity and maintaining ecosystem stability.

The acceleration of warming in China from 1960 to 2019, reaching 0.27 °C/10 a, was greater than the global trend for the same period, and there was an obvious regional difference [18]. That is, the increasing temperature value in the northern region was greater than in the southern region [19]. The Heilongjiang River Basin is crucial for biodiversity conservation, the functional maintenance of carbon sinks, and the protection of black soil productivity [20]. Climate warming is most pronounced in Heilongjiang Province, the first northernmost province of China to react to climate change [21]. In addition, in an area that, through human activities, had lost the structure, function, species composition, and productivity normally associated with a natural forest type expected on the site, a large area of secondary forest formed from natural regeneration in the Xiaoxing’an Mountain forest region [22]. Under the dual pressures of future climate change and human disturbances, the restoration and succession of secondary forests will become more severe [23]. So far, the main body of forest resources in China is secondary forests, of which research has received increasing attention, mainly focusing on the growth and regeneration of secondary forests [24,25,26,27]; the composition, structural characteristics, and distribution pattern of secondary communities [28,29]; the degradation characteristics of secondary forests [30]; the impact of foster management on secondary forests [31,32]; and the changes or impacts of soil and soil microbes during secondary succession [33,34,35]. However, little information is available regarding the succession patterns of secondary forests in northern China under the influence of climate warming.

Studying the succession for secondary communities contributes to the restoration and reconstruction of ecosystems, as species diversity is closely related to community succession. The dynamic characteristics of community succession are reflected in species diversity, which is important for community information. Therefore, researching the species diversity of communities is more beneficial for understanding the composition, structure, function, and dynamics of communities and for grasping their general rules [36]. Yan et al. [37], Xiao et al. [38], and Yuan et al. [39] explored the effects of plant diversity in secondary forests on the physical and chemical properties of soil. Li et al. [40] studied the effects of plant diversity and soil properties on microbial communities in secondary broadleaf forests under different management densities. Many scholars have analyzed the plant diversity of secondary forests for different tree species in tropical and subtropical regions [36,41,42,43,44,45]. However, the responses of secondary communities to climate change in the high-latitude northern regions of China are still unclear.

Animals and plants adapt to climate change by changing their distributional ranges and altering the periods of growth or reproduction [49]. Therefore, one of the most important indices for explaining the distribution of species and changes in diversity is climate heterogeneity, among which changes in temperature and precipitation are extremely critical for plant diversity and ecosystem diversity. From 2015 to 2021, the temperature of the study area increased in spring and autumn, whereas precipitation also increased in spring, summer, and autumn. Yang et al. [50] stated that temperature is a key climatic factor constraining changes in the phenological period of deciduous trees in Xi’an Province, China. The faster the temperature returns in spring, the earlier the tree leaf blooming period. Similarly, an increase in temperature in autumn delays leaf browning. As the northernmost province in China, Heilongjiang Province responds more significantly to climate warming. The important values of coniferous tree species in the study area, such as L. gmelinii, A. nephrolepis, Pinus koraiensis, and other species, showed a decreasing trend from 2015 to 2021. However, the important values of many broadleaf tree species increased, such as A. ukurunduense, A. tegmentosum, S. reticulata, S. taraikensis, U. macrocarpa, U. davidiana var. japonica, P. amurense, A. elata, etc. Moreover, five tree species, A. japonica, A. tataricum, S. matsudana, S. alnifolia, and M. baccata, were found in the quadrats until 2021, and all were broadleaf tree species. The range of suitable habitats for broadleaf tree species expanded within the study area. This result is consistent with the research of Zhang et al. [51], who simulated the effects of climate change on eastern Eurasian forests, and Li [52], who studied the vulnerability of six typical deciduous broadleaf tree species to future climate change in China, which indicates that the range of suitable habitats for broadleaf tree species will gradually expand under the effect of future climate warming. However, the decrease in the importance values of coniferous tree species or some local broadleaf tree species in the study area showed that the distribution of coniferous tree species or broadleaf tree species in the unique region will be reshaped. The limiting factors for some local tree species that require stricter climate conditions will be more stringent with global climate change [53,54].

According to the FAO (2010), 57% of the forests worldwide result from natural regeneration. Angela et al. [55] presented that secondary forests play an important role in biodiversity conservation. Changes in species composition inevitably lead to changes in community structure, affecting plant diversity and ultimately leading to completely different directions of secondary succession. Although there was no significant difference in the annual variation of diversity for each secondary forest community type in the study area from 2015 to 2021, the trend of variation for the three secondary forest types (pure forest, broadleaf mixed forest, and conifer–broadleaf mixed forest) was interesting. For the secondary pure forests, the upper and lower limits fluctuated significantly, and the diversity showed a further downward trend because of the single structure of the tree species and the poor overall stability of the stand. Numerous studies have shown that mixed forests have a high species diversity and can significantly improve ecosystem stability [56,57,58,59,60]. This study also found that the species richness, Shannon–Wiener index, and Simpson index of conifer–broadleaf mixed forests were significantly higher than those of pure forests in different years, with smaller fluctuations between 2015 and 2021. Moreover, the Pielou index showed a higher level of convergence with annual changes, indicating that the distribution of various tree species in the quadrats of conifer–broadleaf mixed forests tended to be average under conditions of climate change, and the overall structure of the forest stand was complete and stable. However, it is interesting to note that, for both types of mixed forests, the diversity of broadleaf mixed forests significantly differed from the conifer–broadleaf mixed forests, showing a downward trend with obvious fluctuations between 2015 and 2021 (Figure 6). The Pielou index of the broadleaf mixed forest was significantly lower than that of the conifer–broadleaf mixed forest in 2021. This might be because of the positive response of broadleaf tree species to northern climate warming, leading to the encroachment of the living space of coniferous tree species in the original secondary broadleaf mixed forest by broadleaf tree species. Similarly, Rajesh et al. [61] reported that the vegetation shift from coniferous forest to broadleaf forest is seen as more dominant.