**1. Introduction**

Nepalese alder (*Alnus nepalensis*), a plant of the alder genus *Alnus* in the family of Betulaceae, is described in the Flora of China as growing in river beach wetlands or gully terrace forests (Figure 1) at a wide range of altitudes between about 700 and 3600 m. It grows fast in warm and humid environments, with a height exceeding 13 m over 5 years [1], and it has a strong carbon sequestration capacity [2]. This species is resistant to barrenness, and is often used for riverside berms and barren mountain beautification. Thus, it has been selected as an ideal tree species for ecological shelter forest and mixed afforestation [3,4]. At the same time, as a high-quality papermaking raw material, the Nepalese alder is an important tree species used to create short-cycle industrial wood raw material forests. Therefore, this species is listed in China's "Forest Resources Development and Protection Project" (FRDPP), and it is the main broad-leaved tree species for the

**Citation:** Xia, C.; Zhao, W.; Wang, J.; Sun, J.; Cui, G.; Zhang, L. Progress on Geographical Distribution, Driving Factors and Ecological Functions of Nepalese Alder. *Diversity* **2023**, *15*, 59. https://doi.org/10.3390/d15010059

Academic Editors: Thomas Fickert and Michael Wink

Received: 30 November 2022 Revised: 24 December 2022 Accepted: 28 December 2022 Published: 4 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

construction of papermaking industries [5]. The characteristics of Nepalese alder as a pioneering species, its rapid growth, and its high-quality fuel carbon are inseparable from its nitrogen fixation performance by the symbiotic nodules where the Frankia is attached [6]. This nitrogen fixation performance increases soil organic matter and nitrogen and phosphorus nutrient content [7,8], promotes soil microbial reproduction, and increases soil enzyme activity [9]. To some extent, the improvement of soil properties also promotes the growth and development of other plants within the same area, so Nepalese alder is often introduced into agroforestry to enhance the biological production of target species. In the eastern Himalayas, agroforestry ecosystems dominated by cardamom (*Amomum subulatum*) and Nepalese alder are specially created [10], and Sharma et al. (2002b) found that the energy conversion efficiency and net energy increase of alder–cardamom were significantly higher than those of an ordinary cardamom system, and the production potential of the ecosystem was also optimized [11]. In addition, studies have pointed out that the new branches and leaves of Nepalese alder reflected high moisture content and are not prone to canopy fires. Therefore, it can be cultivated in large quantities as a biological fireproof tree species [12]. In all, the Nepalese alder plays an important role in maintaining the stability of forest ecosystems, agroforestry ecosystems, and the sustainable development of forestry.

**Figure 1.** Community structure and organ characters of Nepalese alder (Motuo, Tibet).

According to the Flora of China, the distribution of Nepalese alder spans a large altitudinal gradient near 3000 m, which indicates that this species is well-adapted to different environments. In addition, the clear altitude limit (especially the upper limit) indicates that there is an obvious hydrothermal constraint. However, at present, research on the correlation between alder and environmental factors mainly focused on the effects of nitrogen- and phosphorus-addition treatments on the growth of alder seedlings in Nepal [13,14], whereas research concentrating on the relationships between alder and hydrothermal factors such as temperature and precipitation is still lacking on a large scale [15]. In addition, Frankia attached to the roots of Nepalese alder are closely related to the nitrogen fixation ability of the tree itself, and as an actinomycete in microorganisms, its activity is also affected by environmental factors [16]. Thus, the response of Frankia to environmental factors will also affect the growth of Nepalese alder. Accordingly, this paper first discusses the global geographical distribution and limiting factors of Nepalese alder by querying and collecting the relevant literature of Nepalese alder from both domestic and foreign specimen databases, and then it analyzes the relevant research hotspots and development trends based on Citespace software and literature synthesis, respectively. We further reviewed the main physio-ecological characteristics and ecological functions of Nepalese alder, which might provide a reference for in-depth exploration of the ecological adaptability and sustainable use of alder under the background of climate change. At the same time, this paper may offer a theoretical reference for regional forestry production practice and vegetation restoration.
