**4. Discussion**

#### *4.1. Factors and Reasons Affecting the Distribution of Sphaeropteris lepifera during Life History*

We found that the rarity of *S. lepifera* may be due to the relatively large influence of environmental factors, especially temperature and available moisture, which appear critical to the transition between the gametophyte and sporophyte stages. *S. lepifera* generates spores twice yearly, at the end of April and the end of October. The process from spore dispersal to the formation of a sporophyte takes about three months so that the sporophytes should be formed in July-August and January-February. However, it happens that, within most of the native range of the species, January has the lowest temperature of the year (ca. about 14 ◦C) and represents the driest period. We observed in our germination experiments that lower temperatures and dry conditions led to lower rates of sporophytes production. Moisture is required for sperm to successfully fertilize archegonia, such that the moisture requirement is easily understood.

The role of temperature in the reproductive success of *S. lepifera* and its alteration of generations may be more complex. Although greenhouse-grown plants responded better to higher temperatures (25 ◦C) than lower ones (18 ◦C), we observed in the field that the Renhua area of Shaoguan, where the average monthly temperature is the lowest (Table 4), had more surviving plants than the other two places. This may be due to watering frequency by the management, as watering (and other maintenance) is well-known to improve plant growth of ferns introduced (or reintroduced) into the wild [49]. However, several species of the Cyatheaceae fern family, to which *S. lepifera* belongs, are known to be sensitive to phosphorus and have very specific requirements for this nutrient [50]. Phosphorus requires a higher energy investment to take up under higher temperatures [51], and while this may not be limiting for greenhouse-grown plants, it may have affected those reintroduced in wild areas. Thus, the interplay and balance between higher temperature preferences and investments in nutrient uptake at reintroduction should be examined in future studies.

#### *4.2. Factors and Causes of Community Composition Affecting the Distribution of Sphaeropteris lepifera*

Although we observed that *Acacia confusa* and *Pinus massoniana* were the dominant trees on Nan'ao Island, they were not the dominant species within the plot centered around mature individuals of *S. leipifera*. In fact, the 20 × 20 m plot contained no *Pinus massoniana* and only one *Acacia confusa*. Normally, *Acacia confusa* is 15 m in height while *Pinus massoniana* can reach 45 m. This is in contrast to *S. lepifera,* which reaches only ca. 8 m. The demand for light by mature *S. lepifera* would make it hard for these species to compete with tall trees. Overall, the average height of the community around the *S. lepifera* individuals was ca. 10 m, with the tallest species being *Machilus chinensis* and *Cunninghamia lanceolata*. These species are not normally so short in stature nor is the *Acacia confusa* also found in the plot. Their small stature may relate to the occurrence of the community on a south-facing slope. Notably, shrub species within the community, such as *Archidendron lucidum*, were taller than normal. This suggests that slope and aspect may also be critical factors affecting suitable habitat for *S. lepifera*, which may be better able to compete within environments that tend towards supporting lower canopy heights, either through recruitment of mainly short-stature trees or by limiting the growth of typically taller ones. This is also consistent with prior studies, which have shown that tree ferns commonly colonize light gaps within a canopy resulting from disturbances such as high wind, landslides, or other natural or anthropogenic causes [52,53].

#### *4.3. Suitable Habitat for Reintroduction within the Chinese Mainland and Near-Shore Islands*

In recent years, due to the rarity of wild *S. lepifera* on the China mainland, some plants were introduced at botanical gardens for protection. For example, South China Botanical Garden, Chinese Academy of Sciences and Xiamen Botanical Garden have successfully cultivated wild-sourced *S. lepifera* and possess thriving adults that are generating new fiddleheads and spores. Despite these successes with ex-situ conservation, in-situ conservation offers the advantages of facilitating long-term population health through natural selection and of supporting natural ecosystem services [54].

We base our recommendations for reintroduction of *S. lepifera* on our results and the prior work of Wei et al. [55] regarding the successful reintroduction of the endangered orchid, *Bletilla striata* (Thunb.) Rchb.f. This orchid has similar habitat requiremnets to *S. lepifera*, so we believe that protocols leading to its successful reintroduction may also be applicable to *S. lepifera*. Ideally, areas for reintroduction of *S. lepifera* in the wild should meet the following basic conditions: (1) be far away from villages and highways and relatively inaccessible to people to reduce anthropogenic disturbance; (2) have trees with average heights not be more than 10 m with small trees and shrubs as the dominant species and (3) be a moist environment, such as with a nearby water source, ideally at the foot of a shady slope. During reintroduction, we recommend the following steps to promote greater success: (1) plant in a scattered pattern and not too densely; (2) water frequently during dry periods from January to February or July to August or during drought, especially when drought occurs in the first six months after planting; and (3) perform preliminary in-situ experiments prior to large-scale transplanting, focusing on whether there are animal, disease, or other factors potentially affecting the survival of transplanted individuals in the local area.

Both ex-situ and in-situ conservation have advantages and disadvantages. For in-situ conservation, the disadvantages are that it requires a greater investment of time and money and, as evidenced from the recommendations above, the procedures are often cumbersome. However, given the aforementioned advantages, a model that utilizes both in situ and ex situ strategies for *S. lepifera* may be beneficial for the species. Considering the ornamental, economic, and medicinal value of *S. lepifera* [10,13–15], it could be widely promoted for cultivation in Guangdong Province, such as in parks or within urban landscaping, especially with orchids, for which its trunk is an important substrate for epiphytic growth.

The reason why many endangered plants cannot be cultivated at a large scales is that their unique, strict environmental requirements and the growing conditions lead to high maintenance costs. However, our work may not only provide a framework to support in-situ conservation of *S. lepifera*, but also finding suitable habitats for it and maintaining it within urban green spaces. Thus, our findings may help to reduce the costs of ex-situ conservation efforts.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/horticulturae7110496/s1, Table S1. Distribution information of *Sphaeropteris lepifera*.

**Author Contributions:** Conceptualization, X.W., A.H. and F.W.; methodology, Y.C. and A.H.; software, Y.C.; validation, A.H., F.W., Y.C. and X.W.; formal analysis, X.W. and A.H.; investigation, F.W., Y.D., H.H., X.Y. and R.J.; resources, F.W. and X.W.; data curation, X.W., A.H. and F.W.; writing— original draft preparation, X.W., A.H. and Y.C.; writing—review and editing, X.W., A.H. and F.W.; visualization, X.W.; supervision, F.W.; project administration, F.W.; funding acquisition, F.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by grants from the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDA13020601), Agricultural Biodiversity Investigation and Protection Technology System Construction Project (grant number 0835-210Z32104471), Guangdong Province Enterprise Science and Technology Special Commissioner Project (grant number GDKTP2020047500) and Guangdong Wildlife Protection and Management Project (grant number Y936061001).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author (F.-G.W.). It can be downloaded from the supporting files.

**Acknowledgments:** The authors would like to thank Jinfang Cai, Shaohui Wang and Jiyun Li of Shantou Nan'ao County Bureau of natural resources for their assistance with the field work, and Zhiqiang Song of South China Botanical Garden, Chinese Academy of Sciences for his assistance with the experiments observation.

**Conflicts of Interest:** The authors declare no conflict of interest.
