3.4. The variation of Species_estimated, Singletons, Uniques, ACE, ICE, and Chao2

Species_estimated (Figure 5): Transects A and C approached a gentle level earlier than others. The estimated species curve reaches zero prematurely indicating a homogeneous habitat among the plots investigated. There were three species in transect A:Aleuritopteris squamosa , Selaginella stauntoniana , andS. pseudopaleifera ; while there were two species in transect C,Pteris Vittata and A. squamosa . Species increase in transect A and C stopped at the plots 3 and 2 separately. There were no new species discovered in the following plots. In contrast, species accumulation curves in the other four transects keep going up from plot 1 to plot 10, showing a high possibility to find new species in the next plot. This indicates a heterogeneity of habitat among the plots.

Singletons (Figure 5): It was zero in transects A and C because no species grew as a single individual. In transect F, the Singletons approached 1, and there was only one species (Histiopteris incise) found in plot 4. Unlike transects A, C, and F, Singletons increased quickly without a turnover in transect B, D, and E. The maximum of Singletons happens in transect B bearing approximately nine species, Pteris vittata, P. ensiformis ,P. excelsa , P. fauriei , Lygodium japonicum, Selaginella delicatula , Adiantum philippense , Microlepia speluncae, and Phymatosorus scolopendria . These species are generally distributed worldwide in a broad niche. However, none of them colonize the whole plot. Instead, other species take advantage. Extreme water reliance takes charge of this. Water suspended in the air and absorbed in the living substance is vital to the growing and regeneration process.

Uniques (Figure 5): According to the curves, transects A and C were in low Unique . Transect A was riverine; plots in this transect had similar habitats. Transect C was a ravine along a mountain slope, and it featured fragmented rocks and bare surface soil from plot 1 at the bottom to plot 10 at the top. Plots in this transect had similar habitats too. Therefore, there were very few species found in these two transects, and these species were presented in the plots from 1-10 evenly. In opposite to transects A and C,Unique increased in the plots of transects B, D, E, and F, indicating that more species and habitats were expected in the next plot. Transects D and F, for example, were riverine transects, the same with transect A, but the growing substance and canopy coverage made the habitat more suitable for fern and fern allies. When the investigated plots increased continuously, new records were found occupying a single plot or sharing a plot with other species, e.g .,Selaginella helferi , S. biformis , S. doederleinii ,Histiopteris incisa , and Equisetum hyemale .

ACE, ICE, and Chao2 (Figure 5):They verified each other by displaying similar curve shapes. Species richness in transect B, D, and F were much higher than other transects. The maximum richness was 22 species detected in transect B whereas only two species were detected in transect C.

3.5. Indicator species

Eight species are screened out to represent the Yuanjiang dry-hot valley as shown in Table 4. All indicators perched at the altitudes 490 ~ 690 m although some of them distributed down to 260 m. All indicators require a humid environment although they grow in different river segments (Figure 6).

Table 4. Indicator candidates in Yuanjiang dry-hot valley.