FIGURE 6 Indicator species in different segments of Yuanjiang
dry-hot valley. (A) Upper stream, transect A; (B) Middle stream,
transect B; (C) Downstream, transect F; (D) Aleuritopteris
squamosa indicator of transect A; (E) Sinephropteris delavayiindicator of transect B; (F) Selaginella pseudopaleiferaindicator of transect F.
4. Discussion
4.1. Variation of thermal and moisture altitudinally
The importance of altitudes in plant distribution is usually ignored.
However, some studies have paid attention to altitudinal influence on
vascular plants. In the tropical area, studies reveal a hump-shaped
pattern with the highest diversity at mid-altitudes and then decrease
toward both high and low altitudes (Hernández-Rojas et al .,
2018). The altitude where maximum fern and fern allies diversity occurs
differs somehow among mountain ranges. For example, maximum diversity
occurs around 1,800 m in both Costa Rica (Kluge et al ., 2006) and
Mount Kinabalu, Borneo (Kessler et al ., 2001), 2,000 m in Bolivia
(Salazar et al ., 2015), and 2,400 m on Mount Kilimanjaro,
Tanzania (Kessler et al ., 2011). Climatically, these gradients
corresponded to the upper parts of tropical gradients where richness
also declined. Species were comparable between these data sets at the
same mean annual temperature. In temperate regions, richness was
reported to decline continuously with elevation or remains roughly
constant, such as New Zealand or North America (Brock et al .,
2016). In subtropical regions, a study from the Fanjingshan of Guizhou,
China revealed a strong negative correlation between altitude and
species (Wang et al ., 2008).
The altitudinal transects along mountain slope were located from
360~690 m a.s.l . (table 1). The annual
temperature was over 22.3 OC, and annual precipitation
was less than 984 mm (Supplementary data: Table A). Species regeneration
was difficult for most species. Generally, fern and fern allies were
rarely discovered, in which two species were discovered in transect C,
and three species were in transect E. Transect B was different from C
and E. In contrast, it was rich in fern and fern allies with 22 species
and 971 individuals (Supplementary data: Table B). Such a difference
could be explained with topographic features. The altitude of transects
C and E was around 970 m a.s.l ., where annual temperature and
precipitation were 20.8 OC and 785 mm, respectively.
In contrast, the altitude of transect B was around 1750 m a.s.l .,
where annual temperature and precipitation were 16.8 ~
17.3 OC and 1211 ~ 1312 mm,
respectively (Supplementary data: Table A). Rainfall drained into the
ravine and supports the plants inside. Water supply in transect B was
better than others because there was plenty of water supply coming from
the higher elevation. Relative humidity was recorded at above 90% in
transect B, which was more humid than other transects (around 72%).
Comparing to the drought in transects C and E, much more shrubs
(Salix myrtillacea, Ilex cornuta, Buddleja officinalis, Solanum
verbascifolium etc .) and trees (Phyllanthus emblica, Trema
tomentosa, Grewia biloba var. parviflora, Broussonetia papyrifera etc. )
were developed. A closed canopy was composed in densely vegetates.
Diversity in the closed ravine was higher than an open ravine or a
ridge.
4.2. Water condition determines the distribution of fern and fern allies
The population of species acquiring high water supply had shrunk in
Yuanjiang dry-hot valley for centuries (Xu, 1985). Undoubtedly, fern
populations were distributed broadly in the past compared to the mosaic
habitats nowadays. Water condition has traditionally been considered a
decisive factor for fern and fern allies (Kluge et al ., 2006;
Yang et al ., 2011). The fern and fern allies flourishment is
interpreted as a reflection of environmental humidity (Yang et
al ., 2011) (or an optimal combination of mild temperature and humidity
(Kluge et al ., 2006). Compared to ground-living fern and fern
allies, epiphytic species strongly rely on water air in the
closed-canopy (Kreft et al ., 2010). In this study, ground-living
fern and fern allies did not display a different water requirement.
Instead, both of them performed similar positive correlations to the
water condition. It turns out that 1980 individuals (98%) were growing
well in humid transects, i.e., A, B, D, and F (A, D, F benifited from
river flow, while transect B benifited from inner cycling
precipitation), whereas 36 individuals (2%) lived in dry transects.
Considering transect B, the plot at the bottom preserved 54 individuals
of 3 species. It increased to 364 individuals of 19 species at the top
plot, almost seven times of the bottom. Plots in low altitude were
occupied mainly by worldwide species such as Pteris vittata andAdiantum malesianum . However, it was altered to uncommonly seen
species when the altitude went up, where the dominant species wereSinephropteris delavayi and Phymatosorus cuspidatus . The
reason was ascribed to inner cycling precipitation at high altitude.
4.3. Heterogeneous habitat and fern and fern allies distribution
Generally, to maintain plants growing and population size, fern and fern
allies have to absorb more water from the circumstance surroundings,
e.g., Pteris vittata, Lygodium japonicum, and Microlepia
speluncae . However, other species, such as SelaginellaS
delicatula , S. uncinata , Adiantum capillus-veneris , andA. malesianum , survive in a unique approach, e.g ., slender
plant size and vegetative reproduction by rhizomes. Their rarity in the
plots is ascribed to frequent hot and dry air and habitat deterioration
such as substance rocks fragmentation and surface soil erosion. It
becomes worse in recent decades and has resulted in a
dramatically
declined environment. Reproductive organisms, e.g., spores, gemmae, and
slender rhizomes, suffer and regeneration cannot proceed under such
environmental stress. Habitat determines which plants grow, while
environmental factors define a habitat. According to the habitat
heterogeneity hypothesis (Tuanmu & Jetz, 2015; Hamm & Drossel, 2017),
the living requirement differs in species, and each species has their
living requirements exclusively. Their requirements include living
substances, water, temperature, and nutrition, to name a few. A
complicated and diverse habitat supports more species while a unitary
habitat supports less. Diversity increases in more heterogeneous
habitats (Negrão et al ., 2017). For an arid ecosystem,
disturbances with varying intensities and spatial scales are responsible
for habitat patterns (Torimaru et al ., 2018). As regards the fern
and fern allies distribution in Yunnan, China, heterogeneity was
reported to be critical to the diversity of species and to be related to
biogeographic zonation (Zhang et al ., 2017). In Yuanjiang dry-hot
valley, Bray-Curtis index revealed that transects C and E were
unitary habitats, while transects A, B, D, and F were more complicated
and diverse. In the field survey, only three species were recorded in
the unitary transects C and E, whereas 33 species were found in
heterogeneous transects A, B, D, and F (Supplementary data: Table B). A
Topographic variation on altitudinal and latitudinal gradients and
divergent temperature and water condition generated a mosaic
microclimate on the slopes in the valley.
4.4. Fern and fern allies indicators
Indicator species are correlated to specific environmental factors. The
population sizes usually increase or decrease due to the change of one
or several factors. Their presence or absence is mostly determined by an
environmental factor. They are optimal to indicate the environmental
evolution process
regarding
their diverse habitat requirements in species (Karst et al .,
2005; Yang & Grote, 2018; Yang et al ., 2019). Most fern and fern
allies are strictly limited in their habitats because of their
intrinsic
sensitivity to environmental change. For example, Gonocormus
minutus live on water due to their fragile mesophyll tissue, whileSelaginella tamariscina can resist extreme arid at Yang tribute
due to its resuscitation, etc. They are considered key indicators
of the environment (Silva et al ., 2018).
In this study, screened indicators fit well with the hot-dry or hot-wet
environment (Table 4). Three species were worldwide distributed with
broad ecological amplitude, i.e., Pteris vittata , Lygodium
japonicum , and Adiantum malesianum .
However, five other species were limited in distribution and confined to
narrow amplitude, i.e., Selaginella pseudopaleifera , S.
jugorum , Sinephropteris delavayi , Davallia
trichomanoides , and Aleuritopteris squamosa . Although growing in
different river segments from upper to lower, they all occurred in
relatively humid areas and disappeared in hot and dry places. Similar
results revealed the indicator function in these selected genera, e.g.,
endemic species of genus Selaginella in Philippine are recognised
as indicators of refuge in the geological past (Tan, 2013) andDavallia mariesii is used as an indicator of landforms and
vegetation types (Park et al ., 2019). A. squamosa is
endemic in the upper and middle segment of Yuanjiang-Red River, which
strongly associates to dry-hot climate (Wu, 1981). Meanwhile, a cryptic
species A. argentea , is reported as a pyrophytic species andSinephropteris is monotypic confined to karst in southwest China,
northeast India, and north Burma (Pallvi et al ., 2019).
5. Conclusions
In recent years, climate change is broadly focused, while environmental
disasters have occurred frequently. Significant land degradation and
climate events have been reported for more than decades. Plants are
considered as a serious part of environmental change. In response to
such ecological crises, this research demonstrates such changes in
Yuanjiang valley located in the first half part of a river shared by
China and Vietnam by focusing on fern and fern allies affected by other
environmental factors. From bottom to top of the mountain, precipitation
increased while the temperature decreased. Fern and fern allies are
positively correlated with height as a result of atmospheric conditions.
Species richness increases further accompanied habitat heterogeneity in
the valley. Even with no influence of human disturbance, these current
appearances are not the original pattern of the areas, indicating
another consequence of climate change.
Acknowledgement: This research is supported by the Research
Foundation for Advanced Talents of Yibin Vocational and Technical
College (No. ybzysc20bk03), the Innovation Project of the Department of
Education of Guangdong Province, China (No. 2019KTSCX71), Scientific
Research Platform of Yibin Vocational and Technical College (No.
ybzy21kypt05), Project of Science and Technology Innovation Team (No.
ybzy21cxtd-04) and Yi Minority Culture Research Center of the Key
Research Base of Philosophy and Social Sciences of Sichuan Province (No.
YZWH2101). The authors would like to thank Ms. Hong Anh Thi Nguyen for
general assistance and also acknowledge Faculty of Environment and
Resource studies, Mahidol University’s editing service for support in
the improvement of this manuscript.
Data Accessibility: All sampling data are online available with
https://doi.org/10.5061/dryad.k0p2ngf8h.
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