4 Discussion
The growth and distribution of species and habitat condition, including
soil, climate and topography et al., are inseparable (Piao et al., 2012;
Liu et al., 2014; Liao et al., 2020). In this study, there are no
significant differences for the soil properties between different
topographies (Table 2). However, there are significant differences for
the individual growth between different topographies. The experimental
plots were all located on the same alluvial fan with the same climate
overall. While local modification of the climate by topography and
vegetation produces microclimates at the land–air interface
(Zellweger et al., 2019).
Therefore, the micro-topographic condition was main constraints that
affected the changes of microclimates. And hence the spatial complexity
may be important for adaptations to climate change
(Kooijman et al., 2019). The
warming temperature and the change in precipitation have provided the
positive effects of vegetation dynamics in recent decades in this region
(Li et al., 2015; 2016). Thus, the
unreasonable human activity, such as overgrazing, extensive reclamation
and excessive collecting live plants for firewood, remain the dominant
factor caused aeolian sandy land development severely in 1990s in this
region (Shen et al., 2012; Li et
al., 2016). Ecological restoration programs, including active
restoration (i.e. planting the artificial forest and shrubs, conversion
of cropland to grassland, shrub or forest, and establishing the
sand-protecting barriers) and natural restoration (i.e. forbidding
grazing or collecting firewood and controlling the number of livestock),
have been launched and conducted successfully since 2000. In this study,
the coverage for S. moorcroftiana population shows well after
decades of natural restoration (Fig. 1). Population structure is the
synthetic action of biological characteristics, environmental factors,
and intraspecific interactions which can be used to reflect the
succession dynamics and development trends of a population
(Liu and Zhang, 2018). The
relationship between CPA and height was fitted well by a linear model in
each topography from the field survey (p < 0.01) in
this study. Therefore, the plant height of S. moorcroftianapopulation was regarded as the measure of age grade. This study showed
that the height structure of S. moorcroftiana population
approximated a Gaussian distribution model (Fig. 3). The population
structure was dominant for the seedings in 2011, however the number of
individuals was dominant for the adults of 30-60 cm in 2017. Compared
with the early study in 2011, the age structure of S.
moorcroftiana population increased significantly to 2017. Aggregated
spatial distribution patterns are commonly observed in many restored
ecosystem and small individuals showed higher degree of aggregation than
large individuals (Zhao et al.,
2012). The present study also found that the height class of less than
90 cm (class 1, 2, 3) had higher degree of aggregation than those of
more than 90 cm (class 4, 5) for each topography (Fig. 3), and this
phenomenon of aggregation was observed more clearly with increasing
elevation and slope.
It is commonly agreed upon in the academic community that climate change
can cause alterations to the structure of communities and also cause
changes in the spatial distribution of terrestrial populations
(Gelviz-Gelvez et al., 2015).
Therefore, the micro-topography was also an important driver of
individual distribution. Revealing the spatial distribution of S.
moorcroftiana population contribute to illustrate the effects of
various micro-topography on the dispersal and growth of S.
moorcroftiana . Our results showed that the correlations between the
individual height of S. moorcroftiana population and
micro-topography (elevation and slope) reached the 99% confidence level
(p < 0.01), however there were obviously differences
for the individual distribution on different topographies. Although the
distribution of S. moorcroftiana population decreased along
increasing elevation, there was a peak value at 3624 m (T2) (Fig. 6).
There was similar to the overall trend of spatial distribution forS. moorcroftiana population along increasing slope, with some
different changes minimally happened after approximately 20°. While theS. moorcroftiana population was mainly distributed on the
southwest in T1 and T2 and on the northeast in T3. Diamond et al. (2019)
reported that the spatial distribution of relative water levels is
changed by the wetland micro-topography and affected individual
distribution and growth. However, for sandy ecosystem, the changes of
solar radiation with micro-topography also must be taken into account
(Li et al., 2019). In general, the west would receive excessive solar
radiation and evaporation in the afternoon, and this is not conducive to
survival of plants. Therefore, there was little distribution of
individuals on the west in all of micro-topography in this study.
Generally speaking, the short time of solar radiation and fine condition
of water have been found at the shade slope, north slope of mountain,
and it was beneficial for vegetation growth. Therefore, the moving sandy
land was mainly distributed on the north of Yarlung Zangbo River, while
the fixed sandy land was mostly located in the south of Yarlung Zangbo
River, which was the north slope of mountain. However, there were
minimum density of vegetation at the north slope from the
micro-topography. One conjecture, the effect of low temperature in
alpine valley, was submitted for the difference between overall and
micro-topography scale. Less solar radiation with lower temperature at
the shade slope blocked the survival of plants. Comprehensively
analyzing the structure, distribution and growth of population, we
concluded that the habitat conditions on the southwest slope of
approximately 20°-25° with altitudes of 3593-3643 m most favor to the
natural restoration of S. moorcroftiana population on aeolian
sandy land.