1 Introduction
Ecological restoration is the process of assisting the restoration of a
degraded ecosystem and can provide substantial benefits that enhance
quality of life (Benayas et al.,
2009; Suding et al., 2015; McDonald et al., 2016). It has emerged as a
critical tool to reverse and ameliorate the current loss of functions
and services in the degraded ecosystems
(Mijangos et al., 2015). Natural
and artificial restoration are two important strategies of ecological
restoration (Jin et al., 2014;
Barral et al., 2015).
Since the 1990s, a series of ecological restoration programs have been
widely launched to improve these increasingly devastating degraded
grassland ecosystems in the arid and semi-arid areas of China, including
the establishment of a national ecological security shelter zone and the
active restoration of forest and shrubs on the Qinghai-Tibetan Plateau
(Li et al., 2015; Yan et al.,
2018; Li et al., 2019). Increasingly researching for ecological
restoration, the widely held notion that natural regeneration has
limited conservation value and that active restoration should be the
default ecological restoration strategy has been challenged
(Crouzeilles et al., 2017).
ChaZdon (2008) has reported that
natural restoration can achieve great success in degraded ecosystems.
Deng et al. (2016) have shown that
natural restoration is the better option for maintaining the stability
of water resources in arid and semi-arid regions. Although the natural
restoration are increasingly being implemented throughout the world and
have produced some benefits to improve the degraded ecosystems
(Benayas
et al., 2009; Huang et al., 2019), few scientific reports have been
published on the effect of natural restoration in Tibetan Plateau
(Liao et al., 2020a). In 2015, the
General Office of State Council of China has proposed that the natural
restoration should be recorded as the main strategy during ecological
restoration. In recent decades, natural restoration has been conducted
by the removal of degrading factors and mainly involves the secondary
succession of shrubs and grassland, following prohibition of
overgrazing, extensive reclamation, excessive firewood collection in
Tibetan Plateau. The previous study has found that the natural
restoration has advantaged to maintain the fine particles in aeolian
sandy land, compared with artificial restoration
(Liao et al., 2020a). However,
less research has focused on the changes of plant growth and population
structure in different topographies during the natural restoration,
especially for the population of Sophora moorcroftiana (Benth.)
Baker (Fabaceae).
The Tibetan Plateau, which is generally known as the “Third Pole” of
Earth, is one of the most ecologically vulnerable regions in terms of
grassland degradation (Liao et
al., 2019; Li et al., 2019). In many dune habitats, shrub species are
the crucial element in rehabilitating a degraded grassland ecosystem,
due to greater substrate stability and decreased sand movement
(Liu and Zhang, 2018; Wang et al.,
2019). Shrub species are distributed widely across almost the whole
Tibetan Plateau and exposed to the exclusive harsh environments, such as
low oxygen, drought, salinity and cold in Tibetan Plateau
(Zhang et al., 2016; Li et al.,
2017). While, Sophora moorcroftiana (Benth.) Baker (Fabaceae),
one of drought-resistant endemic and dominant shrub species in Tibet,
has advantage to deal with these multiple stresses. S.
moorcroftiana is a perennial leguminous shrub with blue-purple flowers
and mainly dominates in the valleys of the middle reaches of the Yarlung
Zangbo River (Cheng et al., 2017).
It is a unique Sophora characterized by strong drought, solar
radiation and sand burial resistance, and regarded as an ideal species
for studying acclimatization to climatic factors
(Zhao et al., 2007; Guo et al.,
2014). This species enables the fixation of sand dunes, avoiding the
formation of shifting sands and subsequent desertification, and thus it
plays an important role in ecological restoration
(Liu et al., 2006; Zang and Sun,
2019). Although S. moorcroftiana has had much attention from
researchers, for example the adaptability of S. moorcroftiana to
elevation as well as to sand burial
(Zhao et al., 2007) and the
predictor of biomass for S. moorcroftiana in Tibet
(Zhang et al., 2016), little
information is available on the mechanisms of micro-topography on
natural restoration of S. moorcroftiana population.
For shrubs, topography (micro-topography) is an applicable predictor of
aspect and slope that often correlate with distribution of soil
nutrients and spatial pattern of species
(Azizi and Montazeri, 2018). The
micro-topographic relief is often regarded as one of the main drivers of
fine-scale environmental heterogeneity
(Oddi et al., 2019). Despite the
importance of micro-topography on the species distribution and the
growth of dominant shrubs, the research of its quantification is still
rudimentary on the remote area like the Tibetan Plateau due to cost and
technical challenges (Li et al.,
2019). A more recent active remote sensing technology, terrestrial Light
Detection and Ranging (LiDAR) imaging systems now provide unprecedented
ability to characterize micro-topographic structure and vegetation
parament at very fine scales over large areas
(Davis et al., 2019). The airborne
(ALS) and terrestrial laser scanning (TLS) are two types of laser
scanning systems. While TLS (spatial accuracy within millimeter) is
ground-based LiDAR that produces orders of magnitude higher point cloud
density than ALS (spatial accuracy of 5-10 cm) over smaller areas for
capturing shrub structure and micro-topography
(Stovall et al., 2019). TLS have
been proven as a new powerful tool for extracting vegetation structure
and micro-topography in various fields, such as forest ecology (Davis et
al., 2019), wetland ecology (Stovall et al., 2019), hydrology
(Cassidy et al., 2019), and
geomorphology (Calsamiglia et al.,
2018). However, few studies have been conducted to extract the structure
of population and micro-topographic factors and assess the spatial
distribution of S. moorcroftiana during natural restoration,
based on TLS in southern Tibetan plateau.
Here we conducted a series of
field surveys with terrestrial laser scanning (TLS) to achieve the
variation of micro-topography and spatial pattern of S.
moorcroftiana population, and then analyzed the effect of
micro-topography on S. moorcroftiana population during natural
restoration in southern Tibetan Plateau. This study specifically aims to
address two research questions: (1) How did the individual growths
change in the different topographies of sampling plot of S.
moorcroftiana population. (2) What are the impacts of micro-topographic
factors on population structure and spatial pattern of S. moorcroftiana
for natural restoration in southern Tibetan Plateau.