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.