**1. Introduction**

Due to the imbalance between social and economic development and the natural ecological process [1], as well as the instability, sensitivity and complexity of the natural system of the mountainous area itself, the mountainous area has become the most intense region of global environmental change and ecological degradation in recent years [2], and the conflicts between human and land have become more prominent and complex. In the southwest of China, the terrain is fragmented and sensitive, and the generalized mountain landscape is an important and unique natural—human geography unit composed of "mountain" and "flatland" as the core elements [3,4], which is an important content of the scientific research of human—land systems in mountainous areas. To fully guarantee the ecological security and sustainable development of mountainous areas, it is necessary to take land resources as the main constraint condition [5], coordinate the linkage relationship between "mountain" and "flatland" in the whole region, and study the changes and correlation of "mountain" and "flatland" elements in mountainous landscapes, in order to provide

more scientific and accurate strategies for rural revitalization, mountain development, spatial governance, as well as the maintenance and improvement of ecosystem services.

A landscape pattern mainly refers to the shape, proportion and spatial configuration of the elements that constitute landscape units [6]. It is not only the comprehensive expression of landscape heterogeneity in space [7,8], but also the result of various ecological processes driven by natural and social factors at different scales [9,10]. Landscape pattern change is the change in the landscape's spatial structure on the basis of the changes in various landscape elements, which are closely related to climate change, land use/land cover change and change in biodiversity [11,12]. Understanding the evolutionary characteristics of the landscape pattern is the premise and basis for landscape pattern analysis. The evolution of landscape pattern is a comprehensive reflection of the interaction and influence of natural elements and human factors in a certain region, as well as the different external characteristics and spatial combinations of various factors, which constantly affect the ecological process and marginal effect [13]. Exploring the evolution process of a landscape pattern is helpful for grasping the evolutionary characteristics and rules of the regional landscape, and to provide basic data for the assessment of the sensitivity, vulnerability and ecological risk of ecological degradation [14], which is the basis and important support for decision-makers to formulate reasonable and scientific urban planning [15]. At present, the analysis methods of landscape pattern evolution mainly include spatial statistical analysis, landscape index analysis and pattern dynamic model simulation [16,17]. From the perspective of landscape ecology, landscape index analysis regards the study area as a whole and reveals the changes in the landscape's spatial pattern characteristics in time series through various landscape indices [18]. Landscape indices include a patch level index, a patch type level index and a landscape level index. As a scale of landscape pattern characteristics, the landscape index has been widely used to analyze regional landscape patterns and dynamic evolution [19,20], including urban [21], rural [22], urban fringe areas [23] and economically developed areas [24]. However, the landscape pattern evolution of two different geographical units of "mountain" and "flatland" in the generalized mountain landscape is rarely involved. There are great differences between the physical geographical conditions and human and social activities in the mountainous area and flatland area. What are the evolutionary differences in the landscape pattern on the spatial and temporal scale? Is there a certain correlation between the evolution of the two on the space—time scale? All these need to be explained by landscape pattern analysis.

In addition, the scaling effect of a landscape pattern is also a major feature of landscape pattern analysis [25]. The results of landscape pattern evolution at different scales often vary vigorously, and the landscape index will change with the scale and have a scale effect at a finer scale [26]. Landscape pattern characteristics at different spatial scales are often different, and at the same spatial scale, a landscape pattern at different time scales will also be different [27]. The dynamic landscape characteristics of urban and rural construction land show great differences in scale [28], and the landscape pattern of cultivated land in the middle reaches of the Yangtze River has great differences in quantity, area, aggregation degree and diversity at different scales [29]. Therefore, the small-scale and refined studies on land use dynamic change and landscape pattern evolution at and below the county level have attracted ever increasing attention [30,31]. Some scholars have used the grid scale to carry out their research, and obtained more refined results compared with the scale of watershed and administrative regions [32,33].

In addition to the low level intermountain basins, valleys and depressions, "flatlands" in this study also include lacustrine plains which occupy a certain area in central Yunnan [4]. Yuxi City, with typical mountain—flatland landform features [34], is located in the mountainous area of southwest China. The flatland area is flat, suitable for farming and construction, and is an important carrier of human economic activities, while the natural conditions in the mountainous area are complex and difficult to use [35–37]. Due to the geographical connectivity of "mountains" and "flatlands", the intercrossing of human activities in different time and space has brought about the differences in the evolution of

the landscape pattern on the spatial and temporal scale. Therefore, based on the grid scale and with the help of a landscape pattern index, this paper analyzes the temporal—spatial variation in mountain—flatland landscape patterns in Yuxi City, and discusses the correlation between mountains and flatlands in the evolution process of landscape patterns, so as to provide certain references for regional coordinated and sustainable development.
