**1. Introduction**

In recent decades, driven by intensive human activity and climate change, the function of terrestrial ecosystem has been disturbed and continuously degraded on regional and global scales. The increasing levels of atmospheric CO2 concentrations and climate change have highlighted the need for a better understanding of terrestrial carbon cycling and its responses to climate change. Gross primary production (GPP) represents the capacity of the plants in an ecosystem to capture energy and carbon [1]. Net Primary Productivity (NPP) is defined as the amount of atmospheric carbon that is captured by plants and transformed into biomass [2]. The GPP is the sum of NPP and autotrophic respiration (Ra), and Ra

plus heterotrophic respiration (Rh) comprises ecosystem respiration. GPP, NPP and Ra are the most important and highly related constituents of carbon cycling. The carbon fixed by photosynthesis is allocated to a variety of usages in plants, including growth and maintenance respirations and biomass accumulation [3]. About 50–70% of the carbon fixation is returned to the environment through Ra [4,5]. Carbon allocation among plant processes (e.g., respiration, biomass production) and organs (e.g., leaves, stem) is a key process in the carbon cycle because it determines the residence time and location of carbon in the ecosystem [6,7]. For example, the residence times of the carbon used for maintenance respiration and the carbon allocated to the structural biomass of organs are drastically different, ranging from a few hours to a few years [6]. Therefore, the allocation process of carbon is highly relevant to understanding ecosystem carbon stock and carbon cycles [6].

Carbon use efficiency (CUE) is defined as the ratio of NPP to GPP, which indicates the ecosystem capacity in transferring CO2 into biomass and carbon sequestration [8]. CUE is an important functional parameter of ecosystems and can be used for comparing carbon cycle differences in various ecosystems [9]. The index is intuitive and easy to compare between different vegetation types, and to apply to different time scales [7]. A higher CUE indicates a higher growth transfer per unit of carbon sequestration. In practice, GPP usually represents the total amount of carbon captured through photosynthesis, and NPP is the net carbon stored in plant after the reduction of GPP through by plant respiration [1]. CUE is also a measure of how GPP is partitioned into NPP and Ra [7]. Less Ra may result in larger carbon reserve accumulation. Hence, CUE is related to photosynthetic process, and it is also regarded as an important indicator for characterizing ecosystem functions. How efficiently an ecosystem is able to convert GPP into plant and soil storage greatly determines the carbon sequestration of terrestrial ecosystems, so CUE changes strongly affect ecosystem carbon budgets [10]. Quantitative analysis of spatial-temporal changes of CUE and its influencing factors will help better understand the effects of climate change on carbon processes of ecosystems [11].

Satellite remote sensing provides critical information for investigating large-scale and long-term variability of ecosystem CUE. Piao et al. [12] demonstrated that CUE of di fferent vegetation di ffered greatly from the south temperate to the tropic ecoregions based on a global forest C-flux database, and found that the spatial patterns of forest annual Ra at the global scale were largely controlled by temperature. Zhang et al. [1] reported that CUE exhibited a pattern depending on the climatic characteristics-based upon Moderate-Resolution Imaging Spectroradiometer (MODIS)-derived NPP and GPP data. He et al. [13] investigated spatial variations in CUE from di fferent models and analyzed the responses of CUE to precipitation and temperature. Tang et al. [3] established a global database of site-year CUE based on field observations for five ecosystem types and diagnosed the spatial variability of CUE with climate and other environmental factors (e.g., soil variables). Two prominent gradients of CUE in ecosystem types and latitude were found worldwide. CUE varied with ecosystem types, being the highest in wetland and lowest in grassland. CUE decreased with latitude, showing the lowest values in tropics, and the highest CUE were found in higher-latitude regions. The above studies were based on annual scales and advanced the knowledge of understanding the global pattern of CUE. However, monthly scale analysis of CUE has rarely been studied.

From individual plants to an entire ecosystem, phenology directly or indirectly regulates carbon fluxes (e.g., photosynthesis and respiration) between the land surface and the atmosphere [14] through altering physiological and structural characteristics, including photosynthetic rate, canopy conductance and albedo [14–16]. Vegetation phenological changes are closely related to spatial-temporal dynamics of carbon cycle [17]. The change in the length of the growing season may have an important impact on vegetation growth, which will cause changes in the GPP and NPP [18]. CUE and its relationships with land surface phenology (LSP) deserve attention.

The Songnen Plain (SNP), located in temperate semi-humid to semi-arid transition ecological fragile zone in Northeast China, is highly sensitive to global change. As a key agricultural area and important grain commodity base, the SNP is among the designated ecological red-lines as protected farmland area in China. The natural terrestrial ecosystem acts as an ecological protection barrier for the croplands in the SNP. The productivity and sustainability of terrestrial ecosystems are vital

to maintaining regional and national food and ecological security. Due to the combined e ffects of vulnerable physical conditions and excessive human activities, the SNP su ffered from high risk of land degradation during the past century. Concerns for the aggravation of desertification have led to many measures and managemen<sup>t</sup> actions for ecological and environmental protection. The trend of desertification and exacerbation has gradually slowed down [19]. Previous studies have mostly focused on the land cover/use change and e ffects of agricultural activities on environment [20]. In addition, most studies only focused on the condition of protected farmland, while ignoring the productivity and sustainability of natural terrestrial ecosystems around it. There is a lack of reports about the spatial-temporal patterns of ecosystem-level CUE and their response to phenology and climate change in the SNP region. This study attempts to fill in the gaps in the knowledge regarding biotic and abiotic impacts on CUE of the SNP region.

The objectives of this study are to: (1) estimate CUE of di fferent ecosystems and investigate their monthly and seasonal changes based on MODIS GPP and NPP data from 2001 to 2015; (2) explain how phenology and climatic factors contribute to variations in ecosystem CUE, in order to improve our understanding of the carbon budget in temperate semi-arid and semi-humid transitional zone ecosystems and their driving mechanisms.

#### **2. Materials and Methods**
