# Estimation of Gross Primary Productivity (GPP)
## 1 Introduction  
Short-rotation coppice (SRC) plantations provide multiple economic and ecological benefits [1]. They are ideal for producing biomass for energy [2] and can, therefore, substitute for fossil fuelsto reduce anthropogenic CO\({}_{2}\) emissions [3]. They enrich farm-scale biodiversity [4], increase soil organic carbon stock [5; 6], improve groundwater quality [7] and help in preventing pests and diseases. Over 16% of woodlands in Europe were managed as coppice in 2000, covering a global area of about 23 million ha [8]. In the last 20 years, SRC plantations increased globally to cover an area of 31.8 million ha in 2016 [9]. Poplars (_Populus_ spp.) are widely used as bioenergy plantations and for wood production in the Northern Hemisphere representing 99% (31.4 million ha) of SRC area worldwide. Even though the important role of SRC plantations in bioenergy productions and carbon sequestration is well recognized by the scientific community [10; 11], data on SRC productivity (e.g., Gross Primary Productivity (GPP)) and phenology are scarce and limited to small experimental areas. The ecosystem phenology, also defined as carbon flux phenology, describes the seasonality of the gross photosynthesis (i.e., GPP) of an ecosystem. The starting day of the photosynthetic season (SOS) occurs when an ecosystem turns from a carbon source into a carbon sink in spring. Contrary, the end of the season (EOS) identifies the decrease of photosynthetic activity at the beginning of senescence. A shift in the SOS and EOS of a growing season regulates annual GPP [12; 13]. A longer growing season increases the annual productivity of deciduous forests [14; 15]. The timing of bud bursts and sets depend on local environmental conditions. The plant growth strategy of deciduous trees in temperate regions exploits maximally optimal spring growth conditions [16]. SRC plantations allocate carbon to different tree components at the beginning of the growing season [17] as reported also for beech in [PERSON] et al. (2011). The photosynthetic activity shows a substantial decline by early mid-September in temperate deciduous species [18]. Contrary, high carbon uptake rates were observed until the end of September for SRC plantations [17; 19] and poplar [20]. Furthermore, for agroecosystems such as crops and SRC plantations, management practices also play an important role in determining the annual production. However, few studies focused on the understanding of the effect of coppicing on SRC phenology [21; 22]. A clear understanding of how phenology affects the productivity of SRC plantation can effectively support farmers in optimally managing their fields to obtain the most profit from crop production. In addition, it is necessary to improve the model parameterization of global productivity models. In situ observations provide a robust measurement of plant development and phenology [22], but they can be laborious and expensive when a lot of sites need to be sampled. The eddy covariance (EC) method [23] is suitable for monitoring ecosystem productivity, aspects of phenology and their changes related to abiotic stresses (e.g., drought, management) [24; 25]. However, GPP data derived from the EC method are representative of only a small area, roughly speaking 10 to 30 times the measurement height on average, around and mostly upwind of the flux tower [26] and the technique has rarely been applied at SRC plantations. Both EC and in situ observations thus have limitations when data sampling is required at large spatial scales. Large-scale and continuous observations taken by satellite sensors make remote sensing approaches attractive alternatives for phenology assessment [27; 28]. Spectral vegetation indices (VIs) can be used to derive many biophysical and biochemical vegetation parameters (e.g., chlorophyll content, Leaf Area Index (LAI)) and GPP [29; 30; 31; 32; 33; 34; 35].