Effects of sky conditions on net ecosystem productivity of a subtropical coniferous plantation vary from half-hourly to daily timescales
Graphical abstract
Introduction
As the main body of terrestrial ecosystem, forest is the largest carbon sink among terrestrial ecosystems. It plays an irreplaceable role in global economic development and environmental protection (Pan et al., 2011). Planted forests (plantations) play an increasingly important role in ecosystem goods and services such as climate regulation, water and soil conservation, and biodiversity protection. The carbon sequestration capacity of plantation is also one of the key issues in global change studies (Fang et al., 2001).
Solar radiation drives plant photosynthesis (Monteith, 1972). The dynamic changes of solar radiation and its components have been widely explored in global change studies such as “global dimming” after the 1950s and “global brightening” after the 1990s (Wild, 2009; Stjern and Hansen, 2010). These changes will greatly affect eco-physiological processes, such as carbon sequestration (Wang et al., 2008).
Diffuse radiation is a main component of solar radiation (Şen, 2008). Many studies have paid much attention to the effects of diffuse radiation on ecosystem carbon sequestration and productivity (Gu et al., 1999; Gu et al., 2003; Alton et al., 2007; Knohl and Baldocchi, 2008; Mercado et al., 2009). Although comparative analyses of photosynthetic characteristics under different sky conditions have been conducted, there are still apparent controversies about the effects of diffuse radiation on ecosystem productivity and carbon sequestration capacity (Dengel and Grace, 2010; Urban et al., 2012; Kanniah et al., 2013). Many studies have shown that the increased diffuse radiation will enhance ecosystem light use efficiency (LUE), e.g., an increase of 110% in crops (Choudhury, 2001), and 110–180% in temperate forests (Gu et al., 2002). Rap et al. (2015) also found that an increase of 3.4–6.8% in diffuse radiation resulted in an increase of 1.4–2.8% in net primary production (NPP) of Amazonian forests. However, Alton (2008) recorded a general decrease in NPP across 38 FLUXNET sites from different ecosystems, owing to the dramatic reduction in global radiation when clouds obscure the solar disk.
Previous researches showed that diffuse radiation affects ecosystem productivity by both direct and indirect ways (Kanniah et al., 2012). On the one hand, the increased diffuse radiation enhances photosynthesis of shade leaves within the canopy, and the reduction of direct radiation slows down “light saturation” of the canopy (Knohl and Baldocchi, 2008). On the other hand, coupled with the changing solar radiation, the changes of air temperature (Ta) (Krakauer and Randerson, 2003), volumetric soil water content (SWC) (Rocha et al., 2004), and vapor pressure deficit (VPD) (Law et al., 2002) also have an influence on photosynthesis (Wagle et al., 2017). Nevertheless, most studies do not consider the temporal variations into the effects of the changing solar radiation and environmental variables on ecosystem carbon exchange, e.g., the variations from half-hourly to daily scale and across different seasons. It still needs a clear understanding about the impacts of environmental variables on ecosystem productivity with the changing solar radiation (Wu et al., 2017).
The plantations have expanded greatly with the reforestation in China in recent decades, especially in the subtropical regions. According to the 8th National Forest Resource Inventory Report (2009–2013), the plantation area was 69.33 million ha, ranking 1st in the world, which was dominated by the coniferous plantation in southeast China (CFA, 2015). It has been proved to be one of the important carbon sinks in East Asian subtropical monsoon regions (Yu et al., 2014). The previous studies have indicated that net ecosystem CO2 exchange (NEE) in typical ecosystems of China benefited from the increased diffuse radiation at half-hourly scale in the mid-growing season under cloudy skies compared with sunny skies (Zhang et al., 2010, Zhang et al., 2011). However, the effects of the increased diffuse radiation on NEE at half-hourly scale and their underlying mechanisms under different sky conditions (i.e., sunny, cloudy, and overcast) in subtropical plantations remain unclear.
A subtropical evergreen coniferous plantation in southeast China was selected in this study. It is reported that the plantation in this area has a strong capacity of carbon sequestration with annual mean net ecosystem productivity (NEP) of 354 ± 34 g C m−2 (Yu et al., 2008). However, the plantation usually experiences seasonal droughts in summer (Wen et al., 2010). This study aims to address the following questions: (1) Is there a seasonality in the impacts of various sky conditions on NEP? (2) How do solar radiation and environmental variables jointly affect NEP? and (3) Is the response of NEP to various sky conditions at daily scale the same as that at half-hourly scale? Such analyses can not only enrich the understanding of the mechanism behind ecosystem carbon sequestration in this area, but also provide knowledge for improving ecological models.
Section snippets
Site description
This study was conducted in Qianyanzhou (QYZ) Ecological Research Station of Chinese Academy of Sciences (26°44′48″ N, 115°04′13″ E, and 102 m a.s.l.), which is in Ji'an City, Jiangxi Province, China. The area is strongly affected by the East Asian monsoon climate. The prevailing wind direction is southeast in summer and northwest in winter (Fig. 1b–d). According to meteorological records (1985–2015) of China Ecosystem Research Network (CERN), annual mean precipitation sum and annual mean Ta
Variations of environmental variables, LAI, and NEP
Ig and Id exhibited unimodal variation trends in each year, which both reached the maximum in July. Ig (Id) was 4192.96 (2592.68) in 2010, 4292.40 (2695.39) in 2011, and 4078.76 (2585.12) MJ m−2 in 2012, respectively. The annual mean kd (i.e., the ratio of Id to Ig) was 61.8% in 2010, 62.8% in 2011, and 63.4% in 2012, respectively, and 62.7% on average across the three years (Fig. 5a).
Similar to the variation patterns of radiation and Ta, VPD varied roughly in unimodal mode. The annual mean VPD
Effects of diffuse radiation on ecosystem photosynthesis
Previous studies have shown that Id can increase ecosystem photosynthesis (Dengel and Grace, 2010; Urban et al., 2012; Kanniah et al., 2013). This study also demonstrated that α, Pn150, and Pn750 under cloudy conditions were usually greater than those under sunny conditions at half-hourly scale; especially, α and Pn150 under overcast conditions were much greater than those under sunny conditions at half-hourly scale; the effects of Id on α showed intra-seasonal and interannual variability (Fig.
Conclusions
Effects of sky conditions on NEP of a subtropical coniferous plantation were evaluated by using 2010–2012 EC data. We found that cloudy and overcast sky conditions were better for light response parameters (i.e., α, Pn150, and Pn750) than sunny sky conditions at half-hourly scale across seasons. The most α, Pn150, and Pn750 under cloudy conditions were greater than those under sunny conditions at half-hourly scale; especially, α and Pn150 under overcast conditions were much greater than those
Acknowledgement
This study was supported by the National Key Research and Development Program of China (Grant No. 2017YFC0503801 and 2016YFD0600202).
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