Patterns and Controls of Light Use Efficiency in Four Contrasting Forest Ecosystems in Yunnan, Southwest China

Abstract

Ecosystem light use efficiency (LUE) is a critical parameter in estimating CO 2 uptake by vegetation from climatological and satellite data. However, the spatiotemporal dynamics and biophysical regulations of ecosystem-level LUE are not well understood, resulting in large uncertainties in the estimation of gross primary productivity (GPP) using LUE-based models. In this study, we used eddy covariance to explore spatiotemporal variations and controls of LUE in four contrasting forest ecosystems (savanna, tropical rainforest, subtropical evergreen forest, and subalpine coniferous forest). Based on 27 site years of data, we found that (1) the multiyear mean LUE was 0.063, 0.251, 0.247, and 0.140 g C· mol photon −1 in the four contrasting ecosystems, respectively; (2) the LUE in the wet season (May-October) was higher than that in the dry season in all studied ecosystems; (3) the leaf area index controlled GPP and LUE significantly and explained 74%, 29%, 54%, and 36% of the variation in GPP and 51%, 19%, 41%, and 54% of the variation in LUE in the four contrasting ecosystems, respectively; (4) path analysis revealed the critical roles of GPP and vapor pressure deficit in controlling LUE in these four forest ecosystems; and (5) under warming scenarios, LUE may decrease in savanna but increase in the other three ecosystems, while decreasing precipitation (P) may reduce LUE in the ecosystems studied. This study improves our understanding of the influence of biophysical factors on LUE and demonstrates how LUE changes with variations in temperature, soil moisture, and leaf area index, thereby improving estimations of large-scale carbon exchange/cycling.

Global Patterns and Climate Controls of Terrestrial Ecosystem Light Use Efficiency

“…Variations in LUE were also known to be associated with climate (e.g., Hilker et al, 2008), but studies differed in reporting the dominant climatic factors regulating LUE variations and in the impacts of the same climatic factor. For example, Garbulsky et al (2010) found precipitation as the dominant controlling factor of LUE variations based on observations from 35 EC sites over the Northern Hemisphere, while solar radiation and temperature were found to be the most important climatic factor in southwestern China and Canada, respectively (Fei et al, 2019; Schwalm et al, 2006). The impacts of temperature were assessed most in previous studies, but the results were so different that some found it to be significant (e.g., Kergoat et al, 2008; Schwalm et al, 2006), while some found it not (e.g., Jenkins et al, 2007; Turner et al, 2003).…”

“…Light use efficiency (LUE), as a popular concept in plant photosynthesis studies, describes the ability of vegetation to convert absorbed light energy to photosynthetic carbon uptake. As vegetation carbon uptake has become a critical ecosystem service that slows down the anthropogenic increase in greenhouse gas concentrations, there is an urgent need to accurately quantify LUE, from leaf to ecosystem scales (e.g., Fei et al, 2019; Kergoat et al, 2008; Li et al, 2016). At leaf scale, LUE can be measured as the proportion of energy used in the photochemical quenching to that directed through nonphotochemical quenching and chlorophyll fluorescence (Coops et al, 2010; Hilker et al, 2008).…”

Threshold of vapour–pressure deficit constraint on light use efficiency varied with soil water content

“… where NEE , α , PAR , P max represent net ecosystem exchange, initial quantum yield, photosynthetically active radiation, and the maximum photosynthetic rate, respectively; where a and b are constants, and Reco and Ts represent ecosystem respiration and the soil temperature at a depth 5 cm, respectively. We then calculated GPP following Fei et al (2019): There are different definitions of LUE (either the ratio of GPP to absorption of photosynthetically active radiation (APAR) or the ratio of GPP to photosynthetically active radiation (PAR)). In this study, ecological LUE was used to represent ecosystem LUE, which was defined as follows (Fei et al, 2019): VPD was calculated with the following equation (Venturini et al, 2011): where Ta and RH represents air temperature and relative humidity, respectively.…”

Investigating the Patterns and Controls of Ecosystem Light Use Efficiency with the Data from the Global Farmland Fluxdata Network

“…Besides, the assimilation of 1 mol of CO 2 requires 8 moles of photons at the leaf scale under ideal conditions [45]. However, the measured ELUE values are much lower than the theoretical ELUE values because they are constrained by many photosynthetic reactions [21,46]. Photosynthetic productivity usually saturates far below the maximum solar light intensity; in those conditions, many absorbed photons and the resulting electronic excitations of the pigment molecules can no longer be utilized for photosynthesis.…”

“…Path analysis is a typical multivariate statistical analysis approach to study the relationship between multiple variables [19,20]. For example, Fei et al (2019) revealed the critical roles of GPP and vapor pressure deficit in controlling ELUE by path analysis [21]; Jiang et al (2020) quantified the effects of climatic factors and the leaf area index on crop canopy water consumption and carbon sequestration [22]. Therefore, the path coefficients in the analysis as standardized partial regression coefficients can allow us to examine the possible causal link between the independent variables and their relative effects on the dependent variable.…”