Study site and species
The data for this study was collected in 2011 at Wan-Long Farm, a lowland former sugarcane plantation owned by the Taiwan Sugar Corporation in Sinpi Township, Pingtung, Taiwan (120° 36ʹ 30ʺ E, 22° 31ʹ 26ʺ N, 69 m above sea level). Soil were classified as Entisols with over 60% parent material of sandstone and 45–55% gravel content, and the soil profile is shallow (< 40 cm depth) (Chen et al. 2012; Yu et al. 2012). In the study site, 14 species were used for afforestation from 2002 to 2005, with Z. serrata widely planted near the centre of the farm in 2003. Z. serrata is a pioneer species frequently used in lowland afforestation, having the second largest plantation area in Taiwan. The initial stand density was 1,500 seedlings ha-1, with constant pruning and weed removal. A study plot (20 × 25 m) with 27 trees was set in 2010, where the average diameter at breast height and tree height were 3.89 ± 1.58 cm and 3.46 ± 1.28 m, respectively. The surrounding of the study plot was covered by the same species and size with the 77.4% of crown density in average.
Microclimate data were collected by a microclimate station from 400 m southeast of experimental plot with a temperature and relative humidity probe (HMP45C, Vaisala, Finland). The region has a typical tropical monsoon climate, with a high frequency of typhoons and afternoon thundershowers during summer. However, the annual precipitation in 2011 was lower (1,929 mm) than 2010 (2,848.5 mm) and 2012 (3,144.5 mm), and represented an intense drought event. In 2011, the January and June mean air temperature were 16.6 and 28.0 °C, respectively; annual precipitation was concentrated from May to September (wet season). The monthly variation of precipitation, soil water content and air temperature in 2011 is shown in Fig. 1. Data on soil water content were collected by the microclimate station using a time-domain reflectometer (TDR, CS616, Campbell Scientific Inc., Logan, UT, USA) and showed a similar pattern to that of precipitation, being lowest in March (8.8%) and highest in July (19.4%) in 2011.
Measurements of gas exchange and leaf area
Scaffolds (1.7 m height) were set near the sample trees with tripods so the leaf clamp reached the canopy leaves. Diurnal variations of leaf gas exchange were measured by portable photosynthesis systems (LI-6400; LI-COR, Lincoln, NE, USA) with a clear chamber bottom (LI-6400-08; LI-COR) one day per month for each sample tree during 2011 from three intact, fully expanded mature leaves on the same side of the canopy. Juvenile and ageing leaves were excluded from sampling. Measurements were taken hourly from 8:00 a.m. to 4:00 p.m. (mean solar time), and net photosynthesis rate (Pn), stomatal conductance (gs), transpiration rate (E), and intercellular CO2 concentration (Ci) were recorded. The flow rate was set to 500 µmol s-1 and the air inlet of the LI-6400 was connected to a plastic tube (2–3 m in length), with the end set away from the operator to prevent the influence of human activities, and maintain CO2 concentration in the leaf chamber changing with ambient atmosphere. The air temperature (Ta), leaf temperature (Tl), vapour pressure deficit (VPDa), and leaf vapour pressure deficit (VPDl) were also recorded. The data gap in September 4 was caused by rain during the afternoon.
The photosynthesis response to different light intensity gradients was measured to construct photosynthetic light response curves. Measurements were taken for three leaves per sampled tree in each season using an LI-6400 with an artificial LED light chamber (LI-6400-02B; LI-COR). The leaf selection criteria were the same for the diurnal measurement. Sample leaves were exposed to 500 µmol m-2 s-1 artificial PPFD for a few minutes before measurements were taken to induce leaf stomatal opening. The artificial light intensities (photosynthetic photon flux density, PPFD) were set at 0, 5, 10, 20, 50, 100, 200, 500, 750, 1000, 1500, and 2000 µmol m-2 s-1 in sequence with about 10–20 minutes stabilization time in each light intensity. Seasonal measurements of photosynthetic light response curves were used to calculated maximum assimilation rate (Amax) and shape parameter (θ). Simultaneously, quantum efficiency (α) and dark respiration rate (Rd) were calculated from the initial slope of the photosynthetic light response curve, at light intensities lower than 50 µmol m-2s-1 [15,16].
Total leaf area of the tree was estimated from the leaf area index (LAI), measured monthly at dusk from the top and under the canopy of three sample trees. LAI was measured by a plant canopy analyser (LAI-2200; LI-COR) in four directions under each sample tree. The sensor was covered with a lens cap (90° opening) to prevent overestimation of the canopy leaf area owing to shading by the main branch and trunk. The total leaf area of each sample tree was calculated as LAI multiplying canopy projected area of an individual tree’s canopy. The canopy projected area was calculated as an ellipse area by measuring major and minor axis in two perpendicular directions as crown projection.
Data analysis
The single tree total leaf area was calculated by multiplying LAI by the canopy projection area, which was estimated as an ellipse. The crown projection area was calculated by measuring major and minor axes in two perpendicular directions.
According to several studies, the light response curve can demonstrate the effects of environmental factors such as nutrient levels, temperature, and water variables on leaf photosynthesis (Sands 1995; Biswas et al. 2014). Therefore, the leaf carbon assimilation (A, µmol m-2 s-1) was calculated from the following equation:
where Amax (µmol m–2 s–1) is the maximum net assimilation of CO2, α is the quantum efficiency, Il (µmol m–2 s–1) represents leaf-level light intensity, which is derived from the measurements at the canopy top according to Beer’s Law, and θ is the shape parameter of the photosynthetic light response curve, calculated as the slope of the tangent at the light saturation point. The model has been well-tested and extensively applied in several leaf carbon assimilation studies (Sands 1995; Thornley 2002; Biswas et al. 2014; Chen et al. 2019)
Several models to scale-up carbon assimilation from leaf to canopy level are already in use (Lambers et al. 1998). After calculating the leaf assimilation rate, we upscaled from leaf- to canopy-level assimilation using LAI as shown in (Chen et al. 2019). The estimation formulas were as follow:
Total daily canopy assimilation (Ac) can be calculated by considering total leaf area (L), day length (h), and leaf carbon assimilation (A) from formula (1). Total night canopy respiration (Rc) can be calculated by considering total leaf area (L), night length (n), and dark respiration rate (Rd) from light response curves. The variations in air temperature during the nights of the study period were less than 6 °C and 3 °C in winter and summer, respectively. Additionally, we calculated monthly assimilation and respiration by considering days in each month, and annual carbon assimilation of Z. serrata was calculated by summing up monthly carbon assimilations.
SAS 9.4. statistical software (SAS Inc., Cary, NC, USA) was used to analyse the Pearson correlation between gas exchange and environmental variables. Correlation analysis data were collected from monthly measurement of diurnal variation. Significance was set at P < 0.05. Data in figures are presented as means ± standard error.