Verification of a numerical growth model of Pistia stratiotes L. using field data from tropical and subtropical sites

Figures & data

Figure 1. Meteorological characteristics (NASA-POWER 2016) driving the simulations at different latitudes: 07° 26′ N, 03° 53′ E (Nigeria) and 27° 30′ N, 80° 30′ W (USA). (A) Daily solar radiation (MJ m⁻² day⁻¹), (B) daily average air temperature (°C).

Table 1. Characteristics of sampling sites supporting Pistia stratiotes populations at different latitudes.

Parameter	Awba Lake^a	South Florida^b
Country	Nigeria	USA
Latitude	07° 26' N	27° 30' N
Longitude	03° 53' E	80° 30' W
Climate	Tropical	Subtropical
Mean daily air temperature (°C)	23.9–26.9	19.4–26.7
Mean daily solar radiation (MJ m^−2)	14.0–20.7	11.5–21.6
Study period	February–May 1979	November 1985–April 1987
Initial shoots biomass (g DM m^−2)	49.5	23.5
Initial roots biomass (g DM m^−2)	7.1	6.3
Area (m^2)	94,500	170
Depth (m)	5.5	1.0
Water characteristics
Type	Eutrophic	Eutrophic
pH	6.6–8.4	6.7–7.0
Total solids (mg L^−1)	66–838
Dissolved oxygen (mg L^−1)	0.9–7.9	1.2–2.1
NH4 (mg L^−1)	0.44–1.16	1.23–1.54
P (mg L^−1)	0.04–0.18^c	0.51–20.88^d

^a Sharma and Sridhar (1981).

^bDeWald and Lounibos (1990).

^cWater soluble P.

^dTotal P.

Figure 2. Simulated results (lines) and measured data (symbols; data from Sharma & Sridhar 1981) of the biomass and leaf area index (LAI) of Pistia stratiotes populations at 07° 26′ N, 03° 53′ E. Total biomass = shoots biomass + roots biomass.

Figure 3. Simulated results (lines) and measured data (symbols; data from DeWald & Lounibos 1990) of the biomass and leaf area index (LAI) of Pistia stratiotes populations at 27° 30′ N, 80° 30′ W. Total biomass = shoots biomass + roots biomass.

Table 2. The deviations of simulated results reproduced by Pistia-model from observed field data at different latitudes.

					Regression analysis				Wilcoxon signed-rank test
Latitude	Longitude	Country	Parameter	Standard deviation (%)	r	P	df	slope	z-value	P
07° 26′ N	03° 53′ E	Nigeria	Total biomass (g DM m^−2)	6.8	0.889	0.000	13	0.939	−1.350	0.177
			Shoots biomass (g DM m^−2)	6.9	0.894	0.000	13	1.008	−1.412	0.158
			Roots biomass (g DM m^−2)	18.9	0.710	0.004	13	0.422	−0.408	0.683
27° 30′ N	80° 30′ W	USA	Total biomass (g DM m^−2)	16.5	0.884	0.000	10	0.943	−1.511	0.131
			Shoots biomass (g DM m^−2)	17.7	0.885	0.000	10	0.788	−0.445	0.657
			Roots biomass (g DM m^−2)	33.9	0.668	0.025	10	2.155	−0.800	0.424
			Leaf area index (m^2 m^−2)	17.1	0.850	0.001	10	0.923	−0.978	0.328

Figure 4. Production and seasonal fluxes of dry matter of Pistia stratiotes populations at 07° 26′ N, 03° 53′ E (Nigeria) constructed using simulated quantities. Production is expressed as g DM m⁻² and fluxes expressed as g DM m⁻² per year.

Figure 5. Production and seasonal fluxes of dry matter of Pistia stratiotes populations at 27° 30′ N, 80° 30′ W (USA) constructed using simulated quantities. Production is expressed as g DM m⁻² and fluxes expressed as g DM m⁻² per year.

Figure 6. Daily solar radiation (MJ m⁻² day⁻¹) and daily average air temperature (°C) sums at 07° 26′ N, 03° 53′ E (Nigeria) and 27° 30′ N, 80° 30′ W (USA).

Table

Rates of change (g DM m^−2 day^−1) of the state variables (g DM m^−2)

Shoots Growth – Shoots Respiration – Shoots Mortality – Supply of photosynthesized materials for Roots Growth

= Supply of photosynthesized materials for Roots Growth – Roots Respiration – Roots Mortality

Main processes (g DM m^−2 day^−1)^a

Shoots Growth = Gross Photosynthesis rate20. Organsh. f (T)

Shoots Respiration = Shoots Respiration rate20. Organsh. f (T)

Shoots Mortality = Shoots Mortality rate20. Organsh. f (T)

Supply of photosynthesized materials for Roots Growth (Gamage & Asaeda 2004) = . Organrt. f (T)

Roots Respiration = Roots Respiration rate20. Organrt. f (T)

Roots Mortality = Roots Mortality rate20. Organrt. f (T)

Functions^b

f (T) = θ^(^T ^– 20)

The gross photosynthesis rate of macrophytes is described by a form of the Michaelis–Menten equation (Jørgensen et al. 1981) as Gross Photosynthesis rate20 (g gross photosynthesis g^−1 shoots biomass day^−1) =

The relationship of shoots biomass (g DM m^−2) and leaf area index (LAI) was developed by Eid et al. (2016) (r = 0.969, P < 0.001) using the measured data: LAI = 0.314 + 0.018 × Shoots biomass.

^a Organ_sh is shoots biomass (g DM m⁻²); Organ_rt is roots biomass (g DM m⁻²); Rate₂₀ is the rate (day⁻¹) at 20 °C; f (T) is the temperature function correcting the rates for the daily averaged air temperature (T, °C).

^b θ is the Arrhenius constant; K_co is the conversion constant of CO₂ to dry matter (g g⁻¹ CO₂); P_max is the maximum photosynthetic growth rate (g CO₂ g⁻¹ day⁻¹); PAR is the photosynthetic active radiation (MJ m⁻² day⁻¹); K_PAR is the half saturation constant for the photosynthetic active radiation (MJ m⁻² day⁻¹); K_le is the light extinction coefficient within the canopy (m² (ground) m⁻² (leaf)); LAI is the leaf area index of the stand (m² (leaf) m⁻² (ground)); N is the concentration of total nitrogen in water (mg N L⁻¹); K_N is the half saturation constant for nitrogen uptake (mg N L⁻¹); P is the concentration of total phosphorus in water (mg P L⁻¹); K_P is the half saturation constant for phosphorus uptake (mg P L⁻¹); G_rt is the maximum roots growth rate (g g⁻¹ day⁻¹); Age_rt is the age of roots from the start of growth (days); K_rt is the half saturation constant of the effect of age (days) on the roots growth rate.

Table

Parameter	Symbol	Value	Unit	Reference
Shoots respiration rate	βsht	0.01	g g^−1 day^−1	Lorber et al. (1984)
Roots respiration rate	βrt	0.01	g g^−1 day^−1	Lorber et al. (1984)
Shoots mortality rate	γsht	0.001	g g^−1 day^−1	Eid et al. (2016)
Roots mortality rate	γrt	0.001	g g^−1 day^−1	Eid et al. (2016)
Arrhenius constant	θ	1.07		Soetaert et al. (2004)
Conversion constant of CO2 to dry matter	Kco	0.83	g g^−1 CO2	Perdomo et al. (2008)
Maximum photosynthetic growth rate	Pmax	0.283	g CO2 g^−1 day^−1	Eid et al. (2016)
Half saturation constant for the photosynthetic active radiation	KPAR	11.1	MJ m^−2 day^−1	Sale et al. (1985)
Light extinction coefficient within the canopy	Kle	0.019	m^2 m^−2	Eid et al. (2016)
Half saturation constant for nitrogen uptake	KN	0.15	mg N L^−1	Hu et al. (1998)
Half saturation constant for phosphorus uptake	KP	0.01	mg P L^−1	Hu et al. (1998)
Maximum roots growth rate	Grt	0.15	g g^−1 day^−1	Tucker (1983)
Half saturation constant of the effect of age on the roots growth rate	Krt	12.0	Days	Eid et al. (2016)

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