Phenotypic plasticity of two invasive alien plant species inside a deciduous forest in a strict nature reserve in Italy

Figures & data

Figure 1. Trend of mean air temperature (T_mean), minimum air temperature (T_min), maximum air temperature (T_max) and rainfall during the study period (May–October).

Table 1. List of the analyzed leaf traits at morphological, anatomical, and physiological level in Ailanthus altissima and Robinia pseudoacacia.

Leaf trait		unit
Leaf surface area	LA	cm^2
Leaf dry mass	DM	mg
Specific leaf area	SLA	cm^2 g^−1
Total leaf thickness	LT	µm
Palisade parenchyma thickness	PP	µm
Spongy parenchyma thickness	SP	µm
Adaxial cuticle and epidermis thickness	CETad	µm
Abaxial cuticle epidermis thickness	CETab	µm
Net photosynthetic rates	PN	µmol(CO2) m^−2 s^−1
Stomatal conductance	gs	mol(H2O) m^−2 s^−1
Transpiration rates	E	mmol(H2O) m^−2 s^−1
Leaf respiration rates	RD	µmol(CO2) m^−2 s^−1
Ratio between leaf respiration rates and net photosynthetic rates	RD/PN
Relative chlorophyll content	RCc	SPAD unit
Maximum PSII photochemical efficiency	Fv/FM
Actual quantum yield of photosynthesis of light-adapted leaves	ɸPSII
Electron transport rates	ETR	µmol(e-) m^−2 s^−1

Table 2. Soil characterization in sun and shade conditions. SWC – soil water content; N – total soil nitrogen content; C/N – ratio between carbon and nitrogen content; SOM – soil organic matter content. Mean values (± SE) are shown (n = 3).

Parameter	Sun	Shade
SWC[%]	17.0 ± 1.0	25.2 ± 3.3
pH	5.15 ± 0.03	5.50 ± 0.05
N[%]	0.25 ± 0.05	0.17 ± 0.06
C/N	9.2 ± 1.3	17.8 ± 0.8
SOM [%]	3.3 ± 0.3	5.2 ± 0.3

Table 3. Morphological and anatomical leaf traits of sun and shade conditions in Ailanthus altissima and Robinia pseudoacacia. CET_ad – adaxial cuticle and epidermis thickness; CET_ab – abaxial cuticle and epidermis thickness. Mean values (± SE) are shown (n = 50). Different letters indicate significant differences between sun and shade conditions within each species (T-test, p < 0.05).

	A. altissima		R. pseudoacacia
	sun	shade	sun	shade
	Mean ± SE	Mean ± SE	Mean ± SE	Mean ± SE
Leaf area (cm^2)	18.3 ± 0.5a	21.1 ± 0.5b	47.5 ± 1.2a	203.2 ± 5.5b
Specific leaf area (cm^2 g^−1)	192.6 ± 13.0a	327.2 ± 15.4b	135.1 ± 1.2a	344.7 ± 18.8b
Leaf thickness (µm)	144.9 ± 1.5a	130.8 ± 4.7b	150.5 ± 2.2a	92.2 ± 4.5b
Palisade parenchyma thickness (µm)	69.7 ± 1.2a	54.6 ± 1.5b	101.5 ± 2.1a	43.1 ± 3.4b
Spongy parenchyma thickness (µm)	47.9 ± 1.1a	48.4 ± 3.3a	27.9 ± 2.1a	32.1 ± 3.6a
CETad (µm)	15.7 ± 0.6a	15.3 ± 1.5a	12.0 ± 0.4a	9.1 ± 0.2b
CETab (µm)	11.7 ± 0.5a	10.9 ± 0.3a	9.1 ± 0.7a	8.0 ± 0.7b

Figure 2. Regression analysis between specific leaf area (SLA) and total leaf thickness (LT). Regression equation, determination coefficient (R²) and significance level (p) are shown. (∆ = Ailanthus altissima shade condition; ◊ = Robinia pseudoacacia shade condition; ▲ = Ailanthus altissima sun condition; ♦ = Robinia pseudoacacia sun condition). The perpendicular, broken line separates the values of shade leaves (upper left part) and sun leaves (lower right part).

Figure 3. Trend of (a) net photosynthetic rates (P_N), (b) leaf dark respiration (R_D) during the study period in sun and shade conditions for Ailanthus altissima and Robinia pseudoacacia. Each point is the mean (±S.E.) of 15 leaves in three sampling days per month. Different letters indicates significant differences (ANOVA, p ≤ 0.05) during the study period.

Figure 4. Regression analysis between net photosynthetic rates (P_N) and relative chlorophyll content (RCc). Regression equation, determination coefficient (R²) and significance level (p) are shown.

Figure 5. Trend of A) maximum PSII photochemical efficiency (F_V/F_M) B) quantum yield of photosynthesis of light-adapted leaves (ɸ_PSII) during the study period in sun and shade conditions for Ailanthus altissima and Robinia pseudoacacia. Each point is the mean (±S.E.) of 15 leaves in three sampling days per month. Different letters indicates significant differences (ANOVA, p ≤ 0.05) during the study period.

Table 4. Plasticity index for the anatomical, morphological, and physiological leaf traits of Ailanthus altissima and Robinia pseudoacacia. The anatomical (PI_a), morphological (PI_m), physiological (PI_p) plasticity index, are shown. PP – palisade parenchyma thickness; SP – spongy parenchyma thickness; CET_{ad –} adaxial cuticle and epidermis thickness; CET_{ab –} abaxial cuticle and epidermis thickness; LT – total leaf thickness; LA – leaf area; DM – dry mass; SLA – specific leaf area; E – transpiration rate; g_s – stomatal conductance; P_N – net photosynthetic rate; R_D – respiration rate; R_D/P_N – ratio between R_D and P_N; RCc – relative chlorophyll content; F_V/F_M – maximal quantum yield of PSII photochemistry; Φ_PSII – effective quantum yield of PSII photochemistry; ETR – electron transport rate. Bold font indicates leaf traits that significantly differed (p < 0.05) between sun and shade conditions within each species.

	A. altissima	R. pseudoacacia
Anatomical leaf traits	Plasticity index	Plasticity index
PP	0.22	0.58
SP	0.01	0.13
CETad	0.03	0.24
CETab	0.06	0.12
LT	0.10	0.39
Mean PIa	0.08	0.29
Morphological leaf traits
LA	0.13	0.77
DM	0.33	0.42
SLA	0.41	0.61
Mean PIm	0.29	0.60
Physiological leaf traits
E	0.28	0.61
gs	0.75	0.80
PN	0.74	0.80
RD	0.46	0.23
RD/PN	0.52	0.74
RCc	0.23	0.04
FV/FM	0.04	0.07
ɸPSII	0.22	0.21
ETR	0.81	0.93
Mean PIp	0.45	0.49