Table VI. Mechanistic trait analysis.

Standardized slopes, expressed as percentages, for relationships between leaf anatomical traits and K_ox are shown. The mech values are mechanistic effects (i.e. relationships obtained by varying individual parameters in isolation), whereas corr values are for correlations between parameters and K_ox across species. Sig (corr) is the significance of across-species correlations (***, P < 0.001; **, P < 0.01; *, P < 0.05; and ns, P > 0.05). r² (corr) is the correlation coefficient for correlation slopes. For palisade and spongy thicknesses, mechanistic effects were assessed using the palisade-spongy thickness ratio; likewise, mechanistic effects for distances between the BS and epidermis were assessed using the ratio of distances above/below the BS.

Parameter	Symbol	Slope (mech)	Slope (corr)	Sig (corr)	r2 (corr)
Cell wall thicknesses
BS cell wall thickness	tabs	15	−8	ns	0.00
Epidermal cell wall thickness (lower)	tael	0	30	ns	0.01
Epidermal cell wall thickness (upper)	taeu	0	37	ns	0.02
Palisade cell wall thickness	tap	6	23	ns	0.00
Spongy cell wall thickness	tas	36	13	ns	0.00
BSE cell wall thickness	tax	13	51	*	0.06
Cell-scale parameters
Height of individual palisade cell	hp	−5	−64	*	0.05
BS cell perimeter	pbsc	−32	−47	ns	0.03
Palisade radius	rp	−2	−55	**	0.10
Spongy radius	rs	−30	−73	***	0.19
Width of epidermal cells (lower)	wel	0	−48	**	0.10
Width of epidermal cells (upper)	weu	0	−21	ns	0.01
Width or height of one BSE cell	wx	−12	6	ns	0.00
Tissue-scale parameters
Distance from BS to lower epidermis	hxltot		−31	**	0.09
Distance from BS to upper epidermis	hxutot		48	**	0.09
Total perimeter of vascular bundle	pbs	22	−3	ns	0.00
Epidermis thickness lower	tel	−18	13	ns	0.00
Epidermis thickness upper	teu	11	1	ns	0.00
Palisade thickness	tp		10	ns	0.00
Spongy thickness	ts		−22	ns	0.03
Total width of BSE	wxtot	14	39	**	0.08
Dimensionless parameters
Maximum palisade horizontal connectivity	fcph	0	−1	ns	0.00
Palisade vertical connectivity	fcpv	0	−1	ns	0.00
Spongy mesophyll connectivity	fcs	24	−90	***	0.17
Leaf airspace fraction in palisade	pp	3	−37	**	0.07
Leaf airspace fraction in spongy	ps	22	31	ns	0.02
Ratio of palisade to spongy thickness	–	−4	67	***	0.14
Ratio of distances above/below BS	–	34	59	***	0.21
Leaf-scale parameters
Total leaf thickness	–	−18	−4	ns	0.00
Vein length per unit area	VLA	110	121	***	0.47