Table 3.

Percentage of roots with secondary development and number of growth rings for individual root orders (I–V) across nine North American tree species growing in a common garden contrasting in shade tolerance

Family	Species	Shade tolerance	I		II		III		IV		V
			Secondary development	No. of rings	Secondary development	No. of rings	Secondary development	No. of rings	Secondary development	No. of rings	Secondary development	No. of rings
Sapindaceae
	Acer negundo	Low	–	–	–	–	–	–	100	0.6	100	2.3
	Acer rubrum	Moderate	–	–	–	–	30	–	100	0.5	100	1.9
	Acer saccharum	High	–	–	20	–	100	0.3	100	1.5	100	3
Betulaceae
	Betula lenta	Low	–	–	29	–	87	–	100	–	100	–
	Betula alleghaniensis	Moderate	–	–	–	–	10	–	90	–	100	1.5
Fagaceae
	Quercus rubra	Low–moderate	–	–	–	–	20	–	100	–	100	0.8
	Quercus alba	Moderate	–	–	–	–	60	–	100	0.4	100	2.2
Pinaceae
	Pinus virginiana	Low	–	–	–	–	100	0.4	100	0.7	100	1.3
	Pinus strobus	Moderate	–	–	–	–	100	–	100	–	100	1

Ten roots from each branch order from plots were selected for observations.

(–) indicates that no roots in the designated root order had secondary development or growth rings. Mean square error was ±0.16 for secondary development based on percentile data and ±0.32 for no. of rings.