Poplar hybrids were grown with irrigation in a large-scale plantation
to investigate the mechanisms underlying clonal differences in drought
resistance. Beginning in spring 1992, Populus trichocarpa x P. deltoi
des (TD) and P. deltoides x P. nigra (DN) cuttings received 46, 76, or
137 cm year(-1) of irrigation to supplement the 18-20 cm of annual pr
ecipitation, and all trees received the same fertilization regime. Ste
m volume, assessed as the square of stem diameter at breast height tim
es tree height ((DH)-H-2), and water relations of the trees were studi
ed from the end of their second growing season until the end of their
fifth growing season. By the end of the second growing season, stem vo
lume of Clone TD was 40-146% larger than that of Clone DN, but stem vo
lume growth was independent of irrigation in excess of 46 cm year(-1)
in both clones. During the third growing season, stem volume growth of
both clones was limited by both the 46- and 76-cm irrigation treatmen
ts, so that by the end of the third growing season trees in the 46-cm
irrigation treatment were only half the size of trees in the 137-cm ir
rigation treatment. These treatment differences were maintained throug
h the fifth growing season. Although stem volumes of Clone TD trees in
the 76- and 137-cm irrigation treatments were larger than the corresp
onding values for Clone DN trees at the end of the third growing seaso
n (1994), these clonal differences gradually decreased in subsequent y
ears and were not detectable after 5 years, because stem volume relati
ve growth rate of Clone DN was greater than that of Clone TD in all tr
eatments. Although both clones exhibited similar predawn leaf water po
tentials, Clone DN typically maintained higher midday leaf water poten
tials, suggesting better stomatal control of water loss. Clonal and tr
eatment differences in osmotic potential at full turgor were minimal a
nd could not explain the clonal differ ences in drought resistance. Ro
ot density and root density to stem volume ratio increased more in res
ponse to moderate drought in Clone DN than in Clone TD, resulting in e
nhanced drought resistance (high stem volume growth rate under moderat
e drought conditions) and an increased capacity to withdraw water from
the soil. We conclude that the greater drought resistance of Clone DN
compared with Clone TD was the result of the maintenance of a more fa
vorable water balance by stomatal regulation and greater carbon alloca
tion to roots during the early stages of drought. However, the low roo
t density to stem volume ratio in Clone DN growing in the 46-cm irriga
tion treatment suggests that severe water limitation restricted the pr
eferential allocation of carbon to belowground tissues, so that both r
oot and shoot growth were constrained by severe drought.