DROUGHT RESISTANCE OF 2 HYBRID POPULUS CLONES GROWN IN A LARGE-SCALE PLANTATION

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.