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Field observations indicate that conditions at the stolen/soil surface interface are important for the survival of white clover (Trifolium repens) during drought. Mini-swards of clonal material of two contrasting white clover cultivars (Grasslands Tahora, small-leaved, densely branched; Dusi, large-leaved, tap-rooted) were established in boxes (300 x 420 x 290 mm deep) for 6 months, then subjected to a 6-week drought. Stolen/soil surface conditions were modified to have stolons either, 1) fully exposed, 2) shaded, or 3) buried. Numbers and, characteristics of surviving plants were measured. The major effect was on the number of plants surviving. More plants survived under shading (55%) than burial (25%) and full exposure (17%). Numbers of plants surviving drought was related to pre-drought plant density, hence Tahora had higher plant survival than Dusi, particularly when fully exposed. Although initially Dusi plants were 25 times heavier, with 5 times the number of at the expense of leaf growth (Turner 1990, 1991). As severity of the drought increases, stolons tend to die back to strongly rooted nodes, and the frequency of plant death increases, resulting in a decline in population density and long-term losses. These effects are more severe if the drought occurs early in the season (Brock 1988), when plants are still small and recovering from their normal mid-spring fragmentation (Brock et al. 1988). For instance, in Canterbury, Vartha & Hoglund (1983) reported a 30% loss of white clover production from a summer drought (January-February), but a 50% loss from a spring drought (October).in two consecutive seasons. Again, a 6-week drought in late spring-early summer (November-December) in the Manawatu, (Brock 1988) caused a 70% loss in production and a 85% loss of stolen material in a rotationally grazed pasture which subsequently took 3 years to recover, but by contrast, in continuously set stocked pastures the white clover was largely unaffected (apart from loss of leaf), with immediate recovery following rain. Differences in pasture architecture affecting the microenvironment of stolons were thought to be responsible stolons-and-roots-of-Tahora,suwiviving-plants-of-for.~is.m~agement effect. Highpasture cover produced by set stocking intercepted direct solar radiation, shading both cultivars were small and similar in dry weight, although Tahora had higher numbers of growing points and roots thanDusi. The tap-rooted character of Dusi did not. confer any advantage during drought. It was concluded that the main factor influencing drought survival of white clover appears to be grazing management. Systems that can produce dense white clover populations and maintain high pasture density (grass) capable of providing shelter from direct solar radiation during drought stress, are superior. stolons and cooling the soil surface (4O”C), whereas the low density rotationally grazed pastures, with the large areas of bare soil exposed after grazing, resulted in higher soil surface temperatures (50°C) and high stolen death. Under rotational grazing, surviving clover plants were associated with the shelter of large grass clumps (Brock & Hay 1993).

Influence of the stolonkoil surface interface and plant morphology on the survival of white clover during severe drought