Established perennial grasses reduce the growth of emerging subterranean clover seedlings through competition for water, light, and nutrients

Abstract

The effect of defoliating mature phalaris (Phalaris aquatica L.) plants on the growth and turgor of subterranean clover (Trifolium subterraneum L.) seedlings was examined at high and low rates of watering in tubs under a rain shelter with and without root competition. Clover germination was stimulated by applying 36 mm of water to seeds planted in the interspatial areas between phalaris plants, followed by either 12 mm (high) or 3 mm (low) of water every 2 days to simulate good or poor moisture conditions following emergence. The effect of 3 grasses (Phalaris aquatica, Dactylis glomerata L., and Danthonia richardsonii Cashm.) in drying the soil surface was confirmed in a field experiment following 41 mm of rainfall in early March. The percentage of light reaching the clover was twice as high (P < 0· 001) at seedling emergence in defoliated phalaris than undefoliated phalaris. The proportion of light reaching the clover increased from 40% to 70% during the experiment in the undefoliated low water treatment but remained low (33-48%) in the high water treatment. In the presence of root competition, defoliation under conditions of high water resulted in more water for the clover (>17%) in the top 5 cm of soil compared with undefoliated phalaris (13%). Defoliated phalaris with low water also maintained more favourable soil water conditions than undefoliated phalaris, but only for 5 days. Excluding root competition increased soil moisture levels from about 8% to 14% in treatments receiving low water. Defoliation increased (P < 0·05) clover seedling weight at 14 and 21 days after emergence in both the high and low water treatments, by a mean of 0·3 mg/day. In the absence of root competition and at low water, clover seedlings in the defoliated phalaris were twice as large (P < 0·001) 14 days after emergence and had a higher relative water content (RWC) (55% v. 40%) than seedlings in undefoliated swards. At low water, the RWC of clover was higher in the absence of root competition from phalaris than it was in its presence, but competition had no effect on RWC in the high water treatments. Available soil nitrate was significantly (P < 0·001) higher (160 µg N/g) in the absence of phalaris root competition than in its presence (3 µg N/g). The field experiment demonstrated that, in March, the 3 grasses dried the soil more rapidly (P < 0·01) (losing 9·8% of the remaining soil water/day) than pure subterranean clover (6·1%), which had little green biomass. Following later rains in May, when there was >400 kg/ha of green biomass (expressed as dry matter) in both the perennial and annual swards, the surface soil dried at the same rate regardless of whether the sward was perennial or annual. Scavenging for water by the perennial roots was an important factor in drying the surface soil in both March and May and there appeared to be no benefits from shading of the surface by the perennials. The results showed that early growth of clover seedlings in phalaris swards is reduced by a combination of competition for water, soil nitrate, and light. Defoliation of phalaris reduces the loss of water from the topsoil, increases clover seedling size, and, where water is limiting, increases seedling turgor.