Competition between two desert varieties of Medicago laciniata (L.) Mill. under controlled conditions

Two varieties of Medicago laciniata common in the Negev desert of Israel reveal a distinct pattern of distribution within two different plant communities: M. laciniata var. laciniata occupies northern slopes and wadi beds within the association of Artemisia herba-alba, and M. laciniata var. brachyacantha occupies more arid southern slopes within the Zygophylletum dumosi association. Recent field trials have shown that interaction within the plant community restricts distribution of the two varieties, particularly on the northorn slopes.Using the de Wit model on competition, an attempt was made to determine whether the distribution found in nature can be explained on the basis of competition for space. Intervariety competition was measured under controlled conditions in a phytotron under two water regimes. It was found that under both "wet" and "dry" conditions, the two varieties utilize the same space (RYT=1) and similar quantities of water, and both produce similar amounts of dry matter. According to the total dry matter, the varieties have about the same competitive ability under wet as well as under dry conditions.However, under the dry treatment the relative crowding coefficient, based on seed yield, is very different from one: k _lb>2. The relative reproductive rate α_lb < 1, found under both water regimes, indicates that var. brachyacantha will replace var. laciniata through competition for the same space. This is not in accordance with the observations that var. brachyacantha is absent in the less arid northern slope. A possible explanation is discussed.Reciprocal thinning of the one variety from various mixtures grown under the "dry" regime, when followed by irrigation to field capacity, sharply increases the consumption of water by the plants left, proportional to the number of plants removed. This does not occur in cultures grown under the "wet" regime. It is suggested that such competition for water under optimal water conditions may be due to the occurrence of associations of roots. The possible formation of root associations and its ecological significance of their effect on water consumption are discussed.