Sago (Metroxylon sagu Rottb.) growth is affected by weeds in a tropical peat swamp in Thailand

Abstract

The effect of weeds on sago palms (Metroxylon sagu Rottb.) growing in tropical peat soils was examined in the field and in pots. In the field, 31 species of weeds were found, however, Fimbristylis umbellaris (Cyperraceae) and Leersia hexandra (Gramineae) were the most abundant species. Sago heights and weed populations seemed to be correlated. The field experiment showed that high dense L. hexandra was less positively correlated to sago growth, while high dense F. umbellaris was moderately negatively correlated. A study of the effects of F. umbellaris and L. hexandra was required because of the complexity of the weed composition in the field, and also because of the difficulties in evaluating which species have individually affected sago growth. In a pot experiment, L. hexandra accumulated high amounts of P and Zn, and markedly suppressed the growth of sago palms compared with F. umbellaris. In contrast, sago withstood weed competition by increasing root dry matter, however, yellowing of the leaves occurred in response to the low concentration of N observed in sago tissues. Sago palms planted with L. hexandra, F. umbellaris or weeds themselves showed a significant correlation with N2-fixing symbioses by having higher foliar δ15N than soil δ15N. Leersia hexandra demonstrated the highest correlation with N2-fixation symbioses. This capability of L. hexandra may be the reason that it is a stronger competitor for sago growth when compared with F. umbellaris. In conclusion, N2-fixation symbioses are more important than soil nutrition under N-limited peat soil. Moreover, N2-fixation symbioses could be induced during the competition of plant species rather than in the absence of competitive plants.