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Stilt-Root Walking by an Iriateoid Palm in the Peruvian Amazon

1980; Wiley; Volume: 12; Issue: 1 Linguagem: Inglês

10.2307/2387775

1744-7429

John H. Bodley, Foley C. Benson,

Plant Parasitism and Resistance

The aerial stilt roots of Socratea exorrhiza allow this palm literally to pick itself up and walk out from under fallen limbs and obstacles that are major hazards for immature palms. The lower trunk and older roots rot away and are left behind as the tree sends out new roots and moves away from its germination point. THE IRIARTEOID PALMS of the American tropics form a unique group distinguished by their prominent stilt roots (fig. 1). These roots are particularly well developed in Socratea exorrhiza (Martius) Wendland [Iriartea exorrhizaj.1 In this species (fig. IA) the base of the trunk may be elevated more than a meter from the ground and is supported by open cone of spiney stilt roots. According to Corner (1966: 9192), stilt roots are an unexplained deviation from the normal method of trunk-making, but he hints at a possible relationship to water when he states-They grow in swampy places and elevate the trunk above the normal floods.... To our knowledge, no one has presented convincing evidence that stilts are adaptation to flooding, nor have any other explanations of either the origin or function of these remarkable structures been suggested. During intensive field study of palm cultural ecology in eastern Peru2 we observed that the stilt roots of Socratea exorrhiza allow seedlings and juveniles that are flattened by falling trees, limbs, or palm fronds to right themselves and walk out from under the obstacle. They also support seedlings that lean toward light, as when a plant germinates at the base of a large tree. We apply the term walking to these phenomena because of the leg-like action of the stilts, and because the plant eventually straightens itself at a new location, but we do not mean to attribute purpose to the plants. The special significance of the stilts is that they allow a tree to establish itself at some distance from its original germination point because the oldest roots and lower trunk are left behind as they are broken or rot away. The lower trunk quickly becomes superfluous as Socratea grows and the bulk of the roots supersedes that of the trunk. Each new stilt root emerges from the trunk above the previous root. The soft new roots turn from rufous to chocolate brown, and then gray as they mature. They are very dry and hard at maturity and are covered with sharp spines. In a mature adult of 15 m in height, we observed a root grow average of 70 cm/month over a three-month period until it reached the ground. Three months later it was firmly anchored in the ground. The newest root in a walking juvenile with a trunk length3 of 172 cm grew 22 cm in one month. The most dramatic moves occur when large juveniles are knocked down (see fig. 2C and F and the reconstruction in fig. 4). Here the distance moved is determined by the length of the trunk when it is downed. The trunk arches gradually upward as it grows, bending sharpy at the base of the crownshaft (see fig. 3B). New roots emerge along the horizontal trunk and elevate it off the ground as they elongate. Based on trunk measurements, leaf scar counts, and leaf production measurements4 in undisturbed 1Various names have been applied to this species in the literature. Moore et al. (1960) tentatively identified it as Socratea Orbignyana, and he was followed in this by Jordan (1970). MacBride (1960) and Glassman (1972) have assigned it to Iriartea exorrhiza. We prefer to retain the earlier name Socratea exorrhiza (Martius) Wendland in order to emphasize the striking differences in the stilt roots between this species and Iriartea ventricosa. Locally S. exorrhiza is known as Cashapona. 2Fieldwork was conducted in 1967-1977 in dry tropical forest within a 5 km radius of the Shipibo Indian village of San Francisco on the north end of Lake Yarinacocha, 16 km northwest of the city of Pucallpa in Loreto Department, Peru. 3A11 trunk measurements were taken from the trunk base at the ground to the base of the newest leaf sprout. The crownshaft is included but not emerged leaves. 4This finding assumes average annual leaf production rate of 2.71 leaves obtained from observations of 22 tagged individuals over a six-month period. Trunk and leaf scar measurements were obtained for 42 individuals. All ages given are approximations based on these data. BIOTROPICA 12(1): 67-71 1980 67 This content downloaded from 157.55.39.27 on Mon, 05 Sep 2016 06:14:50 UTC All use subject to http://about.jstor.org/terms