PLASTIC ALLOMETRY IN YOUNG SUGAR MAPLE (ACER-SACCHARUM) - ADAPTIVE RESPONSES TO LIGHT AVAILABILITY

Age, height, number of shoot endings, neighbor density, and overhead c over from local neighbors were recorded for over 1,400 young sugar map le trees under 200 cm tall and growing in open vs. closed (shaded) hab itats from two sites. Up to about age 5 yr, seedlings increase in heig ht but rarely branch. Once branching starts, there begins a general de crease with age in the number of centimeters of height added for every new shoot ending that is produced. The pattern of this allometric rel ationship, however, is plastic-in the closed habitats, this switch to increased branching relative to height growth is delayed compared with trees from the open habitats. Within the closed habitats, branching i s also delayed for trees having dense cover from local neighboring und erstory vegetation compared with trees that are not overtopped by unde rstory vegetation. The oldest and tallest unbranched seedlings were re corded from the closed habitats. Seedlings displaying the first branch , however, were younger in the open habitats than in the closed habita ts, and at one site they were shorter in the open habitat than in the closed habitat. These results suggest that the allometric relationship between height and degree of branching can display adaptive plasticit y depending on light availability: As competition for light decreases (with greater light availability), there is an increasing premium on l ateral growth (branching) to maximize light interception. Conversely, as competition for light increases (under decreasing light availabilit y), there is an increasing premium on vertical growth (through strong and persistent apical dominance) to minimize the chances of being over topped by neighbors.