The mechanisms that determine the spatial structure of ecotones are not well understood. The structure of an alpine treeline ecotone is likely determined by plant physiology and the underlying harshness gradient, and these influences are modified by neighbor interactions. The stress-gradient hypothesis postulates that neighbor interactions are inhibitory at the benign end of the gradient and facilitatory at the harsh end of the gradient. In addition, neighbor interactions can influence different aspects of life history (e.g. establishment and growth) in different ways independently. Therefore, through neighbor interactions the current spatial pattern of a treeline will influence the future pattern. In order to better predict the future structure of a treeline, it is essential to understand the current impacts of neighbor interactions. This project hypothesized that seedling establishment and growth would be influenced by competition in vicinity to adult neighbors at benign portions of the gradient and by facilitation at the harsh portions of the gradient. The study analyzed seedling establishment and growth with respect to canopies of adult trees (derived from a drone based orthomosaic) at a 380m by 90m (divided into four zones) treeline site on Pikes Peak, CO.
Results/Conclusions
The results partially supported the stress-gradient hypothesis. Neighbors influence seedlings establishment and growth differently. In the lower two zones (forest and transition 1) seedlings preferentially established away from adult trees. The number of seedlings established within a 1m buffer around adult trees was less than expected on random (forest: χ²=10.529, p=0.0017 & transition 1: χ²=60.691, p<0.00001). In the transition 2 and the tundra zones, seedlings established randomly (transition 2: χ²=0.0492, p=0.0825 & tundra: χ²=0.8205, p=0.365). Data regarding 1yr seedling growth showed seedlings did not grow differently outside or inside the buffers in any of the zones or across the entire study site. For 10yr growth, seedlings grew somewhat better outside the 1m tree buffer across the entire site (t=-1.804, df=224, p=0.073). In the forest and transition 1 there was no significant difference between seedling growth inside and outside the buffers. In transition 2, seedlings grew significantly better within the buffers (t=2.552, df=33, p=0.016). Across the entire site, the growth in the forest and tundra was lower than the growth in the two transition zones (Oneway ANOVA: F (3,222) = 32.936, p<0.005). The results of this study indicate that signals of neighbor influence are only detectable over longer growth periods.