Treelines can serve as model ecotones in their response to climate change. However, the role of tree architecture at treelines is poorly understood. This paper examines tree architectures at a fast-migrating diffuse treeline in a bowl on the western slope of Pikes Peak (Colorado). Investigating the spatial distribution of the allometric types, the relationship between the growth rate and height for each architecture type, and the impacts of the changing climate on the architectural spatial distribution. The study site was divided into an Upper Zone (UZ) and Lower Zone (LZ). We found multiple distinct architectures within this diffuse treeline. Unexpectedly, tree architectures did not follow a spatial distribution pattern of clustering or avoiding with like and or different architectures. Krummholz and Cone architectures were found growing in close proximity to one another, signifying that the upper climatic boundary at this site has advanced up in elevation. These multiple architectures are able to represent current and past climatic conditions. Advancement is occurring at such rapid rates that tree established architectures are not able to release from their path dependency. To my knowledge, this is the first study that examines multiple tree architectural types within a treeline and how they are distributed in space.
Spatial structure of alpine treeline plays a key role in its response to climate change, yet the processes that are responsible for creating that structure are poorly understood. Here, we describe a treeline on the west side of Pikes Peak with different tools to gain insight of the structure at a local scale and to investigate the potential endogenous mechanisms that appear to influence it. We hypothesized that the trees will be clustered in the system through positive intraspecific interactions and that the treeline is a potential phase transition system with classical criticality. With the classical description of treeline structure, we divided the zone into different sections of increasing elevation range with the assumption that the mechanisms driving the structure of each are the same, and ran Ripley’s K function for cluster analyses. Ripley’s K function showed significant tree clustering through the sections in the study area, especially between the elevation of 3600m and 3680m. Clustering among large-sized trees were significant across the entire treeline. To examine the potential of the treeline as phase transitions, which allows us to treat the system as continuous rather than in sections, we analyzed presence of fractal structure in the treeline. Analysis of the spatial structure reveals significant fractal geometry in size classes, as well as on the edges of big tree clusters which has the potential to evolve into a percolation cluster. The analyses show evidences that the system could more likely be in robust criticality rather than phase transition. This study provides insights into describing the treeline on a local scale and could contribute to the current study of treeline dynamics and treeline as a complex system.
Ecotone transitions offer a rare opportunity to examine these spatial patterns along known stress gradients. This allows us to link specific patterns that exist at the different stages of bifurcation to potential mechanisms that create these complex spatial structures. This paper applied robust criticality theory the alpine forest-tundra ecotone transition to see how treelines can be understood through the framework of critical transitions, and concurrently, how robust criticality theory can be applied to an anisotropic system to predict potential threshold behavior in treeline movement upslope. Spatial structure of an abrupt treeline on Pikes Peak, CO was analyzed using Fragstats and ImageJ. The presence of robust criticality was tested using an AIC model fit test. Model analysis showed that this treeline clearly exhibits a type of robust criticality with the existence of a percolation cluster and deviations of log frequency-size distribution of patches along the elevation gradient from a strict power law. However, it did not conform to the theory perfectly. This likely points to the problem of accounting for multiple stressors that exert their influence over the dynamics of treeline to different extents in different zones. It is this complex matrix of local feedbacks created by endogenous interactions between the harsh environmental conditions and the trees that produces the dynamic spatial structure at this site along the elevation gradient. This has important implications for how the treeline will react in the future to increasing temperatures and decreasing snowpack as predicted by climate change models in the Rock Mountain West.