Treeline is not simply determined by physiological tree limitations as temperature decreases with increasing elevation. Endogenous mechanisms play an important role in shaping the structure and dynamics of treelines. In the harsh alpine environment, positive feedbacks operate on a local scale and create large scale self-organized spatial patterns. The spatial structure of tree canopy clusters provides insight into the internal mechanisms of the system. Since treelines are ecotone boundaries, they can be conceptualized as phase transitions. However, so far, no studies have empirically analyzed treeline ecotones using critical transition theory. We tested for the presence of robust criticality at an abrupt and a diffuse treeline on Pikes Peak, Colorado. We used aerial photogrammetry to analyze tree canopy shape and connectedness. The two sites were split into zones based on elevation and ecologically relevant characteristics. Inverse cumulative frequency versus patch size distributions were used to quantify the spatial structure of tree clusters through the ecotone transition. The presence of robust criticality was tested using an AIC model fit test in R that compared a power law, power law with an exponential tail, and an exponential function to the data.
Results/Conclusions:
Both treelines displayed criticality in a large range of parameter space as evident by the fractal dimensions and power-law distributions of the spatial structure, likely caused by the role of local interactions. Analysis showed that the diffuse treeline closely conformed to the theory of robust criticality. With increasing environmental stress, the power law frequency-size distribution present in the lowest elevation zone disintegrated gradually into a power law with an exponential tail, and finally, into an exponential function at the highest elevation zone. However, the abrupt treeline displayed a more complex story. The underlying forest has a bimodal distribution of patch sizes. A percolation cluster formed at the edge of the forest as the density increased to 60%, which then turned into a power-law with an exponential tail as elevation increased. The differences in the spatial structure of the two treelines could be linked to the strength of the positive feedbacks operating at each site. This has important implications for how the treeline will react to future increases in temperature and decreases in snowpack as predicted by climate change models in the Rock Mountain West, and may inform predictive models of other ecotone boundary dynamics under climate change.