Treeline spatial structure appears to determine whether a treeline is responsive to climate change: abrupt treelines tend to respond slowly/not at all, whereas diffused treelines respond rapidly. Even though treeline structure seems to be a key variable, the mechanisms responsible for creating the spatial structure are poorly understood. Mathematical models indicate the spatial structure can be determined by local neighbor-neighbor inhibitive and facilitative interactions. Here we present a field study on the relationship between growth and distribution of seedlings and saplings in relation to distance from neighbors within different zones of the ecotone and snow depth. Our field site is an abrupt treeline located on Pikes Peak, CO. Based on prior observations, we divided the ecotone into four zones: forest, lower-sheltered, upper-sheltered, and tundra. After mapping all seedlings at the 200m wide field site (n=2,438), we produced drone-derived orthomosaics to digitize sapling and tree canopies, and create an accurate snow-depth raster. Additionally, we measured the growth, damage, and height of all seedlings and saplings in a 30m wide AOI.
Results/Conclusions
Overall, the largest proportion of seedlings are found in the upper-sheltered zone, however the proximity to adult tree neighbors also played a role in their distribution. In the lower-sheltered zone, seedlings were significantly underrepresented in 1m buffers surrounding small and large trees (chi=33.40, p<0.00001 and chi=198.40, p<0.00001, respectively). This pattern remains true in the upper-sheltered zone, showing a weaker underrepresentation of seedlings in 1m buffers surrounding small and large trees (chi=27.54, p<0.00001 and chi=40.44, p<0.00001, respectively). This indicates the prevalence of competitive neighbor-neighbor interactions. Mortality rates increased steadily uphill from the forest (2.43%) to the tundra (16.32%). Additionally, mortality was strongly influenced by snow depth, with lowest mortalities at intermediate snow depths, best shown through a resulting U-shaped distribution curve produced by a logistic regression (R2=0.832, p=0.12, F=12.373, df1=2, df2=5). Snow depth itself varied across the ecotone, with the border between the lower and upper sheltered zones harboring the deepest snowpack. Analysis of tree growth showed that seedlings and saplings were growing slowest in the lower-sheltered zone and fastest in the upper-sheltered zone.