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. 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 marking all living, dead, and dying seedlings in a 200-meter wide AOI (n=2,438) we produced a drone-derived orthomosaic to: map the distribution of the marked seedlings, and digitize canopies of saplings and adult trees through the ecotone. We measured the growth, damage, and height of all seedlings and saplings in a smaller 30-meter wide AOI within the larger AOI.
Results/Conclusions
Our results showed that: seedlings were restricted to the lower and upper-sheltered zones and nearly absent from the tundra, dead seedlings were statistically overrepresented in the lower-sheltered zone. Within the lower-sheltered zone, seedlings and saplings were growing significantly slower compared to other zones. In addition, seedlings were significantly underrepresented in 1m buffers (chi-squared = 257.3, df = 1, p<.0001) and 2m buffers (chi-squared = 319.2, df = 1, p<.0001) around adult trees and saplings. This indicates the prevalence of competitive neighbor-neighbor interactions. Within the upper-sheltered zone seedlings were growing better in comparison to other zones. Saplings were growing better than in the lower-sheltered zone, but similarly well to the tundra. Seedlings were overrepresented in 1m buffers (chi-squared = 31.9, df = 1, p<.0001) around saplings (there were no adult trees), but underrepresented in 2m buffers (chi-squared = 40.7, df = 1, p<.0001). This indicates short-range facilitation and long-range inhibition that may eventually create tree islands in this zone. In the tundra, seedlings were growing less well in comparison to the upper-sheltered zone. The seedlings were overrepresented in the 1m buffers around saplings (chi-squared = 94.6, df = 1, p<.0001).