Upper and lower treeline biogeographic patterns in semi-arid pinyon-juniper woodlands

Aim: Upper and lower treelines are particularly exposed to a changing climate. It has been hypothesized that upper treelines are constrained by growing season temperature, whereas lower tree lines are water limited. We expect different causal mechanisms of upper versus lower tree line formation to generate distinct patterns of spatial heterogeneity. Here, we compare dynamics, spatial patterns and shape complexity of upper and lower tree lines of semi-arid pinyon-juniper woodlands. Location: Toiyabe Range of the Nevada Great Basin (western US). Taxon: Pinus monophylla Torr. & Frém. and Juniperus osteosperma (Torr.). Methods: Within 20 sample plots (10 along the upper and 10 along the lower tree line), we mapped tree canopies through photointerpretation of high-resolution imagery. We performed point pattern analyses to compare the spatial arrangement of trees and used LANDSAT 30-year time series and NDVI to understand the vegetation dynamics of these ecotones. We adopted the surface roughness method to measure tree line shape complexity. Results: Lower tree lines were denser and showed a stronger trend of increasing NDVI change over the 1984–2015 period. Trees at the lower tree line were more strongly aggregated than at the upper tree line at spatial scales ranging from 15 to 65 meters. Shape complexity was higher at upper tree lines, expressed by a higher mean surface roughness; however, the spatial structures of upper and lower tree lines were similar. Main conclusions: Upper tree line expansion of pinyon-juniper woodlands in the study area has been limited and highly variable, but lower tree line downslope expansion into adjacent shrub steppe vegetation was evident. The expected difference between energy- and water-limited tree lines did not manifest in the observed spatial structures. Differences in tree line shape complexity were not significant, although lower tree lines exhibited less complex shapes, likely because they have been more strongly influenced by anthropogenic factors.