A montane species treeline is defined by both temperature and drought effects on growth season length

Abstract

Montane treelines are defined by a threshold low temperature. However, what are the dynamics when the snow-free summer growth season coincides with a 6-month seasonal drought? We tested this fundamental question by measuring tree growth and leaf activity across elevations in Mt. Hermon (2,814 m; in Israel and Syria), where oak trees (Quercus look and Q. boissieri) form an observed treeline at 1900 m. While in theory, individuals can be established at higher elevations (minimum daily temperature > 6.5°C for > 4 months even at the summit), soil drying and vapor pressure deficit (VPD) in summer enforces growth cessation in August, leaving only 2–3 months for tree growth. At lower elevations, Q. look is replaced by Q. cerris (1,300 m) and Q. calliprinos (1,000 m) in accompanying Q. boissieri, and growth season length (GSL) is higher due to an earlier start in April. Leaf gas exchange continues during autumn, but assimilates are no longer utilized in growth. Interestingly, the growth and activity of Q. boissieri were equivalent to that of each of the other three species across the ~ 1 km elevation gradient. A planting experiment at 2100 m showed that seedlings of the four oak species survived the cold winter and showed budding of leaves in summer, but wilted in August. Our unique mountain site in the Eastern Mediterranean introduces a new factor to the formation of treelines, involving a drought limitation on GSL. This site presents the elevation edge for each species and the southern distribution edge for both the endemic Q. look and the broad-range Q. cerris. With ongoing warming, Q. look and Q. boissieri are slowly expanding to higher elevations, while Q. cerris is at risk of future extirpation.

Tell me who your neighbors are: The role of spatial location and tree species identity in determining the ectomycorrhizal community composition of saplings and mature trees in a mixed conifer forest

“…Pinus halepensis is native to Israel and has a widespread distribution across all of Israel. While other Cedrus relatives (i.e., Cedrus libani ) have a long history of inhabiting specific sites in Israel, it has vanished from the pollen record ~ 900 ybp (Bar-On et al, 2022). In contrast, Cedrus deodara is a foreign species with a patchy distribution limited to specific areas where it has been planted.…”