Seasonal Shift in Climatic Limiting Factors on Tree Transpiration: Evidence from Sap Flow Observations at Alpine Treelines in Southeast Tibet

Abstract

Alpine and northern treelines are primarily controlled by low temperatures. However, little is known about the impact of low soil temperature on tree transpiration at treelines. We aim to test the hypothesis that in cold-limited forests, the main limiting factors for tree transpiration switch from low soil temperature before summer solstice to atmospheric evaporative demand after summer solstice, which generally results in low transpiration in the early growing season. Sap flow, meteorological factors and predawn needle water potential were continuously monitored throughout one growing season across Smith fir (Abies georgei var. smithii) and juniper (Juniperus saltuaria) treelines in southeast Tibet. Sap flow started in early May and corresponded to a threshold mean air-temperature of 0°C. Across tree species, transpiration was mainly limited by low soil temperature prior to the summer solstice but by vapor pressure deficit and solar radiation post-summer solstice, which was further confirmed on a daily scale. As a result, tree transpiration for both tree species was significantly reduced in the pre-summer solstice period as compared to post-summer solstice, resulting in a lower predawn needle water potential for Smith fir trees in the early growing season. Our data supported the hypothesis, suggesting that tree transpiration mainly responds to soil temperature variations in the early growing season. The results are important for understanding the hydrological response of cold-limited forest ecosystems to climate change.

Rainy season onset mainly drives the spatiotemporal variability of spring vegetation green-up across alpine dry ecosystems on the Tibetan Plateau

“…1 f,g), the observed green-up dates varied little during 2007–2013 and typically occurred at the end of June (DOY 180) when the daily mean air-temperature was continuously above 0 °C, setting a threshold of minimum growing days that allows plants to have enough time to safely complete their life cycle. This is consistent with our previous reports that the number of days with daily mean soil temperature ≥ 5 °C (at − 10 cm) during June and August is about 80–90 days at 5100–5200 m 45 , with a seasonal mean soil temperature of 6.8–7.3 ℃ 46 being close to the low temperature threshold that significantly limits treeline growth and photosynthesis 47 – 50 .…”

Seasonal Dynamics of Stem Radial Increment of Pinus taiwanensis Hayata and Its Response to Environmental Factors in the Lushan Mountains, Southeastern China

“…However, there is evidence that low soil temperature no longer affects tree transpiration when it rises to ca. 8 • C [53,54]. Thus, our data suggests that the relatively low soil temperature in early spring may induce water deficit in tree stems, resulting in a later growth initiation.…”

“…Such a stimulating effect of soil temperature on initiation of stem growth was also found at mountain forest plots with open sparse canopy [52]. Although soil water is available in early spring, low soil temperatures can not only hamper root growth but also restrict root water uptake and above-ground transpiration [53], which in turn affects the cell turgor and enlargement of newly-produced xylem cells in the stem. However, there is evidence that low soil temperature no longer affects tree transpiration when it rises to ca.…”

The complete chloroplast genome sequence of Juniperus saltuaria (Cupressaceae)

“…It occurs mainly on sunny slope of the Sergyemla (Sygera) Mountains in southeast Tibet (Liu et al. 2016 ). J. saltuaria grows along an altitudinal gradient from 4200 to 4520 m on the slope (Liu et al.…”

“… 2014 ; Liu et al. 2016 ).To facilitate its phylogenetic analysis and conservation genetics of this species and sustainable utilization, we assembled its complete chloroplast genome by Illumina sequencing technology in this study. The chloroplast genome sequence was submitted to GenBank (accession number MT133566).…”