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.
Highlights
Alpine and northern treelines, which are a conspicuous vegetation boundary between forest and tundra ecosystems, are widely thought to be controlled by temperature (Tranquillini, 1979; Körner, 2003)
Many experiments have examined the response of tree transpiration to soil warming, but contradictory conclusions including no effect on black spruce [Picea mariana (Mill.) BSP] in northern Canada (Van Herk et al, 2011) vs. positive impact on Pinus cembra in an Alpine treeline (Wieser et al, 2015) and Scots pine (Pinus sylvestris L.; Mellander et al, 2004) and Norway spruce [Picea abies (L.) Karst.; Bergh and Linder, 1999] in northern Sweden were found
Air temperature and precipitation were generally similar throughout the growing season, while soil warming was delayed into mid-May and early June at the juniper and Smith fir treelines, respectively (Figures 1C–F and 2C–F)
Summary
Alpine and northern treelines, which are a conspicuous vegetation boundary between forest and tundra ecosystems, are widely thought to be controlled by temperature (Tranquillini, 1979; Körner, 2003). Many experiments have examined the response of tree transpiration to soil warming, but contradictory conclusions including no effect on black spruce [Picea mariana (Mill.) BSP] in northern Canada (Van Herk et al, 2011) vs positive impact on Pinus cembra in an Alpine treeline (Wieser et al, 2015) and Scots pine (Pinus sylvestris L.; Mellander et al, 2004) and Norway spruce [Picea abies (L.) Karst.; Bergh and Linder, 1999] in northern Sweden were found Such contradictory results were even apparent among experimental years with varying timing of soil warming (May vs July) within a site (Wieser et al, 2015). Such knowledge is crucial for predicting future hydrological functioning in cold-limited forest ecosystems
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