Identifying what determines the high elevation limits of tree growth is crucial for predicting how treelines may shift in response to climate change. Treeline formation is either explained by a low-temperature restriction of meristematic activity (sink limitation) or by the photosynthetic constraints (source limitation) on the trees at the treeline. Our study of tree-ring stable isotopes in two Tibetan elevational transects showed that treeline trees had higher iWUE than trees at lower elevations. The combination of tree-ring δ13C and δ18O data further showed that photosynthesis was higher for trees at the treeline than at lower elevations. These results suggest that carbon acquisition may not be the main determinant of the upper limit of trees; other processes, such as immature tissue growth, may be the main cause of treeline formation. The tree-ring isotope analysis (δ13C and δ18O) suggests that Tibetan treelines have the potential to benefit from ongoing climate warming, due to their ability to cope with co-occurring drought stress through enhanced water use efficiency.
Tree growth in high-elevation forests may increase as a result of increasing temperatures and CO2 concentrations in the atmosphere (Ca). However, the pattern and the physiological mechanism on how these two factors interact to affect tree growth are still poorly understood. Here, we analyzed the temporal changes in radial growth and tree-ring δ13C for Picea and Abies trees growing in both treeline and lower-elevation forests on the Tibetan Plateau. We found that the tree growth at the treeline has significantly accelerated during the past several decades but has remained largely stable or slightly declined at lower elevations. Further results based on tree-ring δ13C suggest that intrinsic water-use efficiency (iWUE) was generally higher at the treeline than in lower-elevation forests, although increasing trends of iWUE existed for all sites. This study demonstrated that the synergetic effects of elevated Ca and increasing temperatures have increased tree growth at the treeline but may not lead to enhanced tree growth in lower-elevation forests due to drought stress. These results demonstrate the elevational dependence of tree growth responses to climatic changes in high-elevation forests from a physiologically meaningful perspective.
Tree intrinsic water-use efficiency (iWUE) has dramatically
increased in recent decades in global forests. The rising iWUE can be a
result of either enhanced photosynthesis rate (A), or decreased
stomatal conductance (g) or both. The underlying
physiological mechanisms are still not well understood. Here, we
investigated tree-ring isotopes δC and
δO from two tree species in three altitudinal
transects on the Southeastern Tibet. We found that the relationship
between iWUE and leaf water O
(ΔO) was negative at the
low-altitude forests but are positive at the treeline, indicating that
enhanced photosynthesis was the main driver of the increasing iWUE at
the treeline, whereas reduction in stomatal conductance was the prime
regulator at the lower-elevation forests. Furthermore, vapor pressure
deficit (VPD) and precipitation during the main growing season showed
the highest and significant correlations with
ΔO at the low-altitude forests,
suggesting that g was strongly controlled by
moisture conditions in the growing season particularly in the
low-altitude forests. These findings shed light to a better
understanding of the regulatory mechanism of iWUE changes under the
background of rising atmospheric CO and climate change
and serve to improve the reliability of ecophysiological modelling.
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