Tree-Ring Isotopes Provide Clues for Sink Limitation on Treeline Formation on the Tibetan Plateau

Xing Pu,Lixin Lyu,Xiaochun Wang

doi:10.3390/atmos12050540

Abstract

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.

Highlights

Understanding what determines the high elevation limits of trees is crucial for predicting how treelines may shift in response to climate change
The low temperature limitation on tree metabolism is related to either carbon gain or photosynthesis, known as the “source limitation hypothesis” [4,5,6], or tissue formation, known as the “sink limitation hypothesis” [7,8]
If treeline trees are able to acquire photoassimilates more efficiently than they can be used for growth, carbohydrates would accumulate in tissues, and, trees would have more available carbon than sinks are able to consume [7,12]

Summary

Introduction

Understanding what determines the high elevation limits of trees is crucial for predicting how treelines may shift in response to climate change. Low temperature is found to impact the growth and regeneration of forests at the treeline [1,2,3]. If treeline trees are able to acquire photoassimilates more efficiently than they can be used for growth, carbohydrates would accumulate in tissues, and, trees would have more available carbon (non-structural carbon, “NSC”) than sinks are able to consume [7,12]. The sources can be manipulated, either by increasing the CO2 concentration of air [18,19] or by removing the photosynthetically active tissues, such as defoliation [15,16,20,21]

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