Abstract
Tree allometry in semi-arid forests is characterized by short height but large canopy. This pattern may be important for maintaining water-use efficiency and carbon sequestration simultaneously, but still lacks quantification. Here we use terrestrial laser scanning to quantify allometry variations of Quercus mongolica in semi-arid forests. With tree height (Height) declining, canopy area (CA) decreases with substantial variations. The theoretical CA-Height relationship in dynamic global vegetation models (DGVMs) matches only the 5th percentile of our results because of CA underestimation and Height overestimation by breast height diameter (DBH). Water supply determines Height variation (P = 0.000) but not CA (P = 0.2 in partial correlation). The decoupled functions of stem, hydraulic conductance and leaf spatial arrangement, may explain the inconsistency, which may further ensure hydraulic safety and carbon assimilation, avoiding forest dieback. Works on tree allometry pattern and determinant will effectively supply tree drought tolerance studying and support DGVM improvements.
Highlights
Tree allometry in semi-arid forests is characterized by short height but large canopy
We focus on the Height-canopy area (CA)-DBH relationship in semi-arid regions firstly, to test whether it can be represented by the theoretical relationships widely adopted by dynamic global vegetation models (DGVMs)
The allometric growth equations Height = 40 × DBH0.5 and CA = 100 × DBH1.6 used in DGVMs can be combined into an equation representing the CA-Height relationship (CA = 7.5 × 10−4 × Height3.2), which results in a value lower than the minimum CA in our results
Summary
Tree allometry in semi-arid forests is characterized by short height but large canopy. This pattern may be important for maintaining water-use efficiency and carbon sequestration simultaneously, but still lacks quantification. Of the carbon sequestration in European forests and the entire global Fluxnet network[4], showing a significantly higher water-use efficiency than that in other ecosystems[5]. The dry climate found in semiarid regions may limit the height growth of trees[8], thereby creating an extreme allometric pattern with short trees but wide canopy. The allometric growth equations Height = 40 × DBH0.5 and CA = 100 × DBH1.6 used in LPJ9,10 can be combined into a theoretical model of the CA-Height relationship (CA = 0.000747 × Height3.2), implying a steeper decrease in tree canopy size with tree height reduction compared with the actual trees in semi-arid forests. All the involving DGVMs may have systematic bias for semi-arid forests carbon sequestration potential due to the simplistic allometric growth module
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