How do plants change allocation to stem and leaf along climate gradients? – Presenting a Simplified and Accurate Model

It is important to understand how and why plants differ in their allocation strategy along geographic climate gradients.  The Huber Value, vH, is the ratio of sapwood area to leaf area, reflecting the plant allocation to stem and leaf. How plants allocate their resources to sapwood and leaves has a large influence on the land carbon and water cycles via water transport and photosynthesis. In DGVMs, these allocations are treated as fixed values or empirical functions, without considering how they coordinate with other physiological processes.

In our study, we developed a model hypothesis with optimal sapwood: leaf area (based on Whitehead et al.,1984) where it is assumed that the water supply through the stem (Darcy’s law) equals the water loss via leaf/stomata (Fick’s law). For the water supply, we used the compensation effect of hydraulic efficiency (Ks) and the driving force of difference between the water potential of the leaf and soil (ΔΨmax). From the water demand perspective, we hypothesised that the photosynthetic traits determine CO2 fixation, hence water demand. It has been proven that the vH is related to the leaf’s internal CO2 (Ci) and Vcmax (Xu et al., 2021) and we can use the least-cost hypothesis and the coordination hypothesis to represent the Ci and Vcmax respectively.  

To test our hypothesis at the global scale we used the species-level hydraulic trait database with 1727 species covering a total over 48 sites according to three climate variables. We applied multiple linear regression with 5 driving factors of the VH at the site level and compared the observed and our model-predicted sensitivities. There was an insignificant effect of ΔΨmax which may be explained by the significant negative relationship between Ks and ΔΨmax as well as uncertainties in the Ψsoil variation. 

Having removed these uncertainties and repeated the multiple linear regression without ΔΨmax, we observed that vH response to climate and its relationship with Ks matches our theoretical predictions. 

Fig. 1 The partial residual plot shows the relationships between vH and its driving factors. KS is sapwood conductivity, Dmax is the maximum vapor pressure deficit, Iabs is irradiance and Tmax is the maximum temperature during the growing season. The black dots are site-mean values, black lines are fitted and red lines are predicted sensitivities.

Comparison between the observed data and our predictions confirms the hypothesis that plants allocate resources optimally to balance the water supply and water demand. Our model predicts vH variation with only a single parameter, where KS is otherwise shown to account for 45% of the vH variation. We also show that temperature and VPD have opposing effects on vH and plant allocation strategies. This implies that global warming effect may compensate for the effects of rising atmospheric dryness.

It appears incorporating our findings into the DGVMS will improve predictions of plant allocation to leaf and stem with two benefits: It simplifies the underlying relationships among traits by using just a single fitted parameter without the need for calibration. It makes our hypothesis accurate and easy to incorporate into DGVMs without increasing model complexity. And finally, it improves the modelling of plant hydraulic efficiency because vH is easier to measure as compared to collecting KS data. 

This work is going to be submitted. We will provide the DOI when it is accepted.


Whitehead D, Edwards WRN, Jarvis PG. 1984. Conducting sapwood area, foliage area, and permeability in mature trees of Piceasitchensis and Pinuscontorta. Canadian Journal of Forest Research 14(6): 940-947.

Xu H, Wang H, Prentice IC, Harrison SP, Wright IJ. 2021. Coordination of plant hydraulic and photosynthetic traits: confronting optimality theory with field measurements. New Phytologist 232(3): 1286-1296.


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