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, v_H, 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 (ΔΨ_max). From the water demand perspective, we hypothesised that the photosynthetic traits determine CO₂ fixation, hence water demand. It has been proven that the v_H is related to the leaf’s internal CO₂ (C_i) and V_cmax (Xu et al., 2021) and we can use the least-cost hypothesis and the coordination hypothesis to represent the C_i and V_cmax 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 V_H at the site level and compared the observed and our model-predicted sensitivities. There was an insignificant effect of ΔΨ_max which may be explained by the significant negative relationship between Ks and ΔΨ_max as well as uncertainties in the Ψ_soil variation. 

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

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 v_H variation with only a single parameter, where K_S is otherwise shown to account for 45% of the v_H variation. We also show that temperature and VPD have opposing effects on v_H 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 v_H is easier to measure as compared to collecting K_S data. 

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

Reference

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.