Co-determinants of trees’ water uptake
The two major findings of this paper include, (i) the establishment of empirical relationship between canopy stomatal conductance and stomatal density, and (ii) the proposed conceptual model of canopy water flux which considers stem and canopy characteristics.
The basic challenge for tree physiologists is to determine the water uptake of a tree per day. We still lack accurate methods for measuring daily water consumption of trees. Porometers have been extensively used to measure the transpiration rate of individual leaves, but this measuring technique is still a long way from being able to calculate the water use by the whole tree. The heat dissipation technology dating back 30 years provides a convenient method for calculating water consumption of an individual tree, but the measurements usually underestimate the real water consumption, and that is why this technique cannot accurately measure the true level of water used by the whole tree. One of the important reasons is the lack of knowledge about tree canopy characters.
The most frequently used heat dissipation technology estimates the water use of the whole tree by measuring sap flow velocity through the xylem. This process is affected by the structure and physiology of the canopy, such as the stem water recharge. This measurement technique does not take transpiration into consideration, even though water flow through the trunk is constant. Water flow measured in this process cannot be used to calculate water use of the whole tree, because the pores (stomata) of the canopy leaves are not constantly open, and there are times when water is not evaporating through these small pores. For example, in tall trees such as baobabs, water storage of the trunk is substantial, and their tree trunks can store tens of tons of water. Such a large amount of water can even support the trees to survive throughout the whole dry season without absorbing soil moisture, and to complete important physiological processes such as flushing and flowering. Some trees can use water storage of the trunk to carry out transpiration to maintain the rate of CO2 exchange or carbon assimilation. In this case, the stomata of the canopy leaves are open, i.e. active stomata movement is present, but the water flow in the trunk may be inactive. Therefore, we can understand how the trees take up water only if we know both the stem sap flux and the behavior of the canopy.
Fig. 1. A conceptual model illustrating the relationship between wood, leaves, and canopy water flux (reference canopy stomatal conductance, Gsref). For details, see Gao & Tian (2019). Credit: Jianguo Gao
Based on our previous study that combined canopy stomatal conductance dataset with stomatal characteristics, our recent work (1) published in Plant Diversity showed that higher wood density impedes water flow, while higher stomata density promotes water flow, and canopy water flux is determined by both wood density and stomatal density. The two major findings of this paper include, (i) the establishment of empirical relationship between canopy stomatal conductance and stomatal density, and (ii) the proposed conceptual model of canopy water flux which considers stem and canopy characteristics as most important factors that influence the middle and exit parts of a tree on the water flow, and vividly depicts the movement pattern of water in a tree. The first conclusion was quite different from most of the previous studies that have focused on the positive correlation between leaf stomatal conductance and stomatal density. We found that at low stomatal density, the relationship between canopy stomatal conductance and stomatal density was not significant, but there was a positive correlation between them at higher stomatal density. We have done four major scenario analyses of these patterns. Scenario I, titled “lighter wood, denser stomata”, was characterized by rapid growth, but may also a greater risk of embolism; scenario II, titled “heavy wood, less stomata”, was characterized by slower growth, but may have strong drought resistance; and scenarios III and IV (and theoretically numerous other scenarios) were in between the extremes of scenarios I and II. The value of this conceptual model is that it can aid understanding of the diverse water use strategies, with the potential to classify tree water use strategies based on both stem and leaf traits (Fig. 1).
It has been estimated that there are up to 60,065 tree species (2) and 3.04 trillion individual trees (3) in the world (Fig. 2), which inhabit diverse environments, from boreal forests to tropical rainforests. The environment influences the trees’ water use strategies in a way that a trade-off occurs between water flow and trees’ characteristics. If we could classify all the trees in the world into one of the four scenarios of our conceptual model, it could generate interesting results, which could be crucial for accurately estimating the trees’ water use in different functional groups or at local scales.
Fig. 2. This is the global map of tree density at the square-kilometer pixel scale. Credit: Crowther et al., 2015.
Consider this example: if you want a car to reach a certain destination, you need to know the start point and the end point. Similarly, if we know two points of water molecule movement, for example the middle stem point through which water flows, and the final water flow points that are stomata through which water is being evaporated, we can also know the distance the water molecule has traveled. Just as two points determine a straight line, the combined traits of the stem and the leaf can determine the canopy flux of a tree.
As a notable mention, I would like to introduce you to the journal called Plant Diversity that has published our research. This journal is a traditional botanical journal with a good reputation, which is now indexed in Science Citation Index Expanded (SCI-E, Thomson Reuters). The journal is published by the Kunming Institute of Botany, Chinese Academy of Sciences. The predecessor of Plant Diversity is Acta Botanica Yunnanica, which was established in 1979 and has since represented the frontier of China’s plant science research. Its first Editor-in-Chief was Mr. Wu Zhengyi, who was awarded the 2007 Highest Science and Technology Award and is a very honorable and respectable botanist. The current Editor-in-Chief of Plant Diversity is Professor Zhou Zhekun who is a senior scientist focuses on paleobotany.
1. J. Gao, K. Tian, Stem and leaf traits as co-determinants of canopy water flux. Plant Divers. 41, 258–265 (2019).
2. E. Beech, M. Rivers, S. Oldfield, P. P. Smith, GlobalTreeSearch: The first complete global database of tree species and country distributions. J. Sustain. For. 36, 454–489 (2017).
3. T. W. Crowther et al., Mapping tree density at a global scale. Nature. 525, 201–205 (2015).