Around 500 years ago, one mystery of tree architecture has been observed by a great mind, Leonardo da Vinci. He wrote in his notebook about ‘Rule of Trees’: “All the branches of a tree at every stage of its height when put together are equal in thickness to the trunk.” To put it in another way, if you squeeze all the daughter branches into one, like closing a folding fan, you would observe that it’s as thick as the trunk at every height (Fig. 1). This fabulous observation hasn’t been moved forward until 1928 by Bruno Huber who measured the xylem area of branch and expressed it per weight of leaves supported by this branch, called Huber Value (Huber 1928). This definition makes it possible to compare the conducting area between different branches. With the development of technology, it’s easier to measure leaf area. Nowadays, we use leaf area instead of weight to calculate Huber Value (Tyree and Ewers 1991).
Huber Value, an integrative trait, reflects not only the carbon allocation between leaf and stem, but also the balance between water supply and loss. As shown in Fig. 2, leaf exchanges water for CO2 in order to photosynthesize (carbon gain). The continuous water transport from sapwood is the water supply for water loss from leaves. When a plant invests more carbon to increase leaf area, LMA decreases and photosynthesis rate increases, which is acquisitive strategy (Wright et al. 2004). High photosynthesis is accompanied with more water loss through leaves. There is a limit on the amount of leaves that a given sapwood can support due to its limited capacity of water supply. Thus, the plant needs to invest more carbon in sapwood to meet the increasing water loss. We would expect Huber Value itself to represent the coordination between photosynthesis and hydraulics and balance between water supply and demand. These processes outcomes can be reflected by relevant traits; thus, We expect that Huber Value should play a mediating role in the trait coordination network.
In light of the importance of Huber Value, it is a crucial parameter in the vegetation models, which affects the carbon cycle and vegetation response to drought (Trugman et al. 2019). Many studies now focus on the relationships between Huber Value and other traits. For example, plants with lower Huber Value have lower LMA, but higher sapwood hydraulic conductance (Ks) and leaf nitrogen per mass (Mencuccini et al. 2019; Rosas et al. 2019). These relationships indicate that acquisitive leaves with short life span have high photosynthesis rate (Wright et al. 2004), and more water is required to keep exchanging for CO2. Thus, plants need to maintain either high Huber Value or high Ks to cope with the excessive water demand.
However, the empirical analyses haven’t quantified the relationship theoretically. We still need to explore the relationship between Huber value and other traits, and its response to climate variables in theory to better comprehend the underlying processes.
Huber B. Weitere quantitative Untersuchungen über das Wasserleitungssystem der Pflanzen[M]. 1928.
Tyree M T, Ewers F W. The hydraulic architecture of trees and other woody plants[J]. New Phytologist, 1991, 119(3): 345-360.
Wright I J , Reich P B , Westoby M , et al. The worldwide leaf economics spectrum[J]. Nature, 2004, 428(6985):821.
Mencuccini M, Rosas T, Rowland L, et al. Leaf economics and plant hydraulics drive leaf: wood area ratios[J]. New Phytologist, 2019, 224(4): 1544-1556.
Rosas T, Mencuccini M, Barba J, et al. Adjustments and coordination of hydraulic, leaf and stem traits along a water availability gradient[J]. New Phytologist, 2019, 223(2): 632-646.
Thanks my supervisor Dr. Wang for commenting on my Fig. 2 and this blog and previous one (which I didn’t acknowledge for her help, sorry…).