Most flowering plants move water in pipes called vessels. As a tree gets taller, the vessel pathway must also get longer. If nothing else in the system changes, and the only thing that happens is that the vessel pathway gets longer, then the resistance due to friction between the sap and the vessel walls increases. This increase turns out to be linearly related to length, that is, flow rate will drop as a linear (not exponential) function of vessel length increase. This phenomenon can be observed, irritatingly, in my house: the kitchen tap ended up farther away from the water tank than planned, and even though we put in ¾” pipe, the flow is notably lower than in the hose bibb halfway between the kitchen and the tank. For various reasons vessels have to be very narrow at the ends, at the stem tips and in the leaves. So narrow, in fact, that if trees had vessels the lengths of their trunks that were the same diameter as the apical vessels, then they would conduct less and less water they taller they get—a penalty for growing in height.
Fortunately, fluid mechanics has a magical trick up its sleeve: small increases in pipe diameter lead to very large (exponential, to the fourth power to be precise) increases in flow rate. This is why, with the same amount of suction, you get a quick mouthful of your drink through a wide straw and just a little through a narrow one. As it turns out, what happens in trees is that vessels widen from the stem tip to the base. This maintains the hydraulic resistance constant even as trees grow in height and the conductive path becomes longer and resistance, minus widening, would increase. Lots of people have worked on this, but work in my lab and by collaborators is listed at the end.
This tip-to-base widening has potentially important implications for how sap flow in trees generally is studied. This is because the volume of fluid moving past any given point in a pipe remains constant. If the pipe changes in diameter, as we know tree vessels do, then the fluid must speed up as the vessels narrow. So, even along a single tree trunk, the actual speed of sap flow should depend on how far the measurement is taken from the stem tip, because of tip-to-base vessel widening. The amount of sap moved along the length of a trunk should remain constant, but it should be slow at the base and fast at the tip.
This prediction has been hard to test in much detail because in most normal trees, as they get taller they become intricately branched. So as we move down the stem, the tip-to-base widening pattern is complicated by incoming vessels from side branches. This is where Moringa comes in.
This is the perfect situation—a single branch that is 7 meters tall with a single growing point. The vessels are narrow at the tips of these branches and wide at the bases. Vinicio home-made special sensors, and Tommaso hand-carried them to us. Then Alberto and Diana soldered the whole system together and connected the sensors to a datalogger. They installed the sensors in the tree and lovingly built little roofs to protect the sensors from the brutal tropical rain and insulated the trunk to protect the sensors from the brutal tropical sun.
The experiment has just started, but the flow does seem to be slower in the lower sensors. This means that an experiment with Moringa should help provide helpful guidelines for factoring stem length into measurements of sap flow for plants in general. In turn, it will also help unlock the secrets of Moringa’s extraordinary drought resistance.
Anfodillo, T., G. Petit, and A. Crivellaro. (2013). Axial conduit widening in woody species: a still neglected anatomical pattern. IAWA Journal 34(4): 352-364.
Bettiati D, G Petit, T Anfodillo 2012 Testing the equi-resistance principle of the xylem transport system in a small ash tree: empirical support from anatomical analyses. Tree Phys doi:10.1093/treephys/tpr137
Olson, M. E., T. Anfodillo, J. A. Rosell, G. Petit, A. Crivellaro, S. Isnard, C. León-Gómez, L. O. Alvarado-Cárdenas, and M. Castorena. 2014. Universal hydraulics of the flowering plants: vessel diameter scales with stem length across angiosperm lineages, habits and climates. Ecology Letters 17: 988-997.
Olson, M. E., J. A. Rosell, C. León, S. Zamora, A. Weeks, L. O. Alvarado-Cárdenas, N. I. Cacho, and J. Grant. 2013. Convergent vessel diameter-stem diameter scaling across five clades of New- and Old- World eudicots from desert to rain forest. International Journal of Plant Sciences 174(7):1062–1078.
Olson, M. E. and J. A. Rosell. 2013. Vessel diameter–stem diameter scaling across woody angiosperms and the ecological causes of xylem vessel diameter variation. New Phytologist 197: 1204–1213.
Petit G, T Anfodillo 2009 Plant physiology in theory and practice: an analysis of the WBE model for vasuclar plants. Journal of Theoretical Biology 259: 1-4.
Rosell, J. A, and M. E. Olson. 2014. Do lianas really have wide vessels? Vessel diameter-stem length scaling in non self-supporting plants. Perspectives in Plant Ecology, Evolution, and Systematics 16: 288-295.