How Do Tall Trees Get Water To The Top

Why practise behemothic redwoods grow so alpine and then stop? It all has to do with how high water can travel up their branches.

Declension redwood, *Sequoia*
*sempervirens*
Photo courtesy of Allie_caulfield/
Wikimedia Commons

The redwoods of northern California, Sequoia sempervirens, are the tallest trees in the world and can abound to heights of more than 110 g. However, what finally limits their height is notwithstanding debated.

The virtually popular theory is the 'hydraulic limitation hypothesis' (Ryan & Yoder, 1997), which suggests that as trees grow taller, it becomes more difficult to supply water to their leaves. This hydraulic limitation results in reduced transpiration and less photo-synthesis, causing reduced growth.

In tall trees, water supply can exist limited by 2 factors: distance and gravity. Tall trees take a longer path- way of transport tissue – known as xylem – which increases the difficulty of water to travel, something we call hydraulic resistance. In addition, non merely is the xylem pathway long, only the trees are tall and the water has to overcome gravity. Increased strength is necessary to pull the water upward to the highest leaves. This situation differs from a long hosepipe lying along the ground: it would have high resistance due to its length, but not the boosted difficulty of beingness upright.

a) Construction of xylem vessels in flowering plants and tracheids in conifers b) Photograph of ~1 cm bore pine (Pinus strobus) branch c) Scanning electron microscope (SEM) image of cut section
Images courtesy of Boutilier et al.

Fast-growing copse oft have shorter life spans. To achieve their rapid growth, pioneer trees have wider xylem vessels, increasing their hydraulic efficiency only besides increasing the risk of embolisms (air locks). Air locks in xylem vessels preclude water from being able to travel through them.

In contrast, very alpine copse are often very long-lived. It is thought that this is partly because they are more likely to prefer a safe hydraulic design, with multiple narrow xylem vessels instead of a few wider ones.

This increased condom is counteracted past a decreased efficiency of water transport, which consequently limits growth rates. Tree height, therefore, may too be limited by the rubber versus efficiency trade-off in xylem function (Burgess et al, 2006).

The following two activities explore the trade-off that plants make between being efficient with h2o transport and having a safety design. Both activities tin exist adapted for students aged fifteen–18 with a wide range of abilities, but you should appraise whether the students can perform all of the experiments or whether information technology is safer for the teacher to practise the cutting. Each action will take almost 50 minutes.

Table 1: Some example xylem lengths of branches less than 15 cm in diameter, taken from Jacobsen et al (2012)
Species	Mean xylem length/m
Acer saccharum (Sugar maple)	0.0312
Cinnamomum camphora (Camphor tree)	0.1184
Rhododendron_maximum (Corking Rhododendrum)	0.0246
Vitis vinifera (Mutual Grape Vine)	0.1503

Estimating maximum xylem vessel lengths

Comparing the lengths of the xylem vessel will allow students to predict their relative resistance to water flow.

Materials

**Branch collection**
Image courtesy of Clare van
der Willigen

Choice of recently cut branches from a tree or shrub, including whatsoever leaves or side branches, up to 2 yard in initial length. If the experiment is to be performed within a few hours of harvesting, keep the plant material in a plastic handbag to avoid excessive h2o loss.
Safe/silicon tubing
Cablevision ties or jubilee clips
Sharp pruning shears or pair of scissors
60 cm^three syringes
Large basin of tap water
Paw lens
Ruler

Pressurise the air in the
syringe
Image courtesy of Clare van
der Willigen

Process

Cut a length of branch over 1 m, making sure the cut is clean and the cease of the branch is non crushed. The branch volition be much longer than the xylem vessels inside.
Attach a 60 cm3 syringe, filled with air, to the proximal (wider) finish of the co-operative using silicon tubing and cable ties as required.
Pressurise the air in the syringe and branch by compressing the volume of air in the syringe past nigh half (e.m. from 60 cm3 of air to xxx cm3). This pressure must exist maintained through steps four–vi.
Hold the distal end of the branch under water.
Use a manus lens to see if a steady stream of bubbles tin can be detected from the distal end of the branch.
Progressively cut the distal terminate of the branch back by about 1 to 5 cm at a time, making certain each time that the end of the branch is not crushed and has a clean cut.
When a stream of bubbles is observed, the length of the branch gives an approximate maximum length of the xylem vessels.

Safety Note

Students should be warned about the condom precautions necessary when using sharp objects. Encounter also the general safety note.

Follow-up activity

Students could compare maximum xylem vessel lengths in a multifariousness of different plants or dissimilar parts (roots, main and side branches) of the aforementioned institute. It is mutual for fast-growing plants to accept longer xylem vessels and therefore fewer breaks betwixt xylems. Tin the students suggest why this might be?

Nearly what happens

A branch contains several xylem vessels linked together. Between the xylem vessels are perforated wall plates. The fewer of these divisions there are, the lower the resistance and the faster water can travel.

A detailed report of vessel length inChrysanthemum stems (Nijsse et al, 2001) and in a wide range of shrubs and trees (Jacobsen et al, 2012) can be used for cantankerous-reference.

Measuring xylem hydraulic electrical conductivity

Measurements of xylem hydraulic properties testify how well plants tin supply h2o to their leaves. It is possible to measure the hydraulic conductance of stems, branches and roots in the classroom with some simple, inexpensive equipment. To measure hydraulic conductivity, the branch length should be longer than the mean length of the xylem vessels (see previous action).

**Diagram of the appliance setup to measure hydraulic electrical conductivity**
Prototype courtesy of Nicola Graf

Materials

**The whole setup**
Paradigm courtesy of Clare van
der Willigen

Pick of recently cut branches from a tree or shrub investigated in the previous experiment. Ensure that the pieces are longer than the longest xylem vessels measured. If the experiment is to exist performed within a few hours of harvesting, keep the plant material in a plastic bag to avert excessive water loss.
Rubber/silicon tubing
Cable ties or jubilee clips
Sharp secateurs, scissors or a large scalpel
Chopping board
Large basin of water
Metre rule
Reservoir of degassed, distilled water in a container with a tap at the bottom. Degas the h2o by humid information technology or using a vacuum pump for approximately 1h until all the gas has been expelled from the water. Air bubbling in h2o that is non degassed may block the xylem vessels.
Hydrochloric acid
1 cm³ pipette (a pipette with a 90o curve is near constructive. A standard glass pipette tin be aptitude in a very hot flame)
50 cm³ plastic beaker
Antiphon stand up and clamp
Balance (precision of at least 0.01 g)
Stop lookout man or end clock

Cut proximal cease under
water
Image courtesy of Clare van
der Willigen

Procedure

1. Set upwards the apparatus every bit illustrated in the diagram above:

aAdd together hydrochloric acid to the degassed, distilled water to give a final concentration of 0.01 M. For example, add 0.5 cm³ of 0.one M HCl to 5 dmⁱⁱⁱ degassed, distilled h2o. Muriatic acid prevents the long-term pass up in conductance by reducing microbial growth in the xylem.

Safety tip

Remember to e'er add acid to water, not water to acrid.

bFill the reservoir with the acidified water. Insert a piece of tubing, sealed at one finish with a hurl, into the top of the reservoir. The open tubing ensures a abiding pressure head considering even if the water level drops, the constructive meridian of the reservoir will remain the same.

**Cut terminate under water**
Epitome courtesy of Clare van
der Willigen

c To the tap of the reservoir, add some tubing, fill up with h2o from the reservoir, seal the open end and place into the big basin of water.

d Close the tap.

due east Submerge the proximal end of the branch in the large bowl of water. This is the stop of the co-operative that was nearest to the principal stem of the constitute.

f Cutting approximately 3 cm off the proximal end of the branch under water to ensure that no air pockets remain in the xylem. Shave off the end of the cut using a abrupt blade.

one thousand Connect the newly cut end of the branch to the h2o-filled tubing attached to the reservoir nether h2o. If the bawl is very rough, information technology can be stripped back prior to connection. A water-tight seal should exist achieved using cablevision ties or jubilee clips if necessary, nonetheless exercise not over-tighten and shrink the xylem vessels.

Connect tubing to stop nether
h2o
Prototype courtesy of Clare van
der Willigen

h Submerge the other finish of the co-operative in the tub of h2o.

i Cut approximately 3 cm off the stop of the branch under h2o to ensure that no air pockets remain in the xylem. Shave off the finish of the cut using a sharp blade.

jMeasure and tape the length of the branch. Ensure information technology is longer than the maximum xylem vessel length (see previous experience).

thousand Connect the bent pipette to more than rubber tubing and sub- merge into the basin of h2o.

l Connect the newly cut stop of the branch to the water-filled tubing fastened to the pipette as above.

grand Fill the 50 cm³ beaker with water and place on the pan balance.

**Co-operative connected to tubing**
Image courtesy of Clare van
der Willigen

n Have the branch end and pipette out of the bowl of water with the end of the pipette sealed.

o Place the end of the pipette in the fifty cm^three beaker on the balance.

p Use the antiphon stand up and clench to hold the pipette in place. The tip of the pipette should not lean on the bottom of the chalice, simply should be below the water level. This ensures that as the water drips through the co-operative, there is a smooth increase in the mass of h2o in the beaker.

2. Open the tap from the reservoir.

three. Measure the mass of water every 30s for 3 min.

4. Measure out the effective height of the reservoir using the metre rule. This is the height from the bottom of the open tubing in the reservoir to the proximal end of the co-operative.

Safety Annotation

Students should be warned about the rubber precautions necessary when using sharp objects and acids. Encounter as well the general safety annotation.

Mensurate h2o collection on
balance
Epitome courtesy of Clare van der
Willigen

Analysis

Hydraulic electrical conductivity is measured equally the mass of h2o flowing through the arrangement per unit time per unit pressure gradient (Tyree & Ewers, 1991). The hydraulic electrical conductivity of the branch, kh, is calculated using the following formula:

k_h = (flow rate x branch length)/hydrostatic pressure head

where the flow rate is measured in kilograms per second (kg/s); branch length in metres (g); and the pressure head in megaPascals (MPa). To calculate the period rate, plot the mass of h2o (in kg) measured in step 3 against time (in due south). The period rate will be the slope of the line of best fit (in kg/s). Come across table ii and figure 1 for a worked example.

**Measure reservoir height**
Epitome courtesy of Clare van der
Willigen

The hydrostatic pressure caput is institute by multiplying the effective height of the reservoir, measured in step 4, with the density of liquid and the acceleration due to gravity. The density of the acidified water can be assumed to exist 1000 kg/m³ (at room temperature) and a value of 9.81 thousand/southward² can exist used for acceleration due to gravity. Thus, with an constructive elevation of the reservoir of 1m, the hydrostatic pressure level caput would exist 1000 x 9.81 x 1 = 9810 Pa or 0.00981 MPa.

Retrieve, maximum hydraulic conductivity is only achieved if none of the xylem vessels are embolised (filled with air). To try to prevent this, branches can be flushed with water at a pressure of approximately 200 kPa for 20 min before measuring conductivity. Alternatively, ensure that branches are selected from well-watered trees and that the leaves are covered in a large plastic bag prior to measurement.

Table ii. Raw data tabular array to calculate the hydraulic electrical conductivity of a side branch from a Laurel tree (*Laurus nobilis*)
Time (south)	Mass of water (g)	Mass of water (kg)	Branch length (1000)	Constructive reservoir pinnacle (one thousand)
0	0.00	0.00000	0.32	one.five
xxx	0.09	0.00009	0.32	1.5
60	0.21	0.00021	0.32	one.5
90	0.28	0.00028	0.32	i.v
120	0.38	0.00038	0.32	1.5
150	0.49	0.00049	0.32	i.five
180	0.55	0.00055	0.32	ane.five

Figure ane. Worked example of
plot to summate flow charge per unit.
Data from table 2.

Remember, maximum hydraulic conductivity is only achieved if none of the xylem vessels are embolised (filled with air). To endeavour to prevent this, branches can be flushed with h2o at a pressure of approximately 200 kPa for 20 min before measuring electrical conductivity. Alternatively, ensure that branches are selected from well-watered trees and that the leaves are covered in a big plastic bag prior to measurement.

Table 3. Processed data from worked example of a side branch from a Laurel tree (*Laurus nobilis*)
Period charge per unit (kg/s)- see figure 1	Branch length (g)	Hydrostatic pressure caput (MPa)	Hydraulic conductivity kh (kg chiliad/MPa s)
iii x 10-6	0.32	0.0147	6.53 x 10-v

Follow-up experiments

Xylem transports water and minerals through vessel elements and tracheids, which are expressionless at maturity and have a primary and secondary cell wall In pits, the secondary wall is thin or missing, allowing water to flow laterally.
Prototype courtesy of Kelvinsong/Wikimediacommons

Investigations on different levels of h2o stress on the same, or similar, branches would give an indication of plants that are more than vulnerable to cavitation, or air bubbling. Hydraulic conductivity can change de- pending on environmental conditions, and the same species of plant that have adapted to different environments could exist tested in the laboratory or in the field. Compare branch cross-sections of different diameter or those supporting different leaf areas.

Students could observe the effect on hydraulic conductivity of changing the co-operative length and chronicle this to the acme of the plant. They could also investigate the upshot on the flow rate of changing the elevation of the reservoir. The reservoir height (pushing forcefulness) could be considered as equal, but opposite, to the pulling force created by the depression h2o potential in xylem vessels.

Did you know?

one- Cut branch
2- Peel off bark
3- Fasten into tube
iv- Xylem filter
Image courtesy of Boutilier et al.

Xylems are essentially porous filters, and scientists recollect that they could be used to filter water and arrive condom to drink. Earlier this year, a group at the Massachusetts Institute of Engineering science in the USA showed that a 3cmⁱⁱⁱ piece of pine branch could human activity as a filter and remove 99.9 % of leaner from water, at a rate of several litres a day. The technique isn't perfect yet: viruses and chemical contamination tin can't be stopped by twigs, merely the work past Boutilier et al. (2014) suggests a cheap fashion to purify water in developing countries.

References

Boutilier M.Southward.H., Lee J., Chambers V., Venkatesh V., Karnik R. (2014) Water Filtration Using Establish Xylem.PLoS Oneix(2): e89934
Burgess S.S., Pittermann J., Dawson T.E. (2006) Hydraulic efficiency and prophylactic of branch xylem increases with tiptop in Sequoia sempervirens (D. Don) crowns.Establish, Prison cell and Environment29(ii): 229-239. doi: 10.1111/j.1365-3040.2005.01415.ten
Jacobsen A.L., Pratt R.B., Tobin 1000.F., Hacke U.G., Ewers F.Due west. (2012) A global analysis of xylem vessel lengths in woody plants.American Journal of Botany99: 1583-1591 doi: 10.3732/ajb.1200140
Nijsse J., van der Heijden G.Due west.A.Chiliad., van Leperen W., Keijzer C.J., van Meeteren U. (2001) Xylem hydraulic conductivity to conduit dimensions along chrysanthemum stems.Journal of Experimental Phytology52: 319-327 doi: 10.1093/jexbot/52.355.319
Ryan 1000.Yard., and Yoder B.J. (1997) Hydraulic limits to tree elevation and tree growth.BioScience47(4): 235-242 doi: 10.2307/1313077
Tyree Chiliad.T., Ewers F.Westward. (1991) The hydraulic architecture of trees and other woody plants.New Phytologist19: 345-360

Resource

For a scientific analysis of maximum possible tree heights, run across:
- Koch G.Westward., Sillett S.C., Jennings G.K., Davis S.D. (2004) The limits to tree height.Nature 428: 851–854. doi: 10.1038/nature02417; freely available

Author(southward)

Clare van der Willigen has an MSc and a PhD in found physiology from the University of Cape Town, South Africa. Post-obit postdoctoral enquiry on water stress in plants and aquaporins, she pursued her passion for teaching. She has worked in Due south Africa, France, The Netherlands and the United Kingdom, and is currently a senior examiner and instructor of many years' experience.

Review

The commodity describes ii experiments that tin can easily be conducted in science classrooms or laboratories to study water movement in plants.

Although the procedures are easy to carry out, the concepts and knowledge that are explored aren't so elementary, only are appropriate for upper secondary-school students (anile 15-xviii). In my experience, there are non very many procedures that consider water motility for this age group, then many science teachers will welcome this article.

There are likewise relevant opportunities for interdisciplinary educational activity involving mathematics in item. It would be quite interesting to use this experiment equally a starting point to introduce students to the evolution of a database and subsequent statistical analysis (not besides complex). For example, students could guess maximum xylem vessel lengths and mensurate xylem hydraulic conductivity of different plants and at different times (e.m. winter vs. summer). This database could exist extended from year to year with other students. Such a strategy could help students to understand scientific discipline as a collaborative activeness – not but between different disciplines merely also between different 'generations' of scientists.

Betina Lopes, Portugal

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