Plants play a key role in most hydrological fluxes in terrestrial ecosystems, including the capture of precipitation, retention and spatial (re)distribution of water above and below ground, and water loss through evaporation and transpiration. There are considerable differences among species in the extent and manner in which they influence water fluxes. These species-based differences affect not only their own growth and survival, but also influence key ecosystem processes such as growth and nutrient cycling through direct and indirect effects of moisture distribution. This section focuses on plant traits that affect hydrological fluxes external to the plants, and therefore excludes traits related to the flow of water through plants or the storage of water in plants (see Sections 3.3 and 4.4, among others). Differences in the impact on fluxes among species are mainly due to differences in plant architecture, morphology and surface features, which can all be quantified. The effects of litter and small plants on surface runoff and infiltration, and the effect of roots on below-ground movement of water through preferential pathways and altered soil hydraulics, can also be important; however, they are not further discussed here.

A large fraction of incident precipitation hits plant surfaces before reaching the ground. The fate of this water can be (1) free throughfall, (2) retention followed by evaporation or uptake into the leaf, (3) release as throughfall, as a consequence of drainage from saturated surfaces or (4) drainage via twigs and stems (stemflow). Precipitation reaches the ground as throughfall or stemflow. Rainfall interception regulates the amount of water reaching the soil (and plant roots) and may ensure a less erosive water supply during and after heavy showers. Rainfall interception is influenced by the density of the plant crown and its ability to retain water on its surfaces. Water retention on plant surfaces is greatest in low-intensity rainfall in the absence of wind. As vegetation surfaces approach saturation, further interception of precipitation leads to drainage of excess water (but see fog interception under Special cases and extras in the present Section). Plant traits that determine the actual water retention on plant surfaces in a given environment are surface angle and ‘wettability’. Hydrophobic surfaces have sharp contact angles and droplets tend to remain separate, whereas hydrophilic surfaces have small contact angles and water spreads over the surface as a film. The surface traits that most affect wettability are cuticular waxes and trichomes.

How to assess

(1) Gap fraction. The openness or gap fraction of a plant’s crown is an important determinant of the free throughfall fraction (p) (as well as its light interception). For each species, we recommend taking measurements of individual plants, with minimal canopy overlap with neighbours and minimal neighbour foliage below the crown. Gap fraction can be estimated photographically. The lens has to be held neatly horizontally below the canopy and capture the entire crown but not much more. If digital pictures have enough contrast, imaging software can help calculate the fraction of sky (light) relative to foliage cover (dark) within the crown outline. If time or facilities are limited, the gap fraction will be a reasonable predictor of free throughfall. However, p is not necessarily equal to the gap fraction measured, because rain drops do not always fall strictly vertically. See Special cases and extras in the present Section for direct measurement of p.

(2) Stemflow. Stemflow can be quantified by attaching collars or spirals around plant stems that channel the water running down the stems into a collector. Usually, a flexible gutter-shaped form (such as a hose cut in half) is fixed around the stem and sealed with silicone or other sealant. At the bottom of the gutter, a hose leads the captured water into a collector. Collars and collectors need to be able to handle large volumes of water because stemflow concentrates the water running off large areas of foliage. For a quantitative measure of the extent to which different plant species channel precipitation to their stems, it is important to do comparisons under similar rainfall conditions, or to use rainfall simulators. Stemflow varies with plant size and architecture, because these influence e.g. the capacity for water storage on stems, the inclination angles of stems and the extent of branching, among other things. From the perspective of plant traits, stemflow is best expressed as a percentage of the volume of rain falling on the plant, as follows:

stemflow [%] = stemflow [L] / (rainfall [mm] × crown projected area [m2]) × 100%.

(3) Water retention on plant surfaces. This parameter can be estimated at the whole-plant level or for plant parts such as leaves and stems. The procedure involves (1) weighing the plant (or part) without surface water, (2) wetting of the plant (part) and (3) weighing the plant (part) with wet surfaces. The amount of water retained can be expressed in different ways, including water per unit plant surface area or per crown projected area. Wetting can be achieved by immersion or by simulated rainfall or fog. Immersion will tend to give maximum water retention, which is seldom reached in the field. The most realistic estimates are obtained using natural or simulated precipitation with plants in the field. Designs for rainfall or fog simulators can be found in the soil erosion and pesticide spray literature, respectively. Rainfall retention can also be estimated as total interception (rainfall minus throughfall and stemflow) of a discrete rain event that is just large enough to saturate the crown, in a situation of negligible evaporation. This provides estimates of retention where weighing is impractical or impossible.

(4) Leaf wettability. How easily a leaf can get wet is determined by measuring contact angles between water droplets and the leaf surface. Droplets on water-repellent surfaces have greater contact angles and are more spherical. A droplet of standard volume (2–5 μL) is pipetted onto the leaf surface and observed from the side with a microscope. The microscope can be fitted with a goniometer for direct measurement of the angle, or images can be obtained with a camera. An alternative to the measurement of the contact angle is its calculation based on measurement of the contact area between droplet and leaf.

The sampling strategy depends on the research aims. Choose leaves randomly or select in a standardised way according to leaf position on the plant or leaf age. For most leaves, the upper (usually adaxial) surface is the logical surface for wettability measurements. For others, particularly isobilateral and needle-type leaves, as well as leaves that are often exposed to inclined rainfall, fog or mist, measurements on both sides of the leaf are recommended.

(5) Droplet retention ability. How well water ‘sticks’ to a leaf can be measured by placing a droplet of water on a horizontal leaf surface and measuring the angle of inclination of the leaf at which the droplet first begins to move. It is useful to measure droplet-retention ability on the same leaves as those used for determination of leaf wettability.

Special cases or extras

(1) Free throughfall (p). The parameter p can be estimated graphically as the slope of the regression line that describes the relationship between throughfall and rainfall, using precipitation events that are of insufficient size and intensity to cause drainage from these plants. If precipitation is recorded continuously, observations made immediately after commencement of large precipitation events can be used as well. For small-scale applications, rainfall simulators may be used for comparative studies. In most cases, free throughfall will be slightly overestimated because part of rainfall striking vegetation may not be retained because of the force of the impact or the movement of crowns. Methods for quantification of rainfall and throughfall are described in the hydrological literature. They typically include standard commercial rain gauges (pluviometers or pluviographs) or custom-made collectors such as funnels, troughs or large sheets. Continuous measurements of precipitation utilise devices such as tipping-bucket mechanisms that produce output for dataloggers. Variability of throughfall under plants is very large as a result of clustering of foliage and channelling towards the centre of the plant or towards the outer parts of the crown. It can be useful to take these patterns into account by sampling along radii of the plant. Representative (equal-area) sampling of concentric parts of the crown is achieved by following the rule rn = Ön × r1, where rn is the distance between the nth collector and the centre of the plant.

(2) Drip tips. These are morphological features that influence leaf wetness and interception by accelerating drainage from wet leaves. Water is channelled towards the long and narrow tip at the low end of a hanging leaf, which is unable to retain the accumulating water, thus reducing the duration of leaf wetness. Measurement of drip-tip length involves a decision about the position of the base of the drip tip. It is recommended that this is established by drawing the tip of a normally tapering (acute or obtuse angled) tip.

(3) Water absorption by leaves. A certain fraction of water on leaf surfaces may be absorbed by the leaf. Typically, the water on wet leaves represents ~0.2-mm depth. Although most of this water will have evaporated before it can be taken up, water uptake into the leaf can be significant even in non-arid environments. Water uptake is particularly efficient in plants with specialised trichomes such as certain bromeliads. A simple method to determine rates and amounts of water uptake is by weighing.

(4) Fog interception. Fog consists of small water droplets deposited on plant surfaces through air flow rather than gravity (‘horizontal precipitation’). Its interception can be a net gain because fog would not normally precipitate in the absence of vegetation that captures it. Fog interception can be particularly important to many epiphytes or to plants from dry substrates such as rocks and coastal deserts. The main plant traits affecting the rate of fog interception are the surface area in the direction of air flow (e.g. cross-sectional area of a tree crown) and the dimensions of canopy elements. Narrow structural elements with thinner boundary layers are relatively efficient in capturing fog. Trichomes (see Point 3 above in the present Section) and aerial roots, common for epiphytes, may further increase fog capture.

(5) Epiphyte load, which in some cases can be species-specific but generally is not, can hugely influence water interception of rainfall and fog, and probably stemflow (increasing water retention on the host plant and modifying water fluxes). Wherever epiphyte load is important, it should be accounted for, e.g. as a covariate. Epiphyte load can be assessed as mass, as percentage cover or as counts (for large epiphytes). Then, their interception and retention properties can be quantified as described above.

References on theory, significance and large datasets: Skinner et al. (1989); Brewer et al. (1991); Puigdefábregas and Pugnaire (1999); David et al. (2005); Martorell and Ezcurra (2007).

More on methods: Aston (1979); Herwitz (1985); Veneklaas et al. (1990); Meyer (1994); Domingo et al. (1998); Brewer and Núñez (2007); Burd (2007).