Thursday, December 13, 2012

Drinking the Clouds

I don’t often talk about water movement in plants even though I work in a lab that studies water movement in plants. I should probably ruminate on that but won’t. Instead, I’ll start out by putting a complex problem into the simplest of terms: Plants drink water with their roots. Okay, that’s true but perhaps it is a bit too elementary. I think that, in this case, I need to explain it in more detail so that you can really appreciate why a certain research paper caught my attention.

For water to do all the important things it should, it needs to get in and around the plant. This is one of the most basic plant physiology…no, basic biology mechanisms we know. Water diffuses in near the tip of a growing root (you know, the hairy part) and makes its way to the xylem. The xylem is the vascular tissue that conducts water and dissolved nutrients to all parts of the plant. Considering that plants grow up, this water must be moved against gravity. This is where transpiration, root pressure, and capillary action come in. Transpiration (like evaporation, it is the loss of water vapor from parts of a plant, usually the leaves) causes tension and pressure that pulls water up, and root pressure pushes water up when transpiration is low, the soil is moist, and when the roots are absorbing lots of water. Capillary action helps it all as it allows water to flow up the narrow channels of the xylem. The leaves are where the plant usually loses water. Leaves have many, small structures called stomata that function in gas exchange. Guard cells open and close a stoma, and when the carbon dioxide is let in water vapor can be let out. Plants can lose a lot of water through transpiration and have various methods (that I won’t go in to) to try and combat it. Now, this sounds like a lot of info but I’m really just barely scratching the surface (see some links below if you want to know more) of this process. What makes the paper I read today so interesting is that it adds another layer to what we already know about water movement in plants.

The authors of a new paper, published online in Ecology Letters, take a look how plants function in cloud forests. These forests are unique and super neat! Tropical montane cloud forests (TMCF) are among Earth’s most rare and endangered ecosystems occupying just 1.4 percent of the world’s tropical forest area. They are like rain forests in that they receive high levels of precipitation. Where they differ is that much of this precipitation comes directly from clouds, through the cloud filtering of the trees. Lateral cloud filtering is a process where clouds blow among the trees and the moisture condenses as it touches the leaves, forming water droplets. This process reduces the vapor pressure deficit (VPD) and photsynthetically active radiation (PAR), decreasing plant water demand and suppressing leaf-level transpiration. When regular, vertical precipitation is limited in the dry season these cloud water droplets can become an important water source. The researchers in this study looked a phenomenon referred to as foliar water uptake (foliar uptake), where this condensed water is taken in through the leaves, and its occurrence in TMCFs.

The researchers compared neighboring tropical montane and pre-montane cloud forests along the Pacific slope of the Cordillera de Tilarán mountains near Monteverde, Costa Rica. They characterized the cloud cover using remote sensing and created an environmental characterization of the forest by looking at factors such as humidity, temperature, soil water content, rainfall, and leaf wetness. Then they characterized and compared foliar uptake in situ in both forest types, measuring sap flow on the small branches of woody plants using the heat ratio method. They also measured the capacity for foliar uptake in several woody plant species by collecting leaves, rehydrating them, and measuring them individually in the lab.

The researchers found that during the dry season, the pre-montane cloud forests (which are further west and at a lower elevation) are subject to greater rain shadow and less cloud immersion. These forests experienced, on average, a lower number of hours of leaf wetness per day and a shorter duration of each individual leaf wetness event when compared with the tropical montane cloud forests. The higher cloud cover frequency of the TMCF resulted in more, longer leaf wetting events resulting in foliar uptake. They found the prevalence of foliar uptake to be quite high and widespread among species, occurring independently of phylogeny, morphology or growth habit. This foliar uptake also took a significant role in the plants’ water status both in the field and in the lab, resulting in greater water deficit reductions. Furthermore, the results showed that the TMCF plant community demonstrated a higher capacity for foliar uptake than did the pre-montane plant community. This is interesting because foliar uptake benefits the drier, pre-montane forest more and yet these forests are unable to capitalize on them physiologically to the same extent as the TMCF plant communities. This may be a result of the presence or frequency certain leaf traits that facilitate the uptake (cuticle, trichomes, hydathodes) or the ability of water to enter the stomata.

As with everything we do now, look at this through your climate change lenses. What are the consequences of climate change in these ecosystems? Tropical mountains are projected to experience high rates of climate change, increasing dry season surface air temperatures 3.8°C and decreasing precipitation by 14 percent. All of this could increase cloud frequency and cloud base height causing a higher water demand in these plants.

All in all, this new layer to the water movement mechanisms we all know and love is pretty thought-provoking. I know it set out to shed light on the prevalence and role of foliar uptake, but, with me, it had a greater impact in thinking about its consequences. The effect of drought on plants is a hot topic in the plant physiology world, and this paper is an interesting way to look at it.

ResearchBlogging.orgGoldsmith, G., Matzke, N., & Dawson, T. (2012). The incidence and implications of clouds for cloud forest plant water relations Ecology Letters DOI: 10.1111/ele.12039

More on water movement in plants:
Bellevue College's "Transport in Plants" notes
Univ of Illinois at Chicago's "Transport in Plants" lecture notes
Science Mag's "Transpiration: Water Movement in Plants" (Flash player animations that are good)

About cloud forests:
Community Cloud Forest Conservation
Canopy in the Clouds

On this story:
Science Daily: "Cloud Forest Trees Drink Water Through Their Leaves"
UC Berkeley's "Cloud forest trees drink water through their leaves"

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