Tuesday, March 27, 2012

Green Roof Plants, CAM Photosynthesis Minimizes Drought Effects

In prior columns we discussed how some cold tender CAM plants, such as many of the succulents, can be damaged by freezing temperatures.  CAM plants are called CAM plants because they possess a specific form of metabolism called Crassulacean acid metabolism.

I love to use the CAM plants on green roofs because they can survive very long periods of tropical and sub-tropical drought without much in the way of rainfall.
The CAM metabolic process helps succulents and other CAM plants survive in dry, arid regions by working to keep stomata closed during the day when high temperatures and hot drying winds.  If temperatures are high, the sun is bright and stomata are open the plants can rapidly dehydrate.  

CAM plants open stomata at night when temperatures are cooler and solar radiation minimal.  When stomata are open CAM plants take in carbon dioxide (CO2) and store the CO2 in their leaf cells.  Once the sun rises and temperatures increase, CAM plants close their stomata and take the CO2 absorbed from the evening air and begin photosynthesis, producing the substances the plant requires and also oxygen, O2.

An analogy I like to use in comparing CAM plants to C3 plants is thinking about the difference between a gas guzzling 1970's sedan as compared to a new hybrid-type car with a highly efficient engine.  The C3 plants' metabolism is like the 1970's V8 gas guzzler - they both take in lots of fuel, fire up quickly, get to where they are going/growing quickly but are inefficient with respect to fuel utilization.  In the 1970's V8 wasted, unburned fuel leaves the engine as exhaust.  In the C3 plants we see volatilization and evaporation out of the leaf of photosynthesis substances through numerous opened stomata.  Yet both get where they are going/growing - quickly!

CAM plants though are like the highly efficient hybrid electric/petroleum engine.  In addition to conserving CO2 and preventing desiccation by keeping their stomata closed during the day, they are also very efficient at uptaking and using nutrients like nitrogen.  Because a CAM plant's CO2 is limited, the plants have developed mechanisms to become ultra-efficient at nutrient utilization.  Little is wasted.

Because of CAM plants adaptations to hot, arid, dry and drought-like conditions, they make great green roof plants and have been used historically across Europe as such.

Interestingly, some plants like the sedums can switch back and forth between the C3 and CAM metabolic processes depending upon the amount of water and nutrients available in the environment.  This process is called acclimation and is very similar to what we may imagine a grizzly bear's hibernation may be like.

Sedums are considerably more cold hardier than many of the other succulents that are members of the Crassulacaea family and so are popular in colder climates as green roof plants.  Some sedums are so popular for use on green roofs that they have become pest plants, exotic invasive species displacing some types of native vegetation.

Several CAM plants (Agave) surrounded by C3 plants
However, though CAM plants are excellent at surviving heat, aridity, and bright solar radiation and frost if protected, another climatic condition often is a limiting factor.  Though CAM plants, if protected can survive freezing temperatures and certainly can survive long periods of drought, many are susceptible to humidity-heat combination related issues.

Sedums and other succulents are highly prone to fungal attacks during the summer months when the temperatures range between 90 F and 100 F (35 - 37C).

The Southern Blight fungus, Sclerotium rolfsii, also known by the common names 'crown rot' and 'white mold' can decimate a well established roof of succulents or sedums.  Fortunately, southern blight does not seem to bother the succulents during cooler or dryer months.  However the everyday rainfall and resulting high air water vapor combined with high temperatures found during the summer months produce an environment just right for Sclerotium rolfsii, to proliferate.

CAM plants (Graptopetalum) with C3 and C4 Green Roof Plants
But what does all this technical information about photosynthesis and botany have to do with green roofs?

For the nature irrigated green roof, an understanding of C3, C4 and CAM plants and their advantages and disadvantages, their benefits and their limitations, is critical for a successful design.

Green Roof Succulent Injured by Southern Blight

As we continue our discussion of C3, C4 and CAM plants over the next several days we will begin to clearly see how 'Right Plant, Right Place' is important even on green roofs.

Water supplies are limited across the world.  We cannot continue to rely on irrigation use of potable water on landscapes or green roofs.

Designing a nature irrigated green roof will require utilization of a combination of C3, C4 and CAM plants, planted on the roof according to a number of biophysical variables we will discuss.

And so, to date we now understand that CAM plants are ideal for arid, hot areas and can survive drought.  We also know CAM plants may be susceptible to frost or also to fungal attacks encouraged by hot, humid weather.  We know C3 plants grow quickly.

Finally, as we discuss other important factors about plants suitable for a nature irrigated green roof we will begin to develop a sense of understanding as to the type of plant that will work long term on the green roof if planted in associations with other plants and in the right roof location.

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