Sandy wasteland? Ha!

Our last few forays into botany have been fairly tame, meet-the-Family stuff. Today, we’re going to mix it up and do what can only be described as extreme botany. Plants grow in some rather unlikely environments on Earth, and sand dunes are certainly one of the more unlikely environments out there. Seaside sand dunes present most of the same challenges as a desert, and what benefits may be gained by the more abundant rainfall are almost entirely wiped out by the periodic saltwater floods, and salt-spray laden winds. For the record, most (but not all) plants are not fans of salt. I’ll devote a whole post to the halophytes eventually...

 In order to survive and reproduce successfully, all plants must adapt to local conditions. What exactly distinguishes the conditions of a dune environment from the conditions of, say, a semi-arid grassland, and makes dune plants incredible survivors? Well, there is the salt. Even without the salt, dunes regularly experience high winds, which, in combination with the salt spray and wind-driven sand, have the potential to damage plant tissues. Dune plants experience what could be called a water water everywhere and not a drop to drink situation - the ocean is close, but all that water is saline. Fresh water tends to be in short supply on the dune, even in climates with plentiful rainfall. Sandy soils have the lowest water-holding capacity of all soils, so dune soils tend to be in a perpetual state of water limitation. Nutrient availability is also limited on the dune. Finally, especially on the outermost dunes, there is virtually no shade. Combined with the reflectivity of sand, this makes for extremely high temperatures on sunny days, as anyone who has run across a beach barefoot in high summer can tell you. So, what do dune plants do?

Start with the salt. For dunes, salt typically comes in the form of wind-borne spray, along with sand. Dune plants exhibit adaptations to deal with these conditions - many plants grow low to the ground, staying within the boundary layer of the dune. The boundary layer, in brief, is the air adjacent to a solid surface - friction forces exerted by that surface are strong enough to overcome a certain amount of wind force. The further you go from the surface, the weaker the effect. Boundary layers vary with the size of the object - planet Earth has a boundary layer of several kilometers, while a sand dune might have a boundary layer of two centimeters. The effect isn’t confined to planets or sand dunes - many plants of windy environments, be they tundra, alpine or what have you, grow very low to the ground so as to remain within the boundary layer (and thus experience much lower wind speeds). So, a number of dune plants, like the Dune Sunflower, hug the boundary layer.

 Clever plant

Other dune plants have tough leaves and stems to protect against potentially scouring winds. Then there are more specialized adaptations. Sea oats have tapered, slightly rolled leaves that catch incoming sand and safely funnel it down to the base of the plant.

 Just imagine sand sliding down that.

Actually, sea oats boast an array of interesting adaptations to life on the dune. The noble Uniola paniculata dominates dunes throughout the southeastern United States, and after a closer look, it isn’t hard to see why. Sea oats tolerate salt spray, and even peridioc inundations. The plants boast enormous root systems, which accomplishes a triple purpose. One, deep, widespread roots anchor plants, even in the windy air and loose soils of the dune. Two, a large rooting system aids in acquisition of water - the more area of soil a plant can access via roots, the more water is potentially available. Three, for the same reason, a large rooting system aids in acquisition of nutrients.

Many dune plants, sea oats included, scatter their pollen and, eventually, seeds via wind dispersal. Other plants use specialized leaf structures to avoid drought stress. The sea rocket and the prickly pear cactus, among other plants, have thick, fleshy leaves - a botanical characteristic known as succulence (http://www.desertmuseum.org/programs/succulents_adaptation.php). The point of having succulent leaves or body structures is to maximize the amount of tissue that can be used to store water, while minimizing the amount of surface from which water can potentially be lost (water moves preferentially from areas of higher water concentration to areas of lower concentration. If you are a leaf in the desert, this is not a good thing). The result is a thick, rounded leaf, often with a waxy coating to prevent further water loss. 

                                                               One of my favorite dune plant adaptations is the coating of small, light-colored hairs found on some plant leaves. The main point of these hairs is to increase the reflectivity, or albedo of the leaf. Surfaces with a high albedo reflect back a greater proportion of incoming radiation, and thus absorb less of it as heat. By increasing leaf albedo, plants are able to lessen the heating that a leaf experiences, even in the shadeless environment of the dune.

 Take a close look...

As always, there’s more to say, but if you can think of a challenge to dune life, there is probably a dune plant that has a really cool adaptation. Dune plants also “give back” to the community - the massive rooting structures of sea oats, for example, play a major role in overall dune stability, and those cool, slightly rolled leaves neatly deposit sand at the base of the plant, which aids in sand accumulation and dune growth. Pretty cool, huh?