18 February 2011

How Do They Do It?

The shores of Lake Michigan... certainly not the rich, fertile soils that characterize much of the rest of the cornbelt. In fact, this is one of the harshest and most inhospitable habitats in the Midwest. The coarse-textured sand does not hold much moisture, the sun beats down relentlessly and reflects off of the quartz-based substrate, and winds whip wildly off of the big pond. Yet, somehow, a specific suite of vascular plants are able to thrive in this desert-like community.

The dominant plant in the photograph above is Marram Grass (Ammophila breviligulata), a colonizing species that is thought by many to be almost entirely responsible for the presence of vegetation just off the coasts of the Great Lakes and the Atlantic Ocean. Marram Grass is at its best in dry, windy areas with shifting sands. As it becomes established, its root system stabilizes the sand and its leaves and stems capture moving sand, eventually leading to the formation of foredunes. Marram Grass has adapted to be able to tolerate the dry, windy conditions common where it grows by developing long, narrow leaves that are often rolled or folded to keep the stomata cool and shaded.

Another of those hardy colonizers is Seaside Spurge (Chamaesyce polygonifolia), shown above. This prostrate-growing, sand-loving species is regularly buried by blowing sand, yet it is able to persist in small patches scattered along the beach. To tolerate the windy conditions present along the lake, Seaside Spurge has adapted to grow only a few centimeters tall, keeping the plant from drying out by keeping it mostly out of the elements. It also has somewhat fleshy leaves with milky sap to help keep the plant from drying out.

Farther from the lake but in equally dessicated situations grows one of our most interesting milkweeds, Clasping Milkweed (Asclepias amplexicaulis). Although it often grows in areas with a higher density of vegetation, as seen in the photograph above, it can persist in the driest and finest of sands, and it often grows in disturbed areas. The leaves of this species are often somewhat appressed to the stem, exposing less surface are to the rays of the sun. Like Seaside Spurge, Clasping Milkweed has somewhat fleshy leaves and a milky sap to help resist dessication.

One of the characteristic plants of sand blowouts within the dunes is Beach Heather (Hudsonia tomentosa). Like the other species featured in this post, Beach Heather is able to withstand the arid, brutal environment in which it grows through a series of morphological adaptations: it is a low-growing, mat forming plant; the leaves are small and scale-like to reduce its surface area exposed to the sun and to reduce the amount of water that is loses through evapotranspiration; and as sand is blown over the plant and covers lower parts of the plant, Beach Heather is able to produce roots from the lower stem nodes so that it continually "moves" to ground level.

Finally, the poster child for the deserts of the Great Lakes, Eastern Prickly Pear (Opuntia humifusa). This cactus has many of the same adaptations as the plants above (low-growing, fleshy, etc.) that allow it to survive in the harshest of conditions.

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