Every time John and I botanize through an open scrubby area, such as today, we enjoy an odd-looking species, Coastalplain Golden Asters rising awkwardly from the white sand at varied angles to attain irregular heights. Very martian! To add drama, the funny stems retain a covering of dead withered foliage, more properly called “marcescent” leaves.
Dead leaves usually fall away and decay in most plants, but not always. The Golden Aster appearance always evokes the same old memory for me. Back in the Reagan Administration I had the good fortune to work at high elevations in South America where several unrelated plants resemble Golden Aster by having marcescent leaves covering an otherwise bare stem. Around the world, this life form has evolved repeatedly, usually in exposed habitats where a drying risk is coupled with fluctuating temperature extremes, often intermittent frost alternating short-term with warm temperatures. Of a few examples here in South Florida, Golden Aster is the most striking. (We’ll look at Rabbit Tobacco another day.)
Coastalplain Golden Aster is generally described as a “biennial,” hunkered down the first year as a rosette on the ground, with the stem then rising the second year to flower and fruit. New rosettes form at the stem base. I’m not 100% sure the plant always obeys its biennial characterization.
Here is Espeletia in Ecuador: CLICK
Here is Golden Aster in Stuart, Espeletia Jr.:
We might say blithely, “well, the dead leaves protect the stem.” OK, but exactly how, from precisely what? If anyone has looked into it at a physiological level in Chrysopsis, I can’t find it. But botanist Alan Smith back in the 70s took a hard look at Espeletia in Venezuela, and provides inspiration for a better look at our similar local case. Dr. Smith found the Espeletia habitat to feature strong seasonal differences in rainfall, like us. And there were wide strong short-term temperature fluctuations, like us. The greatest temperature stress and moisture stress occurred during the dry season, likewise the case in Florida if the main temperature stress is frost. (We live near the southern limit of the all-Florida geographic range for Chrysopsis scabrella.)
Smith and other botanists have interpreted marcescent leaf blankets as a buffer against fluctuating temperature extremes. Removal of the dead leaves cost a lot of Espeletias their lives. The main apparent reason was that during times of frosty nights alternating with warm days stems with their dead leaves intact never dipped below freezing, whereas the ones with marcescent leaves removed dipped and died. Those dead old leaves don’t radiate heat at night.
It may seem odd to speculate that frost protection might be the “main” benefit of marcescent leaves, especially in a plant like Golden Aster so obviously exposed to extreme drying. Don’t those dead leaves merely protect the stem from hot dry winds? Maybe, but two reasons suggest otherwise:
A. In general, water loss from stems is not severe. The stem probably does not need much protection from direct drying. (Cacti are all-stem.)
B. Frost stress is drying stress. One of the worst aspects of frost for a plant (in a not-very-frosty borderline setting) is that freezing in the stem diminishes water passage from the roots to the leaves. An plant in a super-dry setting with temperatures hitting 80 degrees by day and dipping below freezing by night has much to fear from Jack Frost. The warm day, especially at dawn, creates high demand for water to the living leaves, but if frost-impaired stem tissue can’t deliver, well that’s tragic. Walking through the scrub in 90 degree weather and 90 percent relative humidity, it takes some faith to see those stem-blankets of dead leaves as possible winter coats.