The effects of submergence and light on two age classes of baldcypress (Taxodium distichum (L.) Richard) seedlings

Abstract

In the early 1900s, baldcypress swamps were harvested en masse in coastal Louisiana, USA. In many areas, natural regeneration did not occur; instead, these areas converted to marsh or open water. One of the factors that may have been responsible for the lack of regeneration was shading of newly-germinated seedlings by herbaceous vegetation. Alternatively, prolonged flooding or complete submergence may have suppressed germination or growth rates of young seedlings and even caused mortality. This study investigated the effects of complete submergence and variable light regime on two age classes of baldcypress seedlings. Newly-germinated seedlings (under two weeks of age) subjected to complete submergence began to show clear signs of stress after approximately one month and substantial mortality following 45 days of submergence. In contrast, one-year-old seedlings submerged for as much as five months experienced up to 75% survival. In a four-way factorial experiment, two age classes of baldcypress seedlings were subjected to five light transmissions (100%, 80%, 50%, 30%, 20%), five flood durations (0 days, 14 days, 25 days, 35 days, 45 days), and two nutrient regimes (fertilized vs. not fertilized). At 100% light transmission, the newlygerminated seedlings suffered complete mortality after 35 days of submergence, whereas the one-year-old seedlings were largely unaffected by prolonged flooding or light regime. Fertilized one-year-old seedlings that were submerged for an entire month had considerably greater growth in height and diameter than seedlings grown under mesic conditions without fertilizer. This is particularly important in coastal Louisiana because several re-introductions (i.e., diversions) of Mississippi River water into declining swamps are planned or underway, and these diversions will periodically increase nutrient and flood levels.