Clear and black polyethylene mulches were placed over raised seed beds in four studies during a 3-yr period. At intervals during treatment times of up to 12 weeks, soil samples were taken and the recovered seed were tested for germination. The number of seed killed and depths to which they were killed varied with species. The study included natural seed populations of annual bluegrass (Poa annua L. ♯3 POAAN), barnyardgrass (Echinochloa crus-galli (L.) Beauv. ♯ ECHCG), annual species of Cyperus, dayflower (Commelina communis L. ♯ COMCO), and goosegrass (Eleusine indica L. Gaertn. ♯ ELEIN). After 40 days exposure under clear mulch, seed of dayflower were killed to a depth of 11 cm. Under the same conditions, seed of annual Cyperus sedges and barnyardgrass were killed only in the upper 3 to 4 cm. After 3 weeks under clear plastic, germinable seed of goosegrass were recovered only at depths greater than 5 cm. Seed in plots under clear polyethylene mulch were killed more quickly than those in plots covered with black polyethylene mulch.

Pteridium caudatum is a fern that frequently invades burnt areas in the Yucatán Peninsula and other neotropical sites. While post-fire regeneration of this fern apparently occurs mainly by vegetative means, little is known about the role of its spores in post-fire regeneration and in colonization of newly invaded fields. Central to these questions is whether bracken fern spores maintain their viability after fires. Here we experimentally evaluate the effect of fire-induced temperatures on Pteridium caudatum spore germination. We used 1200-cm3 blocks containing a constant fuel load of 47.4 g of litter, in which we placed spores at three different depths. The blocks were then ignited, and temperatures at each depth were monitored at 1-min intervals for 2 h. One day after the experimental fires, spores were dug out and cultured at 25 °C and 12-h light/dark cycles. Soil temperatures decreased significantly in relation to depth during fires. Spores on the surface were severely affected by fire, while those buried at 1 and 3 cm showed 77% germination. Germination in unburned controls was 86%. Our results suggest that during fires, Pteridium caudatum spores buried a few centimetres below the surface have a high percentage of viability, which could explain the rapid establishment of this species in burnt fields.