:Xylariaceae: Daldinia

ECOLOGY

Ingold (1946, 1965) was the first to recognize that Daldinia, as represented by D. concentrica, is ecologically and physiologically xerophytic. He noted that stromata, detached from their host and brought indoors, will discharge ascospores for 3 or 4 weeks without additional water. The density of the stroma declines from just above 1.0 to about 0.3, i.e., through loss of water from the stroma. Stromata placed in a desiccator over anhydrous calcium chloride continue to discharge ascospores for many days (Ingold, 1965). He noted that a stroma attached to a tree remains active much longer than a detached one, one stroma on a trunk discharging ascospores from 3 May through 17 September, 138 days. He noted that Daldinia, with a summer discharge period, differs from most other lignicolous pyrenomycetes that discharge their ascospores in autumn when conditions are more humid (Ingold, 1965). This was attributed to the curious water relations of the fungus. Walkey and Harvey (1968) noted that D. concentrica discharged ascospores continuously for 47 days, including 22 days without rain. Ingold (1965) showed that in Daldinia-as in Hypoxylon fuscum (Pers.: Fr.) Fr.-ascospore discharge is nocturnal, but the diurnal rhythm of spore discharge can be manipulated by altering light exposure patterns. Discharge of ascospores at night coupled with observations that ascospores can germinate within a few hours after discharge (JDR), is probably advantageous in that they can take advantage of night dew for hydration, i.e., even in areas where rainfall is sporadic or does not correlate with the life cycle of the fungus. It has also been noted in the literature (e.g., Tulasne and Tulasne, 1863; Panisset, 1929) that ascospores of D. concentrica ooze from ostioles in long cirrhi during wet weather. These spores are probably dispersed by splashing rain, in contrast with those discharged during dry conditions which are disseminated by wind. Daldinia concentrica and some (probably all) other species produce conidia on young stromata and/or on invaded substrates. Conidia are dry and wind-disseminated. They germinate at high percentages in water.

It is our opinion that the concentric rings in Daldinia stromata are the key to its xerophytic habit. When rings are initially formed they tend to be dense and more or less gelatinous. As stromata age the rings tend to lose their density and collapse or at least become riddled with lacunae. The gel part of the rings probably holds water that is in some way released to the fungus as required. The waxy to carbonaceous crust encasing the ringed stroma and bearing the perithecia probably retards evaporation from the interior. While the stroma is still attached to its substrate, water produced during metabolism of cellulose and glucose is likely recycled to the stroma. Moreover, the white-rotted wood itself becomes an effective water-holding reservoir that is potentially available to the fungus. Bayliss-Elliott (1920) produced evidence indicating that the successive rings in stromata of D. concentrica are regions of abortive perithecia. If this is true, then the fungus might be looked upon as sacrificing successive potential crops of perithecia to ring formation in order to assure that the maturing final crop will have sufficient water to discharge its ascospores. However, our studies of teleomorphic D. caldariorum cause us to question the universality of this phenomenon (see NOTES on D. caldariorum). Bayliss-Elliott (1920) also noted that stromata last only one year, being mined by slugs. We also have noted that old Daldinia stromata are inevitably riddled by insects. It is probable that some proportion of the ascospores in old stromata are disseminated by beetles and other animals.

Although records of Daldinia distributions are incomplete, they do seem to indicate the xerophytic nature of the genus. Perhaps the greatest diversity in terms of numbers of Daldinia species occurs in Mexico (San Martín, 1992; Pérez-Silva, 1973). Much of this country is warm and dry. Even those species that occur principally in more mesic hardwood belts are subjected to periods of scarce free moisture. The propensity of D. vernicosa in the US (D. fissa herein) and D. vernicosa in Great Britain (D. caldariorum herein) to colonize burned material (Rhoads, 1918; Whalley and Watling, 1980) indicates that these fungi are present in areas that are dry enough to burn frequently, another indication of their xerophytic lifestyles. Finally, Boddy et al. (1985) found that, in culture, D. concentrica was more tolerant of low water potentials than some potentially competitive basidiomycetes. This, again, suggests the essentially xerophytic nature of this species and the genus as a whole.