Dispersal, establishment and survival of soredia and fragments of the lichen, Hypogymnia physodes (L.) Nyl.

Dispersal, establishment and survival were studied in a population of Hypogymnia physodes (L.) Nyl. growing on an isolated tree (Prunus blireiana L.) at a site in North Seattle, U.S.A. Lichen propagules were trapped daily on adhesive strips pinned to various sites on the tree over a period of 36 days. Both soredia and fragments were deposited on the strips, particularly on the upper branches of the tree, with soredia being considerably more numerous than fragments. Daily variation in total soredia deposited did not correlate with 10 climatic variables including wind speed, relative humidity and average temperature. Establishment and survival of propagules were studied by introducing soredia and fragments into various sites on the bark and by observing the distribution of small thalli in different microsites. Microtopography of bark significantly influenced establishment and survival. Survival was poor on smooth bark compared with survival on algal or lichen crusts and on rough bark. Survival of soredia did not vary significantly at different locations on the tree. It is likely that colonization of the tree by H. physodes occurs largely by soredia. Colonization appears to be limited more by the range of dispersal over the tree than by differential survival over different parts of the tree.

Dispersal in a population of the lichen Hypogymnia physodes

Dispersal of a Hypogymnia physodes (L.) Nyl. population was studied on an isolated Prunus blireiana L. tree at a site in North Seattle, U.S.A. Lichen propagules were trapped on adhesive strips pinned to four sites on the tree for 7 successive days. Soredia of H. physodes were frequently deposited on the strips but thallus fragments were rare. More soredia were deposited on the upper and lower branches than on the trunk, few soredia were deposited on the underside of the branches. The total daily deposition of soredia on the tree was positively correlated with average daily wind speed. Dispersal downwind from the tree was studied with squares of adhesive contact paper pinned to boards and located at intervals up to 25 m from the tree. Soredia and a few thallus fragments were recorded 25 m and 10 m, respectively, downwind on a day when average wind speed was 10.3 m/sec. The dispersal of soredia by wind from four individual thalli was studied over 10 successive days. Soredia were deposited from each thallus on each day mostly within 2 cm of the source. Higher wind speeds were necessary to dispersae soredia on days when the relative humidity was high. Soredia and thallus fragments were also dispersed by splash dispersal. More soredia were splashed furthest at a splash height of 90 cm. These results suggest that initial colonization of the tree by H. physodes may have occurred by wind-dispersed soredia. Subsequent spread probably occurred from established thalli mainly by the dispersal of soredia by wind and rain splash.

The influence of climate on the dispersal of lichen soredia

The relationship between the daily deposition of soredia of Hypogymnia physodes (L.) Nyl. and local climatic records was studied in the field during three periods at a site in Seattle, WA, U.S.A: (1) 11 August – 16 September 1986 (Study A); (2) 16 December – 11 January 1987 (Study B) and (3) 8 July 1988 – 30 January 1989 (Study C). The soredia were trapped on adhesive strips placed at various locations on a Prunus blireiana L. tree for 24 hr periods. A correlation matrix of the data from all three studies revealed a negative correlation between soredial deposition and relative humidity; and a positive correlation with rainfall and temperature. A multiple regression and forward stepwise regression analysis selected relative humidity as the most significant climatic variable, i.e. more soredia tended to be deposited when relative humidity was low. Analysis of individual studies by multiple regression revealed: (1) no significant relationships between soredial deposition and climate in Study A; (2) positive relationships with temperature and wind speed in Study B and (3) positive relationships with wind speed and rainfall in the summer/autumn months of Study C; in the winter months no relationships with climate were found because few soredia were deposited. The data suggest that in the field seasonal photoperiod differences combined with moderately high temperatures and high relative humidity may promote soredial formation and accumulation on thalli prior to soredia dispersal. In addition, low relative humidity may promote soredial release while wind and raindrops may be possible agents of dispersal.