Changes in epiphyte communities as the shrub, Acer circinatum, develops and ages

The Pacific Northwest tall shrub Acer circinatum (vine maple) can host dive rse and abundant epiphyte communities. A chronosequence approach revealed t hat these communities gradually shift in composition as the shrub progresse s through its life cycle. Different epiphytic life for-ms occupy different spatial and temporal niches on shrub stems. These life forms generally shif t upwards along the shrub stem as the stem ages and develops, in accordance ,with the similar gradient hypothesis. We postulate the following sequence of events. An initial wave of colonization occurs as new substrate is laid down. Over rime, superior competitors gradually engulf and overgrow competi tively inferior primary colonizers. Concurrently, shrub stem microclimate c hang rs as shrub stems grow, age. and layer, causing the processes of compe tition and colonization to shift in Savor of different epiphytic life forms during different life stages of the shrub stem. We define four sepal-ate s hrub stem life stages: life classes 1-4 describe, respectively, young uprig ht 'whips'; vigorous, upright, mature sterns; declining stems beginning to bend towards the forest floor; and horizontal, decadent stems. As space on the shrub stem is filled through growth and colonization, interspecific com petition intensifies. Successful competitors persist and spread while poor competitors al-e increasingly restricted to the stem tips, where interspeci fic competition is less intense. m these forests, Usnea, green-algal folios e lichens, and moss tufts excel as the primary colonizers and: become commo n on the outer portions of shrub stems over time, as long as the overstory is nor too dense. Moss mats are also good primary colonizers, but excel as secondary colonizers, often coming to dominate decadent shrub stems. Althou gh all life forms can be primary colonizers, the remaining forms (cyanolich ens, liverworts, and Antitrichia curtipendula) are effective secondary colo nizers Liverworts are also effective Competitors, but less so than the moss mae on the most decadent stems. Cyanolichens appear to benefit from the ag ing and decline of shrub stems. The ability of vine maple to continually ge nerate new young stems through basal sprouting and layering make it a varie d and dynamic substrate for epiphytes. Such shrubs may ace as epiphyte disp ersal agents, with the potential to affect epiphyte continuity within Sores t stands that have experienced large disturbances.