CANOPY GAP CLOSURE IN THICKETS OF THE CLONAL SHRUB, CORNUS-RACEMOSA

We investigated the rates and mechanisms of canopy gap closure and changes in thicket age, stem density, crown size, and canopy closure with height in thickets of the clonal shrub, Cornus racemosa Linnaeus. We conducted the study in a well drained upland old field with nutrient-poor soil and a poorly drained low old field with more nutrient-rich soil in New York's Hudson Valley. As thickets grow taller, canopy closure tends to decrease because lateral growth of canopy stem crowns does not completely offset decreases in canopy stem density. When stems die, they generally die standing and singly, forming gaps in the canopy. Single stem gaps formed by standing-dead stems were closed predominantly by lateral growth of the branches in the crowns of stems surrounding the gaps. Mean closure rates of experimentally created gaps were greater on average in the wetter and more nutrient-rich lowland site (I 5.0 cm/yr) than in the upland site (5.0 cm/yr). Gaps closed by 8 5%/yr at the lowland site and 2 1 %/yr at the upland site. The fates of understory stems differed between sites, more understory stems reached the canopy in upland thickets. Stem lean contributed to both the opening and closing of individual gaps at both sites. On average, lean opened gaps in upland thickets and had no effect on lowland gap closure rates. Stem lean after gap creation increased with canopy stem density. Larger gaps should take longer to close, allowing more opportunity for understory stems to contribute to gap closure.