In a small saucepan sprinkle the gelatin over the lemon juice, let it soften for 10 minutes, and heat the mixture over moderately low heat, stirring, until the gelatin is dissolved. In a food processor purée the avocado with the cream cheese, the horseradish, and the gelatin mixture until the mixture is smooth. Transfer the mixture to a bowl, set the bowl in a larger bowl of ice and cold water, and stir the mixture until it is thickened to the consistency of thick mayonnaise.
Arrange the smoked salmon slices, overlapping them slightly, on a sheet of plastic wrap to form a 17- by 7-inch rectangle and spread them with the avocado mixture, leaving 1/2-inch border. Using the plastic wrap as an aid and beginning with a long side roll up the smoked salmon jelly-roll fashion, wrap it in the plastic wrap, and twist the ends of the plastic wrap closed. Chill the smoked salmon roll for 3 hours, or until it is frim and the filling is set.
With a 1 1/2-inch round cutter cut out rounds from the pumpernickel, reserving the scraps for another use. Remove the plastic wrap from the salmon roll carefully on a cutting board and with and electric knife or other very sharp knife cut it crosswise int 1/4-inch slices. Arrange each salmon roll slice on a pumpernickel round and garnish each canapé with a parsley sprig and 2 short sections of chive. The canapés keep, covered tightly with plastic wrap and chilled, for 2 hours.
