2. In a small heavy saucepan simmer maple syrup, gingerroot, 3 tablespoons lemon juice, soy sauce, garlic, and salt and pepper to taste until reduced to about 1 cup, about 30 minutes, and let cool. (Maple glaze may be made 2 days ahead and chilled, covered. Bring maple glaze to room temperature before proceeding.)
3. Preheat grill to medium-high.
4. In another small saucepan heat half of glaze over low heat until heated through to use as a sauce. Stir in remaining tablespoon lemon juice. Remove pan from heat and keep sauce warm, covered.
5. Put salmon, skin side down, on cedar plank and brush with remaining glaze. Season salmon with salt and pepper and grill until just cooked through, about 12 minutes.