Biomass and Nutrient Distribution and System Nutrient Budget for Western Juniper in Central Oregon

Invasion of western juniper (Juniperus occidentalis Hook.) into big sagebrush (Artemisia tridentata Nutt.) communities of eastern Oregon implies redistribution of nutrients among ecosystem components, with management implications as stands mature. To assess these effects, we measured biomass and nutrients in juniper trees of five size classes ranging in average age from 36 to 161 yr, and in understory vegetation. Understory was measured at 0.0, 0.5, 1, 2, and 4 canopy radii from the bole and in areas devoid of juniper ("no juniper"). Tree size and distance from boles had no effect on biomass or concentrations and amounts of nutrients in understory vegetation. Above-ground tree biomass increased with maturity from 18.4 kg for size class 1 to 741.4 g for class 5. Biomass accumulated rapidly (22.3 kg/tree/yr) between classes 2 and 3. Foliage comprised 41% of tree biomass in class 1 trees, but decreased to 15% for class 5 trees. Concomitantly, percentage of tree biomass in boles and live branches increased from 29 to 47%. From class 1 to class 5, amounts of nutrients increased as follows: nitrogen (N) from 89 to 2117 kg/tree; phosphorus (P) from 12 to 283 g/tree; and sulfur (S) from 10 to 294 g/tree. In open systems (intercanopy and "no juniper" areas), soil contained 89.9 to 99.8% of the organic carbon (C sub(org)), N, P, and S of entire systems. For tree systems (areas occupied by juniper canopies), C sub(org), N, and S in aboveground biomass increased with increasing tree maturity, while P did not change. For the most mature trees, proportions of total system N, S and P allocated to the organic layer and trees was 30, 78, and 5%, respectively. Considering the N and S limited nature of these sites, a conservative harvest management approach that emphasizes retention of organic layers and tree foliage appears prudent.