Attaining Root:Crown Balance in Landscape Trees

'Rootshoot ratio' is term that is often used to describe relationship between belowand above-ground portions of plants. It is defined as the total root system mass divided by total shoot mass, usually on a dry weight basis (17). This description of physical relationship between below- and above-ground parts of plants does not adequately describe important physiological aspects of relationship. The term 'root:crown balance' is more complete. In this paper, root:crown balance is considered as relationship between water and mineral absorption by root system, and utilization by crown. The term 'balance' describes delicate physiological equilibrium between root system and above-ground parts of plants more completely. 'Crown' is used because it describes entire canopy of large landscape plants more appropriately than 'shoot'. The root:shoot ratio is one component of root:crown balance. The rootxrown balance is affected by both physical and physiological changes. Physical disruption of balance is possible by removal of either roots or branches. Physiological changes such as reduced soil moisture uptake, constriction of vascular transport, and excessive transpiration rates can also disrupt balance. Since reduction of water supply will have most immediate and serious effects, reducing root:crown balance (loss of roots or root function relative to crown size) has far more serious implications than increasing it. Root:crown balance is under both genetic and environmental control. When genotypes of Siberian elm (Ulmus pumila) were compared, seedlings from arid climates had higher root:shoot ratios than did those from more mesic climates, when both are grown under identical conditions (4). Cassava {Manihot eschlenta) clones also show large varietal differences in root:shoot ratio developed under identical conditions of culture (3). Variations in environmental conditions can also affect root crown balance, even within crown of a single tree, von der Heide-Spravka and Watson (18) showed that that north side of littleleaf linden (Tilia cordata) had 26 percent more root surface area per unit leaf area than south side. Common arboricultural practices also alter root:crown balance. Transplanting, root pruning along uplifted sidewalks, and root damage resulting from construction activities are all examples of deliberate root system reduction. Urban trees are commonly planted where development of root system is limited by quantity and quality of soil, and risk of elevated transpiration and water stress is high (7), thus decreasing root:crown balance.