Tree Growth after Trenching and Compensatory Crown Pruning

The roots of pin oak (Quercus palustris) trees were severed by trenching on 1, 2, or 3 sides the tree. The distance between the trunk and the trenches was approximately equal to the trunk circumference. Half of the trees also received compensatory crown pruning. All trees survived and significant dieback was noted only on trees that were trenched on 3 sides. Generally, the more extensive the trenching, the greater the growth reduction and dieback, and the longer the reduction persisted. Compensatory pruning increased twig growth for all trenching treatments, but seems to be most beneficial in reducing dieback after severe root loss from trenching. Under certain circumstances and in the absence of other construction impacts, vigorous trees may be able to tolerate and recover from trenching in the root zone. Established trees on construction sites are often subject to mechanical damage and site changes that could eventually result in their decline and death. Damage to the root system is more difficult to evaluate even when easily observed as an open trench or an obvious grade change. In other situations, root damage may go completely unobserved if the site is not evaluated until after construction and final landscaping is completed, obscuring the cause of tree decline. Compacted soil may be just as damaging to root systems as direct mechanical damage (Ziza et al. 1980). Even if the tree root system is carefully protected during construction, other changes on the site, such as drainage, can alter the root environment within the protected zone. It can be difficult for arborists to evaluate separate and combined effects of the many types of treedamaging activities that may occur on a construction site and to predict their effect on the tree and its root system. There is little published research to help specify how much of the root system must be protected for a reasonable chance of long-term survival. Industry practices use above-ground tree features to specify dimensions of root protection zones. Branch spread (dripline) is most commonly used (Olson and Wray 1979; Schoeneweiss 1982; Fazio 1992; Miller et al. 1993 ). Trunk diameter (Morel 1984) and tree height (Miller et al. 1993) are also sometimes used to determine the size of the root protection zone. While these guidelines may be helpful, they have not been tested in controlled studies. One purpose of this experiment was to study the long-term effects of root loss resulting from trenching in the absence of soil compaction, grade changes, and other factors. Specifications for augering near trees (Morel 1984) called for trenches to be no closer to the trunk than 8 to 12 cm for each cm of trunk diameter (8 to 12 in. for each inch) measured at breast height [dbh, 1.4 m (4.5 ft) above ground level]. When the specifications were followed, no growth reduction occurred (Miller and Neely 1993). When trenches were much closer to the trunk, growth was reduced. No trees died from the trenching. Young trees may be stressed by root loss but may be able to survive. The practice of pruning to compensate for root loss has been used for years, but its value is often questioned. Pruning is intended to relieve stress by rebalancing the size of the crown with the reduced root system. But nearly half of the root system can be lost from a single trench near the base of the trunk. The crown cannot be pruned as extensively and still retain the natural form of the tree. Removing a significant number of branches is sometimes thought to eliminate vital carbohydrate production and reserves (Shigo 1993). The 2nd purpose of this experiment was to study the value of compensatory crown pruning after root loss from trenching. 48 Watson: Tree Growth After Trenching and Crown Pruning Materials and Methods The trees used in the study were pin oaks (Quercus palustris), 29 cm (11.4 in.) average dbh, at The Morton Arboretum in Lisle, Illinois. The trees were planted in groups of 3, spaced approximately 2 m (6 ft) between trees in a straight line. Groups were planted on 15 m (50 ft) centers. Five groups were incomplete (only 2 trees). A 3rd tree from an adjacent incomplete group was used in these cases. All trees were in good health at the start of the experiment. Trenching and crown pruning treatments were assigned to the tree groups randomly. Trenches were dug on May 29 and 30, 1991. The trenching was timed to subject the trees to sudden root loss when the leaves were nearly fully expanded, but not yet hardened-off. At this time, stress resulting from root loss was expected to be most severe. Trees were trenched on 1, 2, or 3 sides (designated as treatments 1T, 2T, and 3T, respectively) with a 1 m (3 ft) maximum depth trenching machine. Trenches were 15 cm (6 in.) wide, extending a distance of 30 cm (1 ft ) from the trunk for each 2.5 cm (1 in.) dbh. Each tree in each group of 3 received a different trenching treatment (Figure 1). The distance between the trench and the base of the trunk was approximately equal to the trunk circumference with slight variations caused by machine access and tree spacing. Trenches were refilled with the same soil (all horizons mixed) and allowed to settle naturally. No other traffic or changes were permitted on the site. No trenches were installed near the control (treatment C) groups of trees. Five groups were used as controls (15 trees), so that there would be 5 replicates of each relative position in the control groups (west end, center, and east end) in the event that position within the group affected growth. Compensatory crown pruning was completed on the same days as the trenching (treatments 1TP, 2TP, and 3TP with 1, 2, or 3 trenches, respectively). A commercial tree care firm, Hendricksen, the Care of Trees, pruned the trees as they would do on an actual construction site. Trees were thinned moderately and lateral branches tipped • • Base of tree