Arboriculture & Urban Forestry 47(4): July 2021 increase the time taken before tissue injury occurs (Rasmussen et al. 2009; Varner et al. 2009; Clarke et al. 2013; Maringer et al. 2016; Bär et al. 2019). The fact that root tip growth commenced before epicormic or lignotuberous bud development empha- sises the importance of a healthy root system. The arboricultural management principle of prevention being better than cure can be applied to stressed trees. Proactively managing for the best soil conditions possible prior to, during, and immediately after stress may be a key to successful shoot production and tree recovery. If the use of mulch, irrigation, decompac- tion, or a combination of treatments before and imme- diately after stress can enhance root tip survival, then a more rapid shoot development and higher rate of shoot survival may be achieved, enhancing the chances of tree survival. The rapid rates of height increment and reestab- lishment of a leader after treatment were expected from earlier work (Moore 2015) and are largely explained by the removal of apical dominance (Rasmussen et al. 2009; Rasmussen and Hunt 2010), which is grad- ually reestablished over the recovery period of 10 weeks. The rapid rates of shoot growth allow for the reestablishment of photosynthetic capacity after defo- liation from stresses, such as fire, drought, and heavy grazing. Increases in height growth rates with some significant reductions in dry matter production after treatment suggest that the height increases were due to internodal elongation. This is consistent with hor- monal involvement in shoot production and can be interpreted as allowing the plants in their natural environment to resume competitive growth. Trees that retain healthy, intact leaves after heat stress or decapitation had higher rates of shoot and overall plant survival. The production of photosyn- thate by leaves may be partly responsible for this improvement, but the differences were not signifi- cant, and this aspect of plant response require further research. For arborists managing stressed or damaged trees, parts of trees bearing healthy foliage should be left intact for as long as possible to allow more suc- cessful shoot production as part of post-stress recov- ery, even if they have to be removed later to ensure sound tree structure or human safety. The dry weight data suggest that a double dose of stress (heat and delayed decapitation) reduces dry matter production, and that early removal of heat- damaged tissue both accelerates recovery and increases 145 dry matter production. The decapitation treatment a week after heat treatment is an added stress when seedlings are in the process of recovery, perhaps explaining the negative response. The arboricultural implication of these results is that in managing the urban forest, every effort must be made to avoid dou- ble dosing already stressed trees with another stress. Immediately after treatment where the apical bud has been killed by heat or removed, there is a loss of apical control, and epicormic or lignotuberous shoots develop rapidly from nodal position 1 to 3, free of apical hormonal control (Meier et al. 2012). As shoots develop over the 10-week recovery period, apical control is reestablished, and the rapidly growing plants develop a classic conical shape consistent with the theories of apical dominance and control (Dun et al. 2006). While these experiments demonstrate that some of the effects of heat treatments can be explained in part by decapitation, there seem to be other factors at play. The immediate removal of heat-damaged tis- sue after heating resulted in enhanced height incre- ment, increased dry weight, and earlier commencement of shoot production, all of which are consistent with an inhibitor affecting seedling growth and the concept of translocated heat injury, in which heat damage to one part of a plant results in the pro- duction of inhibitors that are translocated to other parts, where they affect growth and development (Yarwood 1975; Bär et al. 2019). Four sensors have been suggested as triggers for heat stress responses (HSRs), including a plasma membrane channel for an inward calcium flux, a histone sensor in the nucleus, and unfolded protein sensors in the endoplasmic reticulum and cytosol, which activate HSR genes, leading to enhanced thermo tolerance, but the mecha- nisms of action are still unclear (Mittler et al. 2012). Immediate decapitation would remove inhibitors or phytotoxic degradation products and circumvent HSR, while delaying decapitation for a week would not. It can be postulated that the roots hold the key to whole plant recovery. In some heat treatments, tem- peratures and/or durations were insufficient to directly damage lignotuberous tissues, but seedlings died. Sections of these plants showed undamaged lignotu- berous tissues and, in a few cases when roots failed to resume growth, it was discovered that there had been stem damage at the growing medium surface, below the lignotuber, and so whole plant recovery was impossible, as all lignotuberous and epicormic buds ©2021 International Society of Arboriculture
July 2021
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