Arboriculture & Urban Forestry 34(1): January 2008 both English oak and European beech. Consequently, if the pho- tosynthetic system remains intact during periods of waterlog- ging; manifest by higher light absorbance, trapping, and electron transport values, then the plant has the ability to repair damaged tissue and grow and survive after the cessation of stress. Greater energy dissipation fluxes and inactivation of reaction centers per CS of photosystem II associated with non-N-supplemented English oak and European beech indicate a switch from photo- chemical work (light energy absorbed by leaf chlorophyll pig- ments to provide the chemical energy for CO2 fixation) to non- photochemical work, i.e., heat dissipation, a response indica- tive of breakdown of the leaf photosynthetic system (Lawlor 2001). Besides the flexibility during stress periods, the regeneration of normal functioning of plant physiological processes after the stressor disappearance is crucial for survival (Kreuzwieser et al. 2002). After application of N fertilizers at concentrations 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water to the water- logging solution, most physiological parameters used in this in- vestigation of measures of tree vitality (chlorophyll fluorescence PI, photosynthetic rates, leaf chlorophyll content, stomatal con- ductance, leaf, root protein concentration, foliar N content) were comparable to freely drained controls, indicating repairable dam- age to whole tree physiology 10 days after the cessation of 18 days waterlogging. In the case of non-N-supplemented trees or those supplemented with 7.25 g (0.25 oz) N per liter (0.26 gal) of water, values in most instances were significantly lower than trees grown in freely drained growth media indicating im- pairment or partial functioning of a range of physiological pro- cesses. Improvements in the range of physiological measurements discussed previously may account for increased growth as mea- sured by leaf area, shoot, root, and total plant dry weight in trees supplemented with N fertilizers at concentrations 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water at the cessation of 18 days waterlogging and after a 10-day regeneration period. In both tree species, the most pronounced effect of waterlogging on growth was reduced leaf area (Gardiner and Hodges 1996). Leaf abscission caused by waterlogging in the absence and presence of N was observed in both tree species, although less so with increasing N concentration. Leaf area has been shown to be a significant contributor to growth under waterlogging stress be- cause leaves are the major photosynthetic organ responsible for carbohydrate production necessary for the growth and repair of damaged tissue (Farrell et al. 1996). In conclusion, responses recorded in this study indicate the adverse effect of waterlogging can be significantly offset by increasing the N supply to the rooting medium. Marked improve- ments in growth and alterations in the shoot:root ratio for both English oak and European beech by increasing N supply indi- cates there was a nutrient-induced alteration in waterlogging tolerance beneficial to both tree species. Experiment 2: Recovery From Waterlogging Stress Few investigations have recorded the influence of N fertilization on the recovery phase of trees from waterlogging focusing heavily on short term in situ measurements to quantify damage during the waterlogging process (Mclean 1993). Such experi- mentation is limited because deciduous trees possess the ability to recover from severe environmental stress with limited long- 37 term effects, for example, by refoliation, to replenish depleted nutrient reserves (Hermes 2001). Despite severe damage to the leaf photosynthetic apparatus as measured by reduced photosyn- thetic rates, chlorophyll fluorescence, stomatal conductance, and leaf chlorophyll content at day 18 after waterlogging, both test species showed symptoms of recovery by week 6. Regardless of species, highest rates of recovery were associated in trees fertil- ized with 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water. At the cessation of the 6-week recovery phase, photosyn- thetic rates, photosynthetic efficiency, stomatal conductance, and leaf chlorophyll content were in most cases higher than freely drained controls (Figures 3 and 4) and ranged from 20% to 50% higher than non-N-treated trees, indicating regeneration and full functioning of the leaf photosynthetic system and chlorophyll molecules. The extent of resistance to stress has been shown to be closely related to a plant’s capacity for recovery with geno- types that rebound to original or near original physiological lev- els most likely to survive and tolerate stress episodes compared with those that do not or are slower to recover (Aguilera et al. 1997). Although tree vitality measurements were comparable with freely drained controls at the cessation of the 6-week recovery phase, marked effects on growth were recorded with greater growth associated with enhanced N fertilization. During the 18- day waterlogging period, heavy leaf abscission was recorded in both species. At the cessation of the 6-week recovery phase, higher leaf areas were constantly recorded in N-fertilized trees compared with non-fertilized ones. Work by Farrell et al. (1996) using Eucalyptus camaldulensis and that of Pregitzer et al. (1990) using Populus clones reported that leaf production rates were important variables influencing growth under waterlogging stress. More leaves produced during the recovery period as stimulated by greater abundance of N enhanced total leaf area and subsequent photosynthetic area. In turn, growth as measured by total tree dry weight has been positively correlated with net photosynthetic rates (Ort and Boyer 1985). In conclusion, root deoxygenation caused by prolonged wa- terlogging is recognized as a major factor contributing to tree decline in urban landscapes (Hitchmough 1994). Applications of N fertilizers after waterlogging stress would be of benefit to improve tree recovery rates and growth of English oak and Eu- ropean beech. From a practical point of view, N fertilization 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water is tentatively suggested based on the results of this study. LITERATURE CITED Aguilera, C., C.M. Stirling, and S.P. Long. 1997. Genotypic variation within Zea mays for susceptibility to and rate of recovery from chill- induced photoinhibition of photosynthesis. Physiologia Plantarum 106:429–436. Bartels, D. 2001. Untersuchungen zum Stickstoffhaushalt von Esche (Fraxinus excelsior L.) und Stieleiche (Quercus robur L.) in einem Auenwald. PhD thesis, University of Freiburg, Germany. Basra, A.S., and R.K. Basra. 1997. Mechanisms of Environmental Stress Resistance in Plants. Harwood Academic Publishers, London, U.K. Bertani, A., and R. Reggiani. 1991. Anaerobic metabolism in rice roots, pp. 187–200. In Plant Life Under Oxygen Deprivation. Jackson, M.B., Davies, D.D., and Lambers, H. (Eds.). Academic Publishing, The Hague, The Netherlands. ©2008 International Society of Arboriculture
January 2008
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