Complex Above- and Below-Ground Growth Responses of Two Urban Tree Species Following Root, Stem, and Foliage Damage-An Experimental Approach.

Valentina Vitali,Sylvain Delagrange,Guillaume Perrette,Alain Paquette,Jorge A Ramirez,Christian Messier

doi:10.3389/fpls.2019.01100

Valentina Vitali, Sylvain Delagrange + Show 4 more

Open Access

https://doi.org/10.3389/fpls.2019.01100

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Abstract

Urban trees are subjected to numerous biotic and mechanical damages, which can affect their growth rates and health. However, for most species, a systematic analysis of tree above- and below-ground growth reactions to a variety of damages is still lacking. Under a fully factorial experimental setup, using two common urban trees (Celtis occidentalis, Fraxinus pennsylvanica), we tested the effects of various degrees of frequently occurring damage as defoliation, root reduction, and stem injuries for a total of 18 treatments. We hypothesized that (i) an increasing amount of damage would proportionally negatively affect both root and stem growth; (ii) there would be a lag or lasting effect on growth; and (iii) both species would react similarly to the treatments. Contrary to our expectation, increasing levels of single or combined damage did not have an incremental effect on either stem or root growth. Although Celtis was significantly less vigorous than Fraxinus, it did not react strongly to damage treatments compared to the control. Interestingly, Celtis that experienced stem damage alone or in combination with other damages showed higher growth rates than the control. For Celtis, root injury was the treatment having the most impact, decreasing both root and stem growth consistently throughout the 5 years following treatments, whereas defoliation decreased growth only in the first 2 years. All damage treatments negatively affected stem and root growth of Fraxinus trees. Stem growth was affected the most by defoliation in the first year following the treatment, while root injury became the driving factor in subsequent years. For both species, stem injury showed the least influence on growth rates. The control and low-level damage treatments often affected growth rates in a similar way, suggesting that low-intensity stress triggers compensatory reactions stimulating photosynthetic rates and nutrient utilization. The slower-growing tree species, Celtis, showed a less negative reaction to all damage treatments compared to Fraxinus. This study illustrates that various types of above- and below-ground injuries do not have a simple additive effect on tree growth and that trees are capable of compensating for the loss of foliage, roots, or phloem to meet their metabolic demand.

Highlights

Trees growing in urban landscapes are an invaluable asset as they provide numerous environmental, social, cultural, and economic benefits (Konijnendijk et al, 2005; Tanner et al, 2014)
Mixed Models to Evaluate Overall Effects for All Years. Both stem and root growth of Celtis were less affected by the various levels and combinations of damage treatments than Fraxinus, showing few significant growth differences when compared to the control; in some cases, the stem damage treatment stimulated growth (Table S.1 and Figures 4A, C)
Celtis root growth was more strongly impacted by defoliation than by root reduction in the 2 years after the damage treatments and vice versa in the later years, while the impact of defoliation and root reduction on Fraxinus root growth was similar in all years

Summary

Introduction

Trees growing in urban landscapes are an invaluable asset as they provide numerous environmental, social, cultural, and economic benefits (Konijnendijk et al, 2005; Tanner et al, 2014). These trees have to withstand a wide variety of damages and accidental injuries, such as root trenching for road work or sidewalk placement or reparation; trunk scarification damage caused by construction and infrastructure maintenance; and crown reduction or defoliation through pruning, drought stress or insect infestation (Clair-Maczulajtys et al, 1999; Millet and Bouchard, 2003; Smiley, 2008; Jacquet et al, 2012) Such levels of damage and stress, alone or in combination, can dramatically affect tree growth and vitality and lead to mortality, thereby increasing costs for removal and replacement. Tree species that have the capacity to tolerate the most extreme environmental conditions have low plasticity and low growth rates (Niinemets, 2010b), possibly due to the associated carbon cost of maintaining a positive carbon balance during acclimation (Ramirez, 2017)

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