Effects of severe pruning on the microclimate amelioration capacity and on the physiology of two urban tree species

With the constant increase of the world human population, considering the rise of temperature and the frequency of heat waves predictable by climate change, a mitigation of the negative effects on human health in urban context becomes crucial. The green component of a city has a high potential on climate regulation and it becomes a key factor for the planification and sustainability of the urban environment. Incorrect pruning, especially if made by topping cut and associated whit big cuts, can significantly affect total leaf area of trees, their growth pattern as well as their microclimate amelioration capacity. The purpose of this study is to evaluate the effect of topping practice on the microclimate and on the phys- iology of trees. In 2004, 90 uniform diameter trees of Acer platanoides L. (Ap) and Tilia x europaea L. (Te) of the same age were planted in an experimental plot. After twelve years, when they reach 18 cm of average diameter, half of these trees were submitted to a topping operation, following a randomized block configuration, by removing 85 % of the crown to simulate a bad techniques which are alas common in city green management. The evaluation of human comfort was done using two biometeorological indices calculated from humidity and temperature, measured with a sensor under the canopy while the growth of trees was monitored with biometric measurements. Leaf gas exchanges were assessed during the two growing seasons after topping (2017–2018) with an infrared gas analyser and by measuring CO2 assimilation as the function of internal [CO2], A/Ci curves were drawn. From photosynthesis and transpiration values per unit leaf area, total CO2 assimilation and latent heat dissipation through transpiration have been scaled to the whole plant using Big Leaf model. This study shows how topping significantly impacts the capacity of trees to ameliorate the microclimate. Biometeorological indices showed higher values below topped trees, especially in the hottest months. Despite a temporary increase of transpiration, the energy dissipated by topped trees was significantly lower, due to the smaller total leaf area. Topped trees showed an average value of latent heat dissipated by the hole tree 73,5 % lower in Te and 81,1 % in Ap. Topped trees showed different growth patterns, with higher investment in crown growth at the cost of stem enlargement. This strong crown growth reaction is due principally to adventitious watersprouts and root suckers, with higher average leaf area compared to unpruned trees. In the two years after topping, both species showed some level of compensatory photosynthesis, as well as higher stomatal conduc- tance, chlorophyll content and mesophyll conductance to CO2. Even considering this temporary photosynthetic increase, due to the large amount of surface removed, topping caused a loss in the total CO2 assimilation. These results show how a correct management is necessary to assure an effective and efficient microclimate improvement. The maintenance of apical control and apical dominance are key issues to preserve a healthy tree structure, as well as the long-term efficiency of the photosynthetic apparatus.