The effects of rooting volume, greywater irrigation and reduced sunlight on climbing plants for indirect green façades in urban environments

Abstract

Dense urban areas with limited ground space are often deprived of vegetation. By growing climbing plants on buildings as indirect green facades, vegetation can be implemented in a way that uses limited space and seamlessly integrates man-made structures with nature. Benefits of indirect green facades include air purification, noise reduction, micro-climate regulation, provision of natural habitats and improvement of the physical and psychological well-being of urban residents. However, in order to provide effective ecosystem services, indirect green facades need to develop great vegetation coverage to cover buildings, which can be challenging due to small rooting volumes, lack of potable water availability for irrigation and variable light conditions, imposed by site constraints and weight loading of containers on elevated structures. Therefore, it is important to research how indirect green facades can be successfully developed for cities in the presence of unfavorable growing conditions to ensure their sustainability. In this thesis, I have investigated how woody climbing plants respond to different growing conditions by evaluating their morphological and physiological traits. The three objectives of this thesis are: 1. To evaluate the effect of rooting volume on climbing plant growth, coverage and thermal tolerance in the first growing season (Ch 2); 2. To investigate whether greywater is a viable irrigation resource for indirect green facades in cities by assessing changes in substrate chemical properties and the growth response of six climbing plant species (Ch 3); 3. To explore how shade affects leaf traits and thermal tolerance of climbing plant species assisting green facade design and plant selection (Ch 4).

In chapter two, to examine the impacts of rooting volume on climbing plant species during the first growing season for their establishments, three rooting volumes (21, 42 and 63 L) were utilised to grow two climbing plant species; the slower-growing Akebia quinata and the faster-growing Pandorea pandorana, on east- and west-facing aspects. It was hypothesised that the reduced rooting volume will adversely affect growth, wall coverage and thermal tolerance of climbing plant species. The smallest rooting volume (21 L) significantly reduced plant biomass growth, specific leaf area and percentage wall coverage for both species over the measured six-month period. Surprisingly, neither climbing plant species significantly increased in growth and vegetation coverage when provided with the largest rooting volume (63 L) as compared with medium rooting volume (42 L). The shoot:root ratio and thermal tolerance of climbing plant species however remained unchanged across three different rooting volumes. A significant decline in the ratio of variable to maximum fluorescence (Fv/Fm) was observed for all climbing plants on the west-facing aspect due to heat stress, irrespective of rooting volume treatment.

The outcomes of chapter two suggests that, for a wall area of 2.8 m2 with stainless-steel mesh (1.45 m (W) by 1.95 m (H)), 42 L rooting volume is adequate to support plant growth and coverage of indirect green facades for the first growing season. Practically, for every cubic metre of rooting volume, A. quinata and P. pandorana could approximately achieve 10.2 m2 and 21.3 m2 of wall coverage at the end of the first growing season.

In chapter three, an 18-week glasshouse experiment was conducted to evaluate the effects of greywater irrigation on substrate chemical properties (pH and electrical conductivity) and the growth of six climbing plant species. I hypothesised that domestic greywater would change substrate properties by increasing pH and electrical conductivity, and thereby adversely affect the growth and health of climbing plant species. Synthetic greywater formulated to conform with domestic greywater was used in this study. Three irrigation treatments were applied to container-grown climbing plants: (1) potable water irrigation, (2) greywater irrigation, and (3) greywater irrigation with a potable water ‘flushing’ once every three weeks. After 18 weeks, the results showed that substrate pH did not differ among treatments; however, increased substrate electrical conductivity was evident in containers receiving the greywater with potable water flushing treatment.

Both greywater treatments had no significantly detrimental effects on plant biomass, leaf area, gas exchange rates, or water use when compared with potable water irrigation. The outcomes indicated that domestic greywater can be safely used to irrigate climbing plants for indirect green facades, reducing the need for the use of potable water and supporting more sustainable design of indirect green facades in urban environments.

In chapter four, a 15-week field experiment was designed to assess how seven climbing plant species respond to changing light conditions from full-sun to shade, with respect to the adjustment in leaf morphology, physiology, and photochemistry for mature plants. The hypotheses were (1) a change from sun to shade will increase specific leaf area (SLA), leaf expansion rate, and chlorophyll content of climbing plants; whilst decreasing leaf photosynthetic rate (A1500), stomatal conductance (gs), light compensation point (LCP) and thermal tolerance (T50), and (2) deciduous climbing plant species will be more plastic (greater changes in morphology and physiology) in response to shade than evergreen species.

Significant increases in SLA, and unchanged gs, LCPs and T50 were observed in all seven climbing species after being provided shady conditions. In contrast, the variation in species response to reduced light intensity levels was evident in leaf expansion rates, A1500 and Chl a+b. Leaf expansion rate and A1500 remained unchanged for most of the climbing plant species by the shade treatment. However, significant increases in leaf expansion rate were observed in evergreen Gelsemium sempervirens and Jasminum azoricum. In addition, significant decreases in A1500 were recorded in deciduous Ampelopsis brevipedunculata and Vitis ‘Ganzin Glory’ as well as evergreen Pandorea pandorana under the shade treatment. On the other hand, leaf Chl a+b was significantly increased in most of the climbing plant species by the shade treatment, except for deciduous V. ‘Ganzin Glory’ and evergreen G. sempervirens.

Irrespective of light intensity levels, deciduous climbing plant species displayed significantly greater SLA, leaf expansion rates, A1500, gs, and Chl a+b, whilst lower LCP than evergreen species in this study. On the contrary, deciduous climbing plant species were not more plastic than evergreen species in the overall response to the reduced light intensity levels. However, a greater reduction in A1500 in response to shade was observed in deciduous climbing species as compared with evergreen species.

The findings of chapter four demonstrated that these seven climbing plant species can maintain growth and health by adjusting leaf morphology and physiology when experienced changing light conditions from full-sun to shade. The findings suggested that the seven climbing plant species can maintain or increase vegetation wall coverage and provide ecosystem service benefits in cities with variable light conditions.

Overall, this thesis has indicated that climbing plants can grow successfully with relatively small substrate volume (42 L). It was also shown that a lower limit exists for the rooting volume required to support the early growth and vegetation wall coverage of indirect green facade climbing plants. This thesis also raises the possibility that domestic greywater can replace potable water irrigation of building indirect green facades in urban environments, as there was no deleterious effect on either substrate properties or climbing plant growth. Furthermore, the seven climbing species evaluated in this thesis all could adapt to the changing light conditions from full-sun to shade by adjusting leaf traits to maintain growth (leaf expansion). Despite the limitation noted in rooting volume, woody climbing plant species were quite resilient and adaptable to the different growing conditions evaluated in this thesis. This would likely enable them to maintain their function and performance and thereby continue to deliver ecosystem services under fast-changing urban environments. The findings of this thesis have significant implications for the understanding of how woody climbing plants respond to potentially unfavorable growing conditions, providing insights into plant selection for indirect green facades in urban environments. This thesis further supports the possibility to implement indirect green facades as an integrated approach of sustainable urban greening and water management on high-rise buildings. Further research should be undertaken however with a longer experimental period (beyond six months), the inclusion of more climbing plant species and field studies (i.e., existing indirect green facades on buildings). The research should also be conducted with the inclusion of different types of substrates with the application of on-site greywater and various light intensity levels (e.g., lower or higher).