Climate change and urban trees: using conjoint analysis to improve plant selection in botanic gardens and other designed landscapes

Abstract

Selecting suitable plants is crucial to more sustainable designed landscapes. Elevated temperatures and reduced rainfall are threats to urban tree health and survival. In Botanic Gardens, issues of diverse and aging tree populations, declining irrigation systems and complex plant collections, also need to be managed. In these situations, improved tree selection can provide better long-term plant choices. However, ensuring that tree selection considers all relevant criteria in the decision making process can be extremely difficult. For most tree species, there is limited high quality, localised information around environmental tolerances and cultivation experience. To explore this further, the Royal Botanic Gardens Melbourne New Zealand (NZ) collection was selected as a case study. The collection has cultural and conservation significance, and is under threat from climate change. Twenty trees in the collection, and a further twenty potential succession species were chosen for evaluation, which compared the natural habitat of the NZ trees to Melbourne 2090 climate projections, as well as rating plants on aesthetic and cultural considerations. Using pairwise comparison, each criterion was statistically weighted for its relative importance by an expert panel comprising horticulturists from the Royal Botanic Gardens and University of Melbourne. Rainfall was rated of highest importance by the panel, followed by plant distribution and temperature, with cultural and aesthetic values of lower priority. The results of the conjoint analysis assigned all trees a score and an overall ranking. With further research, the method could be refined, providing more certainty to the suitability of species.