Tolerance and acclimation of photosynthesis of nine urban tree species to warmer growing conditions

Key message Tolerance and acclimation of photosynthesis to warmer growing conditions among nine urban tree species was species-specific indicating that individual screening is needed to prepare urban greenspace for a warmer future. Abstract To prepare and manage urban greenspace for a warmer future, we must select trees that can tolerate or acclimate to warmer growing conditions. Here, we compared tolerance and acclimation of photosynthesis to warmer growing conditions among urban trees species in Japan. Two paired experiments were installed where saplings of nine species were grown outside at two locations (warm- and cool-temperate climates) and in a greenhouse with and without passive warming. We compared the temperature where quantum yield declined to 50% of maximum value (T50) due to excess heat (50–60 °C) and the thermal optimum for photosynthesis (Topt) among species. For two deciduous and three evergreen species, T50 was higher for saplings grown in warm- than cool-temperate climate. T50 was also higher under passive warming for three species, of which two evergreen species (Morella rubra, Quercus myrsinifolia) showed marked increases (5–7 °C). Topt was higher under passive warming in three of six species examined, but the acclimation response of photosynthesis and stomatal conductance to warming was highly species-specific. We inferred that M. rubra and Q. myrsinifolia, which acclimated consistently to warmer growing conditions in both experiments, are heat-hardy species. Our results also suggest warm-temperate evergreen species are not necessarily more tolerant of warmer growing conditions than cool-temperate deciduous species. For example, despite its warm-temperate origin, Machilus thumbergii showed no acclimation response and photosynthetic rates were lower under passive warming. Our results indicate species need to be screened individually to assess their physiological tolerance and acclimation potential to warmer climate.