Why do some trees produce flowers before leaves unfold?
When spring arrives, we rejoice as leaves begin to unfold on trees and flowers boom. For some trees, however, this normal sequence is reversed and they produce flowers before their leaves have developed.
Researchers from the Chinese Academy of Sciences and the World Agroforestry Centre have been hoping to better understand this phenomenon by studying 2 very special trees: an apricot (Prunus armeniaca) and a mountain peach (Prunus davidina) growing in the Beijing Summer Palace in China.
Known as hysteranthous, this behavior of flowering first is significant because if a mass of flowers come out together then this is likely to attract more insects, and if at the same time there are no leaves then this facilitates wind pollination.
For fruit trees, the process of breaking dormancy to produce flowers and leaves and ultimately fruit is driven by different chilling and heating requirements. With climate change predicted to increase temperatures, this will have major implications for the way in which fruit trees respond, perhaps even threatening the natural flowering and fruiting of trees such as apricots and peaches.
While many studies have estimated the chilling requirements of flower buds (because insufficient chilling can cause uneven and even failed bloom threatening the production of some horticultural crops), comparative studies on the chilling and heat requirements of leaf and flower buds are rare.
“The heat requirements of plants is equally important for initiating flowering, leaf unfolding, photosynthesis and vegetative growth,” outlines Eike Luedeling, Senior Decision Analyst with the World Agroforestry Centre, who is co-author of a study comparing the chilling and heat requirements of leaf and flower buds on the fruit trees in the Beijing Summer Palace which was published recently in the scientific journal, Plant Diversity and Resources.
“There has been very little research into why some trees are hysteranthous and what climatic requirements determine this sequential occurrence of flowing before leaf unfolding.”
Through their research, Luedeling and colleagues have been able to come up with what they believe is a useful tool for identifying the chilling and heat requirements of leaf and flower buds, provided there are long-term phenological observations.
Because long-term phenological observations, such as when flowers and leaves first appear on trees, were available for the fruit trees in the summer palace, these were chosen for the study. The scientists were also able to obtain daily maximum and minimum temperatures from 1963 to 1988 from the Beijing Meteorological Station, just 2.5km from the palace.
Using a chilling and a forcing model combined with the temperature data, the scientists calculated daily chilling and heat accumulations over the 25-year period. They used Partial Least Squares (PLS) regression to identify the chilling and forcing periods by relating the leaf unfolding and flowering dates of the 2 trees to daily chilling and heat accumulations.
“We found the reason for the sequence of flowering and leaf unfolding was a large difference in heat requirements between leaves and flower buds,” explains Luedeling. “Flowers had much lower heat requirements compared with leaf buds whereas the chilling requirements for leaves and flower buds were about the same.”
The scientists believe the use of daily chilling and heat accumulations instead of just daily temperatures gives a more accurate estimate of chilling and heat requirements.
“If long term phenological observations are available, this approach provides a rapid way to estimate the chilling and heat requirements of plants without extensive experimentation.”
The authors recommend that consistent phenological observations be conducted in China as they are easy to obtain and can help to provide evidence of the impacts of climate change, such as yields, competition among plants, pests and diseases, and pollen dispersal. Phenological observations also help in advising farmers when to sow, irrigate, fertilize or protect crops. They can help to evaluate the risk of frost damage and forecast plant development and harvest dates.
Download the full study:
Guo, L. Luedeling, E. Dai, J-H. Xu, J. 2014 Differences in heat requirements of flower and leaf buds make hysteranthous trees bloom before leaf unfolding. Plant Diversity and Resources 36 (2): 245-253.