Tree phenology

 

What is phenology?

Phenology is the study of the timing of natural events. These events include flushing or budburst, flowering, fruiting and autumn leaf-fall. Although all are relevant to climate change, budburst is most often associated with evidence of the effects of climate change - indeed, budburst of oak is one of the UK Governments indicators of climate change. Flushing dates vary, primarily as a function of temperature, and this is clear from the time series for 2001 shown below, with leaf emergence first observed in south-west England and south Wales, and with the latest observations in the north east of Scotland.

View map of observed budburst in oak based on responses received by the FC/RSPB Wildsquare project in 2001 ^(26K)
 

It is apparent from a leaf emergance data-set for oak at Ashstead in Middlesex [source: Tim Sparks (CEH) and Mrs J Coombes], that flushing is now advanced by about two weeks compared with the 1950s. This effect of climatic warming on natural cycles is commonplace, and a range of phenological observations for a number of species are routinely recorded by the UK Phenology Network using volunteer observers. A summary of the work of the Phenology Network is given in Bulletin 125.

How will budburst be affected by climate change?

A model of budburst has been developed and calibrated for oak, and can be used to predict how flushing dates will further change as a result of climate change. Agreement between observed budburst dates and model predictions is good although, for some years, significant discrepancies are apparent. Both the trends observed in long-term data-sets and the output from models indicate that as the climate warms, flushing will advance. Although this may enhance productivity as a result of longer growing seasons, the potential for frost damage will increase; this is because although milder winters are expected, the chances of 'cold snaps' and frost damage become greater earlier in the year. As described in the page on climate change adaptation, frost tolerance has been shown to be a key determinant of form and timber quality in ash (Cundall et al., 2003). Indeed, the potential for frost damage is a principal arguement against not planting more southerly, non-native provenances. The variation in flushing date with latitude of seed source is currently being investigated in an oak provenance trial, which is part of a much larger country-wide series of experiments.

Climate change and autumn senescence

The likely impacts of climate change on autumn leaf-fall are less clear than for leaf emergance in spring. This is because more environmental factors are involved: summer drought and enhanced ozone pollution accelerate leaf senescence, with the first frost often precipitating leaf-fall; in contrast, mild autumn temperatures delay the onset of leaf senescence. However, the timing of autumn leaf senescence, unless very early and drought-related, has little effect on productivity. This is because of the low energy input from the sun during autumn (September to November) compared with April and May, when flushing typically occurs.

Monitoring the effects of climate change on phenology

To further our knowledge of the effects of climate and climate change on phenology, we have recently re-established a phenology garden at Alice Holt as part as the International Phenology Garden (IPG) Network. In addition, phenological development one of the routine measurements made as part of the Intensive Forest Health Monitoring protocol, while the Environmental Change Network (ECN) site at Alice Holt also provides data on the timing and abundance of a range of birds and invertebrates.