Phenology and climate

Plant phenology is the study of periodic plant life cycle events and how these are influenced by seasonal and inter-annual variations: leaf burst, flowering, fruiting, autumn foliage colour change. These events are linked to certain climatic parameters.

Why observe phenological events?

Phenology is essential to our understanding of how forest ecosystems work, in particular as relates to tree growth. It is also an effective tool to help monitor how plants adapt to changes in the climate.

The Renecofor carries out phenological observations :

• To trace differences in seasonal rhythms (leaf burst, autumn colour change, growing season length) according to species, climate and region
• To trace inter-annual variations in occurrence dates for the different phenological stages (leaf burst, autumn colour change)
• To elaborate mathematical models which connect climatic data and plant phenology and will enable us to simulate the influence of climate change on tree behaviour.

In the spring, leaves do not appear on all tree species at the same time. In the Network stands, the earliest species opens its leaves on average five weeks before the latest one.

Classification of species in ascending order by average leaf burst date :

• Pedunculate oak : April 7 - 9 sites
• Sessile oak : April 9 - 19 sites
• Beech : April 19 - 17 sites
• Maritime pine : April 23 - 6 sites
• Corsican black pine : May 2 - 2 sites
• Douglas fir : May 3 - 5 sites
• Scots pine : May 3 - 11 sites
• European larch : May 9 - 1 site
• European spruce : May 12 - 10 sites
• Silver fir : May 13 - 11 sites.

Growing season

The growing season for deciduous broadleaved trees is defined as the period of the year between the appearance of the leaves in the spring and leaf colour change in the autumn. During this period, the trees are very active (growth, fruiting). The growing season lasts on average 192 days but there is considerable variation among sites (an average of 149 days for beech in the Vercors and 244 days for pedunculate oak in the Landes).

Since oak trees yellow in the autumn on average 5 or 6 days later than do beech trees (pedunculate oak: October 23, sessile oak: October 24; beech: October 18), the growing season is about 2 weeks longer for oaks than it is for beech (pedunculate oak: 199 days, sessile oak: 198 days; beech:183 days). 

In addition to the differences among species, there is also a regional effect. The growing season is longer in the Southwest than in the North, the Northwest and in mountainous areas.

The influence of temperature and altitude on periodic tree life cycle events

For a 1°C increase in spring temperatures, leaf burst occurs 6 days earlier and the growing season lengthens by 10 days.Autumn colour change is delayed by about 5 days for every degree of increase in autumn temperatures.

As altitude increases, leaf burst is delayed due to lower temperatures at higher altitudes; every 100 metres gained equates to a 2-day delay in leaf burst. This explains the different results for broadleaves and conifers in the Network, since conifers are often found in mountainous regions.