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Results based on tree observations

François Lebourgeois (AgroParisTech) takes the floor

Observed tree responses to climatic variations (growth, leaf phenology and fruiting)

Summary

The annual biomass produced by a forest is divided among the trees' different compartments: the trunk, the crown and also the fruit. Due to a tree's long life span, its radial growth is considered to be a major indicator of response to climatic changes. Similarly, leaf phenology is a good indicator of a tree's capacity to survive since leaf dynamics are highly selective. A tree's ability to produce fruit is also essential to the long-term survival of the species. These different environmental markers are now being recorded though the causes of their inter-annual variations have yet to be fully understood.

The data collected by the RENECOFOR network for the past 25 years have been instrumental in helping to understand the links between environmental characteristics (mostly climatic) and radial growth, leaf phenology and fruiting in the major temperate-zone forest tree species.

The diagram below summarises various phenological events and the underlying climatic variables influencing them for three deciduous species (Sessile and pedunculate oak and European beech).

Periods and variables influencing the radial growth, the flushing and fruiting for three deciduous species (Sessile and pedunculate oak and European beech) on RENECOFOR net work.
Periods and variables influencing the radial growth, the flushing and fruiting for three deciduous species (Sessile and pedunculate oak and European beech) on RENECOFOR net work. © François Lebourgeois / AgroParisTech

In oak stands, on average leaf burst begins around mid-April and fall yellowing occurs around mid-October. In eastern France, the growing season is shorter (180-190 days as opposed to 210-220 days in western France) due to later leaf burst (delayed by 2 days for each degree of longitude) and earlier yellowing (5-10 days).


For beech, the growing season starts later, around the end of the third week in April. Fall yellowing occurs at the beginning of October, resulting in an average growing season of 180 days. Temperatures during the months of March and April play a determining role in leaf burst (see Figure). For example, 1°C more in March moves leaf burst forward by two to five days. For fall yellowing, temperatures in October or November have the most influence, with higher temperatures resulting in delayed senescence (Delpierre et al., 2009; Lebourgeois et al., 2008, 2010).

For annual growth, the soil water balance is what most influences the development and width of the ring.
For beech, for instance, an early summer drought (especially during June) results in a thin ring (see Figure). Beech trees growing in lowland plains are particularly sensitive to drought when soils are superficial (maximum available soil water capacity below 100 mm) or annual precipitation is low (less than 700 mm) (Lebourgeois et al., 2005).

For oak, the main limiting factor for growth is a lack of available water over the entire growing season (July to October), with key periods varying greatly depending on the stand and local conditions.

Differences between the two oak species are slight, even though pedunculate oak seems to be more sensitive to unusually severe drought events. Comparatively, sessile oak appears more sensitive to summer drought in hot, dry climates. This explains why sessile oak reacts strongly and negatively to summer drought and warm autumn temperatures in the dry oceanic climate of western France, whereas it is barely affected by summer drought in the cooler, wetter conditions offered by the semi-continental climate in eastern France (P > 800 mm) (Lebourgeois, 2006; Mérian et al., 2011).

Finally, as far as fruiting is concerned, oak stands produce on average 251 kg per ha per year as compared to 171 kg for beech, yet for both oak and beech, fruits represent a mere 5% of the annual litter accumulation. Beechnut production follows a biannual cycle which appears to be synchronised among stands. Oak, on the contrary, has a much more irregular acorn production cycle with different stands producing asynchronously (Lebourgeois et al., submitted).

For oaks, masting depends on autumn temperatures during the previous growing season, on April temperatures and on the amount of carbon accumulated by the end of summer (see Figure). For beech as for oak, April is a key period determining nut production, though winter temperatures also influence masting. At the end of the growing season, favourable autumn conditions (temperatures and high carbon storage) favour acorn production (in terms of both quantity and biomass). For beech, analyses have shown that beechnut production closely follows the quantity of pollen produced (this relationship has not been observed for oak).
Masting also correlates with growth: the correlation is positive for oak (high-growth years correspond to heavy masting years), while it is negative for beech (heavy masting = reduced growth).

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