Press Info item. 31/10/2008

Evolutions affecting the adventive flora of sunflower over 30 years

Special "Biodiversity"

INRA researchers in Dijon, working in collaboration with the Plant Protection Services, have analysed the evolution over 30 years of adventive flora of sunflower in France. Their study has demonstrated a specialisation of the flora present in this crop, resulting in a reduction of functional diversity. Analysis of the biological characteristics of these weeds has made it possible to observe that the species favoured by the more widespread cultivation of this crop present some biological similarities with sunflower. This type of functional approach can help to predict the species that may constitute a threat of crop invasion in the future.

Since the 1950s, the introduction of new crops (rapeseed, soy, sunflower) and associated changes to farming practices have resulted in the progression or regression of certain adventive species, more commonly referred to as "weeds".  Sunflower (Helianthus annus L.) was introduced into France after the Second World War and was still only a minority crop during the 1970s (covering approximately 76,000 ha).  Today, the land used to cultivate sunflower covers  600,000 hectares.  Consequently, this crop is particularly appropriate for a long-term analysis of the development of its specific flora.

Researchers in the INRA/ENESAD/University of Burgundy Joint Research Unit for Weed Biology and Management compared the frequency and abundance of a series of 168 species during two study campaigns covering the periods 1973-1976 and 2002-2006 (records of the "Flora Biovigilance Network").

Comparison of the records for these two campaigns showed that amongst the most common communities of species, two-thirds (69%) changed their status (in progression, stable, in regression).  These results thus confirmed observations at a larger scale concerning the rapidity with which arable crop weeds renew themselves.

Species in progression

Species already common during the 1970s: black nightshade (Solanum nigrum) (photo), French mercury, redshank or prickly sowthistle © INRA j.-P. Longchamp

Species still common: climbing buckwheat (Fallopia convolvulus) (photo), common knotweed, scarlet pimpernel or field mustard © INRA j.-P. Longchamp

Each species was classified according to 17 "traits", representing 10 biological characteristics, two agronomic characteristics and five ecological indicator values.  Species sharing the same traits were placed in functional groups so as to achieve a detailed analysis of species in progression or regression within these groups, and to clarify the potential contribution of each trait to changes in species status.

Five of the 17 traits studied exhibited major differences between species in progression or regression.  Analysis of the species at the scale of a functional group enabled a check that species in progression mainly belonged to the same group, which would explain their development in sunflower fields.

Thus the species in progression were those of large size, without any seed dispersion mechanism,  with considerable seed longevity, emerging in the summer or at any time, with a peak of flowering and seed production between the summer and autumn.  These are species that appreciate the sun (heliophilous) and soils with a high nitrogen content (nitrophilous).  They are also less susceptible to most sunflower herbicides.  The cultivation techniques specific to sunflower appear to constitute the principal motor for the evolutions observed.  The most common weeds synchronize their life cycle with that of sunflower; they adapt to the herbicides employed (trifluralin) or circumvent this selection pressure by means of late germination.

In addition to compiling the list of species in progression, application of an approach based on groups and functional traits was able to highlight determinant characteristics linked to changes in farming practices, and thus detect species that are likely to become more important in the near future, independent of their current status.  The rise or decline of certain weeds could also have consequences regarding the floral biodiversity of cultivated fields because of the close relationships that exist between weeds and certain insects or birds.  As an example, the seeds of the common knotweed and common chickweed – two species regressing in sunflower fields – are known to be a favourite food of seed-eating birds.

• Photo library, UMR BGA, INRA Dijon : http://www2.dijon.inra.fr/bga/phototheque/main.php

Reference :
Fried, G.; Chauvel, B. & Reboud, X. "A functional analysis of large-scale temporal shifts from 1970 to 2000 in weed assemblages of sunflower crops in France", Journal of Vegetation Science 20, 2009
doi: 10.3170/2008-8-18465, published on-line on June 24, 2008

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