Quantification of Quercus petraea Liebl. forking based on a 23-year-long longitudinal survey Francis Colin a,⇑ , François Ningre a,1 , Mathieu Fortin a,1 , Sylvie Huet b,2 a Institut National de la Recherche Agronomique, Unité Mixte de Recherches INRA-AgroParisTech, LERFOB, Equipe Croissance, Production et Qualité des Bois, Nancy, Champenoux 54280 Seichamps, France b Unité de Biométrie UR 341, INRA, Domaine de Vilvert, 78350 Jouy-en-Josas, France article info Article history: Received 11 October 2011 Received in revised form 21 June 2012 Accepted 23 June 2012 Available online 31 July 2012 Keywords: Fork prevalence Fork height Sessile oak Metamorphosis Simultaneous regressions Modelling abstract For Quercus petraea (sessile oak), the origin of forks is either traumatic or related to crown base formation (‘‘metamorphosis’’). The objectives of this study were to quantify forking, distinguish the two origins and test the effect of initial stocking density and tree attributes on the number of forks and height to the low- est one. The experiment took place in a 23-year-old sessile oak plantation in Normandy, France, where three initial planting densities were compared (1333, 2667 and 5333 seedlings per ha). The forks were observed every 3 years after the plantation. Fork number was modelled with a system of simultaneous linear equations that respectively predicted tree girth, height and the log-transformed fork frequency, with age and initial stocking density as explanatory variables. This model 1 was fitted as a seemingly unrelated regression with random effects to account for the tree and plot clustering. For trees having at least one fork, lowest fork height was quantified using a linear mixed model including predictors com- patible with the response variables of model 1, age and stocking. The results showed that the number of forks can be predicted using the expected explanatory variables and it is significantly correlated with height and girth as well. Traumatic and metamorphosis-related ori- gins were not clearly distinguished, but significant differences between the initial seedling densities were found, with the lowest density showing the highest number of forks. Lowest fork height depended on response variables of model 1, age and stocking. The stocking effect was significant but small. This study presents the first statistical quantification of tree forking ever made. From this quantifica- tion, it seemed obvious that the initial density of 2667 seedlings per ha could be a relevant initial density to avoid the emergence of too many forks and the insertion of the lowest forks too low down. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Forks are plant structures composed of a segment of axis sur- mounted by at least two more or less equivalent axes. Predicting fork prevalence could prove to be a major advance in fields such as botany, forestry, horticulture, wood technology and ecology. Forking also has an impact on external stem morphology by induc- ing stem deviations, bayonet-joints and crooked stems, ramicorn persistence, and thick laterals (Temel and Adams, 2000). It also af- fects trunk length with dead ramifications (Hein, 2004 on Fraxinus and Acer) and internal wood structure through the distribution of ring width (Dhôte et al., 2000; Adu-Bredu et al., 2008) and knots. Internal colouration is expected to be influenced by past and pres- ent fork presence (Knocke, 2003; Wernsdorfer et al., 2006; Kadunc, 2007). In addition, forks are well-known breakage points when trees are subjected to strong winds and gusts (Mattheck and Vorberg, 1991) as well as being entrance points for disease vectors (Terho and Hallaksela, 2005). From an ecological point of view, forking influences forest stands in many ways. In even-aged and two-storey stands, it de- fines the boundary between the environment of the photosyn- thetic active canopy and that of the understorey, which have different microclimatic conditions in terms of light, wind, temper- ature and air moisture (Aussenac and Ducrey, 1977). The location of the lowest fork has been identified as a useful attribute of plant function to distinguish species groups when quantifying the site productivity of mixed forests (Vanclay et al., 1997). The leaf area index, which is related to forest productivity, is also affected by the forking pattern (Kull and Tulva, 2000). Finally, forks constitute favourable habitats for birds and even reptiles, insects, epiphytes, 0378-1127/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2012.06.040 ⇑ Corresponding author. Tel.: +33 3 83 39 40 41/42; fax: +33 3 83 39 40 34. E-mail addresses: colin@nancy.inra.fr (F. Colin), ningre@nancy.inra.fr (F. Ningre), mfortin@nancy.inra.fr (M. Fortin), sylvie.huet@jouy.inra.fr (S. Huet). 1 Tel.: +33 3 83 39 40 41. 2 Tel.: +33 1 34 65 22 24. Forest Ecology and Management 282 (2012) 133–141 Contents lists available at SciVerse ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco