Site quality evaluation by classification tree: an application to cork quality in Sardinia

Abstract

Cork harvesting and stopper production represent a major forest industry in Sardinia (Italy). The target of the present investigation was to evaluate the ''classification tree'' as a tool to discover possible relationships between microsite characteristics and cork quality. Seven main cork oak (Quercus suber) producing areas have been identified in Sardinia, for a total of more than 122,000 ha. Sixty-three sample trees, distributed among different geographical locations and microsite conditions, were selected. A soil profile near each sample tree was described, soil samples were collected and analysed. After debarking, cork quality of each sample tree was graded by an independent panel of experts. Microsites where trees had more than 50% of the extracted cork graded in the best quality class, according to the official quality standard in Italy, were labelled as prime microsites, the others as nonprime microsites. Relationships between a binary dummy variable (0 for nonprime microsites, 1 for prime microsites) and site factors were investigated using classification tree analysis to select the relevant variables and to define the classification scheme. Prime quality microsites for cork production proved to be characterised by elevation, soil phosphorus content and sandiness. Results have been compared with those of the more conventional parametric approach by logistic regression. The work demonstrates the advantages of the classification tree method. The model may be appropriate for classifications at landscape and stand mapping levels, where it is possible to sample a number of microsites and to evaluate distributional characteristics of model output, while its precision is only indicative when estimating the prime quality of single microsites.

Variables influençant l’épaisseur du liège dans les forêts de chênes-lièges espagnoles: une proposition de modélisation

“…Empirical experience has shown that cork oak trees which produce good cork quality, tend to maintain this standard in successive strippings throughout their productive life [7]. In the same way, it has been observed that there are productive areas, where trees tend to have greater cork thickness, and that these areas retain their productivity level throughout the cycle.…”

“…Cork thickness is related to site conditions, as stated by Ferreira et al [14], Corona et al [7] and Montero and Cañellas [32]. Despite this, the site index proposed by Sánchez-González et al [38] is not significantly correlated with random plot effects.…”

“…These factors can be related to microsite or genetics. It is known that cork quality variability is high even under identical site conditions [7,14,18,45], so results suggest a close relationship between cork thickness and genetic aspects. The small correlation distance (< 5 m) detected among tree random components from the same plot may confirm the strong dependence of cork thickness on genetic factors, as trees within a short distance of each other would more than likely belong to the same parent tree or stump sprout.…”

Growth performance of cork oak plantations recently established on farmlands in Sardinia, Italy

“…8). La loro relativa omogeneità nel livellarsi verso il basso indipendentemente dai differenti contesti stazionali e comportamenti imprenditoriali, può essere in parte ricondotta alla modesta capacità di ritenzione idrica dei suoli superficiali e grossolani della Gallura capaci, d'altra parte, di esaltare la qualità del sughero (Dettori et al 1997, Corona et al 2005. La stretta correlazione tra disponibilità idrica annuale, funzionale a entità e distribuzione delle precipitazioni, e accrescimento radiale della quercia è segnalata da numerosi Autori sia per l'intero fusto (Costa et al 2001e 2002 sia in riferimento al sughero (Caritat et al 1996, Fialho et al 2001.…”

“…Poiché l'accrescimento di legno e sughero, e la qualità del secondo, sono legati all'interazione tra fattori genetici e ambientaliselvicolturali (Vieira 1950, Montero & Cañellas 2003, Corona et al 2005, una scelta errata delle provenienze può concorrere all'insuccesso degli imboschimenti, in tempi medi (accrescimenti nella fase giovanile) e lunghi (qualità delle plance). L'elevata domanda di postime quercino scaturita dall'applicazione delle misure comunitarie ha colto impreparate le strutture vivaistiche regionali, sia in termini di quantità che di qualità, con spostamento di materiale di propagazione non solo tra i diversi comprensori sughericoli regionali ma anche sulle lunghe distanze, tanto da doversi registrare l'introduzione di provenienze iberiche.…”