Use of stand density to improve prediction performance of stem taper model for Taurus cedar

Öz

Taurus cedar (Cedrus libani A. Rich.) is economically and ecologically one of the most important forest tree species in Turkey. In this context, knowing the state and limitations of growth and yield of Taurus cedar forests is necessary for improving future management and planning strategies of timber resources. Individual tree volume estimations are one of the most important tools in growth and yield prediction models of the tree species. The use of taper equations in estimating tree volume has recently become useful tool. Taper equations are used to estimate diameters along the bole at any given height and these models are basic pre-requisite to estimating individual tree volumes and product yield. The previous studies showed that, stand characteristics, such as stand density, regeneration method, soil type, and geoclimatic attributes also may have significant impact on tree growth and stem form. Therefore, it makes sense that including stand density information in stem form models should improve model performance in stem diameter and volume estimations. 

In this study, the effects of stand density on stem form were examine and to develop taper equations that incorporate stand density information using nonlinear mixed-effects modeling approach for natural cedar stands in West Mediterranean Region of Turkey. As a result, the predictive accuracy of the model was improved by including stand density as covariate. In addition to, diameter measurements from various stem locations were evaluated for tree-specific calibrations by predicting random-effects parameters. It was found that an upper stem diameter at 9.3 m above ground was best suited for calibrating tree-specific predictions of diameter outside bark.

Anahtar Kelimeler

• Stem form
• Mixed-effects modelling
• Calibration
• Stand density
• Variable-form taper model

Toros sediri için gövde çapı modelinin tahmin performansını iyileştirmek için meşcere sıklığının kullanılması

Öz

Toros sediri (Cedrus libani A. Rich.) Türkiye’nin ekonomik ve ekolojik açıdan en önemli orman ağacı türlerinden birisidir. Bu çerçevede doğal Toros sediri ormanlarının geleceğe dönük yönetim ve planlanması ile ilgili stratejilerinin geliştirilmesinde, türün büyüme ve hasılatına ilişkin mevcut durumun ortaya konması gerekmektedir. Ağaç türlerinin büyüme ve hasılatına ilişkin tahminlerde kullanılan en önemli yapı taşlarından birisi, ağaç hacim tahminleridir. Ağaç hacimlerinin tahmini amacıyla gövde çapı modellerinin kullanımı son yıllarda önemli bir araç olmuştur. Gövde çapı modelleri, bir ağaç gövdesi boyunca farklı yüksekliklerdeki çap değerlerinin tahmin edilmesi, buna bağlı olarak da ağaç hacim tahminleri ve bu hacmin farklı ürün sınıflarına dağılımının belirlenmesi için kullanılmaktadır. Yapılan çalışmalar, ağaçların büyüme ve gövde formu üzerinde; meşcere sıklığı, gençleştirme yöntemi, toprak tipi ve joe-klimatik faktörler gibi bazı karakteristiklerinin önemli etkilere sahip olduğunu göstermiştir. Dolayısıyla, gövde çapı modellerine meşcere sıklığının yardımcı değişken olarak eklenmesi ile modellerin gövde çapı ve hacim tahminlerindeki performansları arttırılabilir. 

Bu çalışma ile Batı Akdeniz yöresi saf doğal sedir meşcereleri için, karışık etkili modelleme tekniği (NLME) kullanılarak, meşcere sıklığının gövde formu üzerindeki etkisi ve meşcere sıklığının yardımcı değişken olarak kullanılması durumunda modelin gövde çapı tahmin performansının nasıl değiştiği araştırılmıştır. Çalışma sonucunda, gövde çapı modeline, meşcere sıklığının eklenmesi ile model tahmin performansının arttığı görülmüştür. Buna ilaveten, ağaç gövdesi üzerinde farklı yüksekliklerdeki çap ölçümleri kullanılarak kalibrasyon alternatifleri de değerlendirilmiş, çap tahminleri için en uygun kalibrasyon seçeneğinin ağaç gövdesinin dipten itibaren 9.3 m’de ölçülen çap değeri olduğu görülmüştür.

Anahtar Kelimeler

• Gövde formu
• Karışık etkili modelleme
• Kalibrasyon
• Meşcere sıklığı
• Değişken şekil gövde çapı modeli

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