Evaluation of the Site Form as a Site Productive Indicator in Temperate Uneven-Aged Multispecies Forests in Durango, Mexico

Abstract

Even though the site index is a popular method for describing forest productivity, its use is limited in uneven-aged multispecies forests. Accordingly, the site form (SF) is an alternative measure of productivity to the site index based on the tree height–diameter relationship. Our study aims to evaluate SF as a measure of productivity in the temperate uneven-aged multispecies forests of Durango, Mexico, applying three methods to estimate SF: (i) as the mean height of dominant trees at a reference diameter (SFH-D); (ii) as the expected mean height of dominant trees at a reference mean diameter (SFMH-MD), and (iii) as the expected height at a reference diameter for a given site (SFh-dbh). We assess the effectiveness of the SF based on two hypotheses: (i) the SF correlates to the total volume production, and (ii) the SF is independent of stand density. The SFH-D and the SFh-dbh showed a high correlation with productivity. However, they also did so with density. Contrary to this, the SFMH-MD had a weak correlation with density and productivity. We conclude that the SF is a suitable approach to describe site quality. Nonetheless, its effectiveness as a site quality indicator may be affected according to the method used.

Site Quality Evaluation Model of Chinese Fir Plantations for Machine Learning and Site Factors

“…This finding indicates that the effect of site factors on the site form is basically the same as the effect on the site, and the site form can better reflect the stand condition; in addition, it can better reflect the relationship between site performance and tree species growth. Overall, the site form of this study can replace the site index as an indicator for fir site quality evaluation, which is consistent with the findings of Vanclay et al [10], Padilla-Martínez et al [12], and Castano-Santamaría et al [14].…”

“…Therefore, the E5 model was selected as the result of the site form model for the follow-up study. Substituting the parameters b of the site index model into Equation ( 1), which has a base age of 20, yields an expression for the E0 model as Equation (11), and substituting the parameters b of the optimal site form and the base DBH into Equation ( 2) yields an expression for the E5 model as Equation (12).…”

“…The site index is the dominant tree height at the base age [36], and the site form is the height of the dominant tree at the base diameter at breast height [12]. In this study, we choose the optimal guidance curve as the base equation for the site index model and the site form model and used the algebraic difference approach (ADA) to derive the site index model and the site form model using the Schumacher equation as an example.…”

“…The site form method has been controversial since it was first proposed. Many scholars have evaluated the site quality of certain areas based on site form and concluded that the method has valid results for measuring the site quality of different forest species, whereas others have come to the opposite conclusion [10][11][12][13][14][15][16]. Vanclay et al [10] were the first to apply site form to the evaluation of heterogeneous coniferous forests in Queensland, Australia; they established a tree height-and-diameter equation based on an exponential relationship.…”

“…Herrera-Fernández et al [11] applied site form to evaluate broadleaf forest site quality in neotropical secondary rainforests and showed that site form is a potentially useful indicator of broadleaf forest site quality in neotropical secondary rainforests. Padilla-Martínez et al [12] applied site form to evaluate sites in a temperate heterogeneous multispecies forest in Durango, Mexico and found that site form can better evaluate site quality. Ahmadi et al [13] used the free statistical programming language "R" to fit a sequential logistic regression model of standing terrain on soil and topographic variables, and the results indicated that standing terrain appears to be a potentially useful indicator of site quality.…”