Self-thinning dynamics in cork oak woodlands: providing a baseline for managing density

Abstract

Aim of study: The study aims to evaluate the maximum potential stocking level in cork oak (Quercus suber L.) woodlands, using the ecologically-based size-density relationship of the self-thinning law.Area of study: The study area refers to cork oak forests in mainland Portugal, distributed along its 18 districts from north to south. Material and Methods:A dataset with a total of 2181 observations regarding pure cork oak stands was collected from the Portuguese Forest Inventory (NFI) databases and from research plots. The dataset was subjected to two filtering procedures, one more restrictive than the other, to select the stands presenting the higher stocking values. The two resulting subsets, with 116 and 36 observations, from 16 and 10 districts of mainland Portugal, respectively, were then used to assess and describe the allometric relationship between tree number and their mean diameter.Main results: The allometric relationship was analysed and modelled using the log transformed variables. A slightly curvilinear trend was identified. Thus, a straight line and a curve were both fitted for comparison purposes. Goodness-of-fit statistics point out for a good performance when the data is set to the uppermost observed stocking values. A self-thinning line for cork oak was projected from the estimated relationship.Research highlights: The self-thinning model can be used as an ecological approach to develop density guidelines for oak woodlands in a scenario of increasing cork demands. The results indicate that the recommendations being applied in Portugal are far below the maximal potential stocking values for the species. It is therefore of the utmost importance to review the traditional silvicultural guidelines and endorse new ones.

Size–Density Trajectory in Regenerated Maritime Pine Stands after Fire

“…For maritime pine, the pattern of the size-density trajectory for the border points clearly deviates from a straight line, as shown in Table 2 and Figure 3, evidencing curvature at both ends of the line. This trend is biologically expected [23] and has been reported for other species [26,27]. It occurs in young stands before the onset of competition begins, and in older stands, due to a decrease in self-tolerance [15].…”

“…2)-to account for a curvature in the size-density trajectory [27]. In Equation 2, the term ln represents the mean value of the transformed variable ln dg.…”

“…The estimation of the relationship of the number of trees over increasing tree size values was performed for the subset of observations retained as border points through linear regression models. Two models were tested-the straight linear model (Equation (1)), and a quadratic model (Equation (2))-to account for a curvature in the size-density trajectory [27]. In Equation (2), the term ln dg represents the mean value of the transformed variable ln dg.…”

“…Some of the references [19][20][21][22] indicate that the change of the trajectory and/or the maximum value of density depends on the location of the stands, as well as on the climate characteristics. Deviance from the straight line is also recurrently mentioned [23][24][25][26][27]. This deviance might occur at both ends of the line-in young stands, before the onset of competition begins, and later on, in older stands, due to the decrease in self-tolerance [15], leading to a curve with a concave shape.…”

The Growing Space Utilization of Main Tree Species in Northern Turkey

“…Another reason might be that, the possibility of obtaining pure black pine and Scots pine stands with lower density is higher in northern Turkey (Odabaşı et al, 2004). Although the relationship between number of trees per unit area and QMD is commonly reported with a straight line on logarithmic scale, it should be noted that the shape of the self-thinning line displays a curvilinear trend (Tang et al, 1994;Charru et al, 2012;Fonseca et al, 2017). Zeide (1987) stated that a deviance from the straight line should occur at both ends is expected.…”