DETERMINING THE AGE OF STREETSIDE TILIA CORDATA TREES WITH A DBH-BASED MODEL

The age of the separate group of trees was determined with an IML E400 Resistograph with a 40 cm (16 in.) boring bit, 1 to 1.3 m (3.3 to 4.3 ft) above the ground. The trees were bored twice, from the northern and the eastern side of the trunk. The bore was set at a right angle to the trunk. The depth of the drilling was equal to half of the premeasured dbh, with a margin of 5 to 10 cm (2 to 4 in.), with the exception of trees with a diameter more than 70 cm (28 in.), where the depth of the drilled hole was limited by the length of the boring bit. The inventory included trees with a dbh ranging between 15 and 37 cm (6 and 14.8 in.).

Data Analysis

For all calculations, a STATISTICA 6.0 package was used, with a nonlinear estimation module (user regression) for preparing the model.

RESULTS AND DISCUSSION

Preparation of the Age/Dbh Curve

The model involved dbh values for 195 trees in 13 age groups (Table 1), whose exact age was found in relevant documents.

Age of Trees Plotted Against Dbh

Plotting tree age against diameter yielded the correlation coefficient r= 0.962 and determination coefficient r^sup 2^ = 0.926. The exponential character of the model curve (Figure 2) reflects a gradual decrease in the dbh growth in proportion to the age of trees. The progressive reduction in annual tree rings in proportion to age has been generally observed as a phenomenon related to tree growth (Pigott 1989; White 1998; Larsen and Kristoffersen 2002). There is a considerable scatter of dbh values within a given age group.

Verification of the Model

To verily the model, 17 trees 34 years old and 15 trees 85 years old (one alley per one age group) (Table 2) were used. Their exact ages were known from the available documents. The calculated age of each tree was then obtained substituting its measured dbh value for p in the model described in Figure 1. This level of variation (Figure 2) makes it impossible to use the model to read the age of an individual tree, but it should not preclude the possibility of assessing the average age of trees from a given site (Table 2). The estimated mean age is very close to the actual age. The difference between the average estimate and actual age did not exceed 10%. The only drawback to this approach is the necessity of measuring of more than one tree.

Comparison of Age Readouts for Common Lime Trees, Estimated with the Model and with a Resistograph

Trace graphs obtained with the Resistograph were analyzed to identify annual tree rings. Tree age was determined based on the number of tree rings observed in a fragment with a known length a by applying the following formula:

W=P* S/(2a),

where W = tree age, P = dbh in centimeters, S = the number of counted tree rings, and a = the length of dbh fragment in which the tree rings were counted.

In instances when the boring holes were made both at north-south (NS) and east-west (EW), we calculated separately for WNS and WRW, and then calculated W as an average, applying the formula

W=(W^sub NS^ + W^sub EW^)/2