Correlation (Spearman’s) analyses showed a significant negative relationship (α=0.01) between spacing and total height until age eight. Height was considerably greater at closer spacing than those of wider spacing (Figure 3), and ANOVA results indicated significant differences (α=0.05) in total height due to spacing until age eight.

Spacing-height growth relationships for ages four to twelve are shown as the ratio of height to maximum height (% of maximum height) to observe relative progressive change of height growth in time (Figure 4). This height/maximum height ratio reached the peaks at younger ages in closer than in wider spacing. For example, at age four the ratio was 99.99 for closer spacing [1.78 m² (1.36 × 1.36 m)], and it was 91.32 for wider spacing [4.86 m² (2.25 × 2.25m)]. However, the highest value of this ratio shifts from closer to wider spacings starting at age eight (Figure 4), so that at age nine the ratio was 99.99 for wider spacing [4.86 m2 (2.25 × 2.25 m)] and 93.00 for closer [1.78 m² (1.36 × 1.36 m)] spacing. This result was consistent with that of found by Scott et al. (1998) for coast Douglas-fir (Pseudotsuga menziesii(Mirb.) Franco var menziesii), by Knowe & Hibbs (1996) for young red alder (Alnus rubra Bong) and by Kerr (2003) for ash (Fraxinus excelsior L.), and with other similar researches.

Larger height values at early ages in closer spacing compared to wider spacing may be due to better micro ecological conditions in closer spacing. It is because wider spacing at early ages appears to increase water loss due to evaporation caused by wind, resulting in less water content of the soil, which leads to growth retardation in trees especially in hot summer days (Kerr, 2003; Harrington et al., 2009). In fact, the experimental sites have typical Mediterranean climate, which already has water deficiency in June-September period. Indeed, long term meteorological data for the period of 1950-2014 for June, July, August and September for the sites show that total monthly precipitation are 7.7, 2.8, 3.1, 13.5 mm, respectively. Monthly mean temperature for the same months are 25.4, 28.4, 28.2, 24.8 ºC respectively (TSMS, 2015), which leads to serious water deficiency in the region. Another effect may be due to more weed suppression on trees at wider spacing, which, in addition to competition for nutrient elements, also affects water budget negatively in early ages (Westfall et al., 2004).

Diameter growth is the most sensitive tree characteristic affected by stand density, such that number of trees and spacing are effective predictors of individual tree diameter growth in even aged stands (Kalıpsız, 1982). ANOVA results showed that mean diameter was significantly (α=0.05) affected by initial spacing following crown closure and thereby the initiation of inter-tree competition about at age eight. This effect was clearer in the years following age eight, depending on severity of competition (Figure 5). These trends showed consistency with previous studies performed on several conifer species (Usta, 1991; Sharma et al., 2002; Hummel, 2000; Harms et al., 2000; Timothy et al., 2009). However, there was a significant positive effect of closer spacing to induce dbh growth before age eight, which was also the case for height growth. Several studies also indicated that there is a strong positive correlation between height and diameter for a given spacing and growing area on forest trees (Kalıpsız, 1984; Husch et al., 1982; Erkan, 1996). Following the beginning of competition at about age eight, wider spacing provides higher dbh growth rate (Figure 5). For instance, at Duacı site at age 12, average dbh is only 77.5 mm for 1.78 m² growing area per tree, while at the same age average dbh is 154.8 mm for 18.54 m² growing area. A similar trend is also observed in the Nebiler site where average dbh for the corresponding spacings are 70.0 mm and 139.0 mm, respectively (Figure 5).

Individual tree stem volume showed similar growth pattern as diameter growth due to the fact that tree volume is strongly correlated with tree diameter. Thus tree volume increases with wider spacing mainly due to decreasing competition (Figure 6).

ANOVA results indicated significant differences (α=0.05) in total individual stem volume, stand volume per hectare and mean annual volume increment following age eight.

Our results showed that individual tree volume increased (Figure 6) while stand volume per hectare decreased as spacing becomes wider (Figure 7). Therefore, before any decision on spacing in plantations, it is necessary to consider not only the volume growth of individual trees, but also production of higher volume (and volume growth) per unit area. Volume growth behavior of individual trees as compared with volume growth of a stand for a given spacing is directly related to area use efficiency and number of trees per unit area (Zhang et al., 1996; Sharma et al., 2002; Harrington et al., 2009; Amateis & Burkhart, 2012). By age twelve, even if the competition has already started at closer spacing, area use by trees was still effective. By age 12^th years, we observed no dead trees due to negative effects of closer spacing. However, in the absence of thinning, the shape of the curve would change due to increasing number of dead trees in advanced ages and for longer rotations. In such cases stem volume per unit area will decrease, being more heavily so, as the spacing becomes closer (Amateis & Burkhart, 2012).

Mean annual volume increment (MAI) per unit area is the most important criterion in terms of forest management when wood production is prioritized. Therefore, MAI has been used to set an optimal rotation age to maximize total wood production in even aged stands (Kalıpsız, 1982; Avery & Burkhart, 1994). Our results showed that MAI decreased with increasing spacing in both test sites by age 12 (Figure 8). A closer spacing provides much more wood production and seems to be advantageous for short rotation forestry. Our overall results are consistent with those of various reports (Harms et al., 2000; Timothy et al., 2009; Amateis & Burkhart, 2012). For instance, Amateis & Burkhart (2012) found that closer spacing (1.22 × 1.22 m), in a loblolly pine plantation, produced more total volume yield through age 13 than the other spacing treatments. However, at age 15 of the same species, volume yield was reduced in closer spacings compared to wider spacings, mostly due to increased mortality.

Empirical yield table prepared by Usta (1991) are based on data obtained from plantations which were established by conventional methods; i.e., without deep soil cultivation and using ordinary seeds collected from natural forests. However, our test sites (Duacı and Nebiler), which were chosen from among good sites (I. class), were established using improved seeds from first generation seed orchards, and the soil was cultivated deeply, which are accepted as essential activities in commercial tree plantations. We compared our volume growth results with Usta’s (1991) yield table presented for good sites (Figure 9).

As illustrated in Figure 9, there were statistically significant (α=0.05) differences in growth performances (MAI) between Usta’s Yield table values and those of the experimental sites. Main differences between the test sites and Usta’s Yield table values may be originated by two reasons: The main reason may arise from the use of improved seeds in the test sites, along with site preparation implemented with deep soil cultivation (Boydak, 1982; Ürgenç, 1986; Westfall et al., 2004). It appears that additional site treatments (such as site preparation, weed control and fertilization practices) prior to plantation establishment would have long term positive effects on growth and increase of final crop (Westfall et al., 2004). The second reason might be that, Usta’s table values are the averages of measurements of 45 good sites, having a range of values around the averages as reflected in Figure 9. As to growth difference between Duacı and Nebiler test sites, which were again significant (α=0.05), it might be due to minor site-quality differences among the test sites. Indeed, top height at age 12 was 10.0 m in Duacı, and it was smaller (8.9 m) in Nebiler.

Carbon (C) storage per hectare in tree biomass (above-ground + below-ground) in a stand changed with spacing (Figure 10). Total C storage (above-ground + below-ground) in a tree decreased drastically from 5.11 to 1.57 tons year^-1 ha^-1 for tree growing areas of 1.78 and 18.54 m², respectively in Duacı site. C storages were 3.63 and 1.18 tonsyear^-1 ha^-1 for the corresponding growing areas in Nebiler.

Forest management that aims for maximizing wood production looks for maximum MAI to determine optimal rotation ages. Carbon sequestration is closely related to yield and particularly to the volume increment of a stand. Although rotations shorter than maximum MAI lead to reduced C sequestration, it can provide earlier revenue that could be invested to increase growth (consequently, C storage) in other forests. Thus, longer rotation may both reduce the long-term mean annual wood production and increase the risk of carbon losses, such as through wildfires (Malmsheimer et al, 2011).