The long-term growth responses of Norway spruce stands to thinning is well known but the initial growth responses and the reasons for them are seldom investigated and discussed. The aim of the studies described in Papers II and III was to examine the initial growth responses of Norway spruce to thinning in relation to needle mass, light, nutrients and water. A heavy storm (“Gudrun”) on the night between the 8th and 9th of January 2005 destroyed the original experimental plan, but also provided opportunities to investigate the growth and resource allocation in more detail.
Material and methods
The experimental stand (2.5 ha) was located at the Tönnersjöheden experimental forest in south-western Sweden (latitude 56º 40’N, longitude 13º 10’E) and was planted in June 1973 with 4-year-old bare-root seedlings at regular 2.0 x 2.0 m spacing. Before thinning in February 2002, the stand density was 2260 trees ha-1, the basal area was 33.6m2ha-1 and the dominant height was 16.4 m, with little variation between the plots. Norway spruce accounted for more than 99% of the basal area before thinning. The thinning from below was done using harvester and forwarder. Three growing seasons were included in the study. The experiment was replicated four times with three treatments (in total twelve 22 x 20 m plots, each containing 11 rows of trees. The treatments were unthinned, normal thinning (30%
basal area removal), and heavy thinning (60% basal area removal). In each thinned plot all trees in the middle row were cut, the area thus cleared was used as a strip-road and selective thinning was carried out between the strip-roads. All harvesting residues were placed in the roads. (See Figure 1 for a map of the stand and the plots).
Figure 1. Description of experimental design. The first number in the plots refers to block 1 to 4 and the second number refers to the treatment (1 = control, 2 = normal and 3 = heavy
All trees were numbered and callipered at breast height before thinning and after each growing season. Band-dendrometers were installed on trees covering the diameter range for each treatment to measure growth during each year (weekly during the growing season). Before initiation of growth in 2002, 12 trees were felled for biomass analyses. After the heavy storm in 2005, a total of 53 trees covering the diameter range for each treatment were taken for biomass calculations and estimations of volume growth. The litter fall was collected and
included in the total above-ground production calculations. In addition to estimates of needle mass, LAI and light transmission were measured (data were presented for both the first and third growing seasons). The soil water content integrated over the first 50 cm of the soil profile was measured weekly during the growing season. Soil temperature and the amount of inorganic nitrogen released in the top soil were also measured. Temperature data were given only for the first and third growing seasons. Data on inorganic nitrogen (NH4+ and NO3-) contents and mineralization were collected three times each year at two-month intervals, starting in spring. The nitrogen content in the needles three years after thinning was analysed.
Results
Both normal and heavy thinning resulted in significant initial volume growth retardations. In the first growing season following heavy and normal thinning the volume growth amounted to 47 and 68% respectively of the growth in the unthinned control treatment. The volume production in the second growing season was still lower in thinned plots, but less markedly, and the differences in this respect between normally thinned and control plots were not significant. In the third growing season the growth was not significantly different between normally and heavily thinned plots, but it was significantly higher in normally thinned than in unthinned plots. Mean growth values in the normally and heavily thinned plots over the whole post-thinning period amounted to 90 and 74% of unthinned control levels, respectively. There was no growth reduction related to the cutting of strip roads following either normal or heavy thinning. On the contrary, in the heavily thinned plots, volume growth per hectare in the parts of the plots including the road and one tree row on either side was twice as high as in the three innermost rows. This may have been, partial at least, because the harvesting residues in strip roads led to increased release of inorganic nitrogen, and the soil water contents were slightly higher in the strip-roads in both the normally and heavily thinned plots than in the controls. The soil water contents were similar in unthinned and normally thinned plots (excluding the roads) but they were higher in the heavily thinned plots, especially in dry periods.
The trees in the heavily thinned stands allocated a higher proportion of their stem growth to their most basal parts, thereby increasing the taper between breast height and the stem base. However, there were no thinning-related changes in stem form in the middle stem section, but a slight reduction in taper above the crown base. The overall stem form of the trees was not significantly altered by thinning.
The total above-ground biomass production of branches increased following thinning. The amount of new needles produced during the study period was higher in unthinned than in thinned plots, but the canopy needle mass in the heavily thinned plots shifted towards that of their unthinned counterparts due to increased longevity of the needles. After thinning in 2002, the needle mass in the canopy layer was reduced by 25 and 59% in the normally and heavily thinned plots compared to the unthinned controls. After three growing seasons the needle mass in the normally and heavily thinned plots amounted to 77 and 71%, respectively, compared to the unthinned controls. The amount of new needles produced in the
control plots equalled the needle losses. The nitrogen content in the needles increased with increasing thinning grade.
In the third growing season, the stand growth efficiency (volume or above ground biomass production per unit needle mass or unit of absorbed light) calculated on the basis of needle mass, LAI or light transmittance, was higher in the heavily thinned than in the normally thinned and unthinned control plots.
The relative basal area growth was affected by both the treatments and the initial basal area of the individual trees. In the control and normally thinned plots, the relative basal area growth of the largest trees was greater than that of the smaller ones. Trees in the heavily thinned plots had similar growth rates regardless of their initial size. The mean growth response of the 100-400 largest trees over three growing seasons in the normally thinned plots was the same as in the control, but growth was significantly higher in the heavily thinned plots.