Finnish Forest Research Institute  Metla

Press release 11.8.2006

Increased needle density of Scots pine correlates with reduced height growth

Annual height growth varies more than the amount of needles developing in a shoot. Therefore, needle density is dependent on the length of annual shoots; in years of low height growth the needle density is high and vice versa.

A study was conducted at Metla to examine the relationship between needle density and height growth of Scots pine and the effects of weather on needle density in pine stands extending from the Arctic Circle to the northern timberline. Only precipitation during April and May affected the ultimate needle density. Height growth was shown to play a central role, with apical extension being the only factor (in addition to age) that could be suggested to explain 50% of the between-year variation in needle density.

A non-linear model was used to describe the correlation between height growth and needle density. The model depicts needle density as normal when height increment varies within the limits of normal growth. However, for years when growth is disturbed the model shows increased needle density.

Needle density is used to express the number of fascicles (dwarf shoots, needle pairs) per shoot centimetre. Between-year variance in needle densities in shoots of different years can be explained by the tree’s height growth, and such details are easily detectable. Needle density and height growth can be used to calculate the annual needle production and to describe height growth in more detail. Needle density can also indicate exceptional years of height growth.

The mean needle density of Scots pine was 7.8 fascicles per centimetre. However, since needle density is one of the stand and tree characteristics, there may be significant between-tree variance in needle densities. The highest measured needle densities were 20-50 and the lowest 3-4 fascicles per centimetre.

The study was performed using the Needle Trace Method (NTM). The NTM is based on the examination of the vascular system connecting dwarf shoots and the shoot pith, visible as a dark brown “dot” or needle trace in the annual rings created when the dwarf shoot was alive. When the dwarf shoot dies, the xylem growth stops. The production and shedding of needles can be traced to provide a time-series throughout the tree’s life span on the basis of the needle traces.

Publication: Salminen, H. & Jalkanen R. 2006. Modelling variation of needle density of Scots pine at high latitudes. Silva Fennica 40(2). 183–194.

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