Abstract
To ensure sustainable forestry, it is important to establish efficient management procedures for seed orchards. We evaluated the effects of thinning, pruning season and girdling intensity on tree growth and vigor in an old Larix kaempferi seed orchard. Even after four years, tree size (crown width and tree height) increased, resulting in light intensity of an un-thinned class decreasing. Thinning counteracted the decrease in light intensity associated with tree growth, but it had no effect on promoting tree growth. Tree crown status was scored based on vigor and health. No significant difference in crown vigor was observed between unpruned and winter-pruned trees, but the summer-pruning class showed significantly lower vigor. Five years after girdling, trees with low and moderate girdling levels maintained high crown vigor, similar to intact trees, while the crown vigor of trees subjected to severe girdling was significantly lower. This is the first study to quantitatively model trends in remaining girdling depth over time, and to show that the half decay period was ca. 2–3 years. Our findings suggested that management procedures that combine moderate-level girdling, winter pruning and thinning to maintain rPPFD at 50% are well-suited to balancing tree vigor and reproduction in seed orchards.
Highlights
Japanese larch (Larix kaempferi (Lamb.) Carr.) often exhibits superior growth compared to other larch species, and the species has been widely used in breeding programs in many countries [1,2]
In a little as five years after girdling, L. kaempferi trees subjected to girdling levels 1 and 2 retained high crown vigor, similar to intact trees, while the vigor of trees subjected to girdling level 3 was significantly lower
This study evaluated the effect of thinning, girdling intensity and pruning season on tree growth and vigor in an old L. kaempferi seed orchard
Summary
Japanese larch (Larix kaempferi (Lamb.) Carr.) often exhibits superior growth compared to other larch species, and the species has been widely used in breeding programs in many countries [1,2]. Hybrids between L. kaempferi and other larches have been used commercially in Europe [2], North America [3] and Asia [4,5]. In Japan, breeding programs for L. kaempferi were initiated in the 1950s, and more than 500 first-generation plus trees were selected [6]. Selection of second-generation trees started in the 2010s, and this is ongoing [6,7]. Reforestation using larches has increased over the last decade, and L. kaempferi is the second most important forestry species in Japanese plantations [8]. Owing to the relatively higher growth rates and carbon fixation ability of this species [6,7], demand for improved seeds and seedlings from orchards is increasing [9]
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