ABSTRACT

Admixtures of birch in Norway spruce plantations are being promoted as a means to increase habitat and species diversity. The implications of this mixture were analysed with regional survey data from southern Sweden. Permanent sample plots from the Swedish National Forest Inventory (NFI), with Norway spruce and admixture of birch, were used to describe the temporal trends in the admixture, regarding species composition and competitive strength. Observations from thinned plots show a higher harvest removal in birch (35%) than for Norway spruce (19%). Observations without thinnings in the period before measurement showed that individual birch tree growth was lower compared to Norway spruce and it decreased even more with increasing stand age and competition. In addition, a complementary field survey, with multiple distributed sample plots in each stand, was used to detect within-stand variation of species composition and density. Although within-stand heterogeneity was larger in mixed stands in terms of species composition, it was not different from Norway spruce monocultures in terms of stand density. These two surveys show that the admixture of birch, for several reasons, decreases over stand age and although birch increases tree species diversity, it does not necessary imply a change in density.

摘要

挪威云杉人工林中的桦木混合物正在得到推广，以增加生境和物种多样性。使用瑞典南部的区域调查数据分析了这种混合物的含义。瑞典国家森林清单（NFI）的永久性样地，挪威云杉和桦木混合物被用来描述混合物的时间趋势，涉及物种组成和竞争强度。从稀疏田地上观察到，桦木的收割率（35％）比挪威云杉的高（19％）。在测量前的一段时间内没有间伐的观察表明，与挪威云杉相比，桦木的单株生长较低，并且随着林分年龄和竞争的加剧，桦木的生长下降幅度更大。此外，还有一项补充实地调查，在每个林分中有多个分布的样地，用于检测林中物种组成和密度的变化。尽管就物种组成而言，混交林中的林间异质性较大，但就林分密度而言，它与挪威云杉单一栽培没有区别。这两个调查表明，由于种种原因，桦木的混合量会随着林分龄的增长而减少，尽管桦木增加了树种的多样性，但这并不一定意味着密度的变化。