Abstract
Aim of study: The objective of this study was to analyse the effect of species mixing of Scots pine and Norway spruce on the productivity at the stand and species level. We also analysed to what extent the mixing effects is modified by drought stress.Area of study: The study was conducted inN-E Poland and based on three experiments located in Maskulińskie, Strzałowo and Kwidzyn Forest Districts.Material and methods: We evaluated long-term mixed-species experiments in Scots pine and Norway spruce which are under continuous survey since more than 100 years. Stand productivity was analysed based on the periodic annual increment and total yield of stem volume. Growth and yield were compared between mixed and neighbouring pure stands. As a substitute for the missing Norway spruce monocultures, we used appropriate yield table data. In order to characterize the effect of water supply on the mixing effects, we correlated the Martonne index of aridity with the ratio of Scots pine growth in mixed versus pure stands.Main results: We found that the mixed stands exceed the weighted mean of the pure stands’ volume productivity on average by 41%. At the species level Scots pine benefits from the mixture by 34% and Norway spruce by 83%. Growth periods with harsh climate conditions reinforce overyielding, while periods with mild conditions reduce the benefit of mixing. The overyielding of mixed stands, especially when growing under unfavourable conditions, is explained by niche complementarity of both species and discussed in view of the stress-gradient-hypothesis.Research highlights: The revealed overyielding of mixed compared with neighbouring pure stands, particularly under harsh weather conditions, substantiates the preferences of Scots pine-Norway spruce mixtures regarding climate change. Keywords: drought resilience; mixed stand; pure stand; facilitation; competition; overyielding; underyielding.
Highlights
Many studies underline the importance of species diversity for most forest functions and services (Zhang et al, 2012; Gamfeldt et al, 2013)
Norway spruce as a late successional species catches up with Scots pine, and presently both species are more approaching in mean size
Notice that Norway spruce as a shade tolerant tree species occupies the upper and lower storey of the canopy
Summary
Many studies underline the importance of species diversity for most forest functions and services (Zhang et al, 2012; Gamfeldt et al, 2013). Recent studies in temperate and boreal forests frequently found overyielding of mixed versus pure stands of 20-30% in terms of stand volume productivity, due to the niche complementary of associated species (Morin et al, 2011; Pretzsch et al, 2010, 2013a). The reviews by Piotto (2008) and Zhang et al (2012) summarize that mixtures are often much more productive than monocultures Such reported findings of advantages of mixed versus pure stands with respect to productivity, decisively influence the forest owners decision in favour of mixed species stands (Olsthoorn et al, 1999). On poor and medium fertile podsolic sandy soils with precipitation around 600 mm, Norway spruce and Scots pine can grow balanced in association, as none of the species gains dominance over the other (see Pukkala et al, 1994; Lindén & Agestam, 2003)
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