Sprucification in protected forests: myth or veracity? – Clues from 60 yrs survey data

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Abstract

Questions:

The purpose of this study was to examine how compositional characteristics of forest reserves in production landscapes change over time and to explore the local site conditions that drive these processes. Our hypothesis was according to the general paradigm that suppression of natural disturbance regimes and lack of active management causes protected forest areas (PFAs) to develop into forests of high density dominated by late successional tree species.

Location:

Boreal and hemiboreal forests of Sweden.

Methods:

We used long-term (60 yrs) data from the Swedish National Forest Inventory (NFI), representing all the PFAs in Sweden, to study changes in tree species composition in relation to site factors and the initial state of the forest.

Results:

During the period of our study the average density expressed by the growing stock (GS) increased from 70 to 140 m3·ha−1. This increase concerned both early and late successional tree species and there was no evidence for a faster increase of the late successional species Norway spruce (Picea abies) and the species composition was stable over time. A net volume increment (NVINCR) in spruce was mostly related to sites where spruce already was present, sites with high productivity and forest of high age. The probability of an increase in Scots pine (Pinus sylvestris) was negatively linked to productivity and stand age, while the NVINCR was higher on more productive sites. The NVINCR of deciduous trees was unrelated to site productivity and age but positively affected by the initial GS, increasing slope and soil depth.

Conclusions:

Our results suggest that the relative composition of species is stable over time and balanced by site productivity, which is in accordance with the resource-ratio hypothesis. An important cause of this stability could be the low number of tree species in northern Europe, which limits the number of potential successional pathways.

Long-term (60 yrs) data from the National Forest Inventory representing all the protected forest in Sweden was used to study changes in tree species composition. Our results indicate that the assumed alterations in disturbance regimes that allowed a doubling of the growing stock have not simultaneously led to changes in tree species composition or the expected increase in late-successional species.

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