In Sweden, Norway spruce is one of the most widespread and important tree species for the provision of economic and ecological benefits. It is a secondary tree species and regenerates naturally under the shelter of larger surrounding trees. However, current forest practices predominantly involve even-aged stands where regeneration is commonly carried out by planting seedlings after final felling of clear-cut stands. Given these practices, late spring frosts have been identified as a major risk to newly replanted Norway spruce forests in southern Sweden, causing damage to the newly exposed shoots which in turn has detrimental effects to the growth, quality and vitality of the trees.
The growth rhythm of Norway spruce is adapted to seasonal changes in day length and temperature, with substantial variation across different regions. Consequently, the timing of bud burst and shoot exposure is clearly related to provenance origin and is under strong genetic control. Thus, to avoid damages of late spring frosts, a general recommendation in southern Sweden is to select forest regeneration material (FRM) with a late bud burst on planting sites which are considered to be frost prone. Suitable FRM for such conditions are stand seed from Eastern Europe and seed orchards with clones specifically selected for late bud burst.
However, these recommendations are very coarse as: (i) it is very difficult to classify the frost risk at a specific planting site; (ii) there is a continuum of FRM available, each with a unique growth rhythm property; (iii) climate change will affect both frost events and growth rhythms. There is therefore a need to develop models that can predict a FRM specific bud burst and associated risk of frost damages, both in the current and future climate.
In this study we used gridded climate data combined with data from 18 Swedish and East-European Norway spruce provenances grown at three different sites in southern Sweden to develop models that can predict timing of bud burst, frost events and the risk of associated damages. Results corroborated that accumulated temperature above a certain threshold value, called temperature sum, is a main determinant for timing of bud burst and that this trait is correlated with the provenance origin along latitudinal and longitudinal gradients. Furthermore, early bud burst was found to be associated with a higher risk of frost damage, but this also depends on the site specific conditions. The importance of local site conditions was highlighted by comparing gridded climate data with data from temperature loggers at ten sites spread across southern Sweden. In particular, gridded climate data often underestimated daily minimum temperatures at 0.5 meters above ground. This indicates that frost risk of young trees that are particularly vulnerable is underestimated in the models developed on gridded data only.
The developed models were used to predict outcomes in a future climate for the periods 2021-2050 and 2071-2100 using the RCP8.5 scenario. There are four RCP-scenarios used to project future climate conditions which represent different greenhouse gas concentrations. In RCP 8.5, emissions continue to rise throughout the 21st century and is generally considered the “worst-case” scenario with respect to temperature increase. As the climate gets warmer the total number of frost events in southern Sweden will decrease. But as bud burst will occur earlier in the year, the number of frost events after bud burst will instead increase as will the associated risk of frost damages. Thus, adapting growth rhythm properties of FRM in future Norway spruce stands will continue to be a highly important task.
The models developed in this study can be used predict a provenance specific risk of frost damage at an arbitrarily selected site if the provenances can be properly characterized with regards to temperature sum requirements for bud burst. Given these features, the models could be included into deployment recommendation tools like Plantval, to improve decision support to forest owners and managers. However, the majority of seedlings planted in Sweden originate from seed orchards, which also have a higher expected areal production than provenances. Thus, to make full use of the models suggested here, a link between the growth rhythm (mainly bud burst) of seed orchards and provenances are needed.
A fuller description of this work is available in: Svystun, T., Lundströmer, J., Berlin M., Westin, J., and Jönsson, A.M. 2021. Model analysis of temperature impact on the Norway spruce provenance specific bud burst and associated risk of frost damage. Forest Ecology and Management, 493, 16p. doi:10.1016/j.foreco.2021.119252