Silver fir stand productivity is enhanced when mixed with Norway spruce: evidence based on large-scale inventory data and a generic modelling approach

Abstract

Questions: How to evaluate the mixture effect on basal area increment in two‐species forest stands? Is a mixed Norway spruce–silver fir stand more productive than pure adjacent stands of either species? How to develop generic modelling approaches to assess mixture effects in forest stands? Location: In addition to a case study on Norway spruce–silver fir stands in French mountain forests, the generic approach used goes beyond local applications. Methods: We took advantage of National Forest Inventory data to develop a unique stand basal‐area‐increment model for pure and mixed stands of Norway spruce and silver fir that responds to ecological site conditions. The database was made up of 284 pure Norway spruce stands, 196 pure silver fir stands, and 323 mixed stands of these species. Results: Pure silver fir basal area increment is strongly influenced by spring climatic conditions, whereas pure Norway spruce is more influenced by soil conditions. The mixture of these species has a positive effect on silver fir, which decreases as the proportion of fir increases. In contrast, the mixture has no noticeable effect on Norway spruce. Conclusion: We developed a stand basal‐area‐increment model evidencing an advantage of the mixture on silver fir basal area increment, but not on Norway spruce. The mathematical formulation of the model developed is generic and can be used in all two‐species mixture situations. It also makes it possible to compare different mixture situations with each other.

Tree light capture and spatial variability of understory light increase with species mixing and tree size heterogeneity

“…This result supports the hypothesis that complementarity of light use could be at the origin of the over-yielding that has been observed in some mixed stands (Toïgo et al, 2015;Pretzsch and Schütze, 2009;Vallet and Pérot, 2011). The difference between the observed values of transmittance in mixed stands and weighted averages originates, at least partly, from the non-linearity of the relationship between light transmittance and stand basal area (Comeau and Heineman, 2003;Lochhead and Comeau, 2012;Sonohat et al, 2004).…”

“…It has been advocated that forests with many tree species and a great heterogeneity in tree sizes are more productive (Pretzsch and Schütze, 2009;Vallet and Pérot, 2011;Toïgo et al, 2015), resistant and resilient (Jactel et al, 2005) to future changes and harbor a greater biodiversity (Huston, 1979) than monospecific forests in which all trees have a similar size. Over the last 50 to 100 years, humans have applied forest management systems over much of the northern hemisphere that tended to simplify both the structure and composition of many forests (Bengtsson et al, 2000).…”

“…Many researchers and forest managers now promote the development of more heterogeneous forests (McElhinny et al, 2005;Puettmann et al, 2009), but our understanding of the sharing and use of resources among species and vegetation strata in heterogeneous forests is limited and impedes our ability to predict the long-term dynamics of such forests. Researchers have usually hypothesized that forest productivity is greater in mixed forests than in the corresponding pure forests, because species mixtures should capture more resources and, in particular, more solar radiation due to the layering of the forest canopy and the complementarity and overlapping of tree crowns (Forrester et al, 2006;Kelty, 2006;Yachi and Loreau, 2007;Pretzsch and Schütze, 2009;Bauhus and Schmerbeck, 2010;Vallet and Pérot, 2011). It is also generally assumed that the structural and compositional heterogeneity of the forest overstory directly affects the amount and variability of light available in the understory for tree regeneration (Kelty, 2006).…”

Tree species mixing can increase maximum stand density

“…Studies based on forest inventory plots (Condés et al 2013;Vallet and Pérot 2011) may cover a certain portion of fully stocked pure and mixed-species stands. However, if the stand history is unknown, it remains open as to which plots are really at maximum stand density.…”

Growth and yield of mixed versus pure stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) analysed along a productivity gradient through Europe

“…Recent empirical studies of mixed forest long-term experiments (Bielak et al 2014;Pretzsch et al 2010Pretzsch et al , 2013a, studies using large-scale data bases (del Río and Sterba 2009;Río et al 2014a;Vallet and Perot 2011), simulation model studies (Morin et al 2011), as well as meta-analyses (Griess and Knoke 2011;Paquette and Messier 2011;Piotto 2008;Zhang et al 2012), showed that the productivity of mixed stands can exceed the weighted mean productivity of pure stands. This overyielding can reach 50 % in mixture with nitrogen-fixing species (Forrester et al 2006) and 20-30 % in other mixtures (Pretzsch et al 2013a).…”