Tree species diversity effects on productivity, soil nutrient availability and nutrient response efficiency in a temperate deciduous forest

• Nutrient limitation was species-dependent with no tree species limited by N.
• Beech growth was facilitated by enhanced levels of P and K in mix-species stands.
• Nutrient response efficiencies can be optimized by selecting species mixtures.

There are contrasting reports whether and how tree diversity influences stand productivity in temperate deciduous forests. Tree species diversity may increase stand productivity in temperate forests through complementary resource use and/or facilitation if the resource considered limits productivity. In unpolluted temperate forests, net primary production is typically limited by nitrogen (N). However, in many parts of Europe high N deposition has alleviated N limitation and there is some evidence that phosphorus (P) and/or potassium (K) limitation has become more widespread. Here, we report on a study where we investigated whether complementarity and/or facilitation increase productivity in a typical German deciduous forest with tree species of beech (Fagus sylvatica), oak (Quercus petraea and Quercus robur), hornbeam (Carpinus betulus) and lime (Tilia cordata and Tilia platyphyllus). We measured biomass production and availability of soil N, P, K, calcium (Ca) and magnesium (Mg) in stands of single species (mono-species stands) and in stands with different combinations of three of the tree species above (mix-species stands). We used nutrient response efficiency (NRE) to evaluate whether a specific nutrient limits tree growth. At a stand level, above-ground net primary productivity did not differ between mono- and mix-species stands. At a tree level, using a neighborhood approach, relative growth rates of beech trees in mono-species stands were smaller than when they were in mix with lime and hornbeam whereas growth of lime trees in mono-species stands was larger than in mix with beech and oak. The NRE curve for beech showed that beech trees in mix-species stands had optimal P and K response efficiencies whereas beech trees in mono-species stands showed P and K limitations. The NRE curve for oak with exchangeable soil K showed that K levels were beyond the optimum NRE and thus K was not limiting oak growth. NRE curves for hornbeam and lime showed no significant relationships with any of the soil nutrients. Hence, nutrient limitation was species-dependent. Our results showed that using both NRE and a neighborhood approach are useful tools in quantifying the effects of individual tree species on a species’ productivity between mono- and mix-species stands. Such tools provide important basis for improving management of typical mix-species, temperate forests.