How do trees respond to species mixing in experimental compared to observational studies?

Abstract

For decades, ecologists have investigated the effects of tree species diversity on tree productivity at different scales and with different approaches ranging from observational to experimental study designs. Using data from five European national forest inventories (16,773 plots), six tree species diversity experiments (584 plots), and six networks of comparative plots (169 plots), we tested whether tree species growth responses to species mixing are consistent and therefore transferrable between those different research approaches. Our results confirm the general positive effect of tree species mixing on species growth (16% on average) but we found no consistency in species‐specific responses to mixing between any of the three approaches, even after restricting comparisons to only those plots that shared similar mixtures compositions and forest types. These findings highlight the necessity to consider results from different research approaches when selecting species mixtures that should maximize positive forest biodiversity and functioning relationships.

Good things take time—Diversity effects on tree growth shift from negative to positive during stand development in boreal forests

“…By contrast, P. sylvestris grew noticeably slower when mixed with B. pendula . The fact that we found such marked differences in the behaviour of the same combination of species growing within a few hundred kilometres of each other is less than promising for future efforts to bridge observational and experimental studies of forest dynamics (Kambach et al., 2019).…”

“…Future work leveraging networks of tree diversity experiments will also help clarify whether some of the other trends we observe in our data—such as the tendency of diversity to negatively influence growth in the early stages of stand development—also emerge across different species and forest types (Kambach et al., 2019). In contrast to our expectations, which were for diversity effects in the earliest stages of stand development to be mostly neutral, seven of the nine species combinations in the FunDivEUROPE plots showed negative effects of diversity on tree growth between ages 5 and 15 (Figure 3).…”

“…The fact that positive diversity effects on tree growth in the FunDivEUROPE plots tended to strengthen with time and only became apparent once stands matured enough to achieve canopy closure may explain why most tree diversity experiments established outside the tropics have so far found little evidence that diverse tree communities are more productive than species‐poor ones (Grossman et al., 2018; Haase et al., 2015; Kambach et al., 2019; Verheyen et al., 2016). Currently, the average duration of the 26 globally distributed tree diversity experiments that form TreeDivNet is 9 years (range 1–20 years, with Satakunta being the oldest; for details see: http://www.treedivnet.ugent.be and Verheyen et al., 2016).…”

“…Future work leveraging networks of tree diversity experiments will also help clarify whether some of the other trends we observe in our data-such as the tendency of diversity to negatively influence growth in the early stages of stand development-also emerge across different species and forest types (Kambach et al, 2019).…”

“…Consequently, overyielding—whereby species in mixture outperform those in monoculture—may take years to manifest in regenerating stands. This may help explain why, in contrast to observational studies conducted in mature forests, most tree diversity experiments established in temperate and boreal forests in the last 5–10 years have so far found little evidence of overyielding (Haase et al., 2015; Verheyen et al., 2016; Grossman et al., 2018; Kambach et al., 2019; although see Williams et al., 2017). The problem is that testing this hypothesis would require long‐term, annually resolved growth records for trees exposed to different levels of diversity, data which are not typically recorded in forests.…”