Species rich forests are biological jack-of-all-trades, but masters of none

The world’s forests form vast, complex ecosystems, which perform an array of services benefitting humanity. These are commonly named ecosystem services, and include, in the example of the forest, purely economic factors on the one hand, such as lumber production and pest resistance, but also environmental concerns on the other, like carbon dioxide sequestering and diversity of birds. But what should a forest look like in order to best generate these services? This is the topic of a new international study, published in the journal Nature communications, investigating how the ability of the ecosystems to provide ecosystem services is affected by species diversity. The study is made by among others Diem Nguyen and Jan Stenlid at the Departmentof Forest Mycology and Plant Pathology at the Swedish University of Agricultural Sciences. 

Selection and complementation

“It is known that species-rich ecosystems is better able to provide these services than monocultures, at least in general terms”, says Diem Nguyen. “This is commonly attributed to two mechanisms: complementation; a particular service is facilitated through an interplay between several species, and selection; the more species, the more likely that one is particularly productive for a certain service. Notwithstanding, note that this does not mean that species diversity automatically benefit every single service; either selection or complementation has to come into play.”

For natural reasons, these mechanisms are mostly relevant to the impact of biodiversity on specific ecosystem functions. It is unclear whether they also contribute to so-called multifunctionality, i.e. the ability of the ecosystem to provide services in general, or if this phenomenon is driven by other mechanisms. The present study investigates this by quantifying ecosystem services at 209 plots throughout forests in Spain, Italy, Romania, Poland, Germany and Finland. The plots were specifically chosen to be as similar as possible, only varying in species composition. They were dominated by between one and five tree species, of a total of sixteen. The ecosystem services were quantified between 0 and 100 %, with 100 % being the highest performance measured for the given function in the entire study.

Diversity promotes low multifunctionality

“We measured multifunctionality across 16 services”, says Diem Nguyen. “These were lumber quality, tree rejuvenation, root, microbe and earthworm biomass, wood and litter decomposition, carbon stock of soil, resistance to drought, mammals, pests and pathogens, and diversity of birds, bats and understory vegetation. We found that crucial to the perceived level of multifunctionality is the required degree of performance. If, for example, a function has to be provided at 50 % of that of the most productive of the 209 plots, then the species rich forests are the most productive. Up to this level, the forest becomes increasingly multifunctional, that is, provides more services the more species it harbors. If a performance over 50 % is necessary, the biodiversity becomes an obstacle and the multifunctionality instead decreases with increasing number of species.”

“Forests with large species diversity does not reach that level of performance for any service”, says Professor Jan Stenlid. “Monocultures do, but only for single services, and in return, underperform in every other aspect. We have described this such that species-rich forests are “jack-of-all-trades”, capable of performing every service to at least some level, but “masters of none”, that is, without doing particularly well in any. The ecological explanation for this is that these services are being performed by single species, and the performance of a particular service thus correlates to the distribution of the species that promotes it. This mechanism thus explains that local diversity is strongly linked to local multifunctionality. For the performance of a single function, the diversity may also be of importance, but only if the complementation or selection mechanisms kick in”, Jan Stenlid concludes.