Moose (Alces alces) is a culturally and economically important game species in Sweden but its browsing on young pine trees in (Pinus sylvestris) cause extensive damage to forestry. Management is therefore posed with the dilemma of how to reduce damage to timber trees, while at the same time support robust moose populations. The proportion damaged trees can be reduced by decreasing the number of moose, but also by increasing the number of pines. However, it is not clear exactly how effective these measures are in general and how their impacts vary across Sweden. Here we analyzed the effects of moose- and pine density on pine damage using yearly data from almost all of Sweden’s moose management areas over the period 2015-2024 (718 observations). We developed a mechanistic model to realistically represent the browsing process and used regression with mixed models to account for variable vulnerability (damage at a given number of moose per pine tree) among MMAs in the statistical analysis. The model explained 53% of the variation in proportion damaged trees and showed that, on average, the relative damage reduction effect of a decreased moose population was larger (25%) than the effect of increased pine density (17%). Vulnerability to browsing varied substantially among MMAs and between years within each MMA, especially in areas with low pine density. This variability prevents reliable predictions of management effects at individual MMA level for most MMAs. Such local predictions may be improved in the future by incorporating longer time series of observations and additional variables, such as alternative food, browsing by other deer species, and snow cover.

Reliable manipulation of soil organic matter (SOM) – a necessity for optimal land management – is constrained by our limited mechanistic understanding of SOM formation. Here we add to existing frameworks a novel mechanistic element that may underpin SOM dynamics, based on evolutionary-ecological rather than chemical or physical limitations to decomposition. We argue that decomposition of some substrates may be ecologically constrained in mycelial fungi, evolutionarily constrained in co-operating bacteria, and geometrically constrained in unicellular microbes. We describe and test a mathematical model based on our framework, providing a proof-of-concept that substrate can plausibly be spared decomposition and accumulate even when it is physically and chemically accessible. Our framework can explain a variety of SOM dynamics, including priming and the suppression of decomposition by nitrogen addition, as well as the typical composition of SOM. An augmented mechanistic framework for understanding SOM dynamics can help guide targeted empirical study, which in turn can contribute to more optimised land management.

The mycorrhizal symbiosis is ubiquitous in boreal forests. Trees and plants provide their fungal partners with photosynthetic carbon in exchange for soil nutrients like nitrogen, which is critical to the growth and survival of the plants. But plant carbon allocation to mycorrhizal symbionts can also fuel nitrogen immobilization, hampering tree growth. Here we present results from field and greenhouse experiments combined with mathematical modelling, showing that mycorrhizal fungi can be simultaneously mutualistic to an individual tree and parasitic to the networked community of trees. Mycorrhizal networks connect multiple plants and fungi, and we show that each tree gains additional nitrogen at the expense of its neighbors by supplying more carbon to the fungi. But this additional carbon supply eventually aggravates nitrogen immobilization in the shared fungal biomass. Individual trees may thus independently benefit from increasing their carbon investment to mycorrhiza, while causing a decline in nitrogen availability for the whole plant community. We illustrate the evolutionary underpinnings of this situation by drawing on the analogous the tragedy of the commons, and explain how rising atmospheric CO2 may lead to greater nitrogen immobilization in the future.