Abstract
AbstractAimPrevious analyses of remotely sensed data detected the multimodality of the tree‐cover distribution of the boreal forest, and identified areas with potentially alternative tree‐cover states. This paper aims at investigating the causes of multimodality and multistability of the boreal forest, their influence on the asymmetric tree species distribution between Eurasia and North America, and whether multistability could be associated with recent greening trends in leaf area index (LAI) and normalized difference vegetation index (NDVI).LocationEurasian and North American boreal forests.Time period2000–2010.Major taxa studiedBoreal forest plant functional types.MethodsWe employ a conceptual model based on tree species competition to simulate the sensitivity of tree cover to stochastic disturbances and to changes in environmental factors. We include different plant functional types based on survival adaptations, and force the model with remotely sensed environmental data. We analyse the model as a dynamical system. We use metrics from statistics and information theory to compare the detection of alternative tree‐cover states and greening trends in LAI and NDVI.ResultsWe find that multimodality and multistability can emerge through competition between different plant functional types. Additionally, our model is able to reproduce the asymmetry in tree species distribution between Eurasia and North America. Moreover, changes in permafrost distribution can be associated with phenomenological bifurcation points of the model. Finally, we find that the detection of multistable areas is not affected by recent vegetation trends, whereas shifts between alternative states could have affected the greening trends.Main conclusionsTree‐cover multistability in the boreal region can emerge through competition between species subject to periodic disturbances. Changes in permafrost thaw and distribution could be responsible for the asymmetry in tree species distribution between North America and Eurasia. Climate change and permafrost degradation could cause shifts in tree‐cover states and dominant species. Recent vegetation greening trends in multistable areas could have been affected by shifts between alternative states.
Highlights
The tree‐cover distribution of the boreal forest exhibits three alter‐ native modes: low tree cover, open woodland and forest (Scheffer, Hirota, Holmgren, Nes, & Chapin, 2012; Xu, Medvigy, & Rodriguez‐ Iturbe, 2015)
This paper aims at investigating the causes of multimodality and multistability of the boreal forest, their influence on the asymmetric tree species distribution between Eurasia and North America, and whether multistability could be associated with recent greening trends in leaf area index (LAI) and normalized difference vegetation index (NDVI)
Through the use of the mutual information for clusters (MI) metric, we conclude that the detection of multistable areas of the boreal forest is not affected by the recent greening trends of LAI and NDVI
Summary
The tree‐cover distribution of the boreal forest exhibits three alter‐ native modes: low tree cover, open woodland and forest (Scheffer, Hirota, Holmgren, Nes, & Chapin, 2012; Xu, Medvigy, & Rodriguez‐ Iturbe, 2015) These states, corresponding to remotely sensed tree‐ cover fraction values below 20%, between 20 and 45%, and above 45%, respectively, have been suggested to reflect the presence of alternative stable states acting as attractors (Scheffer et al, 2012). Areas with potentially alternative tree‐cover states under the same environ‐ mental conditions have been identified, as, rein‐ forcing the hypothesis of the presence of alternative stable states (Abis & Brovkin, 2017) These areas encompass 1.1 million km, and correspond to possible transition zones with a reduced resilience to disturbances. The mechanisms underlying the ex‐ istence of possible multiple stable tree‐cover states are still unknown
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