Functionally and phylogenetically diverse boreal forests promote sapling recruitment

Abstract

Sapling recruitment in established forest stands is critical to long-term ecosystem functioning but can be constrained due to competition from increased neighbourhood crowding. Tree species diversity has the potential to ameliorate negative crowding effects; however, the mechanisms underlying the relationship between diversity and sapling recruitment remain unclear. We used data from spatially mapped individual stems, which were repeatedly measured at 5-year intervals between 1986 and 2010 from 173 permanent plots in Canada. We analyzed individual sapling recruitment probabilities over 5-year census intervals as a function of neighbourhood crowding index, stand age, neighbourhood diversity from shade tolerance dissimilarity and phylogenetic dissimilarity, and their interactions. We show that sapling recruitment probability on average decreases with neighbourhood crowding but increases with stand age and neighbourhood shade tolerance and phylogenetic dissimilarities. The positive effect of shade tolerance dissimilarity was strongest in young stands, while phylogenetic dissimilarity had a positive effect only in older stands. Moreover, sapling recruitment probability in neighbourhoods with high shade tolerance and phylogenetic dissimilarities was less negatively affected by crowding than in more similar neighbourhoods. Our study highlights the mechanisms that promote sapling recruitment in diverse neighbourhoods are context-dependent, with niche complementary being more important in young stands and the feedback from pests and pathogens playing a greater role in old boreal stands. Our results provide direct evidence that neighbourhood diversity helps ameliorate the negative neighbourhood crowding effect on sapling recruitment, particularly in old-growth boreal forests. The mixtures of broadleaves and conifers increase sapling recruitment and are particularly important in the late-successional stages of boreal forests.