Impacts of tree plantation-driven forest fragmentation on native soil bacteria and fungi: similar patterns, but contrasting mechanisms

A global expansion of tree plantations, particularly in developing tropical and subtropical countries, has fragmented natural forests. Despite the importance of microorganisms in ecosystem services, it remains unclear whether plantation-driven fragmentation threatens microbial biodiversity. We hypothesize that bacterial and fungal diversity patterns on remnant forest “islands” surrounded by a “sea” of tree plantations follow the traditional island species–area relationship (SAR), i.e., larger fragments harbor more species. We examined the SAR patterns of soil bacteria and fungi and explored the mechanisms underlying the observed SAR using a cluster of 17 remnant natural forest fragments scattered across a “sea” of tree plantations in Yachang Forest Farm, Guangxi, China. As expected, both fragment-scale bacterial and fungal diversity increased with fragment area; however, neither bacterial nor fungal diversity was influenced by fragment isolation. The positive SARs for both bacteria and fungi were characterized by greater heterogeneity in community composition among localities on larger fragments. However, the mechanisms underlying the positive SARs differ between bacteria and fungi. The greater heterogeneity of bacterial community composition on larger fragments was associated with higher levels of soil pH heterogeneity, while the greater heterogeneity of fungal community composition was linked to increased geographic distances among localities on larger fragments. Our results suggest that the SAR for bacteria is primarily driven by habitat heterogeneity, whereas the SAR for fungi is primarily driven by dispersal limitation. These findings highlight urgent need for conservation efforts to maintain large, continuous remnant natural forest fragments to mitigate fragmentation-induced biodiversity loss.