Responses of plant–pathogen interactions to precipitation: Implications for tropical tree richness in a changing world

Abstract

Tropical tree‐species richness is positively correlated with annual precipitation, but the mechanisms remain unclear. Phytopathogens promote tree‐species coexistence by disproportionately afflicting seedlings of locally abundant species, generating a rare species advantage. We consider whether increased plant–pathogen interactions in humid conditions favourable for phytopathogens could drive the precipitation‐richness relationship by accentuating the rare species advantage. Support for this mechanism requires that increases in disease under humid conditions disproportionately affect locally abundant species without spreading to rarer species. This criterion would be augmented by either increased phytopathogen host‐specificity under humid conditions, or increased asynchronicity in germination of different tree species. Research suggests that precipitation increases the rare species advantage. Increased precipitation enhances phytopathogen transmission, making escape from specialist pathogens more difficult. Additionally, drought stress predisposes plants to disease, especially by opportunistic pathogens. As seasonality in wet forests decreases, scope for asynchronous germination among species increases, potentially concentrating disease transmission within species. Synthesis. The pathways we identify could drive the precipitation‐richness relationship, but finding direct evidence for them remains a priority. Researching these pathways is especially important because decreasing precipitation due to climate change could disrupt key species coexistence mechanisms and erode tree‐species richness.

Limited contributions of plant pathogens to density‐dependent seedling mortality of mast fruiting Bornean trees

“…Seedlings experienced below average rainfall during the first month of the experiment. Drier conditions may have contributed to weaker recorded C-NDD, as increased moisture availability can enhance transmission of both above-and below-ground soil pathogens (Dorrance et al, 2003;Rossi & Caffi, 2012), with reduced precipitation limiting their ability to cause early mortality (Milici et al, 2020). Swinfield et al found increased frequency of watering resulted in significantly greater pathogen-induced seedling mortality, although even the most infrequently watered seedlings experienced substantially greater mortality than observed in our study.…”

“…Notwithstanding methodological issues, variation in the strength of C‐NDD likely reflects differences among species (Carson et al., 2008; Comita et al., 2014), with negative effects of conspecific density posited to more strongly influence rare rather than locally common species (Johnson et al., 2012; Mangan et al., 2010). Species life‐history strategies also affect the strength of C‐NDD (Jia et al., 2020), with trade‐offs between axes of growth and defence, at least partly, governing variation in species susceptibility of pathogens (Milici et al., 2020; Spear et al., 2015), potentially influencing seedling recruitment patterns. Shade‐tolerant species are more susceptible to fungal pathogens (Kobe & Vriesendorp, 2011; McCarthy‐Neumann & Kobe, 2008), particularly in fragmented forests (Krishnadas et al., 2018).…”

“…Species life-history strategies also affect the strength of C-NDD (Jia et al, 2020), with trade-offs between axes of growth and defence, at least partly, governing variation in species susceptibility of pathogens (Milici et al, 2020;Spear et al, 2015), potentially influencing seedling recruitment patterns. Shade-tolerant species are more susceptible to fungal pathogens (Kobe & Vriesendorp, 2011;McCarthy-Neumann & Kobe, 2008), particularly in fragmented forests .…”

When do Janzen–Connell effects matter? A phylogenetic meta‐analysis of conspecific negative distance and density dependence experiments

“…The resulting prediction is stronger J‐C effects in more humid sites (e.g. the tropical lowlands) and sites with higher rainfall (Givnish, 1999; Milici et al ., 2020). However, for herbivore natural enemies, there were no clear latitudinal patterns (Adams and Zhang, 2009; Moles et al ., 2011; Kozlov et al ., 2015; Lim et al ., 2015; Zhang et al ., 2016).…”

Natural Enemies and the Maintenance of Tropical Tree Diversity: Recent Insights and Implications for the Future of Biodiversity in a Changing World

“…Soil moisture and humidity are also well known to influence pathogens, with higher pathogen activity generally associated with wetter conditions (e.g., Brenes-Arguedas et al, 2009). Changing precipitation regimes in tropical forests will likely alter plant-pathogen interactions, although the implications for tropical tree diversity are unclear (see review by Milici et al, 2020). Experimental watering resulted in higher pathogen-caused seedling mortality in a study on one tropical tree species (Swinfield et al, 2012).…”