Abstract
Soil-borne fungi influence coexistence of plant species in mesic environments, but much less is known about their effects on demographic processes relevant to coexistence in arid and semi-arid systems. We isolated 43 fungal strains that naturally colonize seeds of an invasive winter annual (Brassica tournefortii) in the Sonoran Desert, and evaluated the impact of 18 of them on seed germination and mortality of B. tournefortii and a co-occurring native annual (Plantago ovata) under simulated summer and winter temperatures. Fungi isolated from B. tournefortii seeds impacted germination and mortality of seeds of both plant species in vitro. Seed responses reflected host-specific effects by fungi, the degree of which differed significantly between the strains, and depended on the temperature. In the winter temperature, ten fungal strains increased or reduced seed germination, but substantial seed mortality due to fungi was not observed. Two strains increased germination of P. ovata more strongly than B. tournefortii. In the summer temperature, fungi induced both substantial seed germination and mortality, with ten strains demonstrating host-specificity. Under natural conditions, host-specific effects of fungi on seed germination may further differentiate plant species niche in germination response, with a potential of promoting coexistence. Both host-specific and non-host-specific effects of fungi on seed loss may induce polarizing effects on plant coexistence depending on the ecological context. The coexistence theory provides a clear framework to interpret these polarizing effects. Moreover, fungi pathogenic to both plant species could induce host-specific germination, which challenges the theoretical assumption of density-independent germination response. These implications from an in vitro study underscore the need to weave theoretical modeling, reductive empirical experiments, and natural observations to illuminate effects of soil-borne fungi on coexistence of annual plant species in variable desert environments.
Highlights
One major endeavor in ecology is to uncover ecological factors that promote or undermine coexistence of competing species
Our statistical analyses were designed around the following four constraints: 1. seed responses to fungi were first corrected by a response of controls; 2. the data were binomially distributed; 3. each Petri plate was considered as a random effect as seeds of both plant species were placed within; and 4. we aimed to present our results as clear, biologically interpretable effect sizes, preferably without data transformation
We examined the overall effects of each fungal strain on the loss of the seed bank for each plant species, defined by the combined fraction of seed germination and mortality in the summer temperature regime
Summary
One major endeavor in ecology is to uncover ecological factors that promote or undermine coexistence of competing species. Host-specific effects of fungi on seedling recruitment promote plant diversity in a tropical forest by primarily strengthening natural enemy partitioning, a major stabilizing mechanism [9]. Fungal pathogens can generate density dependent feedbacks (i.e., apparent competition) that interfere with the stabilizing effects that arise from resource competition (e.g., resource partitioning and the storage effect due to resource competition) [1,20,21] In this scenario, the presence of fungal pathogens can strengthen or weaken the coexistence potential among plants, an effect that cannot be detected when effects of resource competition are examined alone (e.g., [21])
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