Vegetative sprouting as an additional pathway for a seed size-number trade-off: a field-parameterised simulation approach
Studies of perennial plants generally search for a seed size vs. seed number trade-off. Surprisingly, the fact that perennials may replace an investment in large seeds by the allocation to vegetative propagation has not yet been investigated as an additional pathway enabling species coexistence. We focused on the mechanisms of coexistence in Carex elata and C. elongata, two co-occurring clonal sedges dominant in European swamp alder forests. We asked the following questions: i) Is the number of germinated seeds a better predictor of species coexistence than the total number of seeds? ii) What recruitment conditions and competition rules determine vegetative sprouting to be an alternative to large, competitively superior seeds? We measured several species functional traits related to the colonisation and fitness of perennials. To examine the competitive hierarchy between species and microsite species preferences, we analysed the effects of environmental factors and plant densities on fitness-related traits using Structural Equation Modelling (SEM). Then, using a series of spatially explicit simulations partly parameterised based on the field measurement, we evaluated the importance of seed and ramet propagation and recruitment conditions for long-term species coexistence. SEM indicated a competitive hierarchy and a large overlap in microsite preferences between species. As a response to our initial questions we found that: i) Only differences in the numbers of germinated seeds, allowed the two species to coexist. If we consider only differences in the total number of seeds, the superior competitor (Carex elata) outcompeted the inferior competitor (C. elongata) in all scenarios. This is because the former produced about three-times as many seeds as the latter. ii) We show that vegetative sprouting represents an additional pathway for the seed size-number trade-off when the competitive superiority of species is attributed to vegetative propagation. This is another way that a species deals with the omnipresent seeds of other species. Taken together, our study demonstrates that differences in seed performance, coupled with differences in vegetative propagation related to competitive ability, are an additional mechanism allowing the coexistence of perennial plants.
KeywordsCarex elata Carex elongata Competition-colonisation trade-off Functional traits Sedges Spatially explicit models Structural equation models Wetland forests
Comparative Fit Index
Generalised Linear Mixed Model
Maximum Likelihood Method
Root Mean Square Error of Approximation Index
Structural Equation Modelling
Spatial Eigenvalue Vector Mapping
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