Belowground traits predict juvenile grass demography in microsites

Plant functional traits can be a powerful tool for predicting species demography in response to variable environmental conditions. However, accurate predictions of juvenile plant response require ontogenetically relevant traits that capture response to microsite variability. This is particularly important when considering drivers of seedling emergence, survival, growth, and recruitment of species in the context of population persistence or community assembly. We tested the effect of two different microsites on juvenile demography for eight perennial grass species in a semi-arid system in Colorado along the western edge of the Great Plains. We used seed and root functional traits across multiple life stages to predict these responses and identify mechanisms driving species’ emergence, survival, growth, and recruitment. Contrary to our expectations, we found that microsites with increased soil moisture (i.e., furrows) had a negative effect on grass emergence early in the season but no effect on recruitment at the end of the season. This was likely driven by the increased growth and survival of grass juveniles in furrows compared to grass juveniles on the surface (reduced soil moisture). We also found that species with more acquisitive roots—from more rapid root elongation—benefited from the increased soil moisture early in the season, but this benefit disappeared later in the season, speaking to the value of using life-stage-specific traits to predict early life-stage transitions. Variations in microsites will impact juvenile perennial grass demography differently depending on species’ traits across life stages. While species in this system typically experience wet-to-dry transitions across the growing season, furrows with increased soil moisture reduce the intensity of this dry-down and may alter demographic responses depending on grass functional traits across ontogeny. We found grass species that are adapted to take advantage of resource pulses did best in furrows where moisture was greater, but that species adapted to conserve resources under stress had limited capacity to respond to these resource pulses.

Methods
To determine emergence and recruitment response to microsites, we counted grass abundance throughout the growing season. In addition to using row-level abundances to determine emergence and recruitment, we tracked individual survival and growth of a subset of individuals across three life stages. We used seed and seedling trait data obtained from the greenhouse trait sampling effort described in (Larson, 2021). To quantify the effects of the furrows on microsite conditions, we measured soil moisture, soil temperature, and surface temperature within these rows with TOMST TMS-4 loggers (Wild et al., 2019).