Transplant Experiment and Data Collection
All four species were transplanted into each plot from our regional pool
of seeds. At the corner of each plot, we hammered a 1 x 2.5 cm (diameter
x length) polypipe tube into the ground with a 0.5 cm lip above the soil
surface (Germain et
al. 2022). We left 15 cm around each tube to prevent transplants from
interacting directly, as it is a typical neighborhood size for small
annual plants (Mayfield &
Stouffer 2017). In each tube we planted two seeds of the same species
to increase chances that at least one would germinate (7,200 seeds
total). Using tubes prevents seeds from washing away and allows us to
distinguish our transplanted individuals from individuals that naturally
occur in plots (Germainet al. 2022); the tubes are short enough that they are unlikely
to impede root growth or competition belowground. If we encountered a
rock in locations designated to receive a transplant, we forgoed
planting the seed and marked germination and seed production as ‘zero’
to reflect the unsuitability of this microsite.
At the end of the growing season (May 2020), we returned to quantify
occupancy and persistence. First, to quantify occupancy, we photographed
every block for image analysis. We recorded if our four focal species
occurred naturally (binary), as well as a rough measure of abundance
(i.e., four categories: zero individuals, 1-10 individuals, 11-100
individuals, and >100 individuals per plot). Second, to
quantify persistence, we scored whether transplanted individuals in each
tube germinated (binary), survived to reproduce (binary), and collected
all seed and biomass. We also recorded instances of obvious herbivory
(e.g., lopped off seed head) or if a tube went missing (total of 393
instances, mostly due to birds stealing tubes). We counted every seed
produced by each transplanted individual, as well as noted whether or
not one or both of the two transplanted seeds per tube survived to
produce seed so we could account for it in our analyses. Given that two
seeds were planted into each tube, population growth rates were
calculated per capita (λ ) and were used to create a binary
variable of whether each population was persistent (λ ≥1) or not
(λ <1). Together, these data allow us to identify how
populations experienced the suitability of the landscape along a
productivity gradient.