2.3 Experiment setup and site maintenance
Before planting, each site was treated with a broad-spectrum herbicide
(Roundup, active ingredient Glyphosate, SpecialistSales, Toowoomba, QLD,
Australia) across the entire plot for the DRO and Cow Bay, or along the
planting strips (~1m wide) across the slope at Thiaki.
Saplings ~6 to 12 months (mean height 37 cm, range 5 to
107 cm) were planted at each of the three trial sites in late March to
April 2022 at 3 m x 3 m spacing to provide for an even light environment
as plants grew. This included ten replicate saplings each of the two
provenances (lowland and upland) for 16 species planted at each of the
three trial sites (n = 320 at each site). The order of plants was
randomised within ‘blocks’ with 1 replicate plant from each of 16
species and provenance (upland or lowland) represented in each block
(n = 32 plants per block x 10 blocks at each site). At planting,
soil cores were augered (diameter 200mm, depth ~30 cm)
with each hole receiving 50g of an organic fertilizer pellet (Organic
Life, Terra Firma fertilizers, Beaudesert Qld, Australia), and the
pre-soaked plants replanted and watered in immediately. At the Thiaki
site, there was a lack of significant rainfall in the weeks immediately
after planting, so seedlings were hand-watered every 3 days until the
first rains. After seedling establishment, weed management consisted of
periodic row spraying with a C4-targeted herbicide
(Fuzilade Forte 128C, active ingredient Fluazifop-p, SpecialistSales,
Toowoomba, QLD, Australia).
Measurements of growth and survival
Two months after the initial planting 98.4% survival was observed
across all sites. Supplementary planting of additional replicates was
carried out to maintain the complete planting matrix; these replacement
plants were not included in our survival analyses. Over the experiment,
total plant height (m) and plant health (described below) were recorded:
at planting, and 4, 12, 17, and 24 months after planting. For
multi-stemmed individuals, the total height of the leading stem was
measured. Health was assessed qualitatively using an index of 0 to 5,
with 0 = dead, 1 = main stem dead but with resprouting evident, and
levels 2 to 5 representing; <25%, 25 to 50%, 50 to 75%, and
plants with a health class of 1 or lower were classified as functionally
dead. For analysis of height data, all functionally dead individuals
were excluded.
To account for potential within-site variation in environmental
conditions (i.e. edaphic or light conditions) known to impact tree
growth across the experimental sites (Cheesman et al. 2018) we
tested for spatial autocorrelation present in the tree growth rates,
using Moran’s I. This test indicated significant spatial autocorrelation
in growth data within each plot. As trees were not grouped by species in
the plots, this autocorrelation is likely due to intra-site variation in
soil properties such as rockiness or fertility. Therefore, to account
for this spatial autocorrelation, we built a spatial autologistic term
(Dormann 2007) using height data from the August 2023 campaign
normalised for each site. We generated autologistic terms using focal
windows with diameters of three, five, and seven trees, then selected
the window size that addressed spatial autocorrelation while exerting
the least influence on the model. All three autologistic terms accounted
for spatial autocorrelation in model residuals, thus, we chose the term
with a focal diameter of seven trees as it had the least influence on
the model and was least likely to bias model parameter estimates (Figure
S2).