Results
Short-term effects of ant occupants and herbivores on A.
drepanolobium physiology- In our BACI experiment during the wet season,
on Control trees we found large declines between 2017 and 2018 in
leaf-level photosynthesis (24%) and transpiration (20%), which
significantly differed from minor declines for photosynthesis (4%) and
transpiration (7%) between wet seasons for Transition trees (site x
year terms, photosynthesis: Fig. 1A; transpiration: Fig. 1C). Although
leaf water potential range became more negative by 57% for Control
trees and less negative by 3% for Transition trees from the 2017 to
2018 wet season, the site x year term in our GLMM was only marginally
significant (Fig. 1E).
In our BACI experiment during the dry season, we observed a decline
between 2017 and 2018 dry seasons in leaf-level photosynthesis that was
significantly larger for Transition trees (54%) than for Control trees
(25%) (site x year interaction term, Fig. 1B). Changes in leaf-level
transpiration between dry seasons significantly differed for Transition
trees (1% decline) and Control trees (35% increase) site x year
interaction term, Fig. 1D). Despite these different trends in
photosynthesis and transpiration, we found no significant variation in
leaf water potential range due to any GLMM factors during the dry season
(site x year interaction term, Fig. 1F).
Longer-term effects of ant occupants and herbivores on Acacia
drepanolobium physiology - During wet seasons, we found significant
differences in leaf- and canopy-level photosynthesis and transpiration
as a function of invasion status, vertebrate herbivore exclusion from
Invaded trees, and P. megacephala exclusion (full ANOVA results
in Table S5). Trees in stands which had been invaded for ca . 5
years had 16% lower photosynthesis (Fig. 2A) and 17% lower
transpiration (Fig. S1A) at the leaf-level than trees in uninvaded
stands. Scaled up to the canopy level, Invaded trees had 68% lower
photosynthesis (Fig. 2A), and 54% lower transpiration (Fig. S1A) than
Uninvaded trees. The exclusion of vertebrate herbivores from Invaded
trees was associated with a 19% increase in photosynthesis (Fig. 3A)
and a non-significant 13% increase in transpiration (Fig. S2A) at the
leaf-level, and was also associated with substantially higher
canopy-level photosynthesis (365% higher; Fig. 3C) and transpiration
(305% higher; Fig. 4A). Exclusion of P. megacephala ants did not
significantly affect leaf-level photosynthesis (Fig. 3A) or
transpiration (Fig. S2A), but their exclusion resulted in higher
canopy-level photosynthesis (66% higher, Fig. 3C) and transpiration
(52% higher; Fig. 4A). Despite these large differences between
treatments in leaf and canopy transpiration, we only observed a
significant but minor increase in leaf water potential range due toP. megacephala exclusion (14% less negative; Fig. 4C), and no
significant differences due to invasion status or vertebrate herbivore
exclusion from Invaded trees. Exclusion of C. mimosae and
vertebrate herbivores from Uninvaded trees did not significantly impact
leaf- or canopy-level photosynthesis or transpiration or leaf water
potential range (Note S4).
During dry seasons, at the leaf-level we observed small increases in gas
exchange and water potential range resulting from invasion status,
herbivore exclusion from Invaded trees, and P. megacephalaexclusion, but no significant differences at the canopy level (Table
S5). Invaded trees had significantly higher photosynthesis (13%; Fig.
2B) and transpiration (16%; Fig. S1B) at the leaf level, but we did not
find significant differences in in scaled-up canopy-level estimates of
photosynthesis (Fig. 2B) or transpiration (Fig. S1B). Invaded trees also
had slightly less negative leaf water potential range (7% less
negative; Fig. S1D) during the dry season. The exclusion of P.
megacephala from Invaded trees resulted in a minor but significant
decline (17%) in dry season leaf-level transpiration (Fig. S2B), butP. megacephala exclusion did not significantly affect leaf- or
canopy-level photosynthesis (Fig. 3B & D), canopy-level transpiration
(Fig. 4B), or leaf water potential range (Fig. 4D). Similarly, exclusion
of vertebrate herbivores was associated with a minor but significant
decline (13%) in leaf-level transpiration, but no significant change in
leaf- or canopy-level photosynthesis (Fig 3B & D), canopy-level
transpiration (Fig. 4B), or leaf water potential range (Fig. 4D). In
contrast to trees from longer-term Invaded sites, Uninvaded trees did
not exhibit different gas exchange or leaf water potential traits due to
ant (C. mimosae ) exclusion, herbivore exclusion or their
interaction (Note S4).