Recovery of a cultivation grazer: A mechanism for compensatory growth of Thalassia testudinum in a Caribbean seagrass meadow grazed by green turtles

Recovery of green turtles (Chelonia mydas), mega-herbivores that consume
seagrasses, is resulting in dramatic ecosystem-wide changes as meadows are
returned to a natural grazed state. The green turtle grazing strategy,
with long-term cultivation of meadows and high foraging site fidelity, is
distinct from other terrestrial and aquatic mega-herbivores and may affect
seagrass compensatory growth responses. Identifying mechanisms of
compensatory growth responses to grazing is essential to understanding the
functioning of plant systems under natural grazing regimes. In a naturally
grazed Caribbean seagrass ecosystem, we identify a mechanism for
compensatory growth responses to grazing by evaluating relationships
between Thalassia testudinum morphology and growth, grazing intensity, and
canopy light dynamics in grazed and ungrazed areas. The morphological
characteristics that explain variability in T. testudinum growth differed
between grazed and ungrazed areas. In grazed areas, T. testudinum leaf
linear growth, leaf area growth, and productivity:biomass (P:B)
significantly increased as aboveground biomass decreased; P:B also
increased with shoot density. Mass growth in grazed areas exhibited an
increasing trend with shoot density and was maintained above a threshold
of 2.5 g dry mass m-2 aboveground biomass. In ungrazed areas, trends for
mass growth and P:B with aboveground biomass and shoot density were
opposite to those in grazed areas. In grazed areas, shoot density
significantly increased with grazing intensity, while aboveground biomass
decreased and leaf area index (LAI) was not affected. Light availability
at canopy height was greater in grazed areas than in ungrazed areas, and
canopy light attenuation increased with shoot density in grazed areas.
Synthesis: Grazing removes aboveground biomass, which increases light
availability and stimulates leaf growth and turnover (i.e. compensatory
growth). Shoot density increases with grazing intensity, maintaining LAI
and canopy light harvesting potential. This maximizes the potential for
leaf photosynthetic activity and provides the plant with the capacity to
sustain mass growth and support a compensatory growth response to grazing.
This study presents novel insight for assessing the underlying mechanisms
of plant compensatory growth responses to cultivation grazing and proposes
potential thresholds that may be used to evaluate the sustainability of in
situ grazing pressure by a recovering mega-herbivore.