Local density effects on individual production are dynamic: insights from natural stands of a perennial savanna grass.

Perennial grasses are a dominant component of grasslands, and provide important ecosystem services. However, most knowledge of grasslands' functioning and production comes from plot-level studies, and drivers of individual-level production remain poorly explored. Extrapolation from existing experiments is hampered by the fact that these are mostly concentrated on even-aged cohorts, and/or on the early stages of a plant's life cycle. Here we explored how local density regulates individual production in mono-specific natural grassland, focusing on adult individuals of a perennial savanna grass (Stipagrostis uniplumis). We found individual production to increase with individuals' size, but to decrease with neighbour abundance. A metric of neighbour abundance that considered size was superior to a metric based solely on the number of individuals. This finding is particularly important for studying competitive effects in natural populations, where plants are normally not even-sized. The inferred competition kernel, i.e. the function describing how competitive strength varies with spatial distance from a target plant, was hump-shaped, indicating strongest intraspecific competition at intermediate distances (10-30 cm). The spatial signature of competitive effects changed with time since fire; peak effects moved successively away from the target plant. Our results suggest that inferred competition kernels of long-lived plant populations may have shapes that differ from exponential or sigmoidal decreases. More generally, results underline that competition among neighbouring plants is dynamic. Studies that address density-dependent and density-independent (fire-related) population dynamics of perennial grasses in their fire-prone environment may thus shed new light on the functioning and production of grasslands.