Resilience in the functional responses of Axonopus affinis Chase (Poaceae) to diurnal light variation in an overgrazed grassland

Overgrazed grasslands of Southern Brazil are responsible in part for the degradation, low productivity and loss of biodiversity of the grassland ecosystems submitted to such management. Axonopus affinis Chase, the carpet grass, is a dominant creeping grass species in overgrazed grasslands. The species readily spreads, is highly tolerant of frequent defoliation and to trampling. In any natural environment, different light intensity regimes vary considerably along the day. Such phenomenon requires a high level of plasticity, resistance, and resilience from plants. Within this context, the aim of this study was to describe the mechanisms of photochemical activity in Axonopus affinis (Poaceae) in response to natural variations in daylight. Here we report hysteresis related functional behavior in overgrazed grasslands. This study was conducted in grassland vegetation within an experimental overgrazed area from the Embrapa Pecuaria Sul, Bage/Rio Grande do Sul, Brazil (austral spring 2014). By means of preferential sampling, homogeneous areas with a 90% dominance of Axonopus affinis Chase were selected. In the present study 60 sample units were analyzed (young leaves, non-damaged and fully expanded). Photochemical variables and OJIP curves were sampled in two distinct conditions along one day, under light intensities labeled as: 1 – high photosynthetic photon flux density (HI), when under full sunlight; and 2 - low photosynthetic photon flux density (LI), when under cloudy days. It was possible to quantify and identify the alternative stable states of A. affinis in response to light resource availability, expanding the interpretation of the multivariate analysis (triangular trajectory) and therefore, contributing to univariate metrics (bimodal trajectory). In this study, HI produced hysteretic behavior while LI of dynamic photoinhibition in A. affinis. The ΔF/Fm′ variable was the most important mechanism to explain the plasticity of A. affinis in HI and LI (R2 > 0.98 in function of the PCoA coordinates). However, the non-photochemical quenching activity was responsible for the stability in the OJIP curve. Therefore, A. affinis presented photochemical mechanisms that contribute to the occupation and dominance of overgrazed grasslands with different light regimes.