Summer tiller population demography in temperate forage grasses growing in subtropical silvopastoral systems

In the face of increasingly hot and dry summers in subtropical climates, which threaten the persistence of cool-season grasses, finding effective strategies to enhance their survival is crucial for sustainable grass-based livestock production systems. We hypothesized that SPS conditions would reduce tiller mortality in grasses that are better adapted to shaded environments. This study evaluated tiller dynamics of two C3 perennial grasses of different shade tolerance (orchardgrass: tolerant, tall fescue: moderately tolerant) and a C3 biannual grass typic of moist environments (Yorkshire fog) in full sunlight and SPS with tree rows-oriented North–South (N–S) and East–West (E–W). Eucalyptus grandis trees significantly altered the understory microclimate, resulting in substantial reductions in photosynthetically active radiation (PAR) and moderated soil temperatures. Orchardgrass emerged as the species with the highest tiller population density in the understory by the end of the first summer, particularly under N–S, benefiting from enhanced tiller survival and continuous emergence compared to tall fescue and Yorkshire fog. In contrast, tall fescue exhibited lower tiller survival and emergence under tree canopies, despite showing high tiller density in full sunlight. Tall fescue showed a significant relationship between tiller population density and survival with PAR and soil temperature, with minimal impact from soil moisture. Yorkshire fog experienced high tiller mortality across all conditions, with complete loss of tillers in the understory by summer’s end. The results suggest that in subtropical climates, silvopastoral systems can enhance orchardgrass persistence during the summer, by improving summer survival and maintaining population density.