PS 43-154 - Do altered precipitation regimes interact with grazing intensity and land use to affect plant productivity and vegetation composition in subtropical humid grazing lands?

Abstract: Changing precipitation regimes threaten multifunctional working grasslands. Understanding of how altered precipitation regimes interact with other drivers such as land use and agricultural management remains limited, with most current research focused on water-limited ecosystems. Our multi-factorial experiment manipulates precipitation and grazing in two land management intensities in a south-central Florida, USA, a subtropical humid grassland utilized for beef cattle production. Here we ask, do altered precipitation quantity and seasonality interact with grazing intensity and pasture management intensity to affect aboveground plant productivity and plant species composition in subtropical humid grasslands?

In 2021, we set up a large-scale precipitation manipulation study in two pasture-types: (1) High intensity Improved pastures (IMP), are dominated by non-native forage grasses, N-fertilized and extensively ditched; and (2) Low intensity Semi-native pastures (SNP), contain a mixture of native and non-native grasses, have never been fertilized, and are less extensively ditched. Precipitation treatments included: (i) increased quantity (IQ:+50% rainfall in wet season, +75% rainfall in dry season), (ii) decreased quantity (RQ: -50% wet season, -75% dry season), (iii) enhanced seasonality (ES:+50% wet season, -75% dry season), (iv) diminished seasonality (DS:-50% wet season, +75% dry season), and an ambient control. Grazing treatments included two annual grazing events, representing high and low stocking densities for IMP and SNP. High grazing reduced vegetation height by 68% in IMP and 43% in SNP and low grazing reduced vegetation height by 47% in IMP and 38% in SNP.

After implementing grazing and precipitation treatments for one year, soil moisture was lower in SNP, in low grazing intensity, and all precipitation treatments differed from the control. Aboveground wet-season productivity was not impacted by precipitation manipulation. Productivity was marginally lower in high-grazing treatments (p=0.09) in both pasture-types, and lower in SNP (p=0.007). Native richness was not impacted by grazing or precipitation treatments. However, exotic richness was significantly affected by a three-way interaction of pasture-type, precipitation, and grazing. This interaction was caused by no effect of grazing or precipitation on exotic richness in SNP, but significant effects in IMP. In high grazing, exotic richness was significantly greater in ES and IQ compared to control, while exotic richness in DS and RQ treatments did not differ from control. Ordination results revealed only pasture-type differences in species composition and no short-term changes in vegetation in response to either grazing or precipitation. Knowledge gained from our work can guide climate adaptation strategies to promote resilient working grasslands.