Interactions among fire, grazing, harvest and abiotic conditions shape palm demographic responses to disturbance

Abstract

Summary1. Determining the drivers of plant demography is integral to understanding the processes that shape plant species abundances and distributions. Despite recognition that interactions among drivers have important effects on demographic processes, few demographic studies test for interactions among multiple drivers in plants.2. We used a factorial‐design experiment to study the interactive effects among three common forms of disturbance in the tropics (fire history, grazing and leaf harvest by humans) on the vital rates of Phoenix loureiri (mountain date palm) in South India. In addition, we tested for interactive effects among these disturbances, abiotic conditions and plant size. We also tested for non‐consumptive effects of grazing and harvest, such as trampling, by measuring the intensities of grazing and harvest in plots open to these disturbances.3. Intensities of leaf harvest and grazing varied with abiotic conditions and disturbance. Leaf harvest decreased with increasing grazing intensity, suggesting that the net effect of harvest on palm populations is less where it co‐occurs with grazing. In areas without fire, plots with lower soil moisture had higher grazing intensities.4. We found multiple significant main and interactive effects of disturbance on palm vital rates. Palm mortality increased with fire and grazing. Grazing and harvest reduced growth, but growth increased following fire. The negative impact of harvest on palm individuals was reduced when harvest occurred in plots with fire.5. We found evidence of non‐consumptive effects of grazing and harvest on palm growth, likely from trampling. Studies inferring the effects of grazing by comparing grazed and ungrazed individuals within an area where grazing occurs will likely underestimate grazing effects.6. Synthesis. Our findings reveal that P. loureiri demographic rates are driven by interactive effects among multiple forms of disturbance and abiotic factors and that the intensities of disturbance are themselves driven by interactions between other forms of disturbance and abiotic factors. These results illustrate that understanding the effects of, and interactions among, multiple drivers will be key in attempts to mitigate the effects of environmental change on plant species declines.