Will nutrients limit the tropical forest carbon sink?

Tropical secondary forests and primary forests play a critical role in the terrestrial carbon sink and can help to slow global warming. However, the carbon sink of tropical forests may be limited by soil nutrients, especially nitrogen and phosphorus which is generally low in tropical soils. To overcome nutrient limitation, trees may adjust above- and belowground carbon allocation and change nutrient composition (ratios of carbon to nitrogen, carbon to phosphorus, and nitrogen to phosphorus) and allocations in tissues. Yet, it is still unclear if and how patterns of nutrient limitation (type and strength) on the forest carbon sink shift over tropical forest succession, and how nutrient limitation affects forest dynamics, including tree growth, recruitment, and mortality. In addition, there has been little investigation into whether and to what extent of trees change their allocation of carbon, nutrient composition, and nutrient allocation to address nutrient limitation over the course of tropical forest succession. To fill these knowledge gaps, I used a large-scale, long-term nutrient manipulation experiment across a tropical successional gradient in Panama, including a mature forest and secondary forests aged 0 (newly regenerating forests), 10, and 30 years following deforestation and cattle ranching. I analysed data from multiple censuses in 76 plots totalling 8.56 ha, computed biomass, and analysed the responses of aboveground biomass and its dynamics (growth, recruitment, and mortality) to nutrient addition. I also assessed the effects of nutrient addition on changes in standing fine root biomass, the ratio of fine root biomass to aboveground biomass, nutrient composition, and nutrient content in each tissue type over forest succession. I found that patterns of nutrient limitation on forest aboveground carbon sink shift across forest succession from strong nitrogen limitation in young secondary forests, to phosphorus limitation in middle stage forests, and to no evidence of nutrient limitation in the mature forests. To address nutrient limitation, trees adjust above- and belowground carbon allocation and change nutrient compositions and allocations in tissues. Following addition of limiting nutrients, trees allocate more carbon aboveground to boost aboveground carbon accumulation, decrease the ratio of carbon to nitrogen and/or phosphorus, and allocate more nutrients from leaves and fine roots to wood. These strategies may successfully address weak nutrient limitation on the carbon sink in mature forests. My project is the first to demonstrate how nutrient limitation on the aboveground carbon sink shifts over tropical forest succession and provides insight for policymakers planning to use reforestation to meet carbon targets. It demonstrates that tropical forests can be nitrogen as well as phosphorus limited, challenging the dominant biogeochemical paradigm. Furthermore, this project will help improve predictions of future forest carbon sinks.