Variation in plant water use and water-use efficiency in planted mixtures and monocultures: Toward proper species selection for reforestation efforts in the seasonally dry tropics
SMART REFORESTATION™ is the process by which land is managed to maximize the flow of different goods and services for the benefit of multiple stakeholders in response to global change. This includes selecting tree species for reforestation that enhance forest-related ecosystem services, while reducing negative impacts of unsustainable land use. In the seasonally dry tropics of the Panama Canal Watershed (PCW), a critical factor to consider in tree species selection is the quantity and efficiency by which trees regulate water and carbon. A better understanding of tree interactions related to water-carbon cycling and trade-offs will enable better decision-making in reforestation and restoration within the PCW, where water can be limiting during the dry season and abundant during the wet season, and forest productivity contributes to local livelihoods and climate change mitigation. Our work addresses two central questions 1) how do different species mixtures and monocultures vary in water and carbon use and use efficiency, and 2) how do mechanisms of species interaction (complementarity, facilitation, competition) explain ecosystem level responses? We use a combination of sapflow, leaf gas exchange, and ecosystem measurements in mixed and monoculture plantings in the PCW, focusing on two key reforestation species, Terminalia amazonia and Dalbergia retusa.
Plant water use and productivity varied significantly by species and diversity treatment. We found that, regardless of treatment, T. amazonia uses more water per unit sapwood area than D. retusa. Interestingly, our results show that T. amazonia in mixtures uses nearly twice as much water as in monocultures, while D. retusa in mixtures uses nearly half as much water as it does in monocultures. Greater water use by T. amazonia can be explained by reduced rooting competition in mixtures and greater aboveground growing space. Instantaneous water-use efficiency (iWUE; μ mol mmol-1), or photosynthesis (A) divided by transpiration (T), varied significantly by both species and treatment. T. amazonia was significantly more water-use efficient in mixtures than monocultures, potentially from enhanced nitrogen availability due to nitrogen fixation by D. retusa. In contract, D. retusa was significantly less water-use efficient in mixtures than monocultures. Ongoing analysis of how water use and iWUE across wet and dry seasons and over time provide important insights regarding species-specific ecophysiological strategies that will affect appropriate selection of species for reforestation efforts. Future work includes using stable isotopes to further explain the mechanisms controlling differences in water use across species and treatments.