Carbon storage in the terrestrial biosphere exceeds that in the atmosphere by a factor of four, and represents a dynamic balance between carbon inputs and losses. This balance is being altered by climate change and land-use, with important impacts on terrestrial carbon storage and, hence, atmospheric CO2 levels. However, the response of terrestrial carbon cycling to warming and interactions with land-use remain poorly quantified. This study will examine how rising temperature and land-use interact to regulate carbon input, allocation, and loss in a model study system, thereby enhancing capacity to predict how terrestrial ecosystems will respond to the interactive effects of warming and land-use. To do this, we will measure aboveground and belowground carbon storage and exchange across a 6°C temperature gradient within the Hawaii Experimental Tropical Forest. At each of four temperature regimes, paired and replicated plots will be established in degraded pasture, secondary native forest, and intact native forest to examine how mean annual temperature and land-use interact to affect carbon input, allocation and loss in forest ecosystems.
In addition to examining how rising temperature and land-use interact to impact terrestrial carbon balance, this study will also: (i) build a model global change research platform that can be used by other scientists; (ii) provide critical data on terrestrial carbon storage to the global change community, including scientists, policy makers, and land managers; (iii) enhance graduate and postdoctoral training and undergraduate science education at a Native Hawaiian serving institution; and (iv) foster collaboration between the University of Hawaii and the USDA Forest Service on the newly established Hawaii Experimental Tropical Forest.
Overview Carbon storage in the terrestrial biosphere exceeds that in the atmosphere by a factor of four, and represents a dynamic balance among carbon input, allocation, and loss. This balance is being altered by climate change, with important implications for terrestrial carbon storage and, hence, atmospheric CO2 levels and global climate. However, the response of terrestrial carbon cycling to warming remains poorly quantified, especially in the tropics. This is particularly important because tropical forests account for a ~40% of global terrestrial carbon storage and ~35% of global terrestrial productivity and, as such, tropical forests play a very important role in regulating global climate. This study examined how rising mean annual temperature will impact carbon input, allocation, loss, and storage in native Hawaiian wet forests along a 5.2°C mean annual temperature gradient. Results from the research along this model ecological gradient greatly enhance capacity to predict how terrestrial ecosystems, in particular tropical forests, will respond to warming over the next century. To address this critical question, we estimated carbon input (net photosynthetic carbon gain, or ‘gross primary production’ (GPP)), carbon loss (soil respiration and aboveground plant respiration), carbon partitioning (fraction of GPP that goes towards production vs. respiration in foliage, aboveground wood, and belowground), and ecosystem carbon storage (live aboveground and belowground biomass, forest floor, coarse woody debris, and mineral soil organic matter to ~1 m depth). This research was conducted in nine permanent tropical montane wet forest plots, established with this funding, that are arrayed across a 5.2°C mean annual temperature gradient (13.0-18.2°C) in the Hawaii Experimental Tropical Forest (State of Hawaii and USDA Forest Service) and the Hakalau Forest National Wildlife Refuge (US Fish and Wildlife Service) on the windward slope of Mauna Kea Volcano, Island of Hawaii. Intellectual Merit Results from this study are providing an increasingly detailed picture of how carbon cycling in tropical wet forest will respond to rising mean annual temperature. First, we found that total carbon input (GPP) increases with mean annual temperature, which supports several prior global, cross-site analyses. Second, we found that all component carbon fluxes increase with mean annual temperature. Third, we found that as temperature increases, the fraction of GPP that is partitioned to belowground decreases, most likely in response to an increase in nutrient cycling and availability at higher mean annual temperatures. This is important because carbon that is partitioned belowground has the greatest chance of being stabilized as long-lived soil carbon, where it can reside for hundreds to thousands of years and buffer atmospheric CO2 concentrations. One common prediction of the impact of rising temperature for terrestrial carbon cycling has been that rising temperatures will increase soil carbon decomposition, and thus result in a positive feedback between warming and increased decomposition of soil carbon. Soil carbon is a particularly important component of forest carbon cycling because soils store more carbon than vegetation and the atmosphere combined on a global scale and, thus, soils are critical in regulating global climate. Importantly, we found that the flux of carbon into (litterfall; belowground carbon flux) and out of (soil respiration) soil increases with mean annual temperature, indicating that soil carbon cycling will increase as temperature rises. However, contrary to prior predictions we found that soil carbon storage did not vary with mean annual temperature, indicating that rising temperature will not result in increased soil carbon decomposition and a positive feedback to climate change, at least in tropical wet forests. Broader Impacts In addition to examining how rising mean annual temperature will impact the carbon balance of the world’s most productive forests (i.e., tropical wet forests), this study also contributed to the broader impacts of the National Science Foundation in a variety of ways. In particular, this research: (i) created a model global change research platform that has attracted multiple national and international collaborators; (ii) provided critical data on the response of tropical forests to rising temperatures to the global change community, including scientists, policy makers, and land managers, in the form of 20 professional presentations and seven peer-reviewed scientific publications to date; (iii) enhanced graduate and postdoctoral training and undergraduate science education at a Native Hawaiian serving institution (this project resulted in research and educational opportunities for twelve research technicians, two postdoctoral researchers, one graduate student, and six undergraduate students); (iv) fostered collaboration between the University of Hawaii and the USDA Forest Service on the recently established Hawaii Experimental Tropical Forest; and (v) resulted in auxiliary funding for the creation and implementation of an outdoor youth education curriculum on climate change for middle and high school students from Native Hawaiian and local schools on the Island of Hawaii.
