Terrestrial C storage in vegetation and soils exceeds that in the atmosphere by a factor of four, and represents a dynamic balance among: (i) ecosystem C input (gross primary production; GPP); (ii) C allocation; and (iii) C loss (ecosystem respiration and export). This balance is predicted to be highly sensitive to climate change, and MAT is expected to rise by up to 6.4 C over the next 100 years. Gross primary production and all component C fluxes increase with MAT in global forest databases. However, key knowledge gaps limit capacity to predict the impacts of rising MAT on terrestrial C cycling and storage. First, rates of nutrient regeneration also likely change with temperature and an understanding of the behavior of coupled carbon and nutrient cycling over temperature and parallel changes in plant allocation patterns currently limits capacity to model the contemporary forest C cycle. Second, tropical forest C allocation studies, especially those examining belowground processes including nutrient regeneration and belowground carbon allocation, are underrepresented in global databases despite the disproportionate influence of tropical forests in global C cycling.
In 2008, we established a model study system to quantify how tropical wet forests respond to rising MAT by examining ecosystem C and nutrient storage and fluxes (Figure 1). The elevation gradient is comprised of nine 20 x 20 m plots forming an 800 m gradient that corresponds to a ~5.2°C MAT gradient (13.0 to 18.2°C). This gradient is located on the northeastern slope of the Mauna Kea Volcano on the Island of Hawaii (Litton et al. 2011). Seven plots are located in the Hawaii Experimental Tropical Forest (HETF; 19°56041.3″ N, 155°15044.2″ W; 600–1800 m.a.s.l) and two high elevation plots are located in the Hakalau Forest National Wildlife Refuge (HFNWR; 19°50031.3″ N, 155°17035.2″ W; 600–2000 m.a.s.l). All plots are located within tropical montane wet forests characterized as Metrosideros polymorpha Gaudich.–Acacia koa A. Gray forests. M. polymorpha and Cheirodendron trigynum (Gaudich.) A. Heller dominate the canopy and midstory, respectively, across all plots (84–97% of basal area excluding tree ferns), while tree ferns (Cibotium spp.) make up approximately half of total stand basal area in all plots (Litton et al. 2011). Soil water balance is relatively constant across all plots because annual precipitation and evapotranspiration declines with increasing elevation (Litton et al. 2011). Substrate in all plots is derived from ~20 ky (14–65 ky) weathered tephra (Giardina et al. 2014). Soils are moderate to well-drained hydrous, ferrihydritic/amorphic, isothermic/isomesic Acrudoxic Hydrudands of the closely related Akaka, Honokaa, Maile, and Piihonua soil series (Soil Survey Staff 2010).
To date, all ecosystem C pools have been sampled, including live biomass, coarse woody debris, and soil C to one meter (Iwashita et al. 2013, Selmants et al. 2014). We have also collected C flux data, including 60 months of litterfall, soil respiration, and aboveground net primary productivity (Litton et al. 2011, Giardina et al. 2014). We have conducted detailed studies on litter decomposition, soil carbon turnover, and total belowground carbon flux (Giardina et al., 2014; Bothwell et al., 2014), as well as inorganic nitrogen bioavailability and ammonia oxidizing archaea (AOA) and bacteria (AOB) abundance (Pierre et al., 2017). Additional studies have examined soil bacterial community structure across the MAT gradient (Selmants et al., 2016, 2017), as well as litter macroinvertebrate diversity (Ritzenthaler et al., 2016, 2017). Future work will examine: 1) macro- and micronutrient dynamics through a detailed analysis of live and sensed foliar nutrient content and nutrient return via litterfall – an index of nutrient cycling and availability (Litton et al. in 2020); and environmental controls on interannual variability in ecosystem carbon fluxes across MAT (Lyu et al. 2021).
Highlights of this ongoing research demonstrate that live biomass C increases and coarse woody debris C decreases with MAT, but that soil and ecosystem C pools do not vary across the MAT gradient. Moreover, component C fluxes (i.e., litterfall, soil respiration, total belowground C flux) all increase with MAT. Increasing MAT increases the cycling and availability of N and K (strong support) and Mg and Zn (moderate support), but decreases the cycling and availability of Mn (strong support) and Cu (moderate support). In addition, increased MAT has little direct effect on the cycling and availability of P, Ca or Fe. However, alterations in ecological stoichiometry with MAT suggest that warming will exacerbate P limitations to tropical forest productivity. Finally, there is strong evidence that interactions of weather, especially cloudiness, with available light and temperature can exert a large influence on litterfall production and belowground processes, with light appearing to limiting C process rates (litterfall, TBCF) during wetter and cooler years, but temperature limiting these rates during sunnier and warmer years.
Figure 1. Array of nine permanent plots across a 5.2°C mean annual temperature gradient (13.0 to 18.2°C) in tropical montane wet forest on the northeastern slope of Mauna Kea Volcano, Island of Hawaii.
Publications (Chronological Order)
Lyi, M., Giardina, C.P., and Litton, C.M. 2021. Interannual variation in rainfall modulates temperature
sensitivity of carbon allocation and flux in a tropical montane wet forest. Global Change Biology, DOI: 10.1111/gcb.15664.
Pierre, S., Litton, C.M., Giardina, C.P., Sparks, J.P., and Fahey, T.J. 2020. Mean annual temperature influences local fine root
proliferation and arbuscular mycorrhizal colonization in a tropical wet forest. Ecology and Evolution 10:9635–9646.
Litton, C.M., Giardina, C.P., Freeman, K.R., Selmants, P.C., and Sparks, J.P. 2020. Impact of mean annual temperature on nutrient availability in a tropical montane wet forest. Frontiers in Plant Science, https://doi.org/10.3389/fpls.2020.00784.
Pierre S. 2018. Drivers of nitrogen availability, cycling, and demand in temperate and tropical forest ecosystems. Ph.D. Dissertation. Cornell University, Ithaca, NY.
Pierre S., Hewson, I., Sparks, J.P., Litton, C.M., Giardina, C., Groffman, P.M., and Fahey, T.J. 2017. Ammonia oxidizer populations vary with nitrogen cycling across a tropical montane mean annual temperature gradient. Ecology 98:1896-1907.
Ritzenthaler, C.A. 2017. The effect of soil micronutrient variation along an elevational gradient in a wet motane forest. M.S. Thesis. Bowling Green State University, Bowling Green, OH.
Ritzenthaler, C.A., Litton, C.M., Giardina, C.P. and Pelini, S.L. 2016. Soil moisture and millipede abundance are more important drivers of macroinvertebrate diversity than temperature in Hawaiian forest. Integrative and Comparative Biology 56:E357.
Selmants, P.C., Adair, K.L., Litton, C.M., Giardina, C.P., Schwartz, E. 2017. Erratum: Increases in mean annual temperature do not alter soil bacterial community structure in tropical montane wet forests. Ecosphere 8:e01904.
Selmants, P.C., Adair, K.L., Litton, C.M., Giardina, C.P., Schwartz, E. 2016. Increases in mean annual temperature do not alter soil bacterial community structure in tropical montane wet forests. Ecosphere 7:e01296.
Bothwell, L.D., Selmants, PC., Giardina, C.P., and Litton, C.M. 2014. Leaf litter decomposition rates increase with rising mean annual temperature in Hawaiian tropical montane wet forests. PeerJ 2:e685 https://dx.doi.org/10.7717/peerj.685
Giardina, C.P., Litton, C.M., Crow, S.E., and Asner, G.P. 2014.Warming-related increases in soil CO2 efflux are explained by increased below-ground carbon flux. Nature Climate Change 4:822-827.
Selmants, P.C., C.M. Litton, C.P. Giardina and G.P. Asner. 2014. Ecosystem carbon storage does not vary with mean annual temperature in Hawaiian tropical montane wet forests. Global Change Biology 20:2927-2937.
Iwashita, D.K., Litton, C.M., Giardina, C.P. 2013. Coarse woody debris carbon storage across a mean annual temperature gradient in tropical montane wet forest. Forest Ecology and Management 291:336-343.
Iwashita, D. K. 2012. Role of coarse woody debris in carbon storage and seedling distribution in Hawaiian montane wet forests. M.S. Thesis. University of Hawaii at Manoa, Honolulu, HI.
Litton, C.M., Giardina, C.P., Albano, J.K., Long, M.S., Asner, G.P. 2011. The magnitude and variability of soil-surface CO2 efflux increase with temperature in Hawaiian tropical montane wet forests. Soil Biology & Biochemistry 43:2315-2323.
Litton, C.M., Giardina, C.P., 2008. Below-ground carbon flux and partitioning: Global patterns and response to temperature. Functional Ecology 22:941-954.