THE CICLOS PROJECT

A study of linkages among carbon, water and nutrient cycles in old growth

rainforest at La Selva Biological Station, Costa Rica

Overview Poster (1 Mb jpeg)

Participants

IPCC | Ameriflux | Global Change

Funded by:

US National Science Foundation (Biocomplexity of the Environment Coupled Biogeochemical Cycles)

Project objectives

To build an unprecedented comprehensive understanding of TRF C cycling by adding to on-going ecosystem C studies at La Selva the quantification of rarely-studied components of the forest C budget. TRF field studies of C cycling have largely been restricted to two components of aboveground production - fine litterfall and aboveground biomass increment. The independently-funded CARBONO research will continue to provide these components, as well as fine woody litterfall and annual inputs of coarse woody debris (CWD), and the TOWERS studies will provide scaled-up estimates of forest C uptake and plant respiration (see Results from Prior Support). In addition, our team will quantify across the landscape and through 4 yrs, two potentially key components of the La Selva C cycle: fine-root production and CWD decomposition. This will be the first time all these aspects of C cycling are studied together in a TRF, enabling a first-time evaluation of the relative roles of these major contributors to total forest C balance.

To produce a landscape-scale TRF hydrological budget and quantify hydrological C and nutrient export. Although it is generally recognized that forest water status could have major impacts on TRF C cycling, quantitative studies of total forest water balance have been lacking for old-growth TRF. We will combine simultaneous eddy covariance measurements of above-canopy water exchange with monitoring of soil moisture across the landscape gradients and with outflow measured at replicate gauged weirs on two forest streams, to quantify the forest's hydrological budget and how it varies through time. We will combine this understanding with continuous sampling of dissolved organic carbon (DOC), for a first estimate of forest carbon losses in streams.

To quantify the effects of spatial and temporal variability in soil N and available P on forest net primary productivity and on the stoichiometry of C:N and C:P ratios in plant tissues and the soil. Little is known about the interactions and feedbacks between TRF C and nutrient cycles. Both phosphorus and nitrogen are likely to affect forest productivity in complex ways. The relationships between soil nutrient availability and plant tissue stoichometry are poorly known in TRF, yet this aspect of complexity can strongly influence carbon cycling via its effects on both potential C uptake (Hungate et al. 2003) and decomposition. We will investigate these connections across La Selva's strong gradients of soil chemistry by combining remotely-sensed hyperspectral data (see below) with plot-based studies of plant and soil chemistry across the landscape and through time.

To explore ways in which the biota (functional composition of invertebrate communities, productivity and reproduction at the tree species level) affect forest-level cycling of carbon and nutrients. We will focus on two key biotic groups in this ecosystem as case studies of how biotic composition can affect carbon and nutrient cycles across a heterogeneous TRF landscape, and through the large interannual climatic variation imposed by the ENSO cycle. Preliminary studies of litter invertebrates have found large changes in the contributions and abundances of predators and detritivores to this guild, across La Selva's soil nutrient gradients; such biotic shifts with nutrient availability could greatly affect carbon and nutrient cycling through their effects on litter decomposition. Similarly, the allocation of fixed carbon and of nutrients to reproduction by the trees is largely unstudied in TRF but could affect other processes in the C and nutrient cycles. We will focus on how these allocations by different tree guilds (palms/understory, canopy & emergent trees) change across the fertility gradients, through seasonal and interannual climatic variation, and with the contributions of to total tree reproductive effort.

To investigate the impact of seasonal and interannual (ENSO) climatic variability, especially with respect to temperature and the water budget, on forest C fluxes and stocks, nutrient availability, and water balance. In El Niño years, both forest water balance and C cycling will be affected in complex and nteracting ways by higher temperatures and the seasonal changes in rainfall regime (rainfall increased in the wetter season, decreased in the drier season). Nutrient availability and plant tissue chemistry are also likely to be significantly affected by the climatic variation related to ENSO. These responses to interannual climatic variation cannot be predicted with any certainty from first principles, and are likely to bring major surprises. They will also be valuable indicators of the forest's integrated responses to on-going climate change. The complex responses and interactions of these forest cycles as affected by climatic variation through an ENSO cycle will be key inputs in our parameterization of the TRF CENTURY process model.

To assess the scale-dependence of carbon and nutrient cycling, probing the correspondence between landscape-scale assessments (remote sensing, eddy covariance) of forest carbon and nutrient cycling to plot-level measurements. Field based investigations will always be limited to relatively small areas of the land surface. Thus linkages among data from detailed field studies and remote sensing investigations are critical to scale from the plot to the region. At the landscape scale, we will use tower-based eddy covariance studies to assess whole-forest physiological responses to climatic variation at daily, seasonal, and annual scales, we will assess whole-forest water balance as it changes seasonally and interannually (see point (3)), and we will use remote sensing to assess canopy nutrient status as it varies across the entire old-growth landscape. Our ground measurements of forest C and nutrient cycling and water status across the landscape's edaphic gradients will provide the more traditional plot-level view of these forest processes. With versions of the CENTURY process model parameterized from each of these levels, we will cross-test our findings to probe the scale-dependence of these TRF processes.

To evaluate through iteratively-refined, empirically-based landscape- and plot-level versions of an ecosystem process model, the complex interactive effects of climatic (temperature, light, water) and edaphic (soil C, N, P) variation on TRF C cycling. With the combined measurements from this study and the complementary CARBONO and TOWERS projects, we will have for La Selva some of the most extensive and long-term measurements of C, nutrient, and hydrological cycling for any TRF. To understand the inter-relationships between these forest cycles and their nutrient and climatic drivers, and to determine the key sensitivities of these cycles, can only be accomplished through intensive process modeling. Thus, a core effort of this Biocomplexity project will be our development of strongly empirically-based and repeatedly ground-tested TRF version of the process-based model CENTURY, developed from both local (plot-level) and whole-forest (landscape-scale) data. This modeling will put our results into a broader context, providing a basis for generalization to the TRF biome.

This material is based upon work supported by the National Science Foundation under Grant No BCE 0421178. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF)

Under construction: Last updated 6 February 2006