The Science
Tropical forests play a critical role in the global carbon cycle. Yet, there is a high level of uncertainty on how these forests will respond to ongoing global environmental changes. This uncertainty is partially attributed to the poor representation of tree mortality in Vegetation Demographic Models. We designed a standardized field protocol to evaluate tree vigor, biomass loss, and factors likely to be associated with future tree death. Advancing the mechanistic inclusion of mortality in vegetation models is a research priority to enable more accurate estimates of terrestrial carbon budgets and predictions of future carbon cycle-climate feedbacks.

The Impact
Although key to predict forest response to global changes, much uncertainty remains on causes and consequences of tropical tree mortality. This study proposes a rapid, repeatable, and inexpensive assessment of individual tree death and damage. It minimizes the effort required at each tree to allow the frequent assessments of more trees. A comprehensive assessment of tree damage coupled with the identification of factors associated with tree death will lead to an improved understanding of the causes of tree mortality and estimates of biomass fluxes in tropical forests.

Summary
Tree mortality drives changes in forest structure and dynamics, community composition, and carbon and nutrient cycles. Since tropical forests store a large fraction of terrestrial biomass and tree diversity, improved understanding of changing tree mortality and biomass loss rates is critical. Tropical tree mortality rates have been challenging to estimate due to low background rates of tree death, and high spatial and temporal heterogeneity. Furthermore, the causes of mortality remain unclear because many factors may be involved in individual tree death, and the rapid decomposition of wood in the tropics obscures evidence of possible causes of tree mortality. We present a field protocol to assess tree mortality in tropical forests. The protocol focuses on the rapid, repeatable and inexpensive assessment of individual tree death and damage. The protocol has been successfully tested with annual assessments of >62,000 stems in total in several ForestGEO plots in Asia and the Neotropics. Standardized methods for the assessment of tree death and biomass loss will advance understanding of the underlying causes and consequences of tree mortality.

Contact: Daniel Zuleta, Forest Global Earth Observatory, Smithsonian Tropical Research Institute, dfzuleta@gmail.com

Funding
This research was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research.

Publications
Arellano, D. Zuleta & S.J. Davies, “Tree death and damage: a standardized protocol for frequent surveys in tropical forests”. Journal of Vegetation Science (2020).  [https://doi.org/10.1111/jvs.12981]

Related Links
Frequent Tree Damage and Death Assessment, Forest Global Earth Observatory, Smithsonian Tropical Research Institute.

The Science
Whether trees are in the canopy or not has long been recognized as a critical determinant of tree performance. However, the structural complexity of many tropical forests makes it difficult to determine canopy positions. The integration of remote sensing and ground-based data enabled this determination and measurements of how canopy and understory trees differ in structure and dynamics in the Central Amazon. Researchers found that canopy trees constituted 40% of the inventoried trees with diameter at breast height > 10 cm, and accounted for ~70% of aboveground carbon stocks. Diameter growth was on average twice as large in canopy trees as in understory trees and the size distribution also differed.

The Impact
The combination of high-resolution drone imagery and ground-based field work has great potential to improve the understanding of the structure and dynamics of old-growth tropical forests having dense understories. These results help scientists understand the proportion of trees in canopy and understory in relation to tree size, the contributions of canopy and understory trees to carbon stocks and wood productivity, and differences in stem growth and size distributions between canopy and understory trees.

Summary
Canopy trees constituted 40% of the inventoried trees with diameter at breast height (DBH) > 10 cm, and accounted for ~70% of aboveground carbon stocks and wood productivity. The probability of being in the canopy increased logistically with tree diameter, passing through 50% at 23.5 cm DBH. Diameter growth was on average twice as large in canopy trees as in understory trees. Growth rates were unrelated to diameter in canopy trees and positively related to diameter in understory trees, consistent with the idea that light availability increases with diameter in the understory but not the canopy. The whole stand size distribution was best fit by a Weibull distribution, whereas the separate size distributions of understory trees or canopy trees > 25 cm DBH were equally well fit by exponential and Weibull distributions, consistent with mechanistic forest models.

Contacts (BER PM): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)

PI Contact: Jeffrey Chambers, Lawrence Berkeley National Laboratory, jchambers@lbl.gov

Funding
The study was financed by the INCT – Amazonian Woods (FAPEAM/CNPq) and by the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under Contract DE-AC02-05CH11231. Fellowship support for the first author was provided from Coordination for the Improvement of Higher Education Personnel (CAPES).

Publications
Araujo, R.F., J.Q. Chambers, C.H.S. Celes, H.C. Muller-Landau, A.P.F. dos Santos, F. Emmert, G.H.P.M. Ribeiro, B.O. Gimenez, A.J.N. Lima, M.A.A. Campos, N. Higuchi (2020), Integrating high resolution drone imagery and forest inventory to distinguish canopy and understory trees and quantify their contributions to forest structure and dynamics. PLoS ONE 15(12): e0243079. [DOI:10.1371/journal.pone.0243079]

The Science
Soil moisture plays a key role in hydrological, biogeochemical and energy budgets of terrestrial ecosystems. Accurate soil moisture measurements in the Amazon are difficult due to logistical constraints. Realistic soil moisture data in the Amazon are greatly needed for improving our understanding of ecohydrological processes within tropical forests and for improving models of these systems in the face of changing environmental conditions.

The Impact
1. First Time Domain Reflectometers (TDR) soil moisture calibration in an old-growth forest in the Central Amazon.
2. Interannual thirty-minute observations of soil moisture up to 14.2-m depth.
3. The Topp model underestimated volumetric water content by 22 to 42%.
4. Dry season morning-night fluctuation of soil moisture were observed

Summary
Depth-specific Time Domain Reflectometers (TDR) were calibrated using local soils in a controlled laboratory producing a novel calibration. The sensors were later installed to their specific calibration depth in a 14.2 m pit. The widely used Topp model underestimated the site-specific volumetric water content (θv) by 22-42% indicating significant error in the model when applied to these well-structured, clay rich tropical forest soils. The calibrated wet- and dry-season θv data showed a variety of depth and temporal variations highlighting the importance of soil textural differentiation, root uptake depths, as well as event- to seasonal-precipitation effects.

Contacts (BER PM): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)

PI Contact: Robinson Negron-Juarez, LBNL, Robinson.inj@lbl.gov

Funding
This research was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, under contract number DE-AC02-05CH11231. Laura Borma would like to acknowledge Go-Amazon (2013/50531-2) for retrofitting the pit structure.

Publications
Negrón-Juárez, S. Ferreira, M. Mota, B Faybishenko,  M. Monteiro, L. Candido, R. Ribeiro, R. de Oliveira, A. de Araujo, J. Warren, B. Newman,  B. Gimenez, C. Varadharajan, D Agarwal, L. Borma, J. Tomasella, N. Higuchi, J. Chambers. Calibration, Measurement and Characterization of Soil Moisture Dynamics in a Central Amazonian Tropical Forest. Vadose Zone Journal, 2020.  DOI:10.1002/vzj2.20070

Related Links
Data available at http://dx.doi.org/10.15486/ngt/1602141

The Science
Tropical forests vary widely in woody productivity, tree mortality, and biomass carbon stocks, even for forests of the same age. Reviewing previous studies, we find that productivity is highest in warm, wet forests on fertile soils, whereas mortality is higher at higher soil fertility and higher disturbance. This in turn means that biomass is higher at higher rainfall and temperature, lower disturbance, and intermediate soil fertility.

The Impact
A mechanistic understanding of tropical forest biomass, productivity, and mortality is critical to accurately representing these processes in global vegetation models and predicting responses to global change. By reviewing empirical studies, we describe the state of current knowledge regarding the mechanisms driving carbon dynamics in tropical forests and highlight areas requiring further study.

Summary
We searched the literature for studies of among-site variation in our focal variables (biomass, productivity, and woody residence time) in mature, unlogged tropical forests, or in secondary forests when controlling for stand age.  Woody productivity and biomass decrease from wet to dry forests and with elevation.  Within lowland forests, productivity and biomass increase with temperature in wet forests, but decrease with temperature where water becomes limiting. Woody productivity increases with soil fertility, whereas residence time decreases, and biomass responses are variable, consistent with an overall unimodal relationship. Areas with higher disturbance rates and intensities have lower woody residence time and biomass.  These environmental gradients all involve both direct effects of changing environments on forest carbon fluxes and shifts in functional composition – including changing abundances of lianas — that substantially mitigate or exacerbate direct effects. Biogeographic realms differ significantly and importantly in productivity and biomass even after controlling for climate and biogeochemistry, further demonstrating the importance of plant species composition. Capturing these patterns in global vegetation models requires better mechanistic representation of water and nutrient limitation, plant compositional shifts, and tree mortality.

Contact: Helene C. Muller-Landau, Staff Scientist, Smithsonian Tropical Research Institute, MullerH@si.edu

Funding
KCC was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research.

Publications
Muller-Landau, H.C., Cushman, K.C., Arroyo, E.E., Martinez Cano, I., Anderson-Teixeira, K.J., and B. Backiel (2020). Patterns and mechanisms of spatial variation in tropical forest productivity, woody residence time, and biomass. New Phytologist,  https://doi.org/10.1111/nph.17084.

A logging module added in FATES allows the impacts of forest degradation be represented in Earth system models.

The Science
A team of scientist in NGEE-Tropics developed a selective logging module for the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), a new generation ecosystem demography model for representing vegetation dynamics in Earth system models. This module represents logging practices that span the range from complete clear-cuts to highly selective logging treatments practices. Benchmarking of the module against selective logging experiments in the Tapajós National Forest in Brazil showed that the model is able to capture important effects of logging on forest structure and carbon, water, and energy fluxes.

The Impact
Tropical forest degradation not only alters carbon stocks and carbon fluxes, but also impacts physical land surface properties. Such impacts are poorly quantified to date due to difficulties in accessing and maintaining observational infrastructures, as well as the lack of proper modeling tools. The new module creates a foundation for future model developments to include a wider set of land use transitions and regionally- and temporally-resolved variation in logging practices to better simulate the effects of forest degradation in the Earth system.

Summary
The module is designed to mimic the ecological, biophysical, and biogeochemical processes at a landscape level following a logging event. This is achieved by specifying the timing and aerial extent of logging events, splitting the logged forest patch into disturbed and intact patches, determining the survivorship of cohorts in the disturbed patch, and modifying the biomass and necromass pools following logging. The logging module is parameterized to reproduce a selective logging experiment at the Tapajós National Forest in Brazil and benchmarked against available field measurements.  Our results suggest that the model permits the coexistence of early and late successional functional types following disturbance. The model also realistically characterizes the seasonality of water and carbon fluxes and stocks, the forest structure and composition, and the ecosystem succession after disturbance.

Contacts (BER PM): Daniel Stover, Terrestrial Ecosystem Science, Daniel.Stover@science.doe.gov
PI Contact: Jeff Chambers, Lawrence Berkeley National Laboratory, jchambers@lbl.gov
Ruby Leung, Pacific Northwest National Laboratory, Ruby.leung@pnnl.gov

Funding
This research has been supported by the U.S. Department of Energy, Office of Science (grant no. 66705), the São Paulo State Research Foundation (grant no. 2015/07227-6), the National Aeronautics and Space Administration (grant no. 80NM0018D004), and the National Science Foundation (grant no. 1458021).

Publications
Huang, M., Xu, Y., Longo, M., Keller, M., Knox, R. G., Koven, C. D., and Fisher, R. A.: Assessing impacts of selective logging on water, energy, and carbon budgets and ecosystem dynamics in Amazon forests using the Functionally Assembled Terrestrial Ecosystem Simulator, Biogeosciences, 17, 4999–5023, https://doi.org/10.5194/bg-17-4999-2020, 2020.

The Science
Soil structure and topographic features explain within-species and local-scale variability in tree growth. Using percolation theory, it is shown that root growth into the surface soil layers (0–2 m) tends to follow paths with minima in resistance, which in turn maximizes water flow and nutrient delivery rates that regulate growth and transpiration rates.

The Impact
This analysis generates the hypothesis that within-species and local-scale variability in transpiration, and hence tree growth, can be reliably predicted by soil structure because root growth into the surface soil layer (0–2 m) tends to follow paths with minima in resistance, which in turn maximizes water flow and nutrient delivery rates. It is shown that the variability in tree height may be accounted for by a single analytical equation for the growth of plants, which is governed by the optimal root length within the radial extent of the roots in the surface soil layer.

Summary
It is well established that tree growth rates are proportional to transpiration rates. This study investigates whether variability in tree growth on local scales and within species is related to constraints placed on transpiration by soil structure and other environmental conditions and not by constraints related to available soil water. Percolation theory is applied to explain how root-soil interactions regulate transpiration where root growth into the surface soil layer (0–2 m) tends to follow paths with minima in resistance, which in turn maximizes water flow and nutrient delivery rates that regulate growth. It is shown that the variability in tree height may be accounted for by a single analytical equation for the growth of plants, which is governed by the optimal root length within the radial extent of the roots in the surface soil layer. The scientific results are based on testing several scenarios of different topographic features (curvature, slope, and aspect), soil characteristics, and climate ranges. The results can be used for analytical calculations of NPP within the scope of the NGEE-Tropics project.

Contacts (BER PM): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)

PI Contact: Boris Faybishenko, Lawrence Berkeley National Laboratory, bafaybishenko@lbl.gov

Funding
This research was partially supported by the NGEE-Tropics and DEDUCE projects, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, and Office of Advanced Scientific Computing under contract DE-AC02–05CH11231.

Publications
Hunt AG, Faybishenko B, Powell TL (2020) A new phenomenological model to describe root-soil Interactions based on percolation theory. Ecological Modelling. doi: 10.1016/j.ecolmodel.2020.109205.