Seasonal LAI in slash pine estimated with LANDSAT TM

The leaf area index (LAI, total area of leaves per unit area of ground) of most forest canopies varies throughout the year, yet for logistical reasons it is difficult to estimate anything more detailed than a seasonal maximum LAI. To determine if remotely sensed data can be used to estimate LAI seasonally, field measurements of LAI were compared to normalized difference vegetation index (NDVI) values derived using LANDSAT Thematic Mapper (TM) data, for 16 fertilized and control slash pine plots on 3 dates. Linear relationships existed between NDVI and LAI with R(sup 2) values of 0.35, 0.75, and 0.86 for February 1988, September 1988, and March, 1989, respectively. This is the first reported study in which NDVI is related to forest LAI recorded during the month of sensor overpass. Predictive relationships based on data from eight of the plots were used to estimate the LAI of the other eight plots with a root-mean-square error of 0.74 LAI, which is 15.6 percent of the mean LAI. This demonstrates the potential use of LANDSAT TM data for studying seasonal dynamics in forest canopies

Limits on aboveground net primary production, leaf area, and biomass in vegetational zones of the Pacific Northwest

Samples of mature vegetation from 8 of 12 major vegetational zones in Oregon and Washington, representing about 80% of the area of the two states, were studied along a latitudinal transect from the Pacific Coast to the east slopes of the Cascade Mountains. Six stands were in forest zones, one in woodland and one in the shrub-steppe. Aboveground net primary production (NPP, estimated as the sum of stem, branch, and foliage production) ranged from 0.3 to 14.7 t ha-1 yr-1, aboveground biomass from 3 to 1500 t ha-1, and area of all sides of leaves from 1 to 40 ha ha-1, with minimums in the shrub-steppe zone and maximums in the coastal forest zone. Average day air temperatures were less than -2°C only 2% of the winter at the coast, contrasted to 87% in the shrub-steppe. Although annual precipitation ranged from 20 cm in the shrub-steppe to 260 cm at the coast, it was a relatively poor predictor of stand structure and production. Maximum leaf areas were closely related to a simple growing season water balance in seven vegetational zones. In the subalpine conifer zone, leaf area appeared limited by temperature. Of the water balance components, evaporative demand alone accounted for 95% of the variance in leaf area. Biomass and NPP increased linearly with leaf area up to a leaf area of 20 ha ha-1. Biomass continued to increase with increasing leaf areas. NPP was also linearly related to minimum January temperatures. Except In the coastal forest zones, NPP for any given leaf area was less than maximum values reported for other mature systems elsewhere in the world

Steering Operational Synergies in Terrestrial Observation Networks: Opportunity for Advancing Earth System Dynamics Modelling

Advancing our understanding of Earth system dynamics (ESD) depends on the development of models and other analytical tools that apply physical, biological, and chemical data. This ambition to increase understanding and develop models of ESD based on site observations was the stimulus for creating the networks of Long-Term Ecological Research (LTER), Critical Zone Observatories (CZOs), and others. We organized a survey, the results of which identified pressing gaps in data availability from these networks, in particular for the future development and evaluation of models that represent ESD processes, and provide insights for improvement in both data collection and model integration. From this survey overview of data applications in the context of LTER and CZO research, we identified three challenges: (1) widen application of terrestrial observation network data in Earth system modelling, (2) develop integrated Earth system models that incorporate process representation and data of multiple disciplines, and (3) identify complementarity in measured variables and spatial extent, and promoting synergies in the existing observational networks. These challenges lead to perspectives and recommendations for an improved dialogue between the observation networks and the ESD modelling community, including co-location of sites in the existing networks and further formalizing these recommendations among these communities. Developing these synergies will enable cross-site and cross-network comparison and synthesis studies, which will help produce insights around organizing principles, classifications, and general rules of coupling processes with environmental conditions

Shedding light on plant litter decomposition: Advances, implications and new directions in understanding the role of photodegradation

Litter decomposition contributes to one of the largest fluxes of carbon (C) in the terrestrial biosphere and is a primary control on nutrient cycling. The inability of models using climate and litter chemistry to predict decomposition in dry environments has stimulated investigation of non-traditional drivers of decomposition, including photodegradation, the abiotic decomposition of organic matter via exposure to solar radiation. Recent work in this developing field shows that photodegradation may substantially influence terrestrial C fluxes, including abiotic production of carbon dioxide, carbon monoxide and methane, especially in arid and semi-arid regions. Research has also produced contradictory results regarding controls on photodegradation. Here we summarize the state of knowledge about the role of photodegradation in litter decomposition and C cycling and investigate drivers of photodegradation across experiments using a meta-analysis. Overall, increasing litter exposure to solar radiation increased mass loss by 23% with large variation in photodegradation rates among and within ecosystems. This variation was tied to both litter and environmental characteristics. Photodegradation increased with litter C to nitrogen (N) ratio, but not with lignin content, suggesting that we do not yet fully understand the underlying mechanisms. Photodegradation also increased with factors that increased solar radiation exposure (latitude and litter area to mass ratio) and decreased with mean annual precipitation. The impact of photodegradation on C (and potentially N) cycling fundamentally reshapes our thinking of decomposition as a solely biological process and requires that we define the mechanisms driving photodegradation before we can accurately represent photodegradation in global C and N models. © 2012 US Government

Ecosystem and understory water and energy exchange for a mature, naturally regenerated pine flatwoods forest in north

Abstract: Eddy covariance was used to measure energy fluxes from July 2000 -June 2002 above the tree canopy and above the understory in a mature, naturally regenerated slash pine (Pinus elliottii Engelm. var. elliottii) -longleaf pine (Pinus palustris Mill.) flatwoods forest. Understory latent energy (λE) and sensible heat (H) fluxes accounted for 45% and 55% of whole-ecosystem fluxes, respectively, with strong seasonal variation in the proportion of λE attributable to the understory. The partitioning of net radiation (R net ) to λE and H also changed seasonally, with half-hourly mean ecosystem H in the winter peaking at 175 W·m -2 , almost twice as large as λE. In contrast, half-hourly ecosystem λE and H remained almost equal throughout the day in July and August, with mean midday peaks of approximately 200 W·m -2 . Maximum hourly evapotranspiration (ET) in the months of July and August was 0.32 and 0.29 mm·h -1 for 2000 and 2001, respectively. For a variety of environmental conditions, mean daily ET was approximately 2.7 mm in the summer and 1.3 mm in the winter. Annual ET for the first year was 832 mm, or 87% of annual precipitation (956 mm). Although leaf area index was higher in the second year, annual ET was only 676 mm, which is considerably lower than that of the previous year, but it still accounted for approximately the same proportion (84%) of the much lower annual precipitation (811 mm). Canopy conductance declined as soils dried, changing patterns of partitioning of R net to λE. Pour toute une gamme de conditions environnementales, la moyenne journalière de ET était d'environ 2,7 mm pendant l'été et 1,3 mm pendant l'hiver. La valeur annuelle de ET pour la première année était de 832 mm, soit 87 % des précipitations annuelles (956 mm). Même si la valeur de l'indice de surface foliaire était supérieure pendant la deuxième année, la valeur annuelle de ET a atteint seulement 676 mm, ce qui est considérablement plus faible que la valeur de l'année précédente. Cette valeur correspond environ à la même proportion (84 %) des précipitations annuelles (811 mm) de 2001 qui ont été beaucoup plus faibles qu'en 2000. La conductance du couvert forestier a diminué avec l'assèchement du sol, ce qui a changé le patron de réparti-tion de R net sous forme de λE. [Traduit par la Rédaction] Powell et al. 158

Energy dynamics and modeled evapotranspiration from a wet tropical forest in Costa Rica

The effects of albedo, net radiation (Rn), vapor pressure deficit (VPD), and surface conductances on energy fluxes and evapotranspiration (ET) were determined for a wet tropical forest in NE Costa Rica from 1997 to 2000. Sensible heat fluxes (H) were estimated by the combination of eddy-covariance and the change in below-canopy heat profiles. Above-canopy latent heat fluxes (λE) were estimated by the residuals from Rn and H, and below canopy λE fluxes. Surface reflectance (albedo) was ∼12% of incident solar radiation and did not differ seasonally. Rn was significantly different among years and explained ∼79% of the variation in H and λE fluxes. The effects of VPD did not explain any additional variation in heat fluxes. λE fluxes were always greater than H fluxes when Rn\u3e40 W m−2. Understory heat fluxes were small and contributed little towards daily energy exchange, but may be significant when Rn is small. A dimensionless coefficient (Ω) was used to determine the relative importance of aerodynamic conductance (ga) and bulk canopy conductance (gb) on λE flux. During the day, Ω was \u3e0.6 and peaked at 0.85 suggesting that the forest was decoupled from physiological controls, λE fluxes are more dependent on Rn than water availability, and ga exerts more control on λE fluxes than gb. Because of these results, both the Priestly–Taylor and the Penman–Monteith models performed well using only Rn. Because the canopy is wet ∼32% of the time, there was better precision in estimating λE fluxes using the Priestly–Taylor model (with an empirically estimated α=1.24), when the canopy was wet. Annual ET were 1892, 2292 and 2230 mm for 1998, 1999 and 2000, respectively. Annual ET ranged from 54 to 66% of bulk precipitation. Using a Rutter-type model, interception losses were 17–18% of bulk precipitation. The overall amount of energy needed for annual ET accounted for ∼88 to 97% of total Rn

contrasting environments: toward a global model

Long-term dynamics of pine and hardwood litter i

Appendix A. Annual carbon fluxes for forests at different latitudes.

Annual carbon fluxes for forests at different latitudes