Timothy A. Martin

Ecosystem carbon dioxide fluxes after disturbance in forests of North America

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER.

Canopy nitrogen, carbon assimilation, and albedo in temperate and boreal forests: Functional relations and potential climate feedbacks

The availability of nitrogen represents a key constraint on carbon cycling in terrestrial ecosystems, and it is largely in this capacity that the role of N in the Earths climate system has been considered. Despite this, few studies have included continuous variation in plant N status as a driver of broad-scale carbon cycle analyses. This is partly because of uncertainties in how leaf-level physiological relationships scale to whole ecosystems and because methods for regional to continental detection of plant N concentrations have yet to be developed. Here, we show that ecosystem CO2 uptake capacity in temperate and boreal forests scales directly with whole-canopy N concentrations, mirroring a leaf-level trend that has been observed for woody plants worldwide. We further show that both CO2 uptake capacity and canopy N concentration are strongly and positively correlated with shortwave surface albedo. These results suggest that N plays an additional, and overlooked, role in the climate system via its influence on vegetation reflectivity and shortwave surface energy exchange. We also demonstrate that much of the spatial variation in canopy N can be detected by using broad-band satellite sensors, offering a means through which these findings can be applied toward improved application of coupled carbon cycle–climate models.

Accuracy of Genomic Selection Methods in a Standard Data Set of Loblolly Pine (Pinus taeda L.)

Genomic selection can increase genetic gain per generation through early selection. Genomic selection is expected to be particularly valuable for traits that are costly to phenotype and expressed late in the life cycle of long-lived species. Alternative approaches to genomic selection prediction models may perform differently for traits with distinct genetic properties. Here the performance of four different original methods of genomic selection that differ with respect to assumptions regarding distribution of marker effects, including (i) ridge regression–best linear unbiased prediction (RR–BLUP), (ii) Bayes A, (iii) Bayes Cπ, and (iv) Bayesian LASSO are presented. In addition, a modified RR–BLUP (RR–BLUP B) that utilizes a selected subset of markers was evaluated. The accuracy of these methods was compared across 17 traits with distinct heritabilities and genetic architectures, including growth, development, and disease-resistance properties, measured in a Pinus taeda (loblolly pine) training population of 951 individuals genotyped with 4853 SNPs. The predictive ability of the methods was evaluated using a 10-fold, cross-validation approach, and differed only marginally for most method/trait combinations. Interestingly, for fusiform rust disease-resistance traits, Bayes Cπ, Bayes A, and RR–BLUB B had higher predictive ability than RR–BLUP and Bayesian LASSO. Fusiform rust is controlled by few genes of large effect. A limitation of RR–BLUP is the assumption of equal contribution of all markers to the observed variation. However, RR-BLUP B performed equally well as the Bayesian approaches.The genotypic and phenotypic data used in this study are publically available for comparative analysis of genomic selection prediction models.

Respiration from the Organ Level to the Stand

Publisher Summary Respiration is a major factor in plant, stand, or ecosystem energy budgets, estimated to consume anywhere from 30–70% of total carbon fixed. Respiration has been an area of particular interest and concern recently because of the possibility that C02-induced global warming might lead to substantial increases in respiration in temperate and boreal ecosystems that could decrease net primary productivity This chapter focuses on respiration. It describes dark respiration as a process by which glucose is enzymatically combined with oxygen to liberate chemical energy and CO2. Most respiration in trees is through the normal cytochrome-mediated pathway, but there is an alternative. Cyanide-resistant or salicylhydroxamic acid-sensitive respiration is a nonphosphorylating respiration pathway that generates only 40–50% as much chemical energy per glucose oxidized. When the respiration costs of producing plant tissue are estimated from tissue analyses, any ATP required for synthesis is produced by cytochromemediated respiration. Photorespiration is a by-product of photosynthesis in C3 plants in which ribulose bisphosphate carboxylase/oxygenase binds to O2 instead of CO2. It manifests as a reduction in the rate of photosynthesis. Considerable effort has been put into developing techniques for scaling individual measurements up to stand level. This chapter focuses on these scaling techniques and how they deal with known sources of variation in respiration.

Quantitative genetic analysis of biomass and wood chemistry of Populus under different nitrogen levels.

The genetic control of carbon allocation and partitioning in woody perennial plants is poorly understood despite its importance for carbon sequestration, biofuels and other wood-based industries. It is also unclear how environmental cues, such as nitrogen availability, impact the genes that regulate growth, biomass allocation and wood composition in trees. We phenotyped 396 clonally replicated genotypes of an interspecific pseudo-backcross pedigree of Populus for wood composition and biomass traits in above- and below-ground organs. The loci that regulate growth, carbon allocation and partitioning under two nitrogen conditions were identified, defining the contribution of environmental cues to their genetic control. Sixty-three quantitative trait loci were identified for the 20 traits analyzed. The majority of quantitative trait loci are specific to one of the two nitrogen treatments, demonstrating significant nitrogen-dependent genetic control. A highly significant genetic correlation was observed between plant growth and lignin/cellulose composition, and quantitative trait loci co-localization identified the genomic position of potential pleiotropic regulators. Pleiotropic loci linking higher growth rates to wood with less lignin are excellent targets to engineer tree germplasm improved for pulp, paper and cellulosic ethanol production. The causative genes are being identified with a genetical genomics approach.

DEVELOPMENTAL PATTERNS AND NUTRITION IMPACT RADIATION USE EFFICIENCY COMPONENTS IN SOUTHERN PINE STANDS

A number of contemporary forest productivity models use some variation of a growth efficiency (e) approach. Typically, these models predict production (aboveground net primary production, ANPP, in units of dry mass biomass per unit of area per unit of time) as the product of two terms: radiation use efficiency (e, in units of dry mass biomass per megajoule of photosynthetically active radiation [PAR] intercepted or absorbed by the plant canopy) and the sum of PAR intercepted or absorbed by the canopy (Φpar, in units of megajoules per unit of radiation area per unit of time). Predicting productivity in a biologically realistic manner requires an understanding of how model components are affected by natural and anthropogenic environmental factors, as well as other influences such as aging or stand development. We measured or calculated all components of the e model (aboveground woody biomass increment, IWB; foliage biomass increment, IFB; aboveground net primary production, ANPP; leaf area index, LAI; Φpar;...

Controls on carbon dynamics by ecosystem structure and climate for southeastern U.S. slash pine plantations

Planted pine forests (plantations) in the southeastern United States are an important component of the continents carbon balance. Forest carbon dynamics are affected by a range of factors including climatic variability. Multiyear droughts have affected the region in the past, and an increase in the frequency of drought events has been predicted. How this increased climatic variability will affect the capacity of the regions pine plantations to sequester carbon is not known. We used eddy covariance and biometric approaches to measure carbon dynamics over nine years in two slash pine plantations (Pinus elliottii var elliottii Englm) in north Florida, consisting of a newly planted and a mid-rotation stand. During this time, the region experienced two multiyear droughts (1999-2002 and 2006-2008), separated by a three-year wet period. Net ecosystem carbon accumulation measured using both approaches showed the same trends and magnitudes during plantation development. The newly planted site released 15.6 Mg C/ha during the first three years after planting, before becoming a carbon sink in year 4. Increases in carbon uptake during the early stages of stand development were driven by the aggrading leaf area index (LAI). After canopy closure, both sites were continuous carbon sinks with net carbon uptake (NEE) fluctuating between 4 and ; 8M g Cha � 1 � yr � 1 , depending on environmental conditions. Drought reduced NEE by ;25% through its negative effects on both LAI and radiation-use efficiency, which resulted in a larger impact on gross ecosystem carbon exchange than on ecosystem respiration. While results indicate that responses to drought involved complex interactions among water availability, LAI, and radiation-use efficiency, these ecosystems remain carbon sinks under current management strategies and climatic variability. Variation within locations is primarily due to major disturbances, such as logging in the current study and, to a much lesser extent, local environmental fluctuations. When data from this study are compared to flux data from a broad range of forests worldwide, these ecosystems fill a data gap in the warm-temperate zone and support a broad maximum NEE (for closed-canopy forests) between 88C and 208C mean annual temperature.

Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms.

• It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.

Effects of a Prescribed Fire on Understory Vegetation, Carbon Pools, and Soil Nutrients in a Longleaf Pine-Slash Pine Forest in Florida

ABSTRACT: We quantified fire-driven loss and post-fire recovery of understory and soil carbon (C) and nutrient pools for one to three years following a single prescribed fire in a naturally regenerated longleaf (Pinus palustris Mill.) and slash pine (Pinus elliottii Engelm. var. elliottii) forest located in north central Florida. Fire immediately reduced total aboveground understory C and nitrogen (N) pools, but these pools recovered to pre-fire levels after three years. Our results also showed that the effect of fire on the understory composition and structure was only short-lived. Prescribed fire significantly reduced total C and N pools in the forest floor (F and H horizons), and this effect persisted for at least one year postfire. Available NH4+, PO43-, Ca2+, Mg2, and K+ concentrations in the forest floor decreased immediately after fire, but increased in the surface mineral soil (O to 5 cm depth); Ca2+, Mg2+, and K+ remained elevated for the first year after fire. Fire immediately reduced total ecosystem C and N pools by 40% and 27%, respectively, emitting 3860 g C m-2 and 170 g N m-2 to the atmosphere. The pools recovered to 67% and 76% of pre-fire C and N pools, respectively, after one year. Of the pools measured, C and N recovery in forest floor materials was the slowest, and projections of initial recovery rates suggest that it will take more than six years, the previous fire interval, to reach pre-fire levels. This slow recovery may indicate variation in past rates of forest floor accumulation due to management practices as well as effects of the severity of this or previous fires.

Regional carbon dynamics in the southeastern U.S. coastal plain: Balancing land cover type, timber harvesting, fire, and environmental variation

[1] Understanding regional carbon budgets is a leading issue in carbon cycling research, but issues of measurement difficulty, scale, boundaries, and logistics compromise estimates at areas larger than stands or research plots. We studied four 15 15 km sample areas to examine land management and wildfire effects on carbon storage dynamics in the forested southeastern U.S. coastal plain region from 1975 to 2001. Carbon exchange and storage rates were estimated using satellite remote-sensing methods coupled with micrometeorological and biomass measurements. Carbon losses occurred by timber harvesting and fire, and carbon release continued for four years following clearing, suppressing landscape carbon gain proportional to the cleared area. Carbon accumulated at an average rate of 90,000 t C yr 1 in the landscape (total area 900 km 2 ) from 1975–2000, or 1tCh a 1 yr 1 . Interannual variation was related mainly to the magnitude of annual plantation timber harvesting. Wildfires were rare and their effects on carbon balances consequently small, despite having large local impact. Previous studies in the area demonstrated that environmental fluctuations had little direct effect on the net landscape exchange of carbon, although indirect effects included higher probability of fire during droughts and shifts in harvesting to drier sites during wet periods. Although this study was a simple aggregation of carbon cycle components from fine spatial scale (Landsat images) to the landscape, the extrapolation incorporated important spatial and temporal environmental heterogeneities and led to the unexpected suggestion that the industrial forests of the southeast U.S. Coastal plain are a long-term carbon sink. The analysis also revealed specific uncertainties in our scaling efforts that point to future research needs.