Jeffrey Q. Chambers

Ancient trees in Amazonia

The ages of tropical rain forest trees provide critical information for understanding the dynamics of tree populations, determining historical patterns of disturbance, developing sustainable forestry practices and calculating carbon cycling rates. Nevertheless, the ecological life history of most tropical trees is unknown and even the ages of the largest trees remain to be determined. Tree ages are typically measured by counting annual rings, but in tropical forests rings can be non-existent, annual or irregular. In the absence of annual rings, 14C dating is the only way to determine the age of a tree directly. We have 14C-dated twenty large, emergent trees from a central Amazon rain forest and find that, contrary to conventional views, trees in these forests can be more than 1,400 years old.

Carbon sink for a century.

Intact rainforests have a long-term storage capacity.

Global satellite monitoring of climate-induced vegetation disturbances

Terrestrial disturbances are accelerating globally, but their full impact is not quantified because we lack an adequate monitoring system. Remote sensing offers a means to quantify the frequency and extent of disturbances globally. Here, we review the current application of remote sensing to this problem and offer a framework for more systematic analysis in the future. We recommend that any proposed monitoring system should not only detect disturbances, but also be able to: identify the proximate cause(s); integrate a range of spatial scales; and, ideally, incorporate process models to explain the observed patterns and predicted trends in the future. Significant remaining challenges are tied to the ecology of disturbances. To meet these challenges, more effort is required to incorporate ecological principles and understanding into the assessments of disturbance worldwide.

Toward an integrated monitoring framework to assess the effects of tropical forest degradation and recovery on carbon stocks and biodiversity

Tropical forests harbor a significant portion of global biodiversity and are a critical component of the climate system. Reducing deforestation and forest degradation contributes to global climate-change mitigation efforts, yet emissions and removals from forest dynamics are still poorly quantified. We reviewed the main challenges to estimate changes in carbon stocks and biodiversity due to degradation and recovery of tropical forests, focusing on three main areas: (1) the combination of field surveys and remote sensing; (2) evaluation of biodiversity and carbon values under a unified strategy; and (3) research efforts needed to understand and quantify forest degradation and recovery. The improvement of models and estimates of changes of forest carbon can foster process-oriented monitoring of forest dynamics, including different variables and using spatially explicit algorithms that account for regional and local differences, such as variation in climate, soil, nutrient content, topography, biodiversity, disturbance history, recovery pathways, and socioeconomic factors. Generating the data for these models requires affordable large-scale remote-sensing tools associated with a robust network of field plots that can generate spatially explicit information on a range of variables through time. By combining ecosystem models, multiscale remote sensing, and networks of field plots, we will be able to evaluate forest degradation and recovery and their interactions with biodiversity and carbon cycling. Improving monitoring strategies will allow a better understanding of the role of forest dynamics in climate-change mitigation, adaptation, and carbon cycle feedbacks, thereby reducing uncertainties in models of the key processes in the carbon cycle, including their impacts on biodiversity, which are fundamental to support forest governance policies, such as Reducing Emissions from Deforestation and Forest Degradation.

Emissions of putative isoprene oxidation products from mango branches under abiotic stress

Although several per cent of net carbon assimilation can be re-released as isoprene emissions to the atmosphere by many tropical plants, much uncertainty remains regarding its biological significance. In a previous study, we detected emissions of isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) from tropical plants under high temperature/light stress, suggesting that isoprene is oxidized not only in the atmosphere but also within plants. However, a comprehensive analysis of the suite of isoprene oxidation products in plants has not been performed and production relationships with environmental stress have not been described. In this study, putative isoprene oxidation products from mango (Mangifera indica) branches under abiotic stress were first identified. High temperature/light and freeze–thaw treatments verified direct emissions of the isoprene oxidation products MVK and MACR together with the first observations of 3-methyl furan (3-MF) and 2-methyl-3-buten-2-ol (MBO) as putative novel isoprene oxidation products. Mechanical wounding also stimulated emissions of MVK and MACR. Photosynthesis under 13CO2 resulted in rapid (<30min) labelling of up to five carbon atoms of isoprene, with a similar labelling pattern observed in the putative oxidation products. These observations highlight the need to investigate further the mechanisms of isoprene oxidation within plants under stress and its biological and atmospheric significance.

Highly reactive light‐dependent monoterpenes in the Amazon

Despite orders of magnitude difference in atmospheric reactivity and great diversity in biological functioning, little is known about monoterpene speciation in tropical forests. Here we report vertically resolved ambient air mixing ratios for 12 monoterpenes in a central Amazon rainforest including observations of the highly reactive cis-β-ocimene (160 ppt), trans-β-ocimene (79 ppt), and terpinolene (32 ppt) which accounted for an estimated 21% of total monoterpene composition yet 55% of the upper canopy monoterpene ozonolysis rate. All 12 monoterpenes showed a mixing ratio peak in the upper canopy, with three demonstrating subcanopy peaks in 7 of 11 profiles. Leaf level emissions of highly reactive monoterpenes accounted for up to 1.9% of photosynthesis confirming light-dependent emissions across several Amazon tree genera. These results suggest that highly reactive monoterpenes play important antioxidant roles during photosynthesis in plants and serve as near-canopy sources of secondary organic aerosol precursors through atmospheric photooxidation via ozonolysis.

What's the flux? Unraveling how CO2 fluxes from trees reflect underlying physiological processes

Forum Commentary What’s the flux? Unraveling how CO 2 fluxes from trees reflect underlying physiological processes Tree stems and branches emit carbon dioxide (CO 2 ) at rates that per unit area can rival emissions from leaves or the soil surface and summed over a forest stand can comprise 14–30% of the total CO 2 efflux (Chambers et al., 2004; Ryan et al., 2009). Stem CO 2 fluxes have predictable patterns of variation with growth rate, stand age, and elevation (Chambers et al., 2004; Ryan et al., 2009; Robertson et al., 2010). Over the past decade observations of diel covariation of CO 2 efflux with sapflux rates measured in tree stems have led to the conclusion that internal transport of CO 2 within the stem strongly influences the measured CO 2 efflux at the surface (Teskey et al., 2008). In this issue of New Phytologist, Bloemen et al. (pp. 555–565) report on a tracer experiment that demonstrates not only upward transport of 13 CO 2 added to the transpiration stream, and emission of this label along the stem, but also fixation of a significant fraction of the added CO 2 in canopy branches, petioles and, to a minor extent, leaves. The study of Bloemen et al. adds to the growing literature that demonstrates the utility of isotope labeling studies to understand allocation and carbon (C) cycling in trees (Powers & Marshall, 2011; Epron et al., 2012). ‘Dynamic approaches for measuring continuous diurnal CO 2 fluxes and transport in the transpiration stream need to be more widely applied.’ Processes influencing stem CO 2 efflux A number of factors can influence the efflux of CO 2 measured by a flux chamber covering a segment of tree stem (Fig. 1). The cambium is the site of formation of new tissue, that is, of growth, while maintenance respiration produces CO 2 in all living tissues. The C being respired may derive from recent photosynthetic products transported in the phloem (e.g. Powers & Marshall, 2011) and from storage reserves. The pathways for respiration may vary with time or tree species: recently 18 O/ 16 O measurements in oxygen (O 2 ) provided the first evidence for the alternative oxidase pathway contributing to respiration in some tree stems (Angert et al., 2012a). CO 2 may also be locally fixed by photosynthetic tissues found under the bark before it is lost to the atmosphere. O 2013 The Authors New Phytologist O 2013 New Phytologist Trust Low rates of diffusion, especially across the cambium, can cause high CO 2 concentrations in stems, and internal O 2 concentrations can drop to very low levels (Spicer & Holbrook, 2005; Teskey et al., 2008). CO 2 is highly soluble, and will dissolve in (or exsolve from) stem water, depending on local saturation conditions, which in turn are controlled by factors such as temperature and pH. Uptake of CO 2 directly from the soil atmosphere, once thought potentially important, has largely been shown to be minor (see summary in Bloemen et al.). Hence the source of CO 2 emitted to the atmosphere from the bark surface can reflect a combination of local growth and maintenance respiration, other local processes producing CO 2 (including potentially decomposition in heartwood) or CO 2 from respiration in other tissues (e.g. roots) that has been transported into the volume beneath a chamber in solution. However, there can also be net export in the xylem water stream, as indicated by the fate of the tracer added by Bloemen et al. The measured chamber flux at any given time is thus the complex result of transport in, transport out and respiration minus photosynthesis in local tissues. Use of a dark chamber will exclude local photosynthesis. Observations of a relationship between sapflux and CO 2 efflux provide a clue as to whether CO 2 is net imported or exported from the volume of stem under a chamber attached to the stem surface (see Fig. 1, modified from Teskey et al., 2008). Other evidence for net CO 2 transport away from the region of efflux measurement comes from lower-than-expected efflux rates compared with what is expected given the construction costs of wood (Ryan et al., 2009), and potentially from higher efflux rates in canopy branches (Teskey et al., 2008). Changes in local temperature and/or pH can change respiration rates and also cause changes in CO 2 solubility (Kunert & Mercado Ca´rdenas, 2012). Stem anatomy, including bark thickness and tree hydraulics, likely influences the importance of the mechanisms and can help explain observations such as changes in CO 2 efflux with stand age or tree size, or differences between similar trees growing in different environments (Ryan et al., 2009). Bloemen et al. report results from labeling Populus deltoides, the eastern cottonwood tree, which has very high transpiration rates and generally is found in riparian zones. As noted by Ubierna et al. (2009) most studies that have reported relationships between sapflux and CO 2 efflux have been made in tree species with high sapflux rates and small conducting area. By contrast, the large conifer trees investigated by Ubierna et al. (2009), with lower overall sapflux, did not demon- strate such relationships, and even crown removal did not change the rates of CO 2 efflux from stems they studied. What do these results mean for interpretation of other ecosystem CO 2 efflux measurements? A major conclusion of Bloemen et al. is that the transport of the tracer from the tree base to the canopy indicates that root respiration New Phytologist (2013) 197: 353–355 353 www.newphytologist.com

Carbon dioxide emitted from live stems of tropical trees is several years old

Storage carbon (C) pools are often assumed to contribute to respiration and growth when assimilation is insufficient to meet the current C demand. However, little is known of the age of stored C and the degree to which it supports respiration in general. We used bomb radiocarbon ((14)C) measurements to determine the mean age of carbon in CO2 emitted from and within stems of three tropical tree species in Peru. Carbon pools fixed >1 year previously contributed to stem CO2 efflux in all trees investigated, in both dry and wet seasons. The average age, i.e., the time elapsed since original fixation of CO2 from the atmosphere by the plant to its loss from the stem, ranged from 0 to 6 years. The average age of CO2 sampled 5-cm deep within the stems ranged from 2 to 6 years for two of the three species, while CO2 in the stem of the third tree species was fixed from 14 to >20 years previously. Given the consistency of (14)C values observed for individuals within each species, it is unlikely that decomposition is the source of the older CO2. Our results are in accordance with other studies that have demonstrated the contribution of storage reserves to the construction of stem wood and root respiration in temperate and boreal forests. We postulate the high (14)C values observed in stem CO2 efflux and stem-internal CO2 result from respiration of storage C pools within the tree. The observed age differences between emitted and stem-internal CO2 indicate an age gradient for sources of CO2 within the tree: CO2 produced in the outer region of the stem is younger, originating from more recent assimilates, whereas the CO2 found deeper within the stem is older, fueled by several-year-old C pools. The CO2 emitted at the stem-atmosphere interface represents a mixture of young and old CO2. These observations were independent of season, even during a time of severe regional drought. Therefore, we postulate that the use of storage C for respiration occurs on a regular basis challenging the assumption that storage pools serve as substrates for respiration only during times of limited assimilation.

Dynamic Balancing of Isoprene Carbon Sources Reflects Photosynthetic and Photorespiratory Responses to Temperature Stress

Uncoupling between photosynthesis and isoprene emissions with temperature reflects the differential temperature sensitivities of photosynthesis and photorespiration. The volatile gas isoprene is emitted in teragrams per annum quantities from the terrestrial biosphere and exerts a large effect on atmospheric chemistry. Isoprene is made primarily from recently fixed photosynthate; however, alternate carbon sources play an important role, particularly when photosynthate is limiting. We examined the relative contribution of these alternate carbon sources under changes in light and temperature, the two environmental conditions that have the strongest influence over isoprene emission. Using a novel real-time analytical approach that allowed us to examine dynamic changes in carbon sources, we observed that relative contributions do not change as a function of light intensity. We found that the classical uncoupling of isoprene emission from net photosynthesis at elevated leaf temperatures is associated with an increased contribution of alternate carbon. We also observed a rapid compensatory response where alternate carbon sources compensated for transient decreases in recently fixed carbon during thermal ramping, thereby maintaining overall increases in isoprene production rates at high temperatures. Photorespiration is known to contribute to the decline in net photosynthesis at high leaf temperatures. A reduction in the temperature at which the contribution of alternate carbon sources increased was observed under photorespiratory conditions, while photosynthetic conditions increased this temperature. Feeding [2-13C]glycine (a photorespiratory intermediate) stimulated emissions of [13C1–5]isoprene and 13CO2, supporting the possibility that photorespiration can provide an alternate source of carbon for isoprene synthesis. Our observations have important implications for establishing improved mechanistic predictions of isoprene emissions and primary carbon metabolism, particularly under the predicted increases in future global temperatures.

The impacts of tropical cyclones on the net carbon balance of eastern US forests (1851–2000)

In temperate forests of the eastern US, tropical cyclones are a principal agent of catastrophic wind damage, with dramatic impacts on the structure and functioning of forests. Substantial progress has been made to quantify forest damage and resulting gross carbon emissions from tropical cyclones. However, the net effect of storms on the carbon balance of forests depends not only on the biomass lost in single events, but also on the uptake during recovery from a mosaic of past events. This study estimates the net impacts of tropical cyclones on the carbon balance of US forests over the period 1851–2000. To track both disturbance and recovery and to isolate the effects of storms, a modeling framework is used combining gridded historical estimates of mortality and damage with a mechanistic model using an ensemble approach. The net effect of tropical cyclones on the carbon balance is shown to depend strongly on the spatial and temporal scales of analysis. On average, tropical cyclones contribute a net carbon source over latter half of the 19th century. However, throughout much of the 20th century a regional carbon sink is estimated resulting from periods of forest recovery exceeding damage. The large-scale net annual flux resulting from tropical cyclones varies by up to 50 Tg C yr−1, an amount equivalent to 17%–36% of the US forest carbon sink.