Abstract
Land vegetation is currently taking up large amounts of atmospheric CO
2
, possibly due to tree growth stimulation. Extant models predict that this growth stimulation will continue to cause ...a net carbon uptake this century. However, there are indications that increased growth rates may shorten trees′ lifespan and thus recent increases in forest carbon stocks may be transient due to lagged increases in mortality. Here we show that growth-lifespan trade-offs are indeed near universal, occurring across almost all species and climates. This trade-off is directly linked to faster growth reducing tree lifespan, and not due to covariance with climate or environment. Thus, current tree growth stimulation will, inevitably, result in a lagged increase in canopy tree mortality, as is indeed widely observed, and eventually neutralise carbon gains due to growth stimulation. Results from a strongly data-based forest simulator confirm these expectations. Extant Earth system model projections of global forest carbon sink persistence are likely too optimistic, increasing the need to curb greenhouse gas emissions.
The Amazon basin hosts half the planet's remaining moist tropical forests, but they may be threatened in a warming world. Nevertheless, climate model predictions vary from rapid drying to modest ...wetting. Here we report that the catchment of the world's largest river is experiencing a substantial wetting trend since approximately 1990. This intensification of the hydrological cycle is concentrated overwhelmingly in the wet season driving progressively greater differences in Amazon peak and minimum flows. The onset of the trend coincides with the onset of an upward trend in tropical Atlantic sea surface temperatures (SST). This positive longer‐term correlation contrasts with the short‐term, negative response of basin‐wide precipitation to positive anomalies in tropical North Atlantic SST, which are driven by temporary shifts in the intertropical convergence zone position. We propose that the Amazon precipitation changes since 1990 are instead related to increasing atmospheric water vapor import from the warming tropical Atlantic.
Key Points
Intensification of Amazon Hydrological Cycle since 1990
Revealed by both river discharge and precipitation records
In parallel onset of tropical Atlantic warming offering explanation
Rainfall and river levels in the Amazon are associated with significant precipitation anomalies of opposite sign in temperate North and South America, which is the dominant mode of precipitation ...variability in the Americas that often arises during extremes of the El Niño/Southern Oscillation (ENSO). This co-variability of precipitation extremes across the Americas is imprinted on tree growth and is detected when new tree-ring chronologies from the eastern equatorial Amazon are compared with hundreds of moisture-sensitive tree-ring chronologies in mid-latitude North and South America from 1759 to 2016. Pan-American co-variability exists even though the seasonality of precipitation and tree growth only partially overlaps between the Amazon and mid-latitudes because ENSO forcing of climate can persist for multiple seasons and can orchestrate a coherent response, even where the growing seasons are not fully synchronized. The tree-ring data indicate that the El Niño influence on inter-hemispheric precipitation and tree growth extremes has been strong and stable over the past 258-years, but the La Niña influence has been subject to large multi-decadal changes. These changes have implications for the dynamics and forecasting of hydroclimatic variability over the Americas and are supported by analyses of the available instrumental data and selected climate model simulations.
Recent analyses of Amazon runoff and gridded precipitation data suggest an intensification of the hydrological cycle over the past few decades in the following sense: wet season precipitation and ...peak river runoff (since ∼1980) as well as annual mean precipitation (since ∼1990) have increased, while dry season precipitation and minimum runoff have slightly decreased. There has also been an increase in the frequency of anomalously severe floods and droughts. To provide context for the special issue on Amazonia and its forests in a warming climate we expand here on these analyses. The contrasting recent changes in wet and dry season precipitation have continued and are generally consistent with changes in catchment‐level peak and minimum river runoff as well as a positive trend of water vapor inflow into the basin. Consistent with the river records, the increased vapor inflow is concentrated to the wet season. Temperature has been rising by 0.7°C since 1980 with more pronounced warming during dry months. Suggestions for the cause of the observed changes of the hydrological cycle come from patterns in tropical sea surface temperatures (SSTs). Tropical and North Atlantic SSTs have increased rapidly and steadily since 1990, while Pacific SSTs have shifted during the 1990s from a positive Pacific Decadal Oscillation (PDO) phase with warm eastern Pacific temperatures to a negative phase with cold eastern Pacific temperatures. These SST conditions have been shown to be associated with an increase in precipitation over most of the Amazon except the south and southwest. If ongoing changes continue, we expect forests to continue to thrive in those regions where there is an increase in precipitation with the exception of floodplain forests. An increase in flood pulse height and duration could lead to increased mortality at higher levels of the floodplain and, over the long term, to a lateral shift of the zonally stratified floodplain forest communities. Negative effects on forests are mainly expected in the southwest and south, which have become slightly drier and hotter, consistent with tree mortality trends observed at the RAINFOR Amazon forest plot network established in the early 1980s consisting of approximately 150 regularly censused 1ha plots in intact forests located across the whole basin.
Key Points
Amazon hydrological changes—increasing seasonal amplitude and increase in severe floods and dry conditions
Likely caused by warming of tropical North Atlantic
Effect of dry/hot anomalies in forest plot data visible
Volatile isoprenoids regulate plant performance and atmospheric processes, and Amazon forests comprise the dominant source to the global atmosphere. Still, there is a poor understanding of how ...isoprenoid emission capacities vary in response to ecophysiological and environmental controls in Amazonian ecosystems.
We measured isoprenoid emission capacities of three Amazonian hyperdominant tree species – Protium hebetatum, Eschweilera grandiflora, Eschweilera coriacea – across seasons and along a topographic and edaphic environmental gradient in the central Amazon.
From wet to dry season, both photosynthesis and isoprene emission capacities strongly declined, while emissions increased among the heavier isoprenoids: monoterpenes and sesquiterpenes. Plasticity across habitats was most evident in P. hebetatum, which emitted sesquiterpenes only in the dry season, at rates that significantly increased along the hydro‐topographic gradient from white sands (shallow root water access) to uplands (deep water table).
We suggest that emission composition shifts are part of a plastic response to increasing abiotic stress (e.g. heat and drought) and reduced photosynthetic supply of substrates for isoprenoid synthesis. Our comprehensive measurements suggest that more emphasis should be placed on other isoprenoids, besides isoprene, in the context of abiotic stress responses. Shifting emission compositions have implications for atmospheric responses because of the strong variation in reactivity among isoprenoid compounds.
Amazonian tree species emit less isoprene but more heavier isoprenoids in response to abiotic stress, such as heat and drought.
In Amazonia, wetlands constitute about 30% of its entire basin, of which ancient fluvial terraces located in vast interfluvial regions cover a large portion. Although the increased number of ...permanent plots in the recent years has contributed to improved understanding of regional variation in forest dynamics across the Amazon Basin, the functioning of large lowland interfluvial wetlands remain poorly understood. Here we present the first field-based estimate for tree ages, wood biomass productivity and biomass turnover rates for eight 1 ha plots in wetland and non-flooded forests distributed along the BR-319 Highway along a distance of about 600 km crossing the Purus–Madeira rivers interfluvial region in central-southwestern Amazon Basin. We estimate stand age, wood biomass productivity and biomass turnover rates combining tree-ring data and an allometric equation based on diameter, tree height and wood density and relate these structural parameters to physical soil and hydrological restrictions. Wood biomass and productivity varied twofold among the plots, with wood biomass stocks ranging between 138–294 Mg ha−1 and productivity varying between 3.4–6.6 Mg ha−1 yr−1. Soil effective depth, topography, structure and mainly soil water saturation significantly affected stand age (64–103 yr) and forest dynamics in terms of annual biomass turnover rates (2.0–3.2%). On harsher soils characterized by a poor structure, low effective depth and high water saturation, biomass turnover rates were increased and forests stands were younger compared to well-drained sites. We suggest that soil constraints, especially soil water saturation, limit the development of the stand structure, resulting in forests with younger stand ages and higher biomass turnover rates compared to forests growing on well-drained soils. We do not find, however, any relation between physical soil restrictions or hydrology and wood biomass productivity, but there is a trend of increasing wood biomass productivity and phosphorus concentrations at the soil surface. Based on our results we establish hypotheses for different dynamical processes between forests growing on waterlogged and well-drained soils and discuss how these results can be applied in the background of conservation as well as the potential development of forest management plans in this region, which will experience increased deforestation due to the construction of the BR-319 Highway crossing the interfluvial region of the Purus–Madeira rivers.
In this study we use allometric models combined with tree ring analysis to estimate carbon stocks and sequestration in the aboveground coarse wood biomass (AGWB) of wetland forests in the Pantanal, ...located in central South America. In four 1-ha plots in stands characterized by the pioneer tree species Vochysia divergens Pohl (Vochysiaceae) forest inventories (trees ≥10 cm diameter at breast height, D) have been performed and converted to estimates of AGWB by two allometric models using three independent parameters (D, tree height H and wood density ρ). We perform a propagation of measurement errors to estimate uncertainties in the estimates of AGWB. Carbon stocks of AGWB vary from 7.8 ± 1.5 to 97.2 ± 14.4 Mg C ha−1 between the four stands. From models relating tree ages determined by dendrochronological techniques to C-stocks in AGWB we derived estimates for C-sequestration which differs from 0.50 ± 0.03 to 3.34 ± 0.31 Mg C ha−1 yr−1. Maps based on geostatistic techniques indicate the heterogeneous spatial distribution of tree ages and C-stocks of the four studied stands. This distribution is the result of forest dynamics due to the colonizing and retreating of V. divergens and other species associated with pluriannual wet and dry episodes in the Pantanal, respectively. Such information is essential for the management of the cultural landscape of the Pantanal wetlands.
Key message
Site-specific growth modeling based on tree-ring data is demonstrated to be an efficient tool for conservation and sustainable forest management of an economically important tropical tree ...species,
Calophyllum brasiliense
.
One of the main challenges in the sustained management of natural tropical forests is obtaining reliable data on tree growth, which is prerequisite information for determining harvesting volumes and felling cycles. In this study, we apply growth models based on tree-ring data and allometric equations to estimate site-specific management options for timber resources of the commercial species
Calophyllum brasiliense
(Calophyllaceae) comparing 16 wetland sites across different Brazilian ecoregions, the Amazon, Cerrado (savannah), Pantanal and Mata Atlântica (Coastal Atlantic Rainforest). By modeling diameter, height, and volume growth parameters, we estimate site-specific minimum logging diameters (MLD) and felling cycles analyzing a total of 341 trees. Between ecoregions, the mean diameter increments varied slightly between 4.3 ± 1.6 mm year
−1
in the Amazon region (average of six sites), 4.0 ± 0.8 mm year
−1
in the Cerrado and Pantanal (average of seven sites), and 4.5 ± 1.2 mm year
−1
in the Mata Atlântica (average of three sites). However, between sites, we observed significant differences in diameter and volume increment rates, resulting in felling cycles varying from 14 to 63 years and MLDs in the range of 35–81 cm. This clearly indicates that forest management practices in Brazil, which generally applies a feeling cycle of 25 years and a diameter-cutting limit of 50 cm cannot guarantee a sustainable timber harvest. Timber resource management of this species requires site-specific criteria and should be restricted at sites with a low wood productivity. Moreover, long-term monitoring of the population structure and dynamics is necessary for a better understanding of the relationship between environmental factors and population dynamics, especially concerning the regeneration processes.
Key message
First study that demonstrated the formation of annual growth rings in Amazonian Estuary trees. Growth patterns of
Mora paraensis
and specific management criteria, such as felling cycle, ...are presented.
The aim of the present study was to contribute to increased sustainability in the timber management of
Mora paraensis
, through the estimation of minimum logging diameter (MLD) and felling cycle, using volume increment models based on tree-ring analysis and allometric relationships. We collected stem discs from 17 trees of five diameter classes. The diameters and heights of the trees were also measured. We estimated tree ages by ring-counting and the radial increment rates by measuring the ring widths with a digital analysis system. We built growth models based on relationships between age, diameter and tree height to estimate volume increment along the tree’s whole life cycle. The maximum current diameter increment in
M. paraensis
occurs at an age of around 26 years, reaching 4.91 mm year-1. The MLD was 46.4 ± 0.6 cm (standard error), which trees achieve at an age of about 115 years. The felling cycle, estimated by the mean passage time through 10 cm diameter classes until achieving the MLD, was 24.7 ± 1.3 years. These results corroborated the norms of the current Brazilian legislation that regulates forest management of high intensity in the Amazon basin. In future, more specific growth models are needed for other commercial tree species in the Amazonian Estuary to define the optimal harvest rate to maintain sustainable timber resource management practices. Through such practices, the conservation of these ecosystems and their multiple services and functions, as well as the welfare of the forest-dependent human populations can be secured.
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