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
Amazon forests, which store ∼50% of tropical forest carbon and play a vital role in global water, energy, and carbon cycling, are predicted to experience both longer and more intense dry seasons by ...the end of the 21st century. However, the climate sensitivity of this ecosystem remains uncertain: several studies have predicted large-scale die-back of the Amazon, whereas several more recent studies predict that the biome will remain largely intact. Combining remote-sensing and ground-based observations with a size- and age-structured terrestrial ecosystem model, we explore the sensitivity and ecological resilience of these forests to changes in climate. We demonstrate that water stress operating at the scale of individual plants, combined with spatial variation in soil texture, explains observed patterns of variation in ecosystem biomass, composition, and dynamics across the region, and strongly influences the ecosystem’s resilience to changes in dry season length. Specifically, our analysis suggests that in contrast to existing predictions of either stability or catastrophic biomass loss, the Amazon forest’s response to a drying regional climate is likely to be an immediate, graded, heterogeneous transition from high-biomass moist forests to transitional dry forests and woody savannah-like states. Fire, logging, and other anthropogenic disturbances may, however, exacerbate these climate change-induced ecosystem transitions.
Aim: To test the extent to which the vertical structure of tropical forests is determined by environment, forest structure or biogeographical history. Location: Pan-tropical. Methods: Using height ...and diameter data from 20,497 trees in 112 non-contiguous plots, asymptotic maximum height (H AM ) and height—diameter relationships were computed with nonlinear mixed effects (NLME) models to: (1) test for environmental and structural causes of differences among plots, and (2) test if there were continental differences once environment and structure were accounted for; persistence of differences may imply the importance of biogeography for vertical forest structure. NLME analyses for floristic subsets of data (only/excluding Fabaceae and only/excluding Dipterocarpaceae individuals) were used to examine whether family-level patterns revealed biogeographical explanations of cross-continental differences. Results: H AM and allometry were significantly different amongst continents. H AM was greatest in Asian forests (58.3 ± 7.5 m, 95% CI), followed by forests in Africa (45.1 ± 2.6 m), America (35.8 ± 6.0 m) and Australia (35.0 ± 7.4 m), and height—diameter relationships varied similarly; for a given diameter, stems were tallest in Asia, followed by Africa, America and Australia. Precipitation seasonality, basal area, stem density, solar radiation and wood density each explained some variation in allometry and H AM yet continental differences persisted even after these were accounted for. Analyses using floristic subsets showed that significant continental differences in H AM and allometry persisted in all cases. Main conclusions: Tree allometry and maximum height are altered by environmental conditions, forest structure and wood density. Yet, even after accounting for these, tropical forest architecture varies significantly from continent to continent. The greater stature of tropical forests in Asia is not directly determined by the dominance of the family Dipterocarpaceae, as on average non-dipterocarps are equally tall. We hypothesise that dominant large-statured families create conditions in which only tall species can compete, thus perpetuating a forest dominated by tall individuals from diverse families.
Abstract
Various studies report substantial increases in intrinsic water-use efficiency (
W
i
), estimated using carbon isotopes in tree rings, suggesting trees are gaining increasingly more carbon ...per unit water lost due to increases in atmospheric CO
2
. Usually, reconstructions do not, however, correct for the effect of intrinsic developmental changes in
W
i
as trees grow larger. Here we show, by comparing
W
i
across varying tree sizes at one CO
2
level, that ignoring such developmental effects can severely affect inferences of trees’
W
i
.
W
i
doubled or even tripled over a trees’ lifespan in three broadleaf species due to changes in tree height and light availability alone, and there are also weak trends for Pine trees. Developmental trends in broadleaf species are as large as the trends previously assigned to CO
2
and climate. Credible future tree ring isotope studies require explicit accounting for species-specific developmental effects before CO
2
and climate effects are inferred.
Oxygen isotope ratios in tree rings (δ18OTR) from northern Bolivia record local precipitation δ18O and correlate strongly with Amazon basin‐wide rainfall. While this is encouraging evidence that ...δ18OTR can be used for paleoclimate reconstructions, it remains unclear whether variation in δ18OTR is truly driven by within‐basin processes, thus recording Amazon climate directly, or if the isotope signal may already be imprinted on incoming vapor, perhaps reflecting a pan‐tropical climate signal. We use atmospheric back trajectories combined with satellite observations of precipitation, together with water vapor transport analysis to show that δ18OTR in Bolivia are indeed controlled by basin‐intrinsic processes, with rainout over the basin the most important factor. Furthermore, interannual variation in basin‐wide precipitation and atmospheric circulation are both shown to affect δ18OTR. These findings suggest δ18OTR can be reliably used to reconstruct Amazon precipitation and have implications for the interpretation of other paleoproxy records from the Amazon basin.
Plain Language Summary
Developing a good understanding of past climate is important to understanding ongoing climate change. This can be challenging in regions such as Amazonia where weather station data are limited. Here, other means of reconstructing historical climate are needed. Tree rings are an example of a natural climate record, with each ring recording information about the environment during the period of its formation. To use tree ring characteristics as a proxy for past climate, it is important to understand exactly how climate influences the signal stored in the wood. In this study we look at the factors controlling the ratio of light and heavy oxygen atoms in tree rings from northern Bolivia, which have been shown to be a good indicator of rainfall over the whole Amazon basin. We used a model to reconstruct air transport pathways over the continent, and examine large‐scale moisture flow into, and out of, the basin. We show that the dominant factor controlling the tree ring oxygen signal is the amount of rain that falls during air travel. This result is important because it shows that oxygen isotope ratios in tree rings, and other natural archives, can be reliably used to reconstruct Amazon rainfall.
Key Points
The mechanisms driving interannual variation in oxygen isotopes in Amazon tree rings (δ18OTR) have previously not been fully understood
We show that Amazon basin‐intrinsic processes control interannual variation in δ18OTR, with upstream rainout the most important factor
Our results show that δ18OTR can be reliably used to reconstruct Amazon precipitation, with wider implications for other δ18O proxy records
Forest inventory studies in the Amazon indicate a large terrestrial carbon sink. However, field plots may fail to represent forest mortality processes at landscape-scales of tropical forests. Here we ...characterize the frequency distribution of disturbance events in natural forests from 0.01 ha to 2,651 ha size throughout Amazonia using a novel combination of forest inventory, airborne lidar and satellite remote sensing data. We find that small-scale mortality events are responsible for aboveground biomass losses of ~1.7 Pg C y(-1) over the entire Amazon region. We also find that intermediate-scale disturbances account for losses of ~0.2 Pg C y(-1), and that the largest-scale disturbances as a result of blow-downs only account for losses of ~0.004 Pg C y(-1). Simulation of growth and mortality indicates that even when all carbon losses from intermediate and large-scale disturbances are considered, these are outweighed by the net biomass accumulation by tree growth, supporting the inference of an Amazon carbon sink.
Competition among trees is an important driver of community structure and dynamics in tropical forests. Neighboring trees may impact an individual tree’s growth rate and probability of mortality, but ...large-scale geographic and environmental variation in these competitive effects has yet to be evaluated across the tropical forest biome. We quantified effects of competition on tree-level basal area growth and mortality for trees ≥10-cm diameter across 151 ~1-ha plots in mature tropical forests in Amazonia and tropical Africa by developing nonlinear models that accounted for wood density, tree size, and neighborhood crowding. Using these models, we assessed how water availability (i.e., climatic water deficit) and soil fertility influenced the predicted plot-level strength of competition (i.e., the extent to which growth is reduced, or mortality is increased, by competition across all individual trees). On both continents, tree basal area growth decreased with wood density and increased with tree size. Growth decreased with neighborhood crowding, which suggests that competition is important. Tree mortality decreased with wood density and generally increased with tree size, but was apparently unaffected by neighborhood crowding. Across plots, variation in the plot-level strength of competition was most strongly related to plot basal area (i.e., the sum of the basal area of all trees in a plot), with greater reductions in growth occurring in forests with high basal area, but in Amazonia, the strength of competition also varied with plot-level wood density. In Amazonia, the strength of competition increased with water availability because of the greater basal area of wetter forests, but was only weakly related to soil fertility. In Africa, competition was weakly related to soil fertility and invariant across the shorter water availability gradient. Overall, our results suggest that competition influences the structure and dynamics of tropical forests primarily through effects on individual tree growth rather than mortality and that the strength of competition largely depends on environment-mediated variation in basal area.
Understanding the processes that determine above‐ground biomass (AGB) in Amazonian forests is important for predicting the sensitivity of these ecosystems to environmental change and for designing ...and evaluating dynamic global vegetation models (DGVMs). AGB is determined by inputs from woody productivity woody net primary productivity (NPP) and the rate at which carbon is lost through tree mortality. Here, we test whether two direct metrics of tree mortality (the absolute rate of woody biomass loss and the rate of stem mortality) and/or woody NPP, control variation in AGB among 167 plots in intact forest across Amazonia. We then compare these relationships and the observed variation in AGB and woody NPP with the predictions of four DGVMs. The observations show that stem mortality rates, rather than absolute rates of woody biomass loss, are the most important predictor of AGB, which is consistent with the importance of stand size structure for determining spatial variation in AGB. The relationship between stem mortality rates and AGB varies among different regions of Amazonia, indicating that variation in wood density and height/diameter relationships also influences AGB. In contrast to previous findings, we find that woody NPP is not correlated with stem mortality rates and is weakly positively correlated with AGB. Across the four models, basin‐wide average AGB is similar to the mean of the observations. However, the models consistently overestimate woody NPP and poorly represent the spatial patterns of both AGB and woody NPP estimated using plot data. In marked contrast to the observations, DGVMs typically show strong positive relationships between woody NPP and AGB. Resolving these differences will require incorporating forest size structure, mechanistic models of stem mortality and variation in functional composition in DGVMs.
Large‐scale (>500 km) spatial gradients of precipitation oxygen isotope ratios (δ18Op) hold information about the hydrological cycle. They result from the interplay between rainout and ...evapotranspiration along air‐parcel paths, but these counteracting effects are difficult to disentangle, complicating quantification of the effect of land cover change on δ18Op. We show that disentangling can qualitatively be achieved using climate model simulations with a land‐derived precipitation tracer for tropical South America. We then either vary land cover as observed since 1870 or replace Amazon forests with bare land to determine the resulting signals. Our results indicate that effects of historically changing land cover on annual mean δ18O isotope‐ratio gradients are small and unlikely detectable, although there is a noticeable signal during the dry season. Furthermore, the effect of changes in water recycling on Amazon δ18Op in paleo‐records may have been overestimated and need reinterpretation.
Plain Language Summary
Deforestation causes reduction in precipitation downwind because trees act as pumps of water from soils to the atmosphere. This mechanism is primarily important during the dry season. How strong this effect is currently in the Amazon, given that approximately 20% of the forests have been cut, and how important it may be in the future if more forests are being destroyed is of great interest. One indicator of such changes is the east‐west difference in heavy water isotope content of precipitation. While preferential rainout of the heavy isotope along air parcel trajectories enhances this difference, transpiration by forests decreases the difference. This is because forests inject water back into the atmosphere that is more enriched than the overlying water vapor. Records of this difference during the last ice age, in particular, have been interpreted in a previous study as providing information on continental recycling. We apply a land‐derived water tagging approach in model simulations to investigate the effect of continental recycling on precipitation isotope content and to estimate this effect for varying land cover. We find that a 20% deforestation has only a small impact on precipitation isotope content. Even for a complete deforestation, in contrast to a previous interpretation, thus, only some of the isotopic signal observed during the ice age can be attributed to changes in continental recycling.
Key Points
We simulated Amazon δ18Op and continental water recycling with and without forest cover
The impact of forest removal on annual mean δ18Op is small relative to natural variability
The large observed change in observed paleo‐record δ18Op is unlikely due to substantial changes in the Amazon vegetation
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