The atmospheric effects of Amazon deforestation have frequently been studied in the context of small scales (≈1 km) and very large scales (hundreds of kilometers). However, analysis of ...intermediate‐scale deforestation (tens of kilometers) has received less attention, despite the fact that it better represents the contemporary landscape in some parts of the Amazon. In this study, the dynamic and thermodynamic effects of contemporary intermediate‐scale deforestation in Rondônia, Brazil are investigated through variable resolution Global Circulation Model (GCM) simulations carried out with the Ocean‐Land‐Atmosphere Model. In particular, the atmospheric response to surface roughness changes brought about by deforestation is emphasized. This study shows that reductions in surface roughness associated with intermediate‐scale deforestation give rise to a mesoscale circulation. This circulation is capable of convective triggering, but it also weakens the turbulent exchange of energy between land and atmosphere. Furthermore, this mesoscale circulation has distinct impacts on the hydroclimates of the western and eastern halves of Rondônia, increasing shallow cloudiness in the former while suppressing it in the latter. These results show that the atmospheric response to contemporary intermediate‐scale deforestation in Rondônia is likely to be more influenced by differences in surface roughness between forest and forest clearings than by the differences in the surface energy partitioning.
Key Points
Deforestation‐induced surface roughness changes trigger a mesoscale circulationThis induces contrasting hydroclimates in different parts of the deforested areaThis circulation is weakly affected by sensible heat variations
Lianas are thought to be increasing and altering tree growth and ecosystem productivity in tropical forests, but less research has focused on secondary or seasonally dry tropical forest. We report on ...an 11-year study of tree growth and liana presence from Guanacaste, Costa Rica, where we measured the diameter growth and liana presence on more than 1,700 trees in regenerating forest of different ages. We find that the proportion of trees without lianas is decreasing and the number of trees with lianas occupying more than 10% of tree’s crowns is increasing. We also find that lianas are affecting the diameter growth of trees. The 11-year average relative growth rates of trees with lianas in more than 10% of the tree’s crown are lower than the relative growth of trees with no lianas or lianas in less than 10% of their crown. Year-to-year, tree relative growth rate is related to annual precipitation and tree diameter. However, trees that were heavily infested with lianas (i.e., with lianas in more than 50% of their crowns) had lower relative growth and a weaker precipitation-growth relationship. This work underscores the value of long-term longitudinal data in secondary forest and adds critical data on dry forest liana abundance change.
Future projections suggest an increase in drought globally with climate change. Current vegetation models typically regulate the plant photosynthetic response to soil moisture stress through an ...empirical function, rather than a mechanistic response where plant water potentials respond to changes in soil water. This representation of soil moisture stress may introduce significant uncertainty into projections for the terrestrial carbon cycle. We examined the use of the soil moisture limitation function in historical and future emissions scenarios in nine Earth system models. We found that soil moisture‐limited productivity across models represented a large and uncertain component of the simulated carbon cycle, comparable to 3–286% of current global productivity. Approximately 40–80% of the intermodel variability was due to the functional form of the limitation equation alone. Our results highlight the importance of implementing mechanistic water limitation schemes in models and illuminate several avenues for improving projections of the land carbon sink.
Plain Language Summary
Understanding the environmental controls of terrestrial ecosystem productivity is of critical importance because terrestrial ecosystems directly impact the concentration of CO2 in the atmosphere. However, model projections disagree on the future sign and magnitude of terrestrial ecosystem CO2 drawdown, so it is uncertain if terrestrial ecosystems will continue to mitigate climate change in the future. Here we show that the current representation of water‐limited productivity across state‐of‐the‐art vegetation models is a large and uncertain component of terrestrial productivity, comparable in magnitude to current global productivity. Our results provide a foundation for improved projections of climate change impacts on terrestrial ecosystems, ranging from vegetation growth to agricultural productivity.
Key Points
Most global vegetation models represent plant water limitation with a rarely tested empirical function based solely on soil moisture
Carbon cycle uncertainty associated with such soil moisture stress functions is comparable to current global gross primary productivity
Forty to eighty percent of the soil water stress‐driven uncertainty in productivity among models is due to the functional form of the stress equation alone
This study investigates the possibility of changes in daily scale solar radiation and precipitation variability. Coefficients of variation (CVs) were computed for the daily downward surface solar ...radiation product from the International Satellite Cloud Climatology Project and the daily precipitation product from the Global Precipitation Climatology Project. Regression analysis was used to identify trends in CVs. Statistically significant changes in solar radiation variability were found for 35% of the globe, and particularly large increases were found for tropical Africa and the Maritime Continent. These increases in solar radiation variability were correlated with increases in precipitation variability and increases in deep convective cloud amount. The changes in high-frequency climate variability identified here have consequences for any process depending nonlinearly on climate, including solar energy production and terrestrial ecosystem photosynthesis. To assess these consequences, additional work is needed to understand how high-frequency climate variability will change in the coming decades.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Tropical forests provide important ecosystem services in maintaining biodiversity, sequestering carbon and regulating climate regionally and globally. Climate triggers the seasonal transitions of ...vegetation structure and function in tropical forests. In turn, the seasonal cycles of structure and function in tropical forests feed back to the climate system through the control of land-atmosphere exchange of carbon, water and energy fluxes. Large uncertainties exist in the carbon and water budgets of tropical forests, and environmental controls on phenology are among the least understood factors. Although field studies have identified patterns in the environmental controls on local-scale species-level phenology in the tropics, there is little consensus on large-scale top-down environmental controls on whole-ecosystem seasonality. In this paper, we use both optical and microwave remote sensing to investigate the seasonality of vegetation canopy structure and function in three distinct tropical African forest types, and identify environmental triggers or controls of their variability. For most tropical forests that have a closed canopy and high leaf biomass, optical remote sensing (e.g., vegetation indices) captures canopy photosynthetic capacity (i.e., canopy function), while small-wavelength microwave remote sensing characterizes the leaf biomass and leaf water content of the upper canopy (i.e., canopy structure). Our results reveal a strong coupling of canopy structure with canopy function in the tropical deciduous forests and woody savannas, and their seasonalities are both controlled by precipitation rather than solar radiation. By contrast, tropical evergreen forests in Africa exhibit a decoupling of canopy structure from canopy function revealed by different sensors: canopy photosynthetic capacity shown by the optical remote sensing is linked to the seasonal variation of precipitation, while microwave remote sensing captures semi-annual leaf-flushing that is synchronous with peak insolation intensity at the top of the atmosphere, which is bimodal. The differential coupling of canopy structure and function in tropical forests observed from remote sensing highlights differences inherent in distinct vegetation types within the tropics that may originate in the different life histories of their respective floras. This satellite-based finding encourages more field-based studies to clarify the interpretation of these large scale patterns.
In the central Amazon, surrounding Manaus City, Brazil, the landscape dominated by forest and water bodies supports development of river breezes. Seasonal inundations modify this local scenery ...affecting, consequently, the local circulations. However, this effect has not properly been investigated yet. Thus, we carried out numerical experiments for a river breeze case to investigate the seasonal flooding effect on the river breeze and detail the processes involved in the river breeze development. In the numerical experiments, we ran the Catchment‐Based Macro‐scale Floodplain (CaMa‐Flood) model to simulate flooding depth and extent and used the CaMa‐Flood outputs to force the Ocean‐Land‐Atmosphere Model. The seasonal flooding alters the surface energy partitioning causing a temperature decrease over the river region and intensification of the river breezes in the daytime. The intensified river breezes propagate more rapidly through the upland region, take longer to dissipate and promote stronger upward vertical motion altering the heat and mass transport. These novel findings are fundamentally important to the understanding of the local climate variability of the central Amazon. We can infer that river breezes and its consequence to local climate are less (more) pronounced in drought (wet) years.
Key Points
Different energy partitioning of the water and land surface triggers the river breezes in the central Amazon
Seasonal flooding intensifies the river breezes by altering the surface energy partitioning
The roughness variation between the floodplain and the upland regions has no clear influence on the displacement of the river breeze fronts
Soil nutrients, especially nitrogen (N) and phosphorus (P), regulate plant growth and hence influence carbon fluxes between the land surface and atmosphere. However, how forests adjust biomass ...partitioning to leaves, wood, and fine roots in response to N and/or P fertilization remains puzzling. Recent work in tropical forests suggests that trees increase fine root production under P fertilization, but it is unclear whether mechanistic models can reproduce this dynamic. In order to better understand mechanisms governing nutrient effects on plant allocation and improve models, we used the nutrient-enabled ED2 model to simulate a fertilization experiment being conducted in a secondary tropical dry forest in Costa Rica. We evaluated how different allocation parameterizations affected model performance. These parameterizations prescribed a linear relationship between relative allocation to fine roots and soil P concentrations. The slope of the linear relationship was allowed to be positive, negative, or zero. Some parameterizations realistically simulated leaf, wood, and fine root production, and these parameterizations all assumed a positive relationship between relative allocation to fine roots and soil P concentration. Model simulations of a 30-year timeframe indicated strong sensitivity to parameterization and fertilization treatment. Without P fertilization, the simulated aboveground biomass (AGB) accumulation was insensitive to the parameterization. With P fertilization, the model was highly sensitive to the parameterization and the greatest AGB accumulation occurred when relative allocation to fine roots was independent of soil P. Our study demonstrates the need for simultaneous measurements of leaf, wood, and fine root production in nutrient fertilization experiments and for longer-term experiments. Models that do not accurately represent allocation to fine roots may be highly biased in their simulations of AGB, especially on multi-decadal timescales.
Amazonian deforestation causes systematic changes in regional dry season precipitation. Some of these changes at contemporary large scales (a few hundreds of kilometers) of deforestation have been ...associated with a “dynamical mesoscale circulation,” induced by the replacement of rough forest with smooth pasture. In terms of decadal averages, this dynamical mechanism yields increased precipitation in downwind regions and decreased precipitation in upwind regions of deforested areas. Daily, seasonal, and interannual variations in this phenomenon may exist but have not yet been identified or explained. This study uses observations and numerical simulations to develop relationships between the dynamical mechanism and the local‐ and continental‐scale atmospheric conditions across a range of time scales. It is found that the strength of the dynamical mechanism is primarily controlled by the regional‐scale thermal and dynamical conditions of the boundary layer and not by the continental‐ and global‐scale atmospheric state. Lifting condensation level and wind speed within the boundary layer have large and positive correlations with the strength of the dynamical mechanism. The strength of these relationships depends on time scale and is strongest over the seasonal cycle. Overall, the dynamical mechanism is found to be strongest during times when the atmosphere is relatively stable. Hence, for contemporary large scales of deforestation this phenomenon is found to be the prevalent convective triggering mechanism during the dry and parts of transition seasons (especially during the dry‐to‐wet transition), significantly affecting the hydroclimate during this period.
Key Points
Contemporary Amazonian deforestation causes a regional spatial redistribution of precipitation during the local dry and transition months
This phenomenon is controlled by and is positively correlated with regional conditions like boundary layer stability and wind speed
This control of boundary layer conditions on precipitation redistribution is robustly observed at daily and seasonal time scales
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