Mangroves shift from carbon sinks to sources when affected by anthropogenic land‐use and land‐cover change (LULCC). Yet, the magnitude and temporal scale of these impacts are largely unknown. We ...undertook a systematic review to examine the influence of LULCC on mangrove carbon stocks and soil greenhouse gas (GHG) effluxes. A search of 478 data points from the peer‐reviewed literature revealed a substantial reduction of biomass (82% ± 35%) and soil (54% ± 13%) carbon stocks due to LULCC. The relative loss depended on LULCC type, time since LULCC and geographical and climatic conditions of sites. We also observed that the loss of soil carbon stocks was linked to the decreased soil carbon content and increased soil bulk density over the first 100 cm depth. We found no significant effect of LULCC on soil GHG effluxes. Regeneration efforts (i.e. restoration, rehabilitation and afforestation) led to biomass recovery after ~40 years. However, we found no clear patterns of mangrove soil carbon stock re‐establishment following biomass recovery. Our findings suggest that regeneration may help restore carbon stocks back to pre‐disturbed levels over decadal to century time scales only, with a faster rate for biomass recovery than for soil carbon stocks. Therefore, improved mangrove ecosystem management by preventing further LULCC and promoting rehabilitation is fundamental for effective climate change mitigation policy.
We undertook a systematic review to examine the influence of land‐use and land‐cover change (LULCC) on mangrove carbon stocks and soil greenhouse gas effluxes. A search of 478 data points from peer‐reviewed literature revealed a substantial reduction of biomass (82% ± 35%) and soil (54% ± 13%) carbon stocks due to LULCC. The relative loss depended on LULCC type, time since LULCC and geographical and climatic conditions of sites. Regeneration efforts (i.e. restoration, rehabilitation and plantation) led to biomass recovery after ~40 years. However, we found no clear patterns of mangrove soil carbon stocks re‐establishment following biomass recovery.
To date, discourse associated with the potential application of “blue carbon” within real-world carbon markets has focused on blue carbon as a mitigation strategy in the context of avoided ...deforestation (e.g., REDD+). Here, we report structural dynamics and carbon storage gains from mangrove sites that have undergone rehabilitation to ascertain whether reforestation can complement conservation activities and warrant project investment. Replicated sites at two locations with contrasting geomorphic conditions were selected, Tiwoho and Tanakeke on the island of Sulawesi, Indonesia. These locations are representative of high (Tiwoho, deep muds and silty substrates) and low (Tanakeke, shallow, coralline sands) productivity mangrove ecosystems. They share a similar management history of clearing and conversion for aquaculture before restorative activities were undertaken using the practice of Ecological Mangrove Rehabilitation (EMR). Species diversity and mean biomass carbon storage gains after 10 yr of regrowth from the high productivity sites of Tiwoho (49.2 ± 9.1 Mg C·ha-1·yr-1) are already almost of one-third of mean biomass stocks exhibited by mature forests (167.8 ± 30.3 Mg C·ha-1·yr-1). Tiwoho’s EMR sites, on average, will have offset all biomass C that was initially lost through conversion within the next 11 yr, a finding in marked contrast to the minimal carbon gains observed on the low productivity, low diversity, coral atoll EMR sites of Tanakeke (1.1 ± 0.4 Mg C·ha-1·yr-1). These findings highlight the importance of geomorphic and biophysical site selection if the primary purpose of EMR is intended to maximize carbon sequestration gains.
Globally, forests are facing an increasing risk of mass tree mortality events associated with extreme droughts and higher temperatures. Hydraulic dysfunction is considered a key mechanism of ...drought‐triggered dieback. By leveraging the climate breadth of the Australian landscape and a national network of research sites (Terrestrial Ecosystem Research Network), we conducted a continental‐scale study of physiological and hydraulic traits of 33 native tree species from contrasting environments to disentangle the complexities of plant response to drought across communities. We found strong relationships between key plant hydraulic traits and site aridity. Leaf turgor loss point and xylem embolism resistance were correlated with minimum water potential experienced by each species. Across the data set, there was a strong coordination between hydraulic traits, including those linked to hydraulic safety, stomatal regulation and the cost of carbon investment into woody tissue. These results illustrate that aridity has acted as a strong selective pressure, shaping hydraulic traits of tree species across the Australian landscape. Hydraulic safety margins were constrained across sites, with species from wetter sites tending to have smaller safety margin compared with species at drier sites, suggesting trees are operating close to their hydraulic thresholds and forest biomes across the spectrum may be susceptible to shifts in climate that result in the intensification of drought.
Forests face an increasing risk of extreme droughts and high temperatures under a changing climate. To understand the vulnerability of Australia's native forests to drought‐induced hydraulic dysfunction, we conducted a continental‐scale study of physiological and hydraulic traits from tree species growing in contrasting environments. We found (1) aridity strongly influences plant hydraulic traits; (2) hydraulic vulnerability (P50) tightly couples with the plant water stress experienced by each species (Ψmin); and (3) hydraulic safety margins provide little protection, particularly for wet habitat species, suggesting many forest biomes may be susceptible to climate shifts.
Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found ...that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.
Accurate phenological characterization of dryland ecosystems has remained a challenge due to the complex composition of plant functional types, each having distinct phenological dynamics, sensitivity ...to climate, and disturbance. Solar‐Induced chlorophyll Fluorescence (SIF), a proxy for photosynthesis, offers potential to alleviate such challenge. We here explore this potential using dryland systems along the North Australian Tropical Transect with SIF derived from Orbiting Carbon Observatory‐2. SIF identified the seasonal onset and senescence of Gross Primary Production at eddy covariance sites with improved accuracy over Enhanced Vegetation Index and Near‐Infrared Reflectance of terrestrial Vegetation from Moderate Resolution Imaging Spectroradiometer, especially at inland xeric shrublands. At regional scale, SIF depicted both earlier onset and senescence across North Australian Tropical Transect. We hypothesized that SIF outperformed Enhanced Vegetation Index and Near‐Infrared Reflectance of terrestrial Vegetation mainly because, unlike reflectance, it is not contaminated by background soil, and its total signal is contributed by mixed plant species in additive way.
Plain Language Summary
Australian dryland ecosystems are critical in regulating the global land carbon sink dynamics. However, it is challenging to accurately characterize their phenology from spaceborne measurements. On the one hand, tropical savannas and semiarid ecosystems (e.g., grasslands and shrublands) are typically composed of a complex mixture of species (woody trees and C4 grasses) with each having distinct morphologies and physiological responses to climate condition; on the other hand, such ecosystems are highly sensitive to irregular rainfall events and are often subject to disturbances such as fires and storms. In this study, we utilized the North Australian Tropical Transect rainfall gradient as a “natural laboratory” to assess the ability of satellite solar‐induced chlorophyll fluorescence to capture the phenological dynamics of dryland vegetation, in comparison with traditional reflectance‐based vegetation indices, that is, Enhanced Vegetation Index and Near‐Infrared Reflectance of terrestrial Vegetation. Results showed that satellite solar‐induced chlorophyll fluorescence outperformed Enhanced Vegetation Index and Near‐Infrared Reflectance of terrestrial Vegetation for characterizing seasonal onset and senescence along North Australian Tropical Transect and therefore had potential for improving large‐scale mapping of phenology dynamics of dryland ecosystems over traditional remote sensing of reflectance‐based vegetation indices.
Key Points
Satellite SIF matches seasonal phenology of eddy covariance GPP better than EVI and NIRv along a rainfall gradient in northern Australia
Increasing soil exposure in inland xeric shrublands deteriorates the capability of satellite EVI and NIRv for phenology characterization
High‐resolution satellite SIF is needed to accurately depict phenology in heterogeneous landscapes with complex vegetation‐soil mosaic
The phenology of a landscape is a key parameter in climate and biogeochemical cycle models and its correct representation is central to the accurate simulation of carbon, water and energy exchange ...between the land surface and the atmosphere. Whereas biogeographic phenological patterns and shifts have received much attention in temperate ecosystems, much less is known about the phenology of savannas, despite their sensitivity to climate change and their coverage of approximately one eighth of the global land surface. Savannas are complex assemblages of multiple tree, shrub, and grass vegetation strata, each with variable phenological responses to seasonal climate and environmental variables. The objectives of this study were to investigate biogeographical and inter-annual patterns in savanna phenology along a 1100km ecological rainfall gradient, known as North Australian Tropical Transect (NATT), encompassing humid coastal Eucalyptus forests and woodlands to xeric inland Acacia woodlands and shrublands. Key phenology transition dates (start, peak, end, and length of seasonal greening periods) were extracted from 13years (2000–2012) of Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) data using Singular Spectrum Analysis (SSA).
Two distinct biogeographical patterns in phenology were observed, controlled by different climate systems. The northern (mesic) portion of the transect, from 12°S, to around 17.7°S, was influenced by the Inter-Tropical Convergence Zone (ITCZ) seasonal monsoon climate system, resulting in strong latitudinal shifts in phenology patterns, primarily associated with the functional response of the C4 grass layer. Both the start and end of the greening (enhanced vegetation activity) season occurred earlier in the northern tropical savannas and were progressively delayed towards the southern limit of the Eucalyptus-dominated savannas resulting in relatively stable length of greening periods. In contrast, the southern xeric portion of the study area was largely decoupled from monsoonal influences and exhibited highly variable phenology that was largely rainfall pulse driven. The seasonal greening periods were generally shorter but fluctuated widely from no detectable greening during extended drought periods to length of greening seasons that exceeded those in the more mesic northern savannas in some wet years. This was in part due to more extreme rainfall variability, as well as a C3/C4 grass-forb understory that provided the potential for extended greening periods. Phenology of Acacia dominated savannas displayed a much greater overall responsiveness to hydroclimatic variability. The variance in annual precipitation alone could explain 80% of the variances in the length of greening season across the major vegetation groups. We also found that increased variation in the timing of phenology was coupled with a decreasing tree-grass ratio. We further compared the satellite-based phenology results with tower-derived measures of Gross Ecosystem Production (GEP) fluxes at three sites over two contrasting savanna classes. We found good convergence between MODIS EVI and tower GEP, thereby confirming the potential to link these two independent data sources to better understand savanna ecosystem functioning.
•We retrieved wet to dry savanna phenology over North Australia with MODIS.•Understorey dynamics controlled landscape phenology throughout the transect.•We found large inter-annual variations in rainfall & phenology in southern savannas.•MODIS EVI & tower GPP matched well, in both Eucalyptus and Acacia savannas.
Termites are responsible for ∼1 to 3%of global methane (CH₄) emissions. However, estimates of global termite CH₄ emissions span two orders of magnitude, suggesting that fundamental knowledge of CH₄ ...turnover processes in termite colonies is missing. In particular, there is little reliable information on the extent and location of microbial CH₄ oxidation in termite mounds. Here, we use a one-box model to unify three independent field methods—a gas-tracer test, an inhibitor approach, and a stable-isotope technique—and quantify CH₄ production, oxidation, and transport in three North Australian termite species with different feeding habits and mound architectures. We present systematic in situ evidence of widespread CH₄ oxidation in termite mounds, with 20 to 80% of termite-produced CH₄ being mitigated before emission to the atmosphere. Furthermore, closing the CH₄ mass balance in mounds allows us to estimate in situ termite biomass from CH₄ turnover, with mean biomass ranging between 22 and 86 g of termites per kilogram of mound for the three species. Field tests with excavated mounds show that the predominant location of CH₄ oxidation is either in the mound material or the soil beneath and is related to species-specific mound porosities. Regardless of termite species, however, our data and model suggest that the fraction of oxidized CH₄ (f
ox) remains well buffered due to links among consumption, oxidation, and transport processes via mound CH₄ concentration. The mean f
ox of 0.50 ± 0.11 (95% CI) from in situ measurements therefore presents a valid oxidation factor for future global estimates of termite CH₄ emissions.
Despite their size and contribution to the global carbon cycle, we have limited understanding of tropical savannas and their current trajectory with climate change and anthropogenic pressures. Here ...we examined interannual variability and externally forced long‐term changes in carbon and water exchange from a high rainfall savanna site in the seasonal tropics of north Australia. We used an 18‐year flux data time series (2001–2019) to detect trends and drivers of fluxes of carbon and water. Significant positive trends in gross primary productivity (GPP, 15.4 g C m2 year−2), ecosystem respiration (Reco, 8.0 g C m2 year−2), net ecosystem productivity (NEE, 7.4 g C m2 year−2) and ecosystem water use efficiency (WUE, 0.0077 g C kg H2O−1 year−1) were computed. There was a weaker, non‐significant trend in latent energy exchange (LE, 0.34 W m−2 year−1). Rainfall from a nearby site increased statistically over a 45‐year period during the observation period. To examine the dominant drivers of changes in GPP and WUE, we used a random forest approach and a terrestrial biosphere model to conduct an attribution experiment. Radiant energy was the dominant driver of wet season fluxes, whereas soil water content dominated dry season fluxes. The model attribution suggested that CO2, precipitation and Tair accounting for 90% of the modelled trend in GPP and WUE. Positive trends in fluxes were largest in the dry season implying tree components were a larger contributor than the grassy understorey. Fluxes and environmental drivers were not significant during the wet season, the period when grasses are active. The site is potentially still recovering from a cyclone 45 years ago and regrowth from this event may also be contributing to the observed trends in sequestration, highlighting the need to understand fluxes and their drivers from sub‐diurnal to decadal scales.
Long‐term eddy covariance measures over an 18‐year period show positive trends in carbon fluxes, water use and radiation efficiencies (WUE, RUE) as well as an index of growing season length (GS). The climate and phenology of the region are strongly seasonal and variables with significant trends in each season are shown. Trends in fluxes were largest in the dry season, implying that woody components were a larger contributor to the trends in fluxes than the grassy understorey. The trends were also reproduced using a mechanistic model and a simple attribution experiment suggested CO2 fertilization and a weak wetting trend were the likely drivers of the observed trends.
This study investigates the underlying climate processes behind the largest recorded mangrove dieback event along the Gulf of Carpentaria coast in northern Australia in late 2015. Using ...satellite-derived fractional canopy cover (FCC), variation of the mangrove canopies during recent decades are studied, including a severe dieback during 2015-2016. The relationship between mangrove FCC and climate conditions is examined with a focus on the possible role of the 2015-2016 El Niño in altering favorable conditions sustaining the mangroves. The mangrove FCC is shown to be coherent with the low-frequency component of sea level height (SLH) variation related to the El Niño Southern Oscillation (ENSO) cycle in the equatorial Pacific. The SLH drop associated with the 2015-2016 El Niño is identified to be the crucial factor leading to the dieback event. A stronger SLH drop occurred during austral autumn and winter, when the SLH anomalies were about 12% stronger than the previous very strong El Niño events. The persistent SLH drop occurred in the dry season of the year when SLH was seasonally at its lowest, so potentially exposed the mangroves to unprecedented hostile conditions. The influence of other key climate factors is also discussed, and a multiple linear regression model is developed to understand the combined role of the important climate variables on the mangrove FCC variation.
PDF
