The relative importance of dispersal limitation versus environmental filtering for community assembly has received much attention for macroorganisms. These processes have only recently been examined ...in microbial communities. Instead, microbial dispersal has mostly been measured as community composition change over space (i.e., distance decay). Here we directly examined fungal composition in airborne wind currents and soil fungal communities across a 40 000 km2 regional landscape to determine if dispersal limitation or abiotic factors were structuring soil fungal communities. Over this landscape, neither airborne nor soil fungal communities exhibited compositional differences due to geographic distance. Airborne fungal communities shifted temporally while soil fungal communities were correlated with abiotic parameters. These patterns suggest that environmental filtering may have the largest influence on fungal regional community assembly in soils, especially for aerially dispersed fungal taxa. Furthermore, we found evidence that dispersal of fungal spores differs between fungal taxa and can be both a stochastic and deterministic process. The spatial range of soil fungal taxa was correlated with their average regional abundance across all sites, which may imply stochastic dispersal mechanisms. Nevertheless, spore volume was also negatively correlated with spatial range for some species. Smaller volume spores may be adapted to long-range dispersal, or establishment, suggesting that deterministic fungal traits may also influence fungal distributions. Fungal life-history traits may influence their distributions as well. Hypogeous fungal taxa exhibited high local abundance, but small spatial ranges, while epigeous fungal taxa had lower local abundance, but larger spatial ranges. This study is the first, to our knowledge, to directly sample air dispersal and soil fungal communities simultaneously across a regional landscape. We provide some of the first evidence that soil fungal communities are mostly assembled through environmental filtering and experience little dispersal limitation.
•We examined fungal composition of air and soil communities at the regional scale.•Neither airborne nor soil fungal communities varied with geographic distance.•Airborne fungi shifted over time and soil fungi with environmental parameters.•Spatial range of soil fungal OTUs correlated with their average regional abundance.•Fungal spore volume was negatively correlated with spatial range.
Feedbacks between the intertwined water and carbon cycles in semi‐arid mountain ecosystems can introduce large uncertainties into projections of carbon storage. In this study, we sought to understand ...the influence of key mechanisms on carbon balances, focusing on an ecosystem whose complex terrain and large interannual variability in precipitation adds to its vulnerability to warming. We applied a dynamic vegetation‐ecosystem model (Lund‐Potsdam‐Jena General Ecosystem Simulator) to simulate water‐carbon interactions in the 104,512 km2 Mediterranean‐climate ecosystems of California's Sierra Nevada for 1950–2099. Our 48 scenarios include a combination of carbon dioxide (CO2) increase, air temperature change, and varying plant rooting depths. We found that with warming (+2 and +5°C), water limitations on growth and enhanced soil respiration reduce carbon storage; however, CO2 fertilization and associated enhanced water‐use efficiency offset this loss. Using the 4 km model resolution to capture steep mountain precipitation gradients, plus accounting for the several meters of actual root‐accessible water storage in the region, were also important. With warming accompanied by CO2 fertilization our projections show that the Sierra Nevada sequestering at least 200 Tg (2 kg m−2) carbon, versus carbon loss with warming alone. The increase reflects coniferous forests growing at high elevations, and some increase in broadleaved forests at low‐to‐intermediate elevations. Importantly, uncertainty in fire disturbance could shift our finding from carbon sink to source. The improved mechanistic understanding of these feedbacks can advance modeling of carbon‐water interactions in mountain‐ecosystem under a warmer and potentially drier climate.
Plain Language Summary
As we continue to improve our understanding of the North American or even global carbon pool, it is important to ask whether semi‐arid mountain ecosystems, such as California's Sierra Nevada, will be carbon sinks or sources as climate warms. Numerical models used to answer this question can have large uncertainties associated with input data, climate scenarios, and extent to which higher atmospheric carbon dioxide (CO2) enhances vegetation growth. To explore these limitations, we used a fine spatial resolution (4 km) to better represent the steep mountain terrain, accounted for the deep soil‐water storage that sustains forest growth during dry periods, and carried out simulations covering a range of climate scenarios. We found that Sierra Nevada ecosystems are projected to add 200 Tg or more carbon through the end of this century, a small but important increase (∼8%). Coniferous forests at higher elevations dominate the increase of carbon, and broadleaved forests at low‐to‐intermediate elevations also contribute. Soil carbon decreases with warming; however, increases in vegetation carbon offset the decrease through enhanced water‐use efficiency and vegetation growth under elevated CO2. By explicitly accounting for these uncertainties, we project that Mediterranean mountain ecosystems such as the Sierra Nevada can store additional carbon as climate warms.
Key Points
Separating warming effects versus carbon dioxide (CO2) enhancement of water‐use efficiency explain elevation‐dependent mechanisms affecting net carbon uptake
Warming‐driven forest expansion at higher and CO2‐enhancement effects at lower water‐limited elevations dominate increases in carbon uptake
Deep root‐accessible water provides dry‐period carryover storage, supporting carbon uptake that more than offsets respiration increases
The increase in deciduous shrub growth in response to climate change throughout the Arctic tundra has uncertain implications, in part due to a lack of field observations. Here we investigate how ...increasing alder shrub growth in alpine tundra in Interior Alaska corresponds to active layer thickness and soil physical properties. We documented increased alder growth by combining biomass harvests and dendrochronology with the analysis of remotely sensed Normalized Difference Vegetation Index and fire history. Active layer thickness was measured with a tile probe and carbon and nitrogen pools were assessed via elemental analysis. Shallower organic layers under increasing alder growth indicate that nitrogen-rich, deciduous litter inputs may play a role in accelerating decomposition. Despite the observed reduction in organic carbon stocks, active layer thickness was the same under alder and adjacent graminoid tundra, implying deeper thaw of the underlying mineral soil. This study provides further evidence that the widely observed expansion of deciduous shrubs into graminoid tundra will reduce ecosystem carbon stocks and intensify soil–atmosphere thermal coupling.
Vegetation tolerance to drought depends on an array of site-specific environmental and plant physiological factors. This tolerance is poorly understood for many forest types despite its importance ...for predicting and managing vegetation stress. We analyzed the relationships between precipitation variability and forest die-off in California's Sierra Nevada and introduce a new measure of drought tolerance that emphasizes plant access to subsurface moisture buffers. We applied this metric to California's severe 2012-2015 drought, and show that it predicted the patterns of tree mortality. We then examined future climate scenarios, and found that the probability of droughts that lead to widespread die-off increases threefold by the end of the 21st century. Our analysis shows that tree mortality in the Sierra Nevada will likely accelerate in the coming decades and that forests in the Central and Northern Sierra Nevada that largely escaped mortality in 2012-2015 are vulnerable to die-off.
Wildfire modifies the short- and long-term exchange of carbon between terrestrial ecosystems and the atmosphere, with impacts on ecosystem services such as carbon uptake. Dry western US forests ...historically experienced low-intensity, frequent fires, with patches across the landscape occupying different points in the fire-recovery trajectory. Contemporary perturbations, such as recent severe fires in California, could shift the historic stand-age distribution and impact the legacy of carbon uptake on the landscape. Here, we combine flux measurements of gross primary production (GPP) and chronosequence analysis using satellite remote sensing to investigate how the last century of fires in California impacted the dynamics of ecosystem carbon uptake on the fire-affected landscape. A GPP recovery trajectory curve of more than five thousand fires in forest ecosystems since 1919 indicated that fire reduced GPP by Formula: see text g C mFormula: see text yFormula: see text(Formula: see text) in the first year after fire, with average recovery to prefire conditions after Formula: see text y. The largest fires in forested ecosystems reduced GPP by Formula: see text g C mFormula: see text yFormula: see text (
= 401) and took more than two decades to recover. Recent increases in fire severity and recovery time have led to nearly Formula: see text MMT COFormula: see text (3-y rolling mean) in cumulative forgone carbon uptake due to the legacy of fires on the landscape, complicating the challenge of maintaining California's natural and working lands as a net carbon sink. Understanding these changes is paramount to weighing the costs and benefits associated with fuels management and ecosystem management for climate change mitigation.
Spatially explicit predictions of fuel moisture content are crucial for quantifying fire danger indices and as inputs to fire behaviour models. Remotely sensed predictions of fuel moisture have ...typically focused on live fuels; but regional estimates of dead fuel moisture have been less common. Here we develop and test the spatial application of a recently developed dead fuel moisture model, which is based on the exponential decline of fine fuel moisture with increasing vapour pressure deficit (D). We first compare the performance of two existing approaches to predict D from satellite observations. We then use remotely sensed D, as well as D estimated from gridded daily weather observations, to predict dead fuel moisture. We calibrate and test the model at a woodland site in South East Australia, and then test the model at a range of sites in South East Australia and Southern California that vary in vegetation type, mean annual precipitation (129–1404mmyear−1) and leaf area index (0.1–5.7). We found that D modelled from remotely sensed land surface temperature performed slightly better than a model which also included total precipitable water (MAE<1.16kPa and 1.62kPa respectively). D calculated with observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite was under-predicted in areas with low leaf area index. Both D from remotely sensed data and gridded weather station data were good predictors of the moisture content of dead suspended fuels at validation sites, with mean absolute errors less than 3.9% and 6.0% respectively. The occurrence of data gaps in remotely sensed time series presents an obstacle to this approach, and assimilated or extrapolated meteorological observations may offer better continuity.
•Dead fine fuel moisture content (FM) is crucial for modelling wildfire risk.•Vapour pressure deficit (D) is a good predictor of FM.•MODIS land surface temperature is a good predictor of D.•We used MODIS land surface temperature to model FM at a regional scale.•FM predictions of similar accuracy were achieved with gridded weather data.
Climate change is expected to increase drought intensity and frequency, which are commonly predicted will threaten the survival of forests. Most forest die‐off projections assume that recent tree ...mortality will not alter die‐off severity during subsequent droughts. We tested this assumption by comparing die‐off in semi‐arid conifer forest stands in California that were exposed to a single drought in 2012–2015 (“2nd Drought Only”) with forest stands that experienced drought in both 1999–2002 and 2012–2015 (“Both Droughts”). We quantified die‐off severity as a reduction in the satellite observed Normalized Difference Moisture Index, and cumulative moisture deficit as negative 4‐year Precipitation minus Evapotranspiration (4‐year Pr‐ET overdraft). Here we show that recent tree morality reduces die‐off severity in semi‐arid conifer forests exposed to subsequent drought. Stands in the 2nd Drought Only sample experienced severe die‐off associated with extreme 4‐year Pr‐ET overdraft in 2012–2015. Stands in the Both Droughts sample experienced severe die‐off and 4‐year Pr‐ET overdraft in 1999–2002, but comparatively little 2012–2015 die‐off despite continued 4‐year Pr‐ET overdraft. We interpret this as a dampening effect, where prior tree mortality reduces forest die‐off severity during subsequent drought exposure. As forests continue to experience disturbances linked to climate change, dampening effects will impose a transient, and perhaps long‐term, constraint on the impact of repeated drought.
Plain Language Summary
Climate change is expected to make droughts more frequent and drier. Widespread tree death in forests often occurs during particularly dry droughts. This has implications for the survival of forests in dry climates. Most predictions of future tree death assume that recent tree death will not change how forests respond to later drought. To check this assumption, we compared conifer forests in California that were exposed to two droughts separated by 13 years, with forests that were exposed to only the second drought. We used satellites to measure tree death and the amount of water available to trees. We found that forests that experienced only the second of the two droughts exhibited tree death during years with a lack of water availability. In contrast, forests that were exposed to both droughts exhibited a similar lack of water during each drought, with marked tree death only during the first drought. We interpret this as a dampening effect that leads forests with a recent history of drought exposure and tree death to experience less tree death during a later drought. As climate change continues to threaten forests, dampening effects in forests will limit the short‐term and perhaps long‐term impact of repeated drought.
Key Points
We tested the common assumption that repeated drought causes repeated die‐off in semi‐arid conifer forests
Forests exposed to one drought showed die‐off, while forests exposed to two droughts showed die‐off with the first, but not with the second
We interpret this as a dampening effect where forests with prior tree mortality experience decreased die‐off during subsequent drought
Nature‐based climate solutions are a vital component of many climate mitigation strategies, including California's, which aims to achieve carbon neutrality by 2045. Most carbon offsets in ...California's cap‐and‐trade program come from improved forest management (IFM) projects. Since 2012, various landowners have set up IFM projects following the California Air Resources Board's IFM protocol. As many of these projects approach their 10th year, we now have the opportunity to assess their effectiveness, identify best practices, and suggest improvements toward future protocol revisions. In this study, we used remote sensing‐based datasets to evaluate the carbon trends and harvest histories of 37 IFM projects in California. Despite some current limitations and biases, these datasets can be used to quantify carbon accumulation and harvest rates in offset project lands relative to nearby similar “control” lands before and after the projects began. Five lines of evidence suggest that the carbon accumulated in offset projects to date has generally not been additional to what might have otherwise occurred: (1) most forests in northwestern California have been accumulating carbon since at least the mid‐1980s and continue to accumulate carbon, whether enrolled in offset projects or not; (2) harvest rates were high in large timber company project lands before IFM initiation, suggesting they are earning carbon credits for forests in recovery; (3) projects are often located on lands with higher densities of low‐timber‐value species; (4) carbon accumulation rates have not yet increased on lands that enroll as offset projects, relative to their pre‐enrollment levels; and (5) harvest rates have not decreased on most project lands since offset project initiation. These patterns suggest that the current protocol should be improved to robustly measure and reward additionality. In general, our framework of geospatial analyses offers an important and independent means to evaluate the effectiveness of the carbon offsets program, especially as these data products continue improving and as offsets receive attention as a climate mitigation strategy.
Improved forest management (IFM) is a common form of carbon offsetting, where landowners receive credits for maintaining carbon stocks above a business‐as‐usual baseline. We used remote sensing datasets to investigate “additionality” in the IFM projects within California. We developed a framework for comparing project areas to similar control forests to estimate whether the carbon being stored in the projects is additional to what would otherwise occur.
•We present a new approach for modelling dead fine fuel moisture.•We validate our model across a wide range of ecosystem types.•Our model provides a significant improvement over previous models' ...performance.
The moisture content of vegetation and litter (fuel moisture) is an important determinant of fire risk, and predictions of dead fine fuel moisture content (fuel with a diameter <25.4mm) are particularly important. A variety of indices, as well as empirical and mechanistic models, have been proposed to predict fuel moisture, but these approaches have seldom been validated across temporally extensive datasets, or widely contrasting vegetation types. Here, we describe a semi-mechanistic model, based on the exponential decline of fuel moisture content with atmospheric vapor pressure deficit, that predicts daily minimum fuel moisture content. We calibrated the model at one site in New South Wales, Australia, and validated it at three contrasting ecosystem types in California, USA, where 10-h fuel moisture content was continuously measured every 30min over a year. We found that existing drought indices did not accurately predict fuel moisture, and that empirical and equilibrium models provided biased estimates. The mean absolute error (MAE) of the fuel moisture content predicted by our model across sites and years was 3.7%, which was substantially lower than for other, commonly used models. Our model’s MAE dropped to 2.9% when fuel moisture was below 20%, and to 1.8% when fuel moisture was below 10%. Our model’s MAE was comparable to instrumental MAE (3.1–2.5%), indicating that further improvement may be limited by measurement error. The simplicity, accuracy and precision of our model makes it suitable for a range of applications, such as operational fire management and the prediction of fire risk in vegetation models, without the need for site-specific calibrations.
Accurate descriptions of current ecosystem composition are essential for improving terrestrial biosphere model predictions of how ecosystems are responding to climate variability and change. This ...study investigates how imaging spectrometry‐derived ecosystem composition can constrain and improve terrestrial biosphere model predictions of regional‐scale carbon, water and energy fluxes. Incorporating imaging spectrometry‐derived composition of five plant functional types (Grasses/Shrubs, Oaks/Western Hardwoods, Western Pines, Fir/Cedar and High‐elevation Pines) into the Ecosystem Demography (ED2) terrestrial biosphere model improves predictions of net ecosystem productivity (NEP) and gross primary productivity (GPP) across four flux towers of the Southern Sierra Critical Zone Observatory (SSCZO) spanning a 2250 m elevational gradient in the western Sierra Nevada. NEP and GPP root‐mean‐square‐errors were reduced by 23%–82% and 19%–89%, respectively, and water flux predictions improved at the mid‐elevation pine (Soaproot), fir/cedar (P301) and high‐elevation pine (Shorthair) flux tower sites, but not at the oak savanna (San Joaquin Experimental Range SJER) site. These improvements in carbon and water predictions are similar to those achieved with model initializations using ground‐based inventory composition. The imaging spectrometry‐constrained ED2 model was then used to predict carbon, water and energy fluxes and above‐ground biomass (AGB) dynamics over a 737 km2 region to gain insight into the regional ecosystem impacts of the 2012–2015 Californian drought. The analysis indicates that the drought reduced regional NEP, GPP and transpiration by 83%, 40% and 33%, respectively, with the largest reductions occurring in the functionally diverse, high basal area mid‐elevation forests. This was accompanied by a 54% decline in AGB growth in 2012, followed by a marked increase (823%) in AGB mortality in 2014, reflecting an approximately 10‐fold increase in per capita tree mortality from ~55 trees km−2 year−1 in 2010–2011, to ~535 trees km−2 year−1 in 2014. These findings illustrate how imaging spectrometry estimates of ecosystem composition can constrain and improve terrestrial biosphere model predictions of regional carbon, water, and energy fluxes, and biomass dynamics.
This study uses ecosystem composition derived from airborne imaging spectrometry in the Sierra Nevada mountains, California, to constrain terrestrial biosphere model predictions of regional‐scale carbon and water fluxes and biomass dynamics. Over the 2012–2015 Californian drought, net ecosystem productivity (NEP) reduced by 83% and mortality increasing by more than 800%. Ecosystem impacts of drought are associated with both canopy structure and composition with the largest NEP reductions occurring in denser mid‐elevation mixed conifer forests. These results support upcoming imaging spectrometry satellites in providing fine‐scale information on ecosystem composition that can be used to constrain and improve regional‐global scale predictions of ecosystem dynamics.
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