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"Mark A. Adams"
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Litter accumulation and fire risks show direct and indirect climate-dependence at continental scale
Litter decomposition / accumulation are rate limiting steps in soil formation, carbon sequestration, nutrient cycling and fire risk in temperate forests, highlighting the importance of robust predictive models at all geographic scales. Using a data set for the Australian continent, we show that among a range of models, most of the variance in litter mass over a 40-year time span can be accounted for by a parsimonious model with elapsed time, and indices of climate and litter quality, as independent drivers. Climate is an important driver of variation in both the species identity of dominant eucalypts and litter accumulation across the continent. Litter quality shows emergent properties of climate-dependence and contributes to explained variance. Nonetheless, elapsed time dominated explained variance in litter mass (up to 90%) at continental scale. Results provide guidance for future decomposition studies. Algorithms reported here can significantly improve accuracy and reliability of predictions of carbon and nutrient dynamics and fire risk.
Compiled data on litterfall and litter in eucalypt forests and woodlands for the Australian continent shows that litter mass can be robustly predicted using just three independent variables – time, aridity and litterfall quality
Journal Article
Reply to: Re-evaluation of quadratic and exponential models of litter accumulation incorporating climatic and species-specific dependence
[...]we tested a range of straightforward measures of both for their utility as predictors of litter accumulation based on a continental data set. More direct measures of the quality of litterfall components are typically obtained via analysis of nutrients (% N, P, etc.) or lignin (%) relative to total carbon. [...]a considerable proportion of data points for Tsf < 10 is derived from forests in which deliberate, low-intensity burning is used to manage fuel loads. [...]much of the data for such forests includes non-zero litter mass for Tsf = 0.
Journal Article
CO2, nitrogen deposition and a discontinuous climate response drive water use efficiency in global forests
Reduced stomatal conductance is a common plant response to rising atmospheric CO
2
and increases water use efficiency (
W
). At the leaf-scale,
W
depends on water and nitrogen availability in addition to atmospheric CO
2
. In hydroclimate models
W
is a key driver of rainfall, droughts, and streamflow extremes. We used global climate data to derive Aridity Indices (AI) for forests over the period 1965–2015 and synthesised those with data for nitrogen deposition and
W
derived from stable isotopes in tree rings. AI and atmospheric CO
2
account for most of the variance in
W
of trees across the globe, while cumulative nitrogen deposition has a significant effect only in regions without strong legacies of atmospheric pollution. The relation of aridity and
W
displays a clear discontinuity.
W
and AI are strongly related below a threshold value of AI ≈ 1 but are not related where AI > 1. Tree ring data emphasise that effective demarcation of water-limited from non-water-limited behaviour of stomata is critical to improving hydrological models that operate at regional to global scales.
Water use efficiency is a key measure of plant responses to climate change. Here, the authors investigate its control by CO2, nitrogen deposition, and water availability using a global tree-ring dataset. They find an aridity threshold and quantify changes in control over the past 50 years.
Journal Article
Wildfire management in Mediterranean-type regions: paradigm change needed
During the last decades, climate and land use changes led to an increased prevalence of megafires in Mediterranean-type climate regions (MCRs). Here, we argue that current wildfire management policies in MCRs are destined to fail. Focused on fire suppression, these policies largely ignore ongoing climate warming and landscape-scale buildup of fuels. The result is a 'firefighting trap' that contributes to ongoing fuel accumulation precluding suppression under extreme fire weather, and resulting in more severe and larger fires. We believe that a 'business as usual' approach to wildfire in MCRs will not solve the fire problem, and recommend that policy and expenditures be rebalanced between suppression and mitigation of the negative impacts of fire. This requires a paradigm shift: policy effectiveness should not be primarily measured as a function of area burned (as it usually is), but rather as a function of avoided socio-ecological damage and loss.
Journal Article
Leaf day respiration
It has been 75 yr since leaf respiratory metabolism in the light (day respiration) was identified as a low-flux metabolic pathway that accompanies photosynthesis. In principle, it provides carbon backbones for nitrogen assimilation and evolves CO2 and thus impacts on plant carbon and nitrogen balances. However, for a long time, uncertainties have remained as to whether techniques used to measure day respiratory efflux were valid and whether day respiration responded to environmental gaseous conditions. In the past few years, significant advances have beenmade using carbon isotopes, ‘omics’ analyses and surveys of respiration rates in mesocosms or ecosystems. There is substantial evidence that day respiration should be viewed as a highly dynamic metabolic pathway that interacts with photosynthesis and photorespiration and responds to atmospheric CO2 mole fraction. The view of leaf day respiration as a constant and/or negligible parameter of net carbon exchange isnow outdated and it should now be regarded as a central actor of plant carbon-use efficiency.
Journal Article
Empirical and model-based estimates of spatial and temporal variations in net primary productivity in semi-arid grasslands of Northern China
Spatiotemporal variations in net primary productivity (NPP) reflect the dynamics of water and carbon in the biosphere, and are often closely related to temperature and precipitation. We used the ecosystem model known as the Carnegie-Ames-Stanford Approach (CASA) to estimate NPP of semiarid grassland in northern China counties between 2001 and 2013. Model estimates were strongly linearly correlated with observed values from different counties (slope = 0.76 (p < 0.001), intercept = 34.7 (p < 0.01), R2 = 0.67, RMSE = 35 g C·m-2·year-1, bias = -0.11 g C·m-2·year-1). We also quantified inter-annual changes in NPP over the 13-year study period. NPP varied between 141 and 313 g C·m-2·year-1, with a mean of 240 g C·m-2·year-1. NPP increased from west to east each year, and mean precipitation in each county was significantly positively correlated with NPP-annually, and in summer and autumn. Mean precipitation was positively related to NPP in spring, but not significantly so. Annual and summer temperatures were mostly negatively correlated with NPP, but temperature was positively correlated with spring and autumn NPP. Spatial correlation and partial correlation analyses at the pixel scale confirmed precipitation is a major driver of NPP. Temperature was negatively correlated with NPP in 99% of the regions at the annual scale, but after removing the effect of precipitation, temperature was positively correlated with the NPP in 77% of the regions. Our data show that temperature effects on production depend heavily on recent precipitation. Results reported here have significant and far-reaching implications for natural resource management, given the enormous size of these grasslands and the numbers of people dependent on them.
Journal Article
Rainfall drives variation in rates of change in intrinsic water use efficiency of tropical forests
Rates of change in intrinsic water use efficiency (
W
) of trees relative to those in atmospheric [CO
2
] (
c
a
) have been mostly assessed via short-term studies (e.g., leaf analysis, flux analysis) and/or step increases in
c
a
(e.g., FACE studies). Here we use compiled data for abundances of carbon isotopes in tree stems to show that on decadal scales, rates of change (
dW/dc
a
) vary with location and rainfall within the global tropics. For the period 1915–1995, and including corrections for mesophyll conductance and photorespiration,
dW/dc
a
for drier tropical forests (receiving ~ 1000 mm rainfall) were at least twice that of the wettest (receiving ~ 4000 mm). The data also empirically confirm theorized roles of tropical forests in changes in atmospheric
13
C/
12
C ratios (the
13
C Suess Effect). Further formal analysis of geographic variation in decade-to-century scale
dW/dc
a
will be needed to refine current models that predict increases in carbon uptake by forests without hydrological cost.
How the water use efficiency of trees changes with atmospheric CO
2
variations has mostly been studied on short time scales. Here, a newly compiled data set covering 1915 to 1995 shows how rates of change in water use efficiency vary with location and rainfall over the global tropics on a decadal scale.
Journal Article
Sensitivity of plants to changing atmospheric CO₂ concentration: from the geological past to the next century
1077 I. 1078 II. 1079 III. 1080 IV. 1081 V. 1084 VI. 1087 VII. 1088 1089 References 1089 SUMMARY: The rate of CO₂ assimilation by plants is directly influenced by the concentration of CO₂ in the atmosphere, cₐ. As an environmental variable, cₐ also has a unique global and historic significance. Although relatively stable and uniform in the short term, global cₐ has varied substantially on the timescale of thousands to millions of years, and currently is increasing at seemingly an unprecedented rate. This may exert profound impacts on both climate and plant function. Here we utilise extensive datasets and models to develop an integrated, multi‐scale assessment of the impact of changing cₐ on plant carbon dioxide uptake and water use. We find that, overall, the sensitivity of plants to rising or falling cₐ is qualitatively similar across all scales considered. It is characterised by an adaptive feedback response that tends to maintain 1 − cᵢ/cₐ, the relative gradient for CO₂ diffusion into the leaf, relatively constant. This is achieved through predictable adjustments to stomatal anatomy and chloroplast biochemistry. Importantly, the long‐term response to changing cₐ can be described by simple equations rooted in the formulation of more commonly studied short‐term responses.
Journal Article