Biology subjects

Harrison, S. P.

Publications and source records attributed to Harrison, S. P..

9 recordsLinked to original sources

bioRxiv2026

An eco-evolutionary approach to defining wildfire regimes

Fuel availability and fuel dryness are consistently shown to be the primary drivers of wildfire intensity and burnt area. Here we hypothesise that differences in the timing of fuel build up and drying determine the optimal time for wildfire occurrence. We use gross primary production (GPP) as a measure of biomass production and hence fuel availability, and vapour pressure deficit (VPD) as a measure of fuel drying. We use the phase difference in the seasonal time course and magnitude of GPP and VPD to cluster regions that should therefore have distinct wildfire behaviour. We then show that each of the resultant clusters is distinctive in terms of one or more fire properties, specifically number of ignitions, burnt area, size, speed, duration, intensity, and length of the wildfire season. The emergence of distinct regimes as a function of two biophysical drivers reflects the fact that both vegetation and wildfire properties are a consequence of eco-evolutionary adaptions to environmental conditions. We then examine the degree to which human activities or vegetation properties modify these fire regimes within each of these clusters. Variability in GPP and VPD largely explains the within-cluster variation in fire properties. The type of vegetation cover has an influence on burnt area and carbon emissions in particular, while human activities are more important for fire properties such as size, rate of spread and duration largely through their influence of landscape fragmentation. Although both human activities and vegetation properties modify wildfire regimes, the ability to distinguish wildfire regimes using GPP and VPD alone emphasizes that land management, fire use and fire suppression are constrained by environmental conditions. This eco-evolutionary optimality approach to characterising wildfire regimes provides a basis for designing a simple fire model for Earth System modelling.

ecology↗

bioRxiv2024

Thermal acclimation of stem respiration reduces global carbon burden

Stem respiration is a key driver of carbon flux from ecosystems to the atmosphere, yet its response to global warming remains poorly constrained. In particular it has been proposed that stem respiration acclimates to changing temperatures, which could have large implications for carbon cycling under climate change, but no theory exists to predict acclimated respiration rates. Here, we hypothesized that stem respiration is physiologically linked to transpiration in order to maintain hydraulic continuity. We then use that linkage, combined with Eco-evolutionary optimality theory, to develop a theoretical prediction of the temperature sensitivity of both acclimated and instantaneous stem respiration. Leveraging an extensive global dataset, we observe temperature sensitivities of stem respiration across geographical and seasonal variations that are consistent with this prediction. Our findings reveal that stem respiration contributes approximately a quarter of the global above-ground auto-trophic respiration, with an estimated annual emission of around 11.20 {+/-} 5.88 Pg C--comparable to total anthropogenic emissions. Importantly, incorporating thermal acclimation of stem respiration into projections significantly reduces predicted land ecosystem carbon emissions by 4.41 and 9.56 Pg C under the SSP126 and SSP585 scenarios, respectively, for the 21st century.

ecology↗

bioRxiv2023

A unifying principle for global greenness patterns and trends

Vegetation cover regulates the exchanges of energy, water and carbon between land and atmosphere. Remotely-sensed fractional absorbed photosynthetically active radiation (fAPAR), a land-surface greenness metric, depends on carbon allocation to foliage while also controlling photon flux for photosynthesis. Greenness is thus both a driver and an outcome of gross primary production (GPP). An equation with just two (globally) fitted parameters describes annual maximum fAPAR (fAPARmax) as the smaller of a water-limited value, transpiring a constant fraction of annual precipitation, and an energy-limited value, maximizing annual plant growth. This minimalist description reproduces global greenness patterns, and the consistent temporal trends among remote-sensing products, as accurately as the best-performing dynamic global vegetation models. Widely observed greening is attributed to the influence of rising carbon dioxide on the light- and water-use efficiencies of GPP, augmented by wetting in some dry regions and warming in high latitudes. Limited regions show browning, attributed to drying.

ecology↗

bioRxiv2022

Functional traits trade-offs define plant population stability worldwide

O_LIEcological theory posits that temporal stability patterns in plant populations are associated with differences in species ecological strategies. However, empirical evidence is lacking about which traits, or trade-offs, underlie species stability, specially across different ecosystems. C_LIO_LITo address this, we compiled a global collection of long-term permanent vegetation records (>7000 plots from 78 datasets) from a wide range of habitats and combined this with existing trait databases. We tested whether the observed inter-annual variability in species abundance (coefficient of variation) was related to multiple individual traits and multivariate axes of trait variations (PCoA axes). C_LIO_LIWe found that species with greater leaf dry matter content and seed mass were consistently more stable over time (lower variability in species abundance) although other leaf traits played a significant role as well, albeit weaker. Using multivariate axes did not improve predictions by specific traits. C_LIO_LIOur results confirm existing theory, providing compelling empirical evidence on the importance of specific traits, which point at ecological trade-offs in different resource use and dispersal strategies, on the stability of plant populations worldwide. C_LI

ecology↗

bioRxiv2021

LOTVS: a global collection of permanent vegetation plots

Analysing temporal patterns in plant communities is extremely important to quantify the extent and the consequences of ecological changes, especially considering the current biodiversity crisis. Long-term data collected through the regular sampling of permanent plots represent the most accurate resource to study ecological succession, analyse the stability of a community over time and understand the mechanisms driving vegetation change. We hereby present the LOng-Term Vegetation Sampling (LOTVS) initiative, a global collection of vegetation time-series derived from the regular monitoring of vascular plants in permanent plots. With 79 datasets from five continents and 7789 vegetation time-series monitored for at least six years and mostly on an annual basis, LOTVS possibly represents the largest collection of temporally fine-grained vegetation time-series derived from permanent plots and made accessible to the research community. As such, it has an outstanding potential to support innovative research in the fields of vegetation science, plant ecology and temporal ecology.

ecology↗

bioRxiv2021

Ecosystem photosynthesis in land-surface models: a first-principles approach

Vegetation regulates land-atmosphere water and energy exchanges and is an essential component of land-surface models (LSMs). However, LSMs have been handicapped by assumptions that equate acclimated photosynthetic responses to environment with fast responses observable in the laboratory. These time scales can be distinguished by including specific representations of acclimation, but at the cost of further increasing parameter requirements. Here we develop an alternative approach based on optimality principles that predict the acclimation of carboxylation and electron-transport capacities, and a variable controlling the response of leaf-level carbon dioxide drawdown to vapour pressure deficit (VPD), to variations in growth conditions on a weekly to monthly time scale. In the "P model", an optimality-based light-use efficiency model for gross primary production (GPP) on this time scale, these acclimated responses are implicit. Here they are made explicit, allowing fast and slow response time-scales to be separated and GPP to be simulated at sub-daily timesteps. The resulting model mimics diurnal cycles of GPP recorded by eddy-covariance flux towers in a temperate grassland and boreal, temperate and tropical forests, with no parameter changes between biomes. Best performance is achieved when biochemical capacities are adjusted to match recent midday conditions. This model suggests a simple and parameter-sparse method to include both instantaneous and acclimated responses within an LSM framework, with many potential applications in weather, climate and carbon - cycle modelling. Plain Language SummaryVegetation regulates the exchanges of energy, water and carbon dioxide between the land and the atmosphere. Numerical climate models represent these processes, focusing mainly on their rapid variations in response to changes in the environment (including temperature and light) on timescales of seconds to hours. However, plants also adjust their physiology to environmental changes over longer periods within the season. Here we have adapted a simple model that formulates plant behaviour in terms of optimal trade-offs between different processes, so it simulates processes on both time scales. This model correctly reproduces the daily cycle of carbon dioxide uptake by plants, as recorded in different kinds of vegetation. We show that plants optimize their behaviour for midday conditions, when the light is greatest, and adjust to longer-term environmental variations on a timescale of about a week to a month. The model conveniently avoids the need to give specific, fixed values to physiological variables (such as photosynthetic capacity) for different types of plants. The optimality assumptions mean that the model gives equally good results in tropical, temperate and boreal forests, and in grasslands, using the same equations, and a very small number of input variables that are constant across the world. Key PointsO_LIOptimality theory is used to develop a simple model incorporating fast and acclimated responses of photosynthesis and stomatal conductance C_LIO_LIBiogeochemical photosynthetic capacities adjust to midday light conditions C_LIO_LIThe new model simulates gross primary production on sub-daily timesteps across a range of different vegetation types and climate C_LI

ecology↗

bioRxiv2021

Plant hydraulics coordinated with photosynthetic traits and climate

The coupling between water loss and carbon dioxide uptake drives the coordination of plant hydraulic and photosynthetic traits. Analysing multi-species measurements on a 3000 m elevation gradient, we found that hydraulic and leaf-economic traits were less plastic, and more closely associated with phylogeny, than photosynthetic traits. The two trait sets are linked by the sapwood-to-leaf area ratio (Huber value, vH), shown here to be codetermined by sapwood hydraulic conductance (KS), leaf mass-per-area (LMA) and photosynthetic capacity (Vcmax). Substantial hydraulic diversity was related to the trade-off between KS and vH. Leaf drought tolerance (inferred from turgor loss point, -{pi}tlp) increased with wood density, but the trade-off between hydraulic efficiency (KS) and -{pi}tlp was weak. The least-cost optimality framework was extended to predict trait (KS-dominated) and environmental (temperature-dominated) effects on vH. These results suggest an approach to include photosynthetic-hydraulic coordination in land-surface models; however, prediction of non-plastic trait distributions remains a challenge.

ecology↗

bioRxiv2020

Scalable production of tissue-like vascularised liver organoids from human PSCs.

A lack of physiological parity between 2D cell culture and in vivo, has paved the way towards more organotypic models. Organoids exist for a number of tissues, including the liver. However, current approaches to generate hepatic organoids suffer drawbacks, including a reliance on extracellular matrices (ECM), the requirement to pattern in 2D culture, costly growth factors and a lack of cellular diversity, structure and organisation. Current hepatic organoid models are generally simplistic, composed of hepatocytes or cholangiocytes, which renders them less physiologically relevant when compared to native tissue. Here we aim to address these drawbacks. To address this, we have developed an approach that does not require 2D patterning, is ECM independent combined with small molecules to mimic embryonic liver development that produces massive quantities of liver like organoids. Using single-cell RNA sequencing and immunofluorescence we demonstrate a liver-like cellular repertoire, a higher order cellular complexity, presenting with vascular luminal structures, innervation and a population of resident macrophage - the Kupffer cells. The organoids exhibit key liver functions including drug metabolism, serum protein production, coagulation factor production, bilirubin uptake and urea synthesis. The organoids can be transplanted and maintained in mice producing human albumin long term. The organoids exhibit a complex cellular repertoire reflective of the organ, have de novo vascularization and innervation, enhanced function and maturity. This is a pre-requisite for a myriad of applications from cellular therapy, tissue engineering, drug toxicity assessment, disease modeling, to basic developmental biology.

cell biology↗

bioRxiv2019

Introduced species dominate different responses of grassland communities to climate change on serpentine and nonserpentine soils

Introduced species are a common feature of modern plant communities and experience environmental challenges alongside native species. Changes to the environment may reveal distinct species-environment relationships for native and introduced components of plant communities. Extreme environmental change, such as drought, is predicted to result in declines in native species and increased opportunities for invasion, but empirical support for these ideas remains mixed. We tested for differences in the response of native and invaded species to environmental changes by analyzing a longterm dataset of species abundance in California grasslands collected during a period of severe drought. Sampling sites included a combination of stressful serpentine soils, which are resilient against invasion and maintain diverse native species assemblages, and more benign nonserpentine soils, which are heavily invaded and harbor low levels of native species cover. We found a significant correlation between sampling year and species composition for nonserpentine sites, but not for serpentine sites. These patterns were repeated when only introduced species were included in the analysis but no pattern of change was found for native species. The species most strongly associated with directional change on nonserpentine soils were three invasive Eurasian grasses, Bromus hordaceus, Taeniatherium caput-medusae, and Avena fatua. Differences in species composition on both serpentine and nonserpentine soils were significantly correlated with specific leaf area, a trait which has been linked to drought tolerance in these communities, although changes in abundance for the three Eurasian grasses most strongly associated with change did not consistently follow this pattern. Our analyses indicate relatively stable native community composition and strong directional change in introduced species composition, contradicting predictions for how native and introduced species will respond to environmental shifts, but supporting the hypothesis that native and invading species groups have important functional differences that shape their relationships to the environment.

ecology↗