More Like this

1. Reduced global plant respiration due to the acclimation of leaf dark respiration coupled with photosynthesis

   https://doi.org/10.1111/nph.19355  

   Ren, Yanghang ; Wang, Han ; Harrison, Sandy P. ; Prentice, I. Colin ; Atkin, Owen K. ; Smith, Nicholas G. ; Mengoli, Giulia ; Stefanski, Artur ; Reich, Peter B. ( October 2023 , New Phytologist)

   Leaf dark respiration (R_d) acclimates to environmental changes. However, the magnitude, controls and time scales of acclimation remain unclear and are inconsistently treated in ecosystem models.

   We hypothesized thatR_dand Rubisco carboxylation capacity (V_cmax) at 25°C (R_d,25,V_cmax,25) are coordinated so thatR_d,25variations supportV_cmax,25at a level allowing full light use, withV_cmax,25reflecting daytime conditions (for photosynthesis), andR_d,25/V_cmax,25reflecting night‐time conditions (for starch degradation and sucrose export). We tested this hypothesis temporally using a 5‐yr warming experiment, and spatially using an extensive field‐measurement data set. We compared the results to three published alternatives:R_d,25declines linearly with daily average prior temperature;R_dat average prior night temperatures tends towards a constant value; andR_d,25/V_cmax,25is constant.

   Our hypothesis accounted for more variation in observedR_d,25over time (R² = 0.74) and space (R² = 0.68) than the alternatives. Night‐time temperature dominated the seasonal time‐course ofR_d, with an apparent response time scale ofc.2 wk.V_cmaxdominated the spatial patterns.

   Our acclimation hypothesis results in a smaller increase in globalR_din response to rising CO₂and warming than is projected by the two of three alternative hypotheses, and by current models.

    

   more » « less
2. Contribution and consequences of xylem‐transported CO 2 assimilation for C 3 plants

   https://doi.org/10.1111/nph.15907  

   Stutz, Samantha S. ; Hanson, David T. ( June 2019 , New Phytologist)

   Traditionally, leaves were thought to be supplied withCO₂for photosynthesis by the atmosphere and respiration. Recent studies, however, have shown that the xylem also transports a significant amount of inorganic carbon into leaves through the bulk flow of water. However, little is known about the dynamics and proportion of xylem‐transportedCO₂that is assimilated, vs simply lost to transpiration.

   Cut leaves ofPopulus deltoidesandBrassica napuswere placed in eitherKCl or one of three [NaH¹³CO₃] solutions dissolved in water to simultaneously measure the assimilation and the efflux of xylem‐transportedCO₂exiting the leaf across light andCO₂response curves in real‐time using a tunable diode laser absorption spectroscope.

   The rates of assimilation and efflux of xylem‐transportedCO₂increased with increasing xylem [¹³CO₂*] and transpiration. Under saturating irradiance, rates of assimilation using xylem‐transportedCO₂accounted forc.2.5% of the total assimilation in both species in the highest [¹³CO₂*].

   The majority of xylem‐transportedCO₂is assimilated, and efflux is small compared to respiration. Assimilation of xylem‐transportedCO₂comprises a small portion of total photosynthesis, but may be more important whenCO₂is limiting.

    

   more » « less
3. Temporal and stoichiometric patterns of algal stimulation of litter‐associated heterotrophic microbial activity

   https://doi.org/10.1111/fwb.13442  

   Francoeur, Steven N. ; Neely, Robert K. ; Underwood, Savannah ; Kuehn, Kevin A. ( December 2019 , Freshwater Biology)

   Periphyton communities associated with submerged plant detritus contain interacting autotrophic and heterotrophic microbes, and are sites of extracellular enzymatic activity. The strength and nature of these interactions might be expected to change over time as microbial communities develop on plant litter. Microbial interactions and enzymatic activity can be altered by nutrient availability, suggesting that litter stoichiometry could also affect these phenomena.

   We grew wetland plants under ambient and nutrient‐enriched conditions to generate plant litter of differing nutrient content. In two experiments, we investigated: (1) the influence of algal photosynthesis on fungal and bacterial production and the activities of four extracellular enzymes throughout a 54‐day period of microbial colonisation and growth; and (2) the influence of litter stoichiometry on these relationships.

   Ambient and nutrient‐enriched standing‐dead plant litter was collected and then submerged in wetland pools to allow for natural microbial colonisation and growth. Litter samples were periodically retrieved and transported to the laboratory for experiments manipulating photosynthesis using the photosystem II inhibitor DCMU (which effectively prevents algal photosynthetic activity). Algal (¹⁴C‐bicarbonate), bacterial (³H‐leucine), and fungal (¹⁴C‐acetate) production, and β‐glucosidase, β‐xylosidase, leucine aminopeptidase, and phosphatase activities (MUF‐ or AMC‐labelled fluorogenic substrates) were measured under conditions of active and inhibited algal photosynthesis.

   Photosynthesis stimulated overall fungal and bacterial production in both experiments, although the strength of stimulation varied amongst sampling dates. Phosphatase activity was stimulated by photosynthesis during the first, but not the second, experiment. No other enzymatic responses to short‐term photosynthesis manipulations were observed.

   Microbial communities on high‐nutrient litter occasionally showed increased extracellular enzyme activity, fungal growth rates, and bacterial production compared to communities on non‐enriched litter, but algal and fungal production were not affected. Litter stoichiometry had no effects on fungal, bacterial, or enzymatic responses to photosynthesis, but the mean enzyme vector analysis angle (a measure of P‐ versus N‐acquiring enzyme activity) was positively correlated to litter N:P, suggesting that elevated litter N:P led to an increase in the relative activity of P‐acquiring enzymes.

   These results supported the hypothesis that algal photosynthesis strongly influences heterotrophic microbial activity on macrophyte leaf litter, especially that of fungi, throughout microbial community development. However, the strength of this photosynthetic stimulation does not generally depend on small differences in litter nutrient content.

   Stimulation of microbial heterotrophs by algal photosynthesis could drive diurnal shifts in periphyton community and aquatic ecosystem function, as well as linkinggreen(photoautotroph‐based) andbrown(detrital‐based) food webs.

    

   more » « less
4. Long‐term herbivore removal experiments reveal how geese and reindeer shape vegetation and ecosystem CO2 ‐fluxes in high‐ Arctic tundra

   https://doi.org/10.1111/1365-2745.14200  

   Petit Bon, Matteo ; Hansen, Brage B. ; Loonen, Maarten J. J. E. ; Petraglia, Alessandro ; Bråthen, Kari Anne ; Böhner, Hanna ; Layton‐Matthews, Kate ; Beard, Karen H. ; Le Moullec, Mathilde ; Jónsdóttir, Ingibjörg S. ; et al ( September 2023 , Journal of Ecology)

   Given the current rates of climate change, with associated shifts in herbivore population densities, understanding the role of different herbivores in ecosystem functioning is critical for predicting ecosystem responses. Here, we examined how migratory geese and resident, non‐migratory reindeer—two dominating yet functionally contrasting herbivores—control vegetation and ecosystem processes in rapidly warming Arctic tundra.

   We collected vegetation and ecosystem carbon (C) flux data at peak plant growing season in the two longest running, fully replicated herbivore removal experiments found in high‐Arctic Svalbard. Experiments had been set up independently in wet habitat utilised by barnacle geeseBranta leucopsisin summer and in moist‐to‐dry habitat utilised by wild reindeerRangifer tarandus platyrhynchusyear‐round.

   Excluding geese induced vegetation state transitions from heavily grazed, moss‐dominated (only 4 g m⁻²of live above‐ground vascular plant biomass) to ungrazed, graminoid‐dominated (60 g m⁻²after 4‐year exclusion) and horsetail‐dominated (150 g m⁻²after 15‐year exclusion) tundra. This caused large increases in vegetation C and nitrogen (N) pools, dead biomass and moss‐layer depth. Alterations in plant N concentration and CN ratio suggest overall slower plant community nutrient dynamics in the short‐term (4‐year) absence of geese. Long‐term (15‐year) goose removal quadrupled net ecosystem C sequestration (NEE) by increasing ecosystem photosynthesis more than ecosystem respiration (ER).

   Excluding reindeer for 21 years also produced detectable increases in live above‐ground vascular plant biomass (from 50 to 80 g m⁻²; without promoting vegetation state shifts), as well as in vegetation C and N pools, dead biomass, moss‐layer depth and ER. Yet, reindeer removal did not alter the chemistry of plants and soil or NEE.

   Synthesis. Although both herbivores were key drivers of ecosystem structure and function, the control exerted by geese in their main habitat (wet tundra) was much more pronounced than that exerted by reindeer in their main habitat (moist‐to‐dry tundra). Importantly, these herbivore effects are scale dependent, because geese are more spatially concentrated and thereby affect a smaller portion of the tundra landscape compared to reindeer. Our results highlight the substantial heterogeneity in how herbivores shape tundra vegetation and ecosystem processes, with implications for ongoing environmental change.

    

   more » « less
5. Optimal carbon partitioning helps reconcile the apparent divergence between optimal and observed canopy profiles of photosynthetic capacity

   https://doi.org/10.1111/nph.17199  

   Buckley, Thomas N. ( March 2021 , New Phytologist)

   Photosynthetic capacity per unit irradiance is greater, and the marginal carbon revenue of water (∂A/∂E) is smaller, in shaded leaves than sunlit leaves, apparently contradicting optimization theory. I tested the hypothesis that these patterns arise from optimal carbon partitioning subject to biophysical constraints on leaf water potential.

   In a whole plant model with two canopy modules, I adjusted carbon partitioning, nitrogen partitioning and leaf water potential to maximize carbon profit or canopy photosynthesis, and recorded how gas exchange parameters compared between shaded and sunlit modules in the optimum.

   The model predicted that photosynthetic capacity per unit irradiance should be larger, and ∂A/∂Esmaller, in shaded modules compared to sunlit modules. This was attributable partly to radiation‐driven differences in evaporative demand, and partly to differences in hydraulic conductance arising from the need to balance marginal returns on stem carbon investment between modules. The model verified, however, that invariance in the marginal carbon revenue of N (∂A/∂N) is in fact optimal.

   The Cowan–Farquhar optimality solution (invariance of ∂A/∂E) does not apply to spatial variation within a canopy. The resulting variation in carbon–water economy explains differences in capacity per unit irradiance, reconciling optimization theory with observations.

    

   more » « less