Rising plant demand strengthens nitrogen limitation in tidal marsh.

Nitrogen (N) is a limiting nutrient for primary productivity in most terrestrial ecosystems, but whether N limitation is strengthening or weakening remains controversial because both N sources and sinks are increasing in magnitude globally. Temperate marshes are exposed to greater amounts of external N inputs than most terrestrial ecosystems and more than in preindustrial times owing to their position downstream of major sources of human-derived N runoff along river mouths and estuaries. Simultaneously, ecosystem N demand may also be increasing owing to other global changes such as rising atmospheric [CO2]. Here, we used interannual variability in external drivers and variables related to exogenous supply of N, along with detailed assessments of plant growth and porewater biogeochemistry, to assess the severity of N-limitation, and to determine its causes, in a 14-year N-addition × elevated CO2 experiment. We found substantial interannual variability in porewater [N], plant growth, and experimental N effects on plant growth, but the magnitude of N pools through time varied independently of the strength of N limitation. Sea level, and secondarily salinity, related closely to interannual variability in growth of the dominant plant functional groups which drove patterns in N limitation and in porewater [N]. Experimental exposure of plants to elevated CO2 and years with high flooding strengthened N limitation for the sedge. Abiotic variables controlled plant growth, which determined the strength of N limitation for each plant species and for ecosystem productivity as a whole. We conclude that in this ecosystem, which has an open N cycle and where N inputs are likely greater than in preindustrial times, plant N demand has increased more than supply.

Attaining freshwater and estuarine-water soil saturation in an ecosystem-scale coastal flooding experiment.

Coastal upland forests are facing widespread mortality as sea-level rise accelerates and precipitation and storm regimes change. The loss of coastal forests has significant implications for the coastal carbon cycle; yet, predicting mortality likelihood is difficult due to our limited understanding of disturbance impacts on coastal forests. The manipulative, ecosystem-scale Terrestrial Ecosystem Manipulation to Probe the Effects of Storm Treatments (TEMPEST) experiment addresses the potential for freshwater and estuarine-water disturbance events to alter tree function, species composition, and ecosystem processes in a deciduous coastal forest in MD, USA. The experiment uses a large-unit (2000 m2), un-replicated experimental design, with three 50 m × 40 m plots serving as control, freshwater, and estuarine-water treatments. Transient saturation (5 h) of the entire soil rooting zone (0-30 cm) across a 2000 m2 coastal forest was attained by delivering 300 m3 of water through a spatially distributed irrigation network at a rate just above the soil infiltration rate. Our water delivery approach also elevated the water table (typically ~ 2 m belowground) and achieved extensive, low-level inundation (~ 8 cm standing water). A TEMPEST simulation approximated a 15-cm rainfall event and based on historic records, was of comparable intensity to a 10-year storm for the area. This characterization was supported by showing that Hurricane Ida's (~ 5 cm rainfall) hydrologic impacts were shorter (40% lower duration) and less expansive (80% less coverage) than those generated through experimental manipulation. Future work will apply TEMPEST treatments to evaluate coastal forest resilience to changing hydrologic disturbance regimes and identify conditions that initiate ecosystem state transitions.

Rapid plant trait evolution can alter coastal wetland resilience to sea level rise.

Rapid evolution remains a largely unrecognized factor in models that forecast the fate of ecosystems under scenarios of global change. In this work, we quantified the roles of heritable variation in plant traits and of trait evolution in explaining variability in forecasts of the state of coastal wetland ecosystems. A common garden study of genotypes of the dominant sedge Schoenoplectus americanus, "resurrected" from time-stratified seed banks, revealed that heritable variation and evolution explained key ecosystem attributes such as the allocation and distribution of belowground biomass. Incorporating heritable trait variation and evolution into an ecosystem model altered predictions of carbon accumulation and soil surface accretion (a determinant of marsh resilience to sea level rise), demonstrating the importance of accounting for evolutionary processes when forecasting ecosystem dynamics.

Sensitivity of mean canopy stomatal conductance to vapor pressure deficit in a flooded Taxodium distichum L. forest: hydraulic and non-hydraulic effects.

We measured the xylem sap flux in 64-year-old Taxodium distichum (L.) Richard trees growing in a flooded forest using Granier-type sensors to estimate mean canopy stomatal conductance of the stand (G S). Temporal variations in G S were investigated in relation to variation in vapor pressure deficit (D), photosynthetic photon flux density (Q o), and the transpiration rate per unit of leaf area (E L), the latter variable serving as a proxy for plant water potential. We found that G S was only weakly related to Q o below 500 µmol m-2 s-1 (r 2=0.29), but unrelated to Q o above this value. Above Q o=500 µmol m-2 s-1 and D=0.6 kPa, G S decreased linearly with increasing E L with a poor fit (r 2=0.31), and linearly with lnD with a much better fit (r 2=0.81). The decrease of G S with lnD was at a rate predicted based on a simple hydraulic model in which stomata regulate the minimum leaf water potential. Based on the hydraulic model, stomatal sensitivity to D is proportional to stomatal conductance at low D. A hurricane caused an ~41% reduction in leaf area. This resulted in a 28% increase in G S at D=1 kPa (G Sref), indicating only partial compensation. As predicted, the increase in G Sref after the hurricane was accompanied by a similar increase in stomatal sensitivity to D (29%). At night, G Sref was ~20% of the daytime value under non-limiting light (Q o>500 µmol m-2 s-1). However, stomatal sensitivity to D decreased only to ~46% (both reductions referenced to pre-hurricane daytime values), thus having more than twice the sensitivity expected based on hydraulic considerations alone. Therefore, non-hydraulic processes must cause heightened nighttime stomatal sensitivity to D.

Productivity responses of Acer rubrum and Taxodium distichum seedlings to elevated CO2 and flooding.

Elevated levels of atmospheric CO2 are expected to increase photosynthetic rates of C3 tree species, but it is uncertain whether this will result in an increase in wetland seedling productivity. Separate short-term experiments (12 and 17 weeks) were performed on two wetland tree species, Taxodium distichum and Acer rubrum, to determine if elevated CO2 would influence the biomass responses of seedlings to flooding. T. distichum were grown in replicate glasshouses (n = 2) at CO2 concentrations of 350 or 700 ppm. and A. rubrum were grown in growth chambers at CO2 concentrations of 422 or 722 ppm. Both species were grown from seed. The elevated CO2 treatment was crossed with two water table treatments, flooded and non-flooded. Elevated CO2 increased leaf-level photosynthesis, whole-plant photosynthesis, and trunk diameter of T. distichum in both flooding treatments, but did not increase biomass of T. distichum or A. rubrum. Flooding severely reduced biomass, height, and leaf area of both T. distichum and A. rubrum. Our results suggest that the absence of a CO2-induced increase in growth may have been due to an O2 limitation on root production even though there was a relatively deep (approximately 10 cm) aerobic soil surface in the non-flooded treatment.

Twelve testable hypotheses on the geobiology of weathering.

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P(CO2) will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Sap-flux-scaled transpiration responses to light, vapor pressure deficit, and leaf area reduction in a flooded Taxodium distichum forest.

We used 20-mm-long, Granier-type sensors to quantify the effects of tree size, azimuth and radial position in the xylem on the spatial variability in xylem sap flux in 64-year-old trees of Taxodium distichum L. Rich. growing in a flooded forest. This information was used to scale flux to the stand level to investigate variations in half-hourly and daily (24-hour) sums of sap flow, transpiration per unit of leaf area, and stand transpiration in relation to vapor pressure deficit (D) and photosynthetically active radiation (Q(o)). Measurements of xylem sap flux density (J(s)) indicated that: (1) J(s) in small diameter trees was 0.70 of that in medium and large diameter trees, but the relationship between stem diameter as a continuous variable and J(s) was not significant; (2) J(s) at 20-40 mm depth in the xylem was 0.40 of that at 0-20 mm depth; and (3) J(s) on the north side of trees was 0.64 of that in directions 120 degrees from the north. Daily transpiration was linearly related to daily daytime mean D, and reached a modest value of 1.3 mm day(-1), reflecting the low leaf area index (LAI = 2.2) of the stand. Because there was no soil water limitation, half-hourly water uptake was nearly linearly related to D at D < 0.6 kPa during both night and day, increasing to saturation during daytime at higher values of D. The positive effect of Q(o) on J(s) was significant, but relatively minor. Thus, a second-order polynomial with D explained 94% of the variation in J(s) and transpiration. An approximately 40% reduction in LAI by a hurricane resulted in decreases of about 18% in J(s) and stand transpiration, indicating partial stomatal compensation.