A basic understanding of nutrition effects on the mechanisms involved in tree response to drought is essential under a future drier climate.
A large-scale throughfall exclusion experiment was set up ...in Brazil to gain an insight into the effects of potassium (K) and sodium (Na) nutrition on tree structural and physiological adjustments to water deficit.
Regardless of the water supply, K and Na supply greatly increased growth and leaf area index (LAI) of Eucalyptus grandis trees over the first 3 yr after planting. Excluding 37% of throughfall reduced above-ground biomass accumulation in the third year after planting for K-supplied trees only. E. grandis trees were scarcely sensitive to drought as a result of the utilization of water stored in deep soil layers after clear-cutting the previous plantation. Trees coped with water restriction through stomatal closure (isohydrodynamic behavior), osmotic adjustment and decrease in LAI. Additionally, droughted trees showed higher phloem sap sugar concentrations.
K and Na supply increased maximum stomatal conductance, and the high water requirements of fertilized trees increased water stress during dry periods. Fertilization regimes should be revisited in a future drier climate in order to find the right balance between improving tree growth and limiting water shortage.
Forested wetlands are an important carbon (C) sink. Fine roots (diameter < 2 mm) dominate belowground C cycling and can be functionally defined into absorptive roots (order 1–2) and transport roots ...(order ≥ 3). However, effects of microtopography on the function-based fine root dynamics in forested wetlands are poorly understood. We studied fine root biomass allocation and biomass, necromass, mass loss rate, production, mortality and decomposition of absorptive and transport roots in hummocks and hollows in a coastal plain freshwater forested wetland (FFW) in the southeastern USA using dynamic-flow method. Biomass ratios of first- to second-order roots and absorptive to transport roots and the biomass and necromass of absorptive and transport roots were significantly higher in top 0–10 cm organic peat layer than in 10–20 cm muck and mineral layer, and were significantly higher in hummocks than in hollows. The mass loss rate, production, mortality and decomposition were significantly higher in hummocks than in hollows. Absorptive roots did not have a lower mass loss rate than transport roots. Microtopography significantly affected the contributions of absorptive and transport roots to the total production, mortality and decomposition. Production, mortality and decomposition of absorptive roots were higher than those of transport roots in hummocks but lower than those of transport roots in hollows. Total (hummocks plus hollows) fine root production, mortality and decomposition were 455 ± 106 g m⁻² y⁻¹, 475 ± 79 g m⁻² y⁻¹ and 392 ± 60 g m⁻² y⁻¹, respectively. Greater mortality than decomposition resulted in net fine root C input to soil. The observed microtopographic controls on fine root dynamics have great implications for soil C cycling. As sea level rises, the relative area of hollows in coastal plain FFWs will increase, causing a decrease in fine root mass loss rate, biomass, production, mortality and decomposition and it is the balance of these processes that will determine future soil C storage and cycling.
Quantifying the spatial variability of species‐specific tree transpiration across hillslopes is important for estimating watershed‐scale evapotranspiration (ET) and predicting spatial drought effects ...on vegetation. The objectives of this study are to (1) assess sap flux density (Js) and tree‐level transpiration (Ts) across three contrasting zones a (riparian buffer, mid‐hillslope and upland‐hillslope, (2) determine how species‐specific Js responds to vapour pressure deficit (VPD) and (3) estimate watershed‐level transpiration (Tw) using Ts derived from each zone. During 2015 and 2016, we measured Js in eight tree species in the three topographic zones in a small 12‐ha forested watershed in the Piedmont region of central North Carolina. In the dry year of 2015, loblolly pine (Pinus taeda), Virginia pine (Pinus virginiana) and sweetgum (Liquidambar styraciflua) Js rates were significantly higher in the riparian buffer when compared to the other two zones. In contrast, Js rates in tulip poplar (Liriodendron tulipifera) and red maple (Acer rubrum) were significantly lower in the buffer than in the mid‐hillslope. Daily Ts varied by zone and ranged from 10 to 93 L/day in the dry year and from 9 to 122 L/day in the wet year (2016). Js responded nonlinearly to VPD in all species and zones. Annual Tw was 447, 377 and 340 mm based on scaled‐Js data for the buffer, mid‐hillslope and upland‐hillslope, respectively. We conclude that large spatial variability in Js and scaled Tw was driven by differences in soil moisture at each zone and forest composition. Consequently, spatial heterogeneity of vegetation and soil moisture must be considered when accurately quantifying watershed level ET.
Tree transpiration varied by species, tree size and zone with the highest water use occurring in the buffer and mid‐slope. This study improves our understanding of the relationship between species‐specific transpiration and soil moisture. Linking soil moisture, tree water use and climatic variability is critical to refining transpiration estimates and managing the effects of drought. Findings from this project could help landowners decide which trees might be better to maximize the benefits related to water in urban and forested systems. CO, chestnut oak; LP, loblolly pine; NRO, northern red oak; RM, red maple; SG, sweetgum; TP, tulip poplar; VP, Virginia pine; WO, white oak.
Leaf fluorescence can be used to track plant development and stress, and is considered the most direct measurement of photosynthetic activity available from remote sensing techniques. Red and far‐red ...sun‐induced chlorophyll fluorescence (SIF) maps were generated from high spatial resolution images collected with the HyPlant airborne spectrometer over even‐aged loblolly pine plantations in North Carolina (United States). Canopy fluorescence yield (i.e., the fluorescence flux normalized by the light absorbed) in the red and far‐red peaks was computed. This quantifies the fluorescence emission efficiencies that are more directly linked to canopy function compared to SIF radiances. Fluorescence fluxes and yields were investigated in relation to tree age to infer new insights on the potential of those measurements in better describing ecosystem processes. The results showed that red fluorescence yield varies with stand age. Young stands exhibited a nearly twofold higher red fluorescence yield than mature forest plantations, while the far‐red fluorescence yield remained constant. We interpreted this finding in a context of photosynthetic stomatal limitation in aging loblolly pine stands. Current and future satellite missions provide global datasets of SIF at coarse spatial resolution, resulting in intrapixel mixture effects, which could be a confounding factor for fluorescence signal interpretation. To mitigate this effect, we propose a surrogate of the fluorescence yield, namely the Canopy Cover Fluorescence Index (CCFI) that accounts for the spatial variability in canopy structure by exploiting the vegetation fractional cover. It was found that spatial aggregation tended to mask the effective relationships, while the CCFI was still able to maintain this link. This study is a first attempt in interpreting the fluorescence variability in aging forest stands and it may open new perspectives in understanding long‐term forest dynamics in response to future climatic conditions from remote sensing of SIF.
Red and far‐red sun‐induced chlorophyll fluorescence maps were generated from the HyPlant airborne spectrometer over even‐aged loblolly pine plantations in North Carolina. Results showed that red fluorescence yield varies with stand age. Young stands exhibited a nearly twofold higher red fluorescence yield than mature plantations. We interpreted this finding in a context of photosynthetic stomatal limitation in aging loblolly pine stands. At coarse spatial resolution, the relationship between fluorescence and tree age was no longer discernible and a new normalization approach of the fluorescence flux according to the fractional vegetation cover is proposed to account for subpixel heterogeneity.
Hemlock woolly adelgid (HWA) is an exotic insect pest causing severe decimation of native hemlock trees. Extensive research has been conducted on the ecological impacts of HWA, but the exact ...physiological mechanisms that cause mortality are not known.
Water relations, anatomy and gas exchange measurements were assessed on healthy and infested eastern (Tsuga canadensis) and Carolina (Tsuga caroliniana) hemlock trees. These data were then used in a mechanistic model to test whether the physiological responses to HWA infestation were sufficiently significant to induce changes in whole-plant water use and carbon uptake.
The results indicated coordinated responses of functional traits governing water relations in infested relative to healthy trees. In response to HWA, leaf water potential, carbon isotope ratios, plant hydraulic properties and stomatal conductance were affected, inducing a reduction in tree water use by > 40% and gross primary productivity by 25%. Anatomical changes also appeared, including the activation of traumatic cells.
HWA infestation had a direct effect on plant water relations. Despite some leaf compensatory mechanisms, such as an increase in leaf hydraulic conductance and nitrogen content, tree water use and carbon assimilation were diminished significantly in infested trees, which could contribute to tree mortality.
The air-seeding hypothesis predicts that xylem embolism resistance is linked directly to bordered pit functioning. We tested this prediction in trunks, roots, and branches at different vertical and ...radial locations in young and old trees of Pseudotsuga menziesii. Dimensions of bordered pits were measured from light and scanning electron micrographs, and physiological data were from published values. Consistent with observations, calculations showed that earlywood tracheids were more resistant to embolism than latewood tracheids, mainly from earlywood having stretchier pit membranes that can distend and cover the pit aperture. Air seeding that occurs in earlywood appears to happen through gaps between the torus edge and pit border, as shown by the similar calculated pressures required to stretch the membrane over the pit aperture and to cause embolism. Although bordered pit functioning was correlated with tracheid hydraulic diameter, pit pore size and above all pit aperture constrained conductivity the most. From roots to branches and from the trunk base to higher on the trunk, hydraulic resistance of the earlywood pit membrane increased significantly because of a decrease in the size of the pit aperture and size and number of margo pores. Moreover, overall wood conductivity decreased, in part due to lower pit conductivity and a decrease in size and frequency of pits. Structural and functional constraints leading to the trade-off of efficiency against safety of water transport were also demonstrated at the individual pit level, with a positive correlation between pit membrane resistance on an area basis and the pressure differential required to cause membrane stretching, a characteristic that is essential for pit aspiration.
Drought-induced forest dieback has been widely reported over the last decades, and the evidence for a direct causal link between survival and hydraulic failure (xylem cavitation) is now well known. ...Because vulnerability to cavitation is intimately linked to the anatomy of the xylem, the main objective of this study was to better understand the xylem anatomical properties associated with cavitation resistance. An extensive data set of cavitation resistance traits and xylem anatomical properties was developed for 115 conifer species, with special attention given to the micro-morphology of bordered pits. The ratio of torus to pit aperture diameter, so-called torus overlap, increased with increasing cavitation resistance, while the flexibility of the margo does not seem to play a role, suggesting that air-seeding is located at the seal between the aspirated torus and pit aperture. Moreover, punctured tori were reported in various Pinaceae species. Species resistant to cavitation had thicker tracheid walls, while their lumen diameter (conduit size) was only slightly reduced, minimizing the impact on hydraulic conductance. The results also demonstrated (i) the existence of an indirect trade-off between hydraulic safety and mechanical strength; and (ii) a consistency between species distribution and xylem anatomy: species with a wide torus overlap and high valve effects are found in arid environments such as the Mediterranean region.
Large-scale abandoned agricultural areas in Southeast Asia resulted in patches of forests of multiple successions and characteristics, challenging the study of their responses to environmental ...changes, especially under climatic water stress. Here, we investigated seasonal variation in leaf water status and drought tolerance of dominant tree species in three multi-aged tropical forests, ranging from 5 to > 200 years old, with contrasting soil moisture in Thailand. Seasonal variation in leaf water status differed among the forests with trees in young and intermediate sites demonstrating larger differences between seasons than the old-growth forest. Although vulnerability to embolism curves revealed that trees in old-growth forest were potentially more sensitive to declining leaf water status than others, they were predicted to lose < 5% of their hydraulic capacity as opposed to 13% for the trees in the younger sites. Our results suggest that the responses to water stress of tree species in different forest ages greatly vary with a tendency of trees in younger sites to be more resilience than those in older sites. Such information would benefit the selection of tree species that could adapt well to specific environments, thus improving the strategies for managing forests of different ages under a warmer future.
The coordination of plant leaf water potential (ΨL) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. ...There is a general consensus that trees with vulnerable xylem more conservatively regulate ΨL than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh. and Liriodendron tulipifera L., by synthesizing 1600 ΨL observations, 122 xylem embolism curves and xylem anatomical measurements across 10 forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated ΨL less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose ΨL regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought‐prone future.
Summary Statement
Across 10 diverse eastern US stands, anisohydric Quercus alba possessed more embolism‐vulnerable stems than isohydric Acer saccharum and Liriodendron tulipifera. Although oaks are often considered drought‐tolerant species, Q. alba sustains gas exchange by operating with heightened risk for hydraulic dysfunction.
•A 3D soil-root model is coupled with a 1D plant xylem flow and photosynthesis model.•Competition for light and water among multiple trees is investigated numerically.•Hydraulic redistribution and ...canopy shading enhance stand resilience to drought.•Equivalent resistor-capacitor models can reasonably describe stand-level dynamics.
It is generally accepted that resource availability shapes the structure and function of many ecosystems. Within the soil-plant-atmosphere (SPA) system, resource availability fluctuates in space and time whereas access to resources by individuals is further impacted by plant-to-plant competition. Likewise, transport and transformation of resources within an individual plant is governed by numerous interacting biotic and abiotic processes. The work here explores the co-limitations on water losses and carbon uptake within the SPA arising from fluctuating resource availability and competition. In particular, the goal is to unfold the interplay between plant access and competition for water and light, as well as the impact of transport/redistribution processes on leaf-level carbon assimilation and water fluxes within forest stands. A framework is proposed that couples a three-dimensional representation of soil-root exchanges with a one-dimensional description of stem water flow and storage, canopy photosynthesis, and transpiration. The model links soil moisture redistribution, root water uptake, xylem water flow and storage, leaf potential and stomatal conductance as driven by supply and demand for water and carbon. The model is then used to investigate plant drought resilience of overstory-understory trees simultaneously competing for water and light. Simulation results reveal that understory-overstory interactions increase ecosystem resilience to drought (i.e. stand-level carbon assimilation rates and water fluxes can be sustained at lower root-zone soil water potentials). This resilience enhancement originates from reduced transpiration (due to shading) and hydraulic redistribution in soil supporting photosynthesis over prolonged periods of drought. In particular, the presence of different rooting systems generates localized hydraulic redistribution fluxes that sustain understory transpiration through overstory-understory interactions. Such complex SPA dynamics cannot be properly summarized by equivalent ecosystem-scale Resistor-Capacitor (RC) representation. However our results show that, with proper averaging across water flow paths, RC models can provide reasonable estimates of stand-level water and carbon fluxes during inter-storm periods.
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