KEY MESSAGE : The muted wood isotopic signal in slow-growing trees of unthinned stands indicates lower responsiveness to changing environmental conditions compared to fast-growing trees in thinned ...stands. To examine the physiological processes associated with higher growth rates after thinning, we analyzed the oxygen isotopic values in wood (δ¹⁸Ow) of 12 ponderosa pine (Pinus ponderosa) trees from control, moderately, and heavily thinned stands and compared them with wood-based estimates of carbon isotope discrimination (∆¹³C), basal area increment (BAI), and gas exchange. We found that (heavy) thinning led to shifts and increased inter-annual variability of both stable carbon and oxygen isotope ratios relative to the control throughout the first post-thinning decade. Results of a sensitivity analysis suggested that both an increase in stomatal conductance (g ₛ) and differences in source water among treatments are equally probable causes of the δ¹⁸Ow shift in heavily thinned stands. We modeled inter-annual changes in δ¹⁸Ow of trees from all treatments using environmental and physiological data and found that the significant increase in δ¹⁸Ow inter-annual variance was related to greater δ¹⁸Ow responsiveness to changing environmental conditions for trees in thinned stands when compared to control stands. Based on model results, the more muted climatic response of wood isotopes in slow-growing control trees is likely to be the consequence of reduced carbon sink strength causing a higher degree of mixing of previously stored and fresh assimilates when compared to faster-growing trees in thinned stands. Alternatively, the muted response of δ¹⁸Ow to climatic variation of trees in the control stand may result from little variation in the control stand in physiological processes (photosynthesis, transpiration) that are known to affect δ¹⁸Ow.
Watershed integrity, the capacity of a watershed to support and maintain ecological processes essential to the sustainability of services provided to society, can be influenced by a range of ...landscape and in-stream factors. Ecological response data from four intensively monitored case study watersheds exhibiting a range of environmental conditions and landscape characteristics across the United States were used to evaluate the performance of a national level Index of Watershed Integrity (IWI) at regional and local watershed scales. Using Pearson's correlation coefficient (
), and Spearman's rank correlation coefficient (
), response variables displayed highly significant relationships and were significantly correlated with IWI and ICI (Index of Catchment Integrity) values at all watersheds. Nitrogen concentration and flux-related watershed response metrics exhibited significantly strong negative correlations across case study watersheds, with absolute correlations (|
|) ranging from 0.48 to 0.97 for IWI values, and 0.31 to 0.96 for ICI values. Nitrogen-stable isotope ratios measured in chironomids and periphyton from streams and benthic organic matter from lake sediments also demonstrated strong negative correlations with IWI values, with |
| ranging from 0.47 to 0.92, and 0.35 to 0.89 for correlations with ICI values. This evaluation of the performance of national watershed and catchment integrity metrics and their strong relationship with site level responses provides weight-of-evidence support for their use in state, local and regionally focused applications.
To test tree growth sensitivity to temperature under different ambient CO₂ concentrations, we determined stem radial growth rates as they relate to variation in temperature during the last deglacial ...period, and compare these to modern tree growth rates as they relate to spatial variation in temperature across the modern species distributional range. Paleo oaks were sampled from Northern Missouri, USA and compared to a pollen-based, high-resolution paleo temperature reconstruction from Northern Illinois, USA. Growth data were from 53 paleo bur oak log cross sections collected in Missouri. These oaks were preserved in river and stream sediments and were radiocarbon-dated to a period of rapid climate change during the last deglaciation (10.5 and 13.3 cal kyr BP). Growth data from modern bur oaks were obtained from increment core collections paired with USDA Forest Service Forest Inventory and Analysis data collected across the Great Plains, Midwest, and Upper Great Lakes regions. For modern oaks growing at an average CO₂ of 330 ppm, growth sensitivity to temperature (i.e., the slope of growth rate versus temperature) was about twice that of paleo oaks growing at an average CO₂ of 230 ppm. These data help to confirm that leaflevel predictions that photosynthesis and thus growth will be more sensitive to temperature at higher CO₂ in mature trees—suggesting that tree growth forest productivity will be increasingly sensitive to temperature under projected global warming and high-CO₂ conditions.
Dwarf mistletoes, obligate, parasitic plants with diminutive aerial shoots, have long-term effects on host tree water relations, hydraulic architecture and photosynthetic gas exchange and can ...eventually induce tree death. To investigate the long-term (1886-2010) impacts of dwarf mistletoe on the growth and gas exchange characteristics of host western hemlock, we compared the diameter growth and tree-ring cellulose stable carbon (C) and oxygen (O) isotope ratios (δ(13)Ccell, δ(18)Ocell) of heavily infected and uninfected trees. The relative basal area growth of infected trees was significantly greater than that of uninfected trees in 1886-90, but declined more rapidly in infected than uninfected trees through time and became significantly lower in infected than uninfected trees in 2006-10. Infected trees had significantly lower δ(13)Ccell and δ(18)Ocell than uninfected trees. Differences in δ(18)Ocell between infected and uninfected trees were unexpected given that stomatal conductance and environmental variables that were expected to influence the δ(18)O values of leaf water were similar for both groups. However, estimates of mesophyll conductance (gm) were significantly lower and estimates of effective path length for water movement (L) were significantly higher in leaves of infected trees, consistent with their lower values of δ(18)Ocell. This study reconstructs the long-term physiological responses of western hemlock to dwarf mistletoe infection. The long-term diameter growth and δ(13)Ccell trajectories suggested that infected trees were growing faster than uninfected trees prior to becoming infected and subsequently declined in growth and leaf-level photosynthetic capacity compared with uninfected trees as the dwarf mistletoe infection became severe. This study further points to limitations of the dual-isotope approach for identifying sources of variation in δ(13)Ccell and indicates that changes in leaf internal properties such as gm and L that affect δ(18)Ocell must be considered.
Rising atmospheric CO₂, cₐ, is expected to affect stomatal regulation of leaf gas‐exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of ...forests. Researchers have proposed various strategies for stomatal regulation of leaf gas‐exchange that include maintaining a constant leaf internal CO₂, cᵢ, a constant drawdown in CO₂ (cₐ − cᵢ), and a constant cᵢ/cₐ. These strategies can result in drastically different consequences for leaf gas‐exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas‐exchange responses to varying cₐ. The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas‐exchange responses to cₐ. To assess leaf gas‐exchange regulation strategies, we analyzed patterns in cᵢ inferred from studies reporting C stable isotope ratios (δ¹³C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of cₐ spanning at least 100 ppm. Our results suggest that much of the cₐ‐induced changes in cᵢ/cₐ occurred across cₐ spanning 200 to 400 ppm. These patterns imply that cₐ − cᵢ will eventually approach a constant level at high cₐ because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant cᵢ. Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low cₐ, when additional water loss is small for each unit of C gain, and increasingly water‐conservative at high cₐ, when photosystems are saturated and water loss is large for each unit C gain.
Nocturnal increases in water potential (ψ) and water content (θ) in the upper soil profile are often attributed to root water efflux, a process termed hydraulic redistribution (HR). However, ...unsaturated liquid or vapor flux of water between soil layers independent of roots also contributes to the daily recovery in θ (∆θ), confounding efforts to determine the actual magnitude of HR. We estimated liquid (J₁) and vapor (J v ) soil water fluxes and their impacts on quantifying HR in a seasonally dry ponderosa pine (Pinus ponderosa) forest by applying existing datasets of ψ, θ and temperature (T) to soil water transport equations. As soil drying progressed, unsaturated hydraulic conductivity declined rapidly such that J₁ was irrelevant (<2E—05 mm h⁻¹ at 0-60 cm depths) to total water flux by early August. Vapor flux was estimated to be the highest in upper soil (0-15 cm), driven by large T fluctuations, and confounded the role of HR, if any, in nocturnal θ dynamics. Within the 15-35 cm layer, J v contributed up to 40% of hourly increases in nocturnal soil moisture. While both HR and net soil water flux between adjacent layers contribute to θ in the 15-65 cm soil layer, HR was the dominant process and accounted for at least 80% of the daily recovery in θ. The absolute magnitude of HR is not easily quantified, yet total diurnal fluctuations in upper soil water content can be quantified and modeled, and remain highly applicable for establishing the magnitude and temporal dynamics of total ecosystem water flux.
ABSTRACT
Hydraulic redistribution (HR) occurs in many ecosystems; however, key questions remain about its consequences at the ecosystem level. The objectives of the present study were to quantify ...seasonal variation in HR and its driving force, and to manipulate the soil–root system to elucidate physiological components controlling HR and utilization of redistributed water. In the upper soil layer of a young Douglas‐fir forest, HR was negligible in early summer, but increased to 0.17 mm day−1 (20–60 cm layer) by late August when soil water potential was approximately −1 MPa. When maximum HR rates were observed, redistributed water replenished approximately 40% of the water depleted from the upper soil on a daily basis. Manipulations to the soil or to the soil/plant water potential driving force altered the rate of observed HR indicating that the rate of HR is controlled by a complex interplay between competing soil and plant water potential gradients and pathway resistances. Separating roots from the transpiring tree resulted in increased HR, and sap flow measurements on connected and disconnected roots showed reversal of water flow, a prerequisite for HR. Irrigating a small plot with deuterated water demonstrated that redistributed water was taken up by small understorey plants as far as 5 m from the watering source, and potentially further, but the utilization pattern was patchy. HR in the upper soil layers near the watering plot was twice that of the control HR. This increase in HR also increased the amount of water utilized by plants from the upper soil. These results indicate that the seasonal timing and magnitude of HR was strongly governed by the development of water potential differences within the soil, and the competing demand for water by the above ground portion of the tree.
Although tree- and stand-level estimates of forest water use are increasingly common, relatively little is known about partitioning of soil water resources among co-occurring tree species. We studied ...seasonal courses of soil water utilization in a 450-year-old Pseudotsuga menziesii (Mirb.) Franco-Tsuga heterophylla (Raf.) Sarg. forest in southwestern Washington State. Soil volumetric water content (θ) was continuously monitored with frequency domain capacitance sensors installed at eight depths from 0.2 to 2 m at four locations in the vicinity of each species. Vertical profiles of root distribution and seasonal and daily courses of hydraulic redistribution (HR), sap flow and tree water status were also measured. Mean root area in the upper 60 cm of soil was significantly greater in the vicinity of T. heterophylla trees. However, seasonal water extraction on a root area basis was significantly greater near P. menziesii trees at all depths between 15 and 65 cm, leading to significantly lower water storage in the upper 65 cm of soil near P. menziesii trees at the end of the summer dry season. Greater apparent efficiency of P. menziesii roots at extracting soil water was attributable to a greater driving force for water uptake rather than to differences in root hydraulic properties between the species. The dependence of HR on θ was similar in soil near individuals of both species, but seasonal maximum rates of HR were greater in soil near P. menziesii because minimum values of θ were lower, implying a steeper water potential gradient between the upper and lower soil that acted as a driving force for water efflux from shallow roots. The results provide information on functional traits relevant for understanding the ecological distributions of these species and have implications for spatial variability of processes such as soil respiration and nutrient cycling.
•Elevation drove surface water isotope ratios in windward basins.•The elevation-δ18O relationships varied, even between adjacent basins.•Spatial models improved isoscapes compared to non-spatial ...models.•Utility of spatial models was greater where covariates explained little variation.
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![Evidence that higher [CO₂] ...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0029-8549_1432-1939/small "Evidence that higher [CO₂] increases tree growth sensitivity to temperature")
