Increased Needle Nitrogen Contents Did Not Improve Shoot Photosynthetic Performance of Mature Nitrogen-Poor Scots Pine Trees

Future carbon sequestration potential in a widespread transcontinental boreal tree species: Standing genetic variation matters!

Climate change (CC) necessitates reforestation/afforestation programs to mitigate its impacts and maximize carbon sequestration. But comprehending how tree growth, a proxy for fitness and resilience, responds to CC is critical to maximize these programs' effectiveness. Variability in tree response to CC across populations can notably be influenced by the standing genetic variation encompassing both neutral and adaptive genetic diversity. Here, a framework is proposed to assess tree growth potential at the population scale while accounting for standing genetic variation. We applied this framework to black spruce (BS, Picea mariana [Mill] B.S.P.), with the objectives to (1) determine the key climate variables having impacted BS growth response from 1974 to 2019, (2) examine the relative roles of local adaptation and the phylogeographic structure in this response, and (3) project BS growth under two Shared Socioeconomic Pathways while taking standing genetic variation into account. We modeled growth using a machine learning algorithm trained with dendroecological and genetic data obtained from over 2600 trees (62 populations divided in three genetic clusters) in four 48-year-old common gardens, and simulated growth until year 2100 at the common garden locations. Our study revealed that high summer and autumn temperatures negatively impacted BS growth. As a consequence of warming, this species is projected to experience a decline in growth by the end of the century, suggesting maladaptation to anticipated CC and a potential threat to its carbon sequestration capacity. This being said, we observed a clear difference in response to CC within and among genetic clusters, with the western cluster being more impacted than the central and eastern clusters. Our results show that intraspecific genetic variation, notably associated with the phylogeographic structure, must be considered when estimating the response of widespread species to CC.

Components explain, but do eddy fluxes constrain? Carbon budget of a nitrogen-fertilized boreal Scots pine forest

Nitrogen (N) fertilization increases biomass and soil organic carbon (SOC) accumulation in boreal pine forests, but the underlying mechanisms remain uncertain. At two Scots pine sites, one undergoing annual N fertilization and the other a reference, we sought to explain these responses. We measured component fluxes, including biomass production, SOC accumulation, and respiration, and summed them into carbon budgets. We compared the resulting summations to ecosystem fluxes measured by eddy covariance. N fertilization increased most component fluxes (P < 0.05), especially SOC accumulation (20×). Only fine-root, mycorrhiza, and exudate production decreased, by 237 (SD = 28) g C m-2  yr-1 . Stemwood production increases were ascribed to this partitioning shift, gross primary production (GPP), and carbon-use efficiency, in that order. The methods agreed in their estimates of GPP in both stands (P > 0.05), but the components detected an increase in net ecosystem production (NEP) (190 (54) g C m-2  yr-1 ; P < 0.01) that eddy covariance did not (19 (62) g C m-2  yr-1 ; ns). The pairing of plots, the simplicity of the sites, and the strength of response provide a compelling description of N effects on the C budget. However, the disagreement between methods calls for further paired tests of N fertilization effects in simple forest ecosystems.

Allocation Strategies of Carbon, Nitrogen, and Phosphorus at Species and Community Levels With Recovery After Wildfire

Plant stoichiometry and nutrient allocation can reflect a plant's adaptation to environmental nutrient changes. However, the allocation strategies of carbon (C), nitrogen (N), and phosphorus (P) between leaf and fine root in response to wildfire have been poorly studied. Our primary objective was to elucidate the trade-off of elemental allocation between above- and belowground parts in response to the soil nutrient changes after a wildfire. We explored the allocation sloping exponents of C, N, and P between leaf and fine root at the species and community levels at four recovery periods (year 2, 10, 20, and 30) after moderately severe wildfire and one unburned treatment in boreal forests in Great Xing'an Mountains, northeast China. Compared with the unburned treatment, leaf C concentration decreased and fine root C increased at year 2 after recovery. The leaf N concentration at year 10 after recovery was higher than that of unburned treatment. Plant growth tended to be limited by P concentration at year 10 after recovery. Nutrient allocation between leaf and fine root differed between species and community levels, especially in the early recovery periods (i.e., 2 and 10 years). At the community level, the nutrient concentrations of the leaf changed more as compared to that of the fine root at year 2 after recovery when the fine root nutrients changed more than those of the leaf. The different C, N, and P allocation strategies advanced the understanding of plant adaptation to soil nutrient changes during the postfire ecosystem restoration.

Organic nitrogen enhances nitrogen nutrition and early growth of Pinus sylvestris seedlings

Boreal trees are capable of taking up organic nitrogen (N) as effectively as inorganic N. Depending on the abundance of soil N forms, plants may adjust physiological and morphological traits to optimize N uptake. However, the link between these traits and N uptake in response to soil N sources is poorly understood. We examined Pinus sylvestris L. seedlings' biomass growth and allocation, transpiration and N uptake in response to additions of organic N (the amino acid arginine) or inorganic N (ammonium nitrate). We also monitored in situ soil N fluxes in the pots following an addition of N, using a microdialysis system. Supplying organic N resulted in a stable soil N flux, whereas the inorganic N resulted in a sharp increase of nitrate flux followed by a rapid decline, demonstrating a fluctuating N supply and a risk for loss of nitrate from the growth medium. Seedlings supplied with organic N achieved a greater biomass with a higher N content, thus reaching a higher N recovery compared with those supplied inorganic N. In spite of a higher N concentration in organic N seedlings, root-to-shoot ratio and transpiration per unit leaf area were similar to those of inorganic N seedlings. We conclude that enhanced seedlings' nutrition and growth under the organic N source may be attributed to a stable supply of N, owing to a strong retention rate in the soil medium.

Nitrogen and Boron Dosage Effects on Arginine Accumulation in Scots Pine Needles

Free arginine (Arg) content was observed to multiply when the level of nitrogen (N) nutrition was high, and additional fertilization with boron (B) potentiated this effect. Owing to this feature, conifers can be suggested for use as bioproducers of Arg. Concentrations of Arg in relation to N and B fertilization needed to be better understood. The effect of soil fertilization with N and B on accumulation of these elements and free Arg in one-year-old needles of 16-year-old Scots pine (Pinus sylvestris L.) trees was determined in this study. Plantations were fertilized with doses of N from 0 to 1000 kg ha−1 and B from 0 to 6 kg ha−1. Fertilization with 3 kg ha−1 B at N doses of 200–500 kg ha−1 stimulated the accumulation of N in needles of up to 3.1–3.6% dry weight (DW). The level of Arg in needles increased from 74.7 to 175.9 μmol g−1 DW at these levels of N and B.

Limits to photosynthesis: seasonal shifts in supply and demand for CO2 in Scots pine

Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (A_net ) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (g_m ) and its effect on the estimation of carboxylation capacity ( VCmax ). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over A_net . The seasonality of A_net was strongly dependent on VCmax and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that VCmax limited A_net in the spring, whereas daily temperatures and the gradual reduction of light availability limited A_net in the autumn, despite relatively high VCmax . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between g_m and A_net , but not between g_m and stomatal conductance.

Changes in plant nutrient utilization during ecosystem recovery after wildfire

Wildfire is the primary natural disturbance in boreal forest ecosystems. It substantially changes soil nutrient conditions and plant nutrient dynamics. After a wildfire, various plant strategies of nutrient utilization are fundamental to ecosystem recovery processes. Stability of plant nutrients reflects the ability of plants possessing relatively constant elemental concentrations in the face of nutrient changes, which can be calculated by the value of "nutrient homeostasis". However, the mechanism of how nutrient homeostasis mediates plant community recovery in post-fire ecosystems remains unknown. The dominant tree species that survived after fire and the new emergence of regenerated tree species are the important components of a plant community during the recovery process. Our primary objective was to elucidate the nutrient homeostasis trade-off between dominant and regenerated species over years after recovery. Five treatments, namely, 2 year, 10 year, 20 year, 30 years after moderate burning severity, and unburned forests, were designed in the boreal forests of Great Xing'an Mountains, Northeast China. Compared with unburned forests, wildfire lowered the average value of homeostasis of plant nutrients (N and P). Moreover, the mean homeostasis value of the dominant species (i.e., Larix gmelinii) was higher than that of the regenerated species (i.e., Betula platyphylla). The slope of relationship between nutrient homeostasis and recovery years of the regenerated species was higher than that of the dominant species, suggesting that the nutrient homeostasis in the regenerated species recovered more quickly than dominant species after recovery. Compared with the dominant species, changes in the regenerated species' homeostasis can explained more to the changes of species diversity during the years after recovery. This study revealed plant nutrient adaptation in different species and different plant organs with years after wildfire and highlighted the importance of nutrient homeostasis in plant adaptation strategies and the recovery of plant community.

Higher biomass partitioning to absorptive roots improves needle nutrition but does not alleviate stomatal limitation of northern Scots pine

Harsh environmental conditions affect both leaf structure and root traits. However, shoot growth in high-latitude systems is predominately under photoperiod control while root growth may occur for as long as thermal conditions are favorable. The different sensitivities of these organs may alter functional relationships above- and belowground along environmental gradients. We examined the relationship between absorptive root and foliar traits of Scots pine trees growing in situ along a temperate-boreal transect and in trees grown in a long-term common garden at a temperate latitude. We related changes in foliar nitrogen, phosphorus, specific leaf area, needle mass and ¹³ C signatures to geographic trends in absorptive root biomass to better understand patterns of altered tree nutrition and water balance. Increased allocation to absorptive fine roots was associated with greater uptake of soil nutrients and subsequently higher needle nutrient contents in the northern provenances compared with more southern provenances when grown together in a common garden setting. In contrast, the leaf δ¹³ C in northern and southern provenances were similar within the common garden suggesting that higher absorptive root biomass fractions could not adequately increase water supply in warmer climates. These results highlight the importance of allocation within the fine-root system and its impacts on needle nutrition while also suggesting increasing stomatal limitation of photosynthesis in the context of anticipated climatic changes.

Linking canopy-scale mesophyll conductance and phloem sugar δ13 C using empirical and modelling approaches

Interpreting phloem carbohydrate or xylem tissue carbon isotopic composition as measures of water-use efficiency or past tree productivity requires in-depth knowledge of the factors altering the isotopic composition within the pathway from ambient air to phloem contents and tree ring. One of least understood of these factors is mesophyll conductance (g_m ). We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative g_m descriptions and a simple transport of sugars from foliage down the trunk. We parameterised the model for a boreal Scots pine stand and compared simulated g_m responses with weather variations. We further compared the simulated δ¹³ C of new photosynthates among the different g_m descriptions and against measured phloem sugar δ¹³ C. Simulated g_m estimates of the seven descriptions varied according to weather conditions, resulting in varying estimates of phloem δ¹³ C during cold/moist and warm/dry periods. The model succeeded in predicting a drought response and a postdrought release in phloem sugar δ¹³ C indicating suitability of the model for inverse prediction of leaf processes from phloem isotopic composition. We suggest short-interval phloem sampling during and after extreme weather conditions to distinguish between mesophyll conductance drivers for future model development.

Limited vertical CO2 transport in stems of mature boreal Pinus sylvestris trees

Several studies have suggested that CO2 transport in the transpiration stream can considerably bias estimates of root and stem respiration in ring-porous and diffuse-porous tree species. Whether this also happens in species with tracheid xylem anatomy and lower sap flow rates, such as conifers, is currently unclear. We infused 13C-labelled solution into the xylem near the base of two 90-year-old Pinus sylvestris L. trees. A custom-built gas exchange system and an online isotopic analyser were used to sample the CO2 efflux and its isotopic composition continuously from four positions along the bole and one upper canopy shoot in each tree. Phloem and needle tissue 13C enrichment was also evaluated at these positions. Most of the 13C label was lost by diffusion within a few metres of the infusion point indicating rapid CO2 loss during vertical xylem transport. No 13C enrichment was detected in the upper bole needle tissues. Furthermore, mass balance calculations showed that c. 97% of the locally respired CO2 diffused radially to the atmosphere. Our results support the notion that xylem CO2 transport is of limited magnitude in conifers. This implies that the concerns that stem transport of CO2 derived from root respiration biases chamber-based estimates of forest carbon cycling may be unwarranted for mature conifer stands.

Estimating canopy gross primary production by combining phloem stable isotopes with canopy and mesophyll conductances

Gross primary production (GPP) is a key component of the forest carbon cycle. However, our knowledge of GPP at the stand scale remains uncertain, because estimates derived from eddy covariance (EC) rely on semi-empirical modelling and the assumptions of the EC technique are sometimes not fully met. We propose using the sap flux/isotope method as an alternative way to estimate canopy GPP, termed GPP_iso/SF , at the stand scale and at daily resolution. It is based on canopy conductance inferred from sap flux and intrinsic water-use efficiency estimated from the stable carbon isotope composition of phloem contents. The GPP_iso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPP_iso/SF to the GPP derived from PRELES, a model parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPP_iso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPP_iso/SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met.

Diurnal variation in mesophyll conductance and its influence on modelled water-use efficiency in a mature boreal Pinus sylvestris stand

Mesophyll conductance (gm) is a critical variable for the use of stable carbon isotopes to infer photosynthetic water-use efficiency (WUE). Although gm is similar in magnitude to stomatal conductance (gs), it has been measured less often, especially under field conditions and at high temporal resolution. We mounted an isotopic CO2 analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic 13C discrimination (Δ13C) on mature Pinus sylvestris trees. This allowed the calculation of gm, gs, net photosynthesis (Anet), and WUE. These measurements highlighted the asynchronous diurnal behaviour of gm and gs. While gs declined from around 10:00, Anet declined first after 12:00, and gm remained near its maximum until 16:00. We suggest that high gm played a role in supporting an extended Anet peak despite stomatal closure. Comparing three models to estimate WUE from ∆13C, we found that a simple model, assuming constant net fractionation during carboxylation (27‰), predicted WUE well, but only for about 75% of the day. A more comprehensive model, accounting explicitly for gm and the effects of daytime respiration and photorespiration, gave reliable estimates of WUE, even in the early morning hours when WUE was more variable. Considering constant, finite gm or gm/gs yielded similar WUE estimates on the diurnal scale, while assuming infinite gm led to overestimation of WUE. These results highlight the potential of high-resolution gm measurements to improve modelling of Anet and WUE and demonstrate that such gm data can be acquired, even under field conditions.

Nitrogen Nutrition of European Beech Is Maintained at Sufficient Water Supply in Mixed Beech-Fir Stands

Research highlights: Interaction effects of coniferous on deciduous species have been investigated before the background of climate change. Background and objectives: The cultivation of European beech (Fagus sylvatica L.) in mixed stands has currently received attention, since the future performance of beech in mid-European forest monocultures in a changing climate is under debate. We investigated water relations and nitrogen (N) nutrition of beech in monocultures and mixed with silver-fir (Abies alba Mill.) in the Black Forest at different environmental conditions, and in the Croatian Velebit at the southern distribution limit of beech, over a seasonal course at sufficient water availability. Material and methods: Water relations were analyzed via δ13C signatures, as integrative measures of water supply assuming that photosynthesis processes were not impaired. N nutrition was characterized by N partitioning between soluble N fractions and structural N. Results: In the relatively wet year 2016, water relations of beech leaves, fir needles and roots differed by season, but generally not between beech monocultures and mixed cultivation. At all sites, previous and current year fir needles revealed significantly lower total N contents over the entire season than beech leaves. Fir fine roots exhibited higher or similar amounts of total N compared to needles. Correlation analysis revealed a strong relationship of leaf and root δ13C signatures with soil parameters at the mixed beech stands, but not at pure beech stands. While glutamine (Gln) uptake capacity of beech roots was strongly related to soil N in the monoculture beech stands, arginine (Arg) uptake capacities of beech roots were strongly related to soil N in mixed stands. Conclusions: Leaf N contents indicated a facilitative effect of silver-fir on beech on sites where soil total N concentrations where low, but an indication of competition effect where it was high. This improvement could be partially attributed to protein contents, but not to differences in uptake capacity of an individual N source. From these results it is concluded that despite similar performance of beech trees at the three field sites investigated, the association with silver-fir mediated interactive effects between species association, climate and soil parameters even at sufficient water supply.

Photosynthetic refixation varies along the stem and reduces CO2 efflux in mature boreal Pinus sylvestris trees

Trees are able to reduce their carbon (C) losses by refixing some of the CO2 diffusing out of their stems through corticular photosynthesis. Previous studies have shown that under ideal conditions the outflowing CO2 can be completely assimilated in metabolically active, young stem and branch tissues. Fewer studies have, however, been carried out on the older stem sections of large trees and, accordingly, the importance of refixation is still unclear under natural environmental conditions. We investigated the spatial and temporal variation in refixation in ~90-year-old boreal Scots pine (Pinus sylvestris L.) trees by utilizing month-long continuous measurements of stem CO2 efflux (Ec) made at four heights along the bole. Refixation rates were found to vary considerably along the bole, leading to a 28% reduction in long-term Ec in the upper stem compared with a negligible reduction at breast height. This vertical pattern correlated with variation in light availability, bark chlorophyll content and bark type. Analysis of the vertical and diurnal patterns in Ec further suggested that the influence of sap flow on the observed daytime reduction in Ec was small. The areal rates of corticular photosynthesis were much lower than previous estimates of photosynthetic rates per unit leaf area from the same trees, implying that the impact of refixation on tree-scale C uptake was small. However, upscaling of refixation indicated that 23-27% of the potential Ec was refixed by the bole and the branches, thereby significantly reducing the woody tissue C losses. Thus, our results suggest that refixation needs to be considered when evaluating the aboveground C cycling of mature P. sylvestris stands and that breast-height estimates should not be extrapolated to the whole tree.

Temperature responses of photosynthetic capacity parameters were not affected by foliar nitrogen content in mature Pinus sylvestris

A key weakness in current Earth System Models is the representation of thermal acclimation of photosynthesis in response to changes in growth temperatures. Previous studies in boreal and temperate ecosystems have shown leaf-scale photosynthetic capacity parameters, the maximum rates of carboxylation (V_cmax ) and electron transport (J_max ), to be positively correlated with foliar nitrogen (N) content at a given reference temperature. It is also known that V_cmax and J_max exhibit temperature optima that are affected by various environmental factors and, further, that N partitioning among the foliar photosynthetic pools is affected by N availability. However, despite the strong recent anthropogenic influence on atmospheric temperatures and N deposition to forests, little is known about the role of foliar N contents in controlling the photosynthetic temperature responses. In this study, we investigated the temperature dependencies of V_cmax and J_max in 1-year-old needles of mature boreal Pinus sylvestris (Scots pine) trees growing under low and high N availabilities in northern Sweden. We found that needle N status did not significantly affect the temperature responses of V_cmax or J_max when the responses were fitted to a peaked function. If such N insensitivity is a common tree trait it will simplify the interpretation of the results from gradient and multi-species studies, which commonly use sites with differing N availabilities, on temperature acclimation of photosynthetic capacity. Moreover, it will simplify modeling efforts aimed at understanding future carbon uptake by precluding the need to adjust the shape of the temperature response curves to variation in N availability.

Annual climate variation modifies nitrogen induced carbon accumulation of Pinus sylvestris forests

We report results from long-term simulated external nitrogen (N) input experiments in three northern Pinus sylvestris forests, two of moderately high and one of moderately low productivity, assessing effects on annual net primary production (NPP) of woody mass and its interannual variation in response to variability in weather conditions. A sigmoidal response of wood NPP to external N inputs was observed in the both higher and lower productivity stands, reaching a maximum of ~65% enhancement regardless of the native site productivity, saturating at an external N input of 4-5 g N·m^-2 ·yr^-1 . The rate of increase in wood NPP and the N response efficiency (RE_N , increase in wood NPP per external N input) were maximized at an external N input of ~3 g N·m^-2 ·yr^-1 , regardless of site productivity. The maximum RE_N was greater in the higher productivity than the lower productivity stand (~20 vs. ~14 g C/g N). The N-induced enhancement of wood NPP and its RE_N were, however, markedly contingent on climatic variables. In both of the higher and lower productivity stands, wood NPP increased with growing season precipitation (P), but only up to ~400 mm. The sensitivity of the response to P increased with increasing external N inputs. Increasing growing season temperature (T) somewhat increased the N-induced drought effect, whereas decreasing T reduced the drought effect. These responses of wood NPP infused a large temporal variation to RE_N , making the use of a fixed value unadvisable. Based on these results, we suggest that regional climate conditions and future climate scenarios should be considered when modeling carbon sequestration in response to N deposition in boreal P. sylvestris, and possibly other forests.

Symbiosis constraints: Strong mycobiont control limits nutrient response in lichens

Symbioses such as lichens are potentially threatened by drastic environmental changes. We used the lichen Peltigera aphthosa-a symbiosis between a fungus (mycobiont), a green alga (Coccomyxa sp.), and N2-fixing cyanobacteria (Nostoc sp.)-as a model organism to assess the effects of environmental perturbations in nitrogen (N) or phosphorus (P). Growth, carbon (C) and N stable isotopes, CNP concentrations, and specific markers were analyzed in whole thalli and the partners after 4 months of daily nutrient additions in the field. Thallus N was 40% higher in N-fertilized thalli, amino acid concentrations were twice as high, while fungal chitin but not ergosterol was lower. Nitrogen also resulted in a thicker algal layer and density, and a higher δ13C abundance in all three partners. Photosynthesis was not affected by either N or P. Thallus growth increased with light dose independent of fertilization regime. We conclude that faster algal growth compared to fungal lead to increased competition for light and CO 2 among the Coccomyxa cells, and for C between alga and fungus, resulting in neither photosynthesis nor thallus growth responded to N fertilization. This suggests that the symbiotic lifestyle of lichens may prevent them from utilizing nutrient abundance to increase C assimilation and growth.

Basal area growth, carbon isotope discrimination, and intrinsic water use efficiency after fertilization of Douglas-fir in the Oregon Coast Range

Many hectares of intensively managed Douglas-fir (Pseudotsuga menziesii Mirb. Franco) stands in western North America are fertilized with nitrogen (N) to increase growth rates, but only about ⅔ of all stands respond. Understanding the mechanisms of response facilitates prioritization of stands for treatment. The primary objective of this study was to test the hypothesis that the short-term basal area growth response to a single application of 224 kg N ha^-1 as urea was associated with reduced stable carbon isotope discrimination (Δ¹³C) and increased intrinsic water use efficiency (iWUE) in a 20-yr-old plantation of Douglas-fir in the Oregon Coast Range, USA. Increment cores were measured to estimate earlywood, latewood, and total basal area increment over a time series from 1997 to 2015. Stable carbon isotope discrimination and iWUE were estimated using earlywood and latewood stable carbon isotope concentrations in tree-ring holocellulose starting seven years before fertilization in early 2009 and ending seven years after treatment. A highly significant (p<0.01) interaction effect between fertilization treatment and year was found for total basal area growth and earlywood basal area increment. Specifically, fertilized trees showed significant responses (p<0.05) in total basal area growth and earlywood basal area increment in the first (2009) and second (2010) growing seasons after fertilization in 2009. A marginally significant (p<0.10) fertilization effect was found for latewood basal area increment only in the first growing season after treatment. A significant treatment x year interaction was also found for Δ¹³C and iWUE in earlywood and latewood. Fertilization significantly reduced earlywood Δ¹³C and increased earlywood iWUE in the first and second growing seasons after fertilization. Only a marginally significant fertilization effect was detected for latewood Δ¹³C and iWUE in the second growing season after treatment. Previous studies of N fertilization of Douglas-fir forests have reported consistently increased growth and iWUE on low productivity sites treated with relatively high fertilization rates. This study suggested that these responses can also be observed on highly productive sites despite their lower frequency and apparently shorter duration. Other key mechanisms driving growth responses appear less important than iWUE, including an increase in LAI and shift from belowground to aboveground carbon allocation.