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Moose (Alces alces L.) browsing in Gros Morne National Park has damaged its balsam fir (Abies balsamea (L.) Mill.)-dominated forest. A forest estate model was used to evaluate (i) the impacts of moose browsing and woodcutting on forest succession and (ii) strategies of forest restoration through planting and moose population management. The simulation results show that under current heavy browsing pressure growing stock of balsam fir decreases by 38%, but the area of spruce (Picea mariana (Mill.) BSP and P. glauca (Moench) Voss) increases by 32% over a 100-year planning horizon, compared to that under light browsing scenario which is assumed to be similar to the forest outside the Park due to moose population management. Annual allowable cut (AAC) for the Park’s 19 400 ha domestic harvest area is estimated to be around 120 979 m3 in a light browsing scenario, 21% higher than the sustainable harvest level in a heavy browsing scenario. The model forecasts a 97% reforestation of the Park’s 7 194 ha disturbed area by planting in the heavy browsing scenario, leading to an increase in total forest growing stock by 22% and AAC by 12%. Integration of planting with moose population management could be a more efficient way of restoring forest under high browsing pressure in GMNP.

Leaf level gas‐exchange measurements can be made on detached foliage to address the challenge of access to the crown of tall trees. However, detachment may impact leaf gas exchange. This necessitates the study of gas‐exchange characteristics of foliage on detached branches to assess the feasibility of using detached branches for gas‐exchange analysis. We compared photosynthetic parameters and stomatal conductance in foliage of attached and detached branches of balsam fir [Abies balsamea (L.) Mill.] during the growing season. Data were analyzed using a linear mixed‐effect model, with fixed and random effects (branch status and measurement month, and tree number, respectively). Branch detachment had no significant effects on: (i) photosynthesis at the current ambient CO2 concentration (400 µmol mol−1, A400); (ii) maximum rates of Ribulose‐1,5‐bisphosphate (RuBP) carboxylation (Vcmax) and regeneration (Jmax); (iii) the ratio of Jmax to Vcmax (i.e., Jmax:Vcmax), and (iv) stomatal conductance (gs) during the study period (p = 0.120–0.335). There was a strong seasonal effect on all gas‐exchange variables (p ≤ 0.001–0.015). Gas‐exchange measurements made on detached foliage during the warm summer months should be performed with care. Reliable gas‐exchange measurements can be obtained using balsam fir foliage on detached branches 50–80 cm in length, in cooler growing‐season months, up to 30 min after detachment.

Understanding plant resource use efficiencies (RUEs) and their tradeoffs in a desert shrub community, particularly as it concerns the usage of water, light, and nitrogen, remains an ecological imperative. Plant RUEs have been widely used as indicators to understand plant acclimation processes to unfavorable environmental conditions. This study aimed to examine seasonal dynamics in RUEs in two widely distributed plant species in a typical desert shrub community (i.e., Artemisia ordosica and Leymus secalinus) based on in-situ measurements of leaf photosynthesis, specific leaf area (SLA), leaf nitrogen concentration (i.e., Nmass + Narea), and several site-related abiotic factors. Both species exhibited significant seasonal variation in RUEs, with a coefficient of variation (CV) &gt, 30% and seasonal divergence among the various RUEs. Seasonal divergence was largely controlled by variation in stomatal conductance (Gs), which was in turn influenced by variation in soil water content (SWC) and water vapor pressure deficit (VPD). RUEs between species converged, being positively correlated, yielding: (i) r2 = 0.40 and p &lt, 0.01 for WUE, (ii) r2 = 0.18 and p &lt, 0.01 for LUE；and (iii) r2 = 0.25 and p &lt, 0.01 for NUE. RUEs for A. ordosica were mostly larger than those for L. secalinus, but less reactive to drought. This suggests A. ordosica was more conservative in its usage of available resources and was, therefore, better able to adapt to arid conditions. Resource use strategies between species differed in response to drought. Desert shrubs are projected to eventually replace grasses, as drought severity and duration increase with sustained regional climate change.

Climate models predict large climatic variability and more frequent, prolonged droughts in northern China. Still, few studies have examined the seasonal and interannual variation in water use efficiency (WUE) in northern China’s young plantations predisposed to drought. Here, variation in WUE was examined in a young plantation in Beijing over a 6-year period (2012–2017) at several timescales. The study was conducted by examining eddy-covariance measurements of gross ecosystem production (GEP) and evapotranspiration (ET). Results showed that the multi-year mean WUE for this young plantation was 1.71 ± 0.22 g C per kg H2O. Drought reduced WUE in summer, but increased it in autumn, with no apparent effect in spring. Long-term droughts usually had a greater effect on the annual GEP, ET and WUE than short-term droughts. Intense drought was observed to have a long-term legacy effect on ecosystem GEP, ET and WUE of about 3 years. Monthly WUE had a strong positive linear relationship with normalized difference vegetation index (NDVI) and soil volumetric water content (VWC) during the growing season. Seasonal variation in WUE was mainly regulated by the biophysical factors controlling GEP. In contrast, interannual variation in WUE was controlled to a greater extent by vegetation cover (i.e., NDVI), which in turn responded to variation in VWC. These results clearly show the importance of timing and duration of drought in controlling ecosystem WUE. Collectively, WUE was more strongly affected by factors affecting carbon uptake than water consumption. Our findings suggest that future drought, when coupled with increased climate variability, irrigation-support planning and selection of appropriate tree species should be central to afforestation programs in northern China.

Allometric variations in tree height and stem diameter are a genetically controlled trait that reflects the ability of trees to adjust to different environmental conditions. This paper examines the ecological control of abiotic and forest-state variables on height- and stem diameter-growth in oriental beech (Fagus orientalis Lipsky) in an unmanaged, high-elevation forest of northern Iran. Spatially explicit abiotic variables of growing-season mean potential solar radiation, mean air temperature, topographic wetness index (TWI, as a proxy of soil water content) and wind velocity essential to the analysis were generated numerically by computer models. Forest-state variables of total tree height (H) and diameter at breast height (D) and stand basal area (BA) were assessed at individual sample plots. Degree of explanation of observed variation in individual-tree H and plot-level changes in stem D (i.e. dD/dt) by the assemblage of modelled abiotic and observed tree variables was 87.4%, with a root mean squared error (RMSE) and mean absolute error (MAE) of 3.60 and 2.84 m and 67.3%, RMSE = 0.13 cm and MAE = 0.10 cm, respectively. Wind velocity, TWI and mean air temperature provided the greatest overall influence on the calculation of static H and annual D-increment, with relative impact of 38.3, 37.3 and 9.6 and 7.7, 6.4 and 6.7, respectively. The other variables, including BA, had weak to no control on tree-growth response. Methods used here are sufficiently general to address tree-growth response in many other tree species around the world, with or without changes to site conditions.

Models of forest growth and yield (G&amp, Y) are a key component in long-term strategic forest management plans. Models leveraging the industry-standard “empirical” approach to G&amp, Y are frequently underpinned by an assumption of historical consistency in climatic growing conditions. This assumption is problematic as forest managers look to obtain reliable growth predictions under the changing climate of the 21st century. Consequently, there is a pressing need for G&amp, Y modelling approaches that can be more robustly applied under the influence of climate change. In this study we utilized an established forest gap model (JABOWA-3) to simulate G&amp, Y between 2020 and 2100 under Representative Concentration Pathways (RCP) 2.6, 4.5, and 8.5 in the Canadian province of Newfoundland and Labrador (NL). Simulations were completed using the province’s permanent sample plot data and surface-fitted climatic datasets. Through model validation, we found simulated basal area (BA) aligned with observed BA for the major conifer species components of NL’s forests, including black spruce [Picea mariana (Mill.) Britton et al.] and balsam fir [Abies balsamea (L.) Mill]. Model validation was not as robust for the less abundant species components of NL (e.g., Acer rubrum L. 1753, Populus tremuloides Michx., and Picea glauca (Moench) Voss). Our simulations generally indicate that projected climatic changes may modestly increase black spruce and balsam fir productivity in the more northerly growing environments within NL. In contrast, we found productivity of these same species to only be maintained, and in some instances even decline, toward NL’s southerly extents. These generalizations are moderated by species, RCP, and geographic parameters. Growth modifiers were also prepared to render empirical G&amp, Y projections more robust for use under periods of climate change.

There is a need to understand the physio-morphological responses of northern tree species to climate change. The hypothesis of the current study was that provenance and light intensity were both influential in the control of intrinsic water-use efficiency (iWUE). Diameter at breast height (DBH)-increment was hypothesized as being more affected by provenance. Intrinsic water-use efficiency (iWUE), the ratio of photosynthesis (A) to stomatal conductance (gs), was assessed in foliage under two levels of photosynthetically active radiation (PAR, i.e., 300 and 1200 μmol m-2 s-1) in 63-year-old balsam fir [Abies balsamea (L.) Mill.] provenances derived from seed sources from across the species’ natural range (namely, within 44°–51° N latitudes and 53°–102° W longitudes) and cultivated in a common garden in eastern Canada. Diameter at breast height (DBH) of provenances from the common garden were measured when they were 42 and 58 years old (DBH1998, DBH2014). The results confirmed the hypotheses regarding the roles of provenance on iWUE and DBH (p &lt, 0.05), but showed a diminished role of PAR on iWUE. The lowest and highest mean iWUE and DBH among the provenances ranged between 0.028 and 0.031 and 0.079–0.083 μmol mmol−1 and 11.82–12.78 and 16.38–18.44 cm, respectively. Stomatal conductance of balsam fir had a strong relationship with iWUE at both light settings, whereas A had a weaker relationship with iWUE. There were no significant relationships between iWUE at the two light settings and climatic variables at the provenance source (p &gt, 0.05). Diameter at breast height in 2014 was significantly greater than DBH1998 (p &lt, 0.05). The relationships between DBH2014 and climatic variables at the provenance source were statistically significant (p &lt, 0.05). There was a significant positive relationship between iWUE and DBH measured in 2014. Survivorship of provenances was shown to vary with DBH-increment. The results show that for present-day and future forest management, (i) selection in balsam fir, in relation to iWUE should ideally be based on a criterion of intraspecific stomatal conductance, (ii) shade tolerance of balsam fir, population differentiation, and consistent pace of DBH-growth under variable climatic conditions are important factors in the species’ sustained growth under changes in forest dynamics projected to accompany changes in regional climate, (iii) temperature variables are strong indicators of DBH-increment in balsam fir, (iv) the effect of tree size on its survival is maintained under variable climatic conditions, and (v) there is a clear association between iWUE and the species’ radial growth.

Long-term predictions of forest dynamics, including forecasts of tree growth and mortality, are central to sustainable forest-management planning. Although often difficult to evaluate, tree mortality rates under different abiotic and biotic conditions are vital in defining the long-term dynamics of forest ecosystems. In this study, we have modeled tree mortality rates using conditional inference trees (CTREE) and multi-year permanent sample plot data sourced from an inventory with coverage of New Brunswick (NB), Canada. The final CTREE mortality model was based on four tree- and three stand-level terms together with two climatic terms. The correlation coefficient (R2) between observed and predicted mortality rates was 0.67. High cumulative annual growing degree-days (GDD) was found to lead to increased mortality in 18 tree species, including Betula papyrifera, Picea mariana, Acer saccharum, and Larix laricina. In another ten species, including Abies balsamea, Tsuga canadensis, Fraxinus americana, and Fagus grandifolia, mortality rates tended to be higher in areas with high incident solar radiation. High amounts of precipitation in NB’s humid maritime climate were also found to contribute to heightened tree mortality. The relationship between high GDD, solar radiation, and high mortality rates was particularly strong when precipitation was also low. This would suggest that although excessive soil water can contribute to heightened tree mortality by reducing the supply of air to the roots, occasional drought in NB can also contribute to increased mortality events. These results would have significant implications when considered alongside regional climate projections which generally entail both components of warming and increased precipitation.

The paper describes the development of predictive equations of windthrow for five tree species based on remote sensing of wind-affected stands in southwestern New Brunswick (NB). The data characterises forest conditions before, during and after the passing of extratropical cyclone Arthur, July 4–5, 2014. The five-variable logistic function developed for balsam fir (bF) was validated against remote-sensing-acquired windthrow data for bF-stands affected by the Christmas Mountains windthrow event of November 7, 1994. In general, the prediction of windthrow in the area agreed fairly well with the windthrow sites identified by photogrammetry. The occurrence of windthrow in the Christmas Mountains was prominent in areas with shallow soils and prone to localised accelerations in mean and turbulent airflow. The windthrow function for bF was subsequently used to examine the future impact of windthrow under two climate scenarios (RCP’s 4.5 and 8.5) and species response to local changes anticipated with global climate change, particularly with respect to growing degree-days and soil moisture. Under climate change, future windthrow in bF stands (2006–2100) is projected to be modified as the species withdraws from the high-elevation areas and NB as a whole, as the climate progressively warms and precipitation increases, causing the growing environment of bF to deteriorate.

Northern China’s plantations have long played a role in providing ecological services to the people of China. Reliable detection and attribution of ecosystem water and energy exchange is a precondition to the development of strategies for the sustainable management of these plantations. Here, the seasonal and interannual variability in surface energy exchange and evapotranspiration (ET) over a young pine plantation was investigated using eddy-covariance measurements collected over a four-year period (2012–2015). Seasonal patterns in net radiation (Rn), latent (LE) and sensible heat fluxes (H) were largely similar over the four years. H was the dominant energy component, with LE exceeding H only during the mid-growing season. A significant share of Rn was allocated to H during the 2014 summer drought. Energy exchange for the young pine plantation was characterized by high Bowen ratios (β = H/LE; 6.28, 6.42, 5.65, and 5.34 for the four years), high H/Rn (0.37, 0.36, 0.36, and 0.30), and low LE/Rn (0.15, 0.17, 0.17, and 0.16), with mean annual values of 5.92, 0.35, and 0.16 for β, H/Rn, and LE/Rn, respectively. Daily maximum and annual ET were 4.8, 4.5, 4.1, and 3.3 mm day−1 and 328, 371, 290, and 326 mm for the four years, with a mean annual value of 329 mm. Seasonal variation in ET was strongly controlled by biological factors (i.e., bulk surface conductance and vegetation greenness and density, as characterized by the normalized difference vegetation index) regulated by soil water availability and water vapor pressure deficit (VPD). Evapotranspiration varied interannually and the evapotranspiration-to-precipitation ratio (ET/P) ranged from 0.79 to 1.62. Soil water replenishment through precipitation during the non-growing period of the previous year and mid-growing season of the current year was responsible for the interannual variation observed in ET. These results clearly indicate the importance of precipitation timing and soil moisture carry-over from previous years in controlling ecosystem energy and water vapor exchange. Irrigation during the spring and dry periods of the year is highly favorable for growth of plantation trees in northern China.

Soil respiration (Rs) plays an important role in the terrestrial carbon cycle, but how canopy photosynthesis and abiotic drivers interact to affect Rs is poorly understood. This study aimed at examining the degree of control gross ecosystem productivity (GEP), photosynthetically active radiation (PAR), and soil temperature (Ts) and water content (SWC) may have on Rs in a semi-arid shrub ecosystem. We applied wavelet analysis and non-parametric spectral Granger-causality to a multi-year dataset. Wavelet coherence revealed synchronized diel cycles in photosynthesis proxies (GEP and PAR) and Rs. The spectral Granger-causality analysis suggested a possible causal linkage between GEP and Rs at the diel scale during the growing season. Significant wavelet coherence was also observed between GEP and Rs at seasonal to annual scales (200–365-day periods), with Rs lagging GEP by an average of two days. Fluctuations in SWC showed non-continuous temporal covariance with Rs over 4–64-day periods. Apart from direct moisture effects on decomposition processes, SWC also seemed to regulate Rs indirectly by affecting canopy carbon assimilation. Our findings indicate that photosynthesis may modulate Rs at multiple timescales in semi-arid shrub ecosystems. Future studies should combine field manipulations with spectral analysis for a mechanistic understanding of the coupling between photosynthesis and Rs.

Desert regions of northern China have often been reported with climate change signals. Thus, assessments of dryness in space and time have become critical for the development of climate change. Earlier studies have identified long-term patterns of dryness in northern China, but they have usually been of limited value to land-management planning as they ignored local-to-regional-scale climate features. To identify cause-and-effect relationship between aridity and vegetation cover, changes in aridity and vegetation cover were tracked with the assistance of a chronological series of surfaces based on the mapping of aridity and normalized difference vegetation index (NDVI) and convergent cross mapping. By tracking regional-scale variation in precipitation, air temperature, aridity from 1961 to 2013 (53 years), and vegetation cover dynamics from 1982 to 2013 (32 years), we show that precipitation increased in approximately 70% of the greater desert region, including in the Ulanbuh, Tengger, Badain Jaran, Qaidam, Kumtag, Gurbantunggut, and Taklimakan Deserts. This increase was statistically strongest for the Gurbantunggut and Taklimakan Deserts. Our results indicate that air temperature rose in nearly all deserts in northern China at about 2.5 times greater than the global rate. NDVI largely increased in oases of the western desert region and the Mu Us Desert over the 1982–2013. For the most parts, aridity had a moderate control on NDVI, with no indication of vegetation feedback at an 8-km resolution. The wetting trend observed in the western desert regions (increasing aridity index, i.e., AI) coincided with an increase in NDVI; most regional increases in NDVI ranged between 0 and 0.0005 yr−1. In contrast, continued drying in the eastern desert regions (decreasing AI) was mostly associated with a decline in NDVI

The extent of the effect of projected changes in climate on trees remains unclear. This study investigated the effect of climatic variation on morphological traits of balsam fir [Abies balsamea (L.) Mill.] provenances sourced from locations spanning latitudes from 44° to 51°N and longitudes from 53° to 102°W across North America, growing in a common garden in eastern Canada. Lower latitude provenances performed significantly better than higher latitude provenances (p < .05) with regard to diameter at breast height (DBH), height (H), and crown width (CW), a distinction indicative of genotypic control of these traits. There was, however, no significant difference among provenances in terms of survival (p > .05), an indication of a resource allocation strategy directed at survival relative to productivity in higher latitude provenances as seen in their lower DBH, H, and CW compared to the lower latitude provenances. Temperature had a stronger relationship with DBH, H, and CW than precipitation, a reflection of adaptation to local conditions in populations of the species along latitudinal gradients. Both climatic variables had some effect on tree survival. These results suggest that the response of balsam fir to climatic variation will likely not be uniform in the species, but differ based on genetic characteristics between populations located in the northern and southern parts of the species’ range. Population differences in response to climatic variation may be evident earlier in growth traits, compared to survival in balsam fir. The findings of this study will facilitate modeling in the species that is reflective of genetic variation in response to climatic conditions, and guide provenance selection for utilization in terms of productivity or resilience as well as breeding programs directed at obtaining species that possibly combine both traits.

Moose (Alces alces L.) browsing in Gros Morne National Park has damaged its balsam fir (Abies balsamea (L.) Mill.)dominated forest. A forest estate model was used to evaluate (i) the impacts of moo...

The current understanding of acclimation processes in desert-shrub species to drought stress in dryland ecosystems is still incomplete. In this study, we measured sap flow in Artemisia ordosica and associated environmental variables throughout the growing seasons of 2013 and 2014 (May–September period of each year) to better understand the environmental controls on the temporal dynamics of sap flow. We found that the occurrence of drought in the dry year of 2013 during the leaf-expansion and leaf-expanded periods caused sap flow per leaf area (Js) to decline significantly, resulting in transpiration being 34 % lower in 2013 than in 2014. Sap flow per leaf area correlated positively with radiation (Rs), air temperature (T), and water vapor pressure deficit (VPD) when volumetric soil water content (VWC) was greater than 0.10 m3 m−3. Diurnal Js was generally ahead of Rs by as much as 6 hours. This time lag, however, decreased with increasing VWC. The relative response of Js to the environmental variables (i.e., Rs, T, and VPD) varied with VWC, Js being more strongly controlled by plant-physiological processes during periods of dryness indicated by a low decoupling coefficient and low sensitivity to the environmental variables. According to this study, soil moisture is shown to control sap-flow (and, therefore, plant-transpiration) response in Artemisia ordosica to diurnal variations in biophysical factors. This species escaped (acclimated to) water limitations by invoking a water-conservation strategy with the regulation of stomatal conductance and advancement of Js peaking time, manifesting in a hysteresis effect. The findings of this study add to the knowledge of acclimation processes in desert-shrub species under drought-associated stress. This knowledge is essential in modeling desert-shrub-ecosystem functioning under changing climatic conditions.

Monitoring responses of plants to stresses in situ using the chlorophyll-fluorescence (ChlF) technique has been of growing interest, but challenged by lack of stable daytime indicators, limiting the method’s application. Seasonal changes in diurnal responses of effective quantum yield of photosystem II (PSII) photochemistry (Φ PSII ) to photosynthetically active radiation (PAR) have been assumed to reflect changes in physiological status of plants in changing environments. We examined the responses of diurnal Φ PSII to PAR in relation to environmental factors through continuous season-long monitoring of ChlF of PSII in a desert shrub species, Artemisia ordosica . Response data were analysed with a modification of the diurnal regression method by Durako (2012) . Diurnal variation in Φ PSII for A. ordosica was largely controlled by PAR, decreasing with increasing PAR. The rates of the response of diurnal Ф PSII to PAR (slopes) were down-regulated by seasonal changes in environmental factors, in particular, high values for daily mean air temperature ( T a ) and vapor pressure deficit (VPD), and up-regulated with increases in soil water content (SWC). The y-intercepts of diurnal Ф PSII -PAR regressions were linearly related to maximal quantum yield of PSII photochemistry ( F v / F m ) during the growing season, increasing with increasing SWC. Our results confirm the suggestion by Durako (2012) that the y-intercept of diurnal Ф PSII -PAR relationship serves as a good proxy for F v / F m in non-invasive monitoring of the physiological status of plants with the ChlF technique.

Naturally growing vegetation often suffers from the effects of drought. There exists a vast number of drought indices (DI's) to assess the impact of drought on the growth of crops and naturally occurring vegetation. However, assessing the fitness of these indices for large areas with variable vegetation cover is often problematic because of the absence of adequate spatial information. In this study, we compared six DI's to NDVI (the normalized difference vegetation index), a common indicator of vegetation occurrence and health based on satellite-acquired reflectance data. The study area covers an aridity gradient from forests to deserts along a 2,400-km-long section across the Inner Mongolia Autonomous Region of China. On an annual timescale, standardized precipitation index (SPI) was the most appropriate in assessing drought in steppes and deserts. On a seasonal timescale, the self-calibrated Palmer drought severity index (scPDSI) displayed the greatest sensitivity during the summer, but not during the other seasons. On a monthly timescale, scPDSI demonstrated the greatest sensitivity to the various vegetation zones (i.e., forests, steppes, and deserts) in June and July. Further analysis indicated that summer drought had a lag-effect on vegetation growth, which varied from one to six months according to the specific vegetation cover. The mixed response of DI's to NDVI and the lag-effect in transitional vegetation on annual, seasonal, and monthly timescales were ascribed to differences in DI definition and the dominant plant species within the transitional cover. The current study has the potential to inform the drafting of selection criteria of DI's for the study of drought-related impact on naturally growing vegetation at timescales from month to year.

The impact of extreme desert conditions on long-term nighttime sap flow dynamics in desert-dwelling shrubs is poorly understood. Based on heat-balance measurements in a semiarid area of northwest China, we investigated the nighttime sap flow (transpiration) dynamics in Salix psammophila as affected by changes in site biophysical variables over a six-year period (2012-2017). Nocturnally, sap flow rate per leaf area ( J s n ) formed a positive linear relationship with air temperature (Ta), water vapor pressure deficit (VPD) and stomatal conductance (Gs), when Ta J s n during wet and dry years, respectively. Interannually, Ta and Gs were able to explain a significant portion of observed variation in J s n , individually explaining about 74 and 83% of that variation. Interannual control on J s n also varied with SWC, Ta and Gs explaining 84% of the variation. The diminished sensitivity of J s n to both Ta and VPD and increased sensitivity to SWC during dry years, suggested that the shrubs had a physiological capacity to adapt to dry soil conditions. Daytime sap flow rates ( J s d ) were significantly greater than those at night (i.e., J s n ), with an interannual mean difference of 0.15 ± 0.04 g cm−2 d−1 and coefficient of variation of 27%. In addition, J s n formed a positive linear association with J s d during the growing season. Mean J s n contributed to the daily total sap flow (i.e., J s n /Js24-h × 100) by 8-14% over the six years, with an interannual mean of 11%. Our findings underscore the importance of nighttime sap flow in S. psammophila and the importance of temperature and SWC in controlling nighttime water losses through transpiration. It is suggested that the proportion of nighttime sap flow could increase during plant acclimatization in response to increasing temperature and water shortages with future climate change.

Extreme seasonal droughts frequently occur in semiarid and arid areas of the world. They can exert a profound influence on the physiological state of desert plants. Short-term rainfall and long-term water shortages in the Mu Us Desert have differentiated the role of individual species with their distinct ability to resist and recover from drought. This study explores how the scale of rainfall events affects interannual variation in sap flow in three dominant desert-shrub species [i.e., Salix psammophila (SP), Artemisia ordosica (AO), and Hedysarum mongolicum (HM)], and whether the associated changes in soil-water dynamics helped to modify sap flow. As a result, interannual variation in sap flow (Js) was mostly driven by precipitation (PPT), Volumetric soil water content (VWC), and leaf area index (LAI), with their individual influences varying as a function of plant phenophases. The species acclimated to long-term summer drought (> 30 days, i.e., no rain over a 30-day period or longer) by reducing stomatal conductance in the three species by 66.5, 59.5, and 43.5%, respectively, suggesting a physiological adjustment in water conservation in response to drought. Average summer drought (June–July) reduced Js in the three species by 55.5 (in SP), 42 (AO), and 28.5% (HM), more than spring drought (May–June) at 28% in SP and 9.5% in AO. Among those species, HM exhibited the least sensitivity and the highest overall resistance and resilience to drought. The relationship between Js and VWC was modulated by the effect of PPT. Our findings provide compelling evidence that HM is seen to be better suited to future climatic warming, as the species may be capable of accessing deep groundwater reserves replenished by large PPT pulses (5–10 mm), in sustaining its physiological activity over longer periods. These results could help formulate a selection process in determining which shrub species to plant in the sustainable management of desert ecosystems.

Summary Accurate estimates of soil temperature are important for quantifying hydrological and biological processes in hydrological models. Soil temperature predictions in the widely used Soil and Water Assessment Tool (SWAT) have large prediction errors when applied to regions with significant snow cover during winter. In this study, a new physically-based soil-temperature module is developed as an alternative to the empirical soil-temperature module currently used in SWAT. The physically-based module ​simulates soil temperature in different soil layers as a result of energy transfer between the atmosphere and soil (or snow) interface. The modified version of SWAT with the new soil-temperature module in place, introduces only three new parameters over the original soil-temperature module. Both the original and new soil-temperature modules are tested against field data from the Black Brook Watershed, a small watershed in Atlantic Canada. The results indicate that both versions of soil-temperature module ​are able to provide acceptable predictions of temperature in different layers of the soil during non-winter seasons. However, the original module severely underestimates soil temperatures in winter (within −10 to −20 °C), while the new module produces results that are more consistent with field measurements (within −2 to 2 °C). In addition, unlike its counterpart, the new module ​is able to simulate freeze–thaw cycles in the soil profile. Ice-water content variations in winter are reasonably simulated by the new module for different snow cover scenarios. In general, modified-SWAT improves prediction accuracy on baseflow discharge compared with the original-SWAT, due to improved estimates of soil temperature during winter. The new physically-based soil-temperature module has greatly improved the ability of SWAT to predict soil temperatures under seasonal snow cover, which is essential to the application of the model in regions like Atlantic Canada.

In the late 1980s, large forest companies began precommercial thinning (PCT) operations in young northern hardwood cutovers in New Brunswick, Canada. To provide supporting growth and yield information, an industrial experiment was established at residual stand densities of 1300, 1600, 1900, and 2200 stems ha−1. Stand responses were examined for measurements recorded at 0 (1987), 5 (1992), 10 (1997), 16 (2003), and 23 (2010) years after establishment. Average diameter at breast height, quadratic mean diameter, stand basal area, and stand total volume growth increased as stem density decreased from PCT. There were significant linear differences for many of these variables between treatments and time periods (year). No significant differences were detected in tree height between treatments. In 2010, the four PCT thinning treatments did not exhibit any differences in potential sawlogs at 2.4 m (8 ft) and 3.6 m (12 ft) lengths. Significant differences were observed for 4.9 m (16 ft) sawlogs that were p...

The impact of extreme weather events on water-carbon coupling and ecosystem water use efficiency (WUE) in arid to semi-arid conditions is poorly understood. Evapotranspiration (ET) and gross ecosystem production (GEP) were based on continuously eddy-covariance measurements taken over an urban-forest reserve in Beijing, in a 3-year period (2012–2014) to calculate WUE (GEP:ET). Our objective was to investigate the seasonal response of WUE to changing environmental and drought conditions at different timescales. Annually, the forest produced new plant biomass at 2.6 ± 0.2 g C per kg of water loss. Within each season, interactions of surface conductance (gc) and normalized difference vegetation index (NDVI; i.e., gc × NDVI) in spring, net radiation (Rn) and air temperature (Ta; i.e., Rn × Ta) in summer, and Rn and vapor pressure deficit (D; i.e., Rn × D) in autumn were found as the significant variables explaining seasonal variation in WUE. Daily WUE correlated positively with Ta and NDVI during the growing season, but a negative relationship during excessively dry periods (i.e., 2014). Daily WUE decreased during warm and dry days or remained nearly constant at low levels due to proportional decreases in GEP and ET. An extreme drought during the leaf expansion led to a greater decline in GEP than in ET, causing WUE to be lower in 2012 and 2014 than that in 2013. In contrast, an extreme drought during the leaf coloration led to a greater decline in ET than in GEP, causing higher WUE in 2013 and 2014 than that in 2012. We concluded that: (i) high soil water content (SWC) during leaf expansion was more important than high SWC in mid-summer or autumn for maintaining a high seasonal WUE; and that (ii) seasonal water availability combined with variable drought severity and duration during periods of changing Ta, caused seasonal ET and GEP to respond differently, introducing significant variation in seasonal WUE.

A precise prediction of the response in soil respiration (Rs) to climate warming requires the temperature-driven portion of the response to be characterized separately from the global, undecomposed response. Eliminating non-temperature-driven effects, which tend to confound the response in Rs, is possible with Singular Spectrum Analysis (SSA), a non-parametric approach to timeseries analysis. Rs and ancillary data were collected at three microsites in a desert-shrubland ecosystem in northwest China over a three-year period (i.e., 2013–2015). The microsites include desert-sites with non-biocrusted, lichen-crusted, and moss-crusted soil surfaces, or NC, LC, and MC, respectively. Results revealed that temperature sensitivity in Rs (i.e., Q10) based on the original, unprocessed Rs-data and a seasonal equation relating Rs to temperature were similar among microsites, with similar amplitudes in Rs. High-frequency bins of Rs, with sub-signals with a period

With increased usage of plantations in the control of desertification and water shortages in the Mu Us Desert, water use by desert-dwelling shrubs, such as Salix psammophila has become a serious topic in the management of ecologically-important plants. However, our understanding of the unevenness in water consumption across multiple timescales is still limited for many desert shrub species. Interannual variation in water consumption and its environmental controls are scarcely documented. To attend to this deficit, sap-flow measurements were continuously collected in a region of the Mu Us Desert from 2012 to 2017 to investigate the biophysical controls on sap-flow and water-consumption dynamics in S. psammophila. Diurnally, sap flow per leaf area (Js) was shown to be controlled by shortwave radiation (Rs), peaking in summer about two hours ahead of Rs, and about three to five hours ahead of air temperature (Ta) and vapor pressure deficit (VPD), respectively. Seasonal patterns in Js varied with precipitation and subsequent changes in soil volumetric water content (VWC). Soil volumetric water content was observed to modify the influence of the other environmental variables on Js (i.e., Ta, Rs, and VPD), with Js being more responsive to the site variables, when VWC > 0.09 m3 m−3, and less responsive, when

Decision making on water resources management at ungauged, especially large-scale watersheds relies on hydrological modeling. Physically based distributed hydrological models require complicated setup, calibration, and validation processes, which may delay their acceptance among decision makers. This study presents an approach to develop a simple decision support tool (DST) for decision makers and economists to evaluate multiyear impacts of land use change and best management practices (BMPs) on water quantity and quality for ungauged watersheds. The example DST developed in the present study was based on statistical equations derived from Soil and Water Assessment Tool (SWAT) simulations and applied to a small experimental watershed in northwest New Brunswick. The DST was subsequently tested against field measurements and SWAT simulations for a larger watershed. Results from DST could reproduce both field data and model simulations of annual stream discharge and sediment and nutrient loadings. The relative error of mean annual discharge and sediment, nitrate–nitrogen, and soluble-phosphorus loadings were −6, −52, 27, and −16 %, respectively, for long-term simulation. Compared with SWAT, DST has fewer input requirements and can be applied to multiple watersheds without additional calibration. Also, scenario analyses with DST can be directly conducted for different combinations of land use and BMPs without complex model setup procedures. The approach in developing DST can be applied to other regions of the world because of its flexible structure.

Modelling tree habitat suitability (HS) is a common practice to assess tree species distribution across a broad range of spatial scales. However, it is seldom used to test the extent that modelled HS-scores (probability of species occurrence) can represent on-the-ground measures of species-structural characteristics. In this study, we compare four parametric and non-parametric models generated with the R-package, sdm, to assess the potential for these models to estimate tree species distribution of balsam fir [Abies balsamea (L.) Mill.] in naturally-growing forests across an extensive landscape. Central to this development are inventory plot data of species presence-absence and four abiotic factors linked to plant growth and distribution. The study’s abiotic factors include: (1) photosynthetically active radiation; (2) growing degree-days; (3) relative extractable soil water content; and (4) near-surface wind speed, all expressed spatially at 30-m resolution. To gauge whether modelled HS can explain structural characteristics in balsam fir-dominated stands, field-based estimates of site index (SI) and cumulative aboveground biomass (ABG) were compared against independently-derived HS-scores. The results showed that: (i) random forest was the most successful at representing species distribution of balsam fir among the four methods considered; (ii) overall growing conditions for balsam fir was observed to be most favourable on north-facing slopes, particularly in the northwest part of the target landscape, where near-surface air temperatures are cooler, soils are moderately wetter, and wind speeds are lower; (iii) tree-based calculations of SI were partially characterised by patterns in modelled HS-scores, due to scale differences (i.e., from individual tree to 30-m grid cells) and an inadequate number of sample trees; and (iv) patterns of cumulative AGB were more accurately represented by species HS. Modelled HS-scores, as potential indicators of tree species habitat preference, AGB, and species distribution, can offer key ecological information essential to inform forest management and conservation planning at the landscape level.

Quantifying ecosystem water use efficiency (WUE), its spatial variation along an aridity gradient, and its control in response to drought are crucial to understanding regional eco-physiological processes of heterogeneous landscapes. This study examined the magnitude, large-scale spatial patterns in WUE, and underlying drivers by examining published data from 31 eddy-covariance (EC) towers and remote sensing based assessments of normalized difference vegetation index (NDVI) along a southwest-to-northeast aridity gradient in China. Average growing-season WUE at the 31 sites was 1.67 ± 0.98 g C kg−1 H2O. Water use efficiencies among vegetation types were significantly higher for cropland and forest, and lower for shrubland and grassland ecosystems (all p-values

Global climate models project significant changes to the air temperature and precipitation regimes in some regions of the Northern Hemisphere. These meteorological changes will have associated impacts to the surface and shallow subsurface thermal regimes, which are of interest to practitioners and researchers in many disciplines of the natural sciences. For example, groundwater temperature is critical for providing and sustaining suitable thermal habitat for cold-water salmonids. To investigate the surface and subsurface thermal effects of atmospheric climate change, seven downscaled climate scenarios (2046–2065) for a small forested catchment in New Brunswick, Canada were employed to drive the surface energy and moisture flux model, ForHyM2. Results from these seven simulations indicate that climate change-induced increases in air temperature and changes in snow cover could increase summer surface temperatures (range −0.30 to +3.49 °C, mean +1.49 °C), but decrease winter surface temperatures (range −1.12 to +0.08 °C, mean −0.53 °C) compared to the reference period simulation. Thus, changes to the timing and duration of snow cover will likely decouple changes in average annual air temperature (mean +2.11 °C) and average annual ground surface temperature (mean +1.06 °C). The projected surface temperature data were then used to drive an empirical surface to groundwater temperature transfer function developed from measured surface temperature and depth-dependent groundwater temperature. Results from the empirical transfer function indicated that the change in groundwater temperature will exhibit seasonality at shallow depths (1.5 m), but be seasonally constant and approximately equivalent to the change in the mean annual surface temperature at deeper depths (8.75 m). The increases in future groundwater temperature suggest that the thermal sensitivity of baseflow-dominated stream to decadal climate change may be greater than previous studies have indicated. The ecological significance of these findings is discussed.

This paper presents a pre- and post-harvest comparison of stream temperatures collected in five neighbouring streams (sub-catchments) over a period of five years (1994-1998). The aim of the study was to determine whether land cover changes from clear cutting in areas outside forest buffer zones (applied to streams >0.5 m wide) might contribute to an increase in summer mean stream temperatures in buffered streams downslope by infusion of warmed surface and sub-surface water into the streams. Specific relationships were observed in all five forest streams investigated. To assist in the analysis, several spatially-relevant variables, such as land cover change, mid-summer potential solar radiation, flow accumulation, stream location and slope of the land were determined, in part, from existing aerial photographs, GIS-archived forest inventory data and a digital terrain model of the study area. Spatial calculations of insolation levels for July 15th were used as an index of mid-summer solar heating across sub-catchments. Analysis indicated that prior to the 1995 harvest, differences in stream temperature could be attributed to (i) topographic position and catchment-to-sun orientation, (ii) the level of cutting that occurred in the upper catchment prior to the start of the study, and (iii) the average slope within harvested areas. Compared to the pre-harvest mean stream temperatures in 1994, mean temperatures in the three streams downslope from the 1995 harvest areas increased by 0.3 to 0.7°C (representing a 4-8% increase; p-value of normalised temperatures < Keywords: terrain attributes, solar radiation, land cover, forest buffers, New Brunswick regulations, spatial modelling, DEM, forest covertypes

Thermal dissipation of excess excitation energy is an important photoprotective mechanism that plants have evolved to cope with surplus illumination. However, light-energy-partitioning dynamics in an exotic sand-dune willow (Salix psammophila) commonly used in restoring and/or stabilizing sand lands in northwestern China is largely unknown. In this study, chlorophyll fluorescence (ChlF) of photosystem II (PSII) was continuously monitored in situ in Salix psammophila to investigate plant acclimation processes driven by excessive solar radiation and extreme air temperatures (Ta). As part of a heat-regulation mechanism, energy partitioning is shown to vary with prevailing environmental conditions. In this investigation, energy absorbed during periods of moderate photosynthetically active radiation (PAR &lt, 1200 &mu, moL&middot, m&minus, 2&middot, s&minus, 1) was largely allocated towards photochemistry (&Phi, PSII) with nominal amounts to thermal dissipation through reversible thermal dissipation (&Phi, NPQr). In extremely high solar radiation (PAR &gt, 1500 &mu, 1) or in a cold temperature (Ta &lt, 0 &deg, C), more energy was dissipated by way of non-regulated thermal energy (&Phi, f,D) and sustained thermal dissipation (&Phi, NPQs), leading to non-reversible photoinhibition or photodamage. This was mainly as a result of the low utilization and high absorption of light energy by PSII under cold conditions and physiologically-induced vulnerability. It was concluded that Salix psammophila had a clear tolerance to high temperatures and moderate solar radiation, but tended to be more vulnerable to high solar radiation and cold temperature. Based on species sensitivity to extreme environmental conditions, practical application and extension of Salix psammophila for land-restoration purposes should be approached cautiously, especially in high-latitude or high-altitude desert ecosystems commonly affected by events of high solar radiation and cold temperature.