Forest Water Balance Research Articles

Regional patterns and drivers of modelled water flows along environmental, functional, and stand structure gradients in Spanish forests

Abstract. The study of the water cycle in the forest at large scales, such as countries, is challenging due to the difficulty of correctly estimating forest water flows. Hydrological models can be coupled with extensive forest data sources, such as national forest inventories, to estimate the water flow of forests over large extents, but so far the studies conducted have not analysed the role of stand structure variables or the functional traits of the forest on predicted blue and green water flows in detail. In this study, we modelled the water balance of Spanish forests using stand structure and species data from forest inventories to understand the effects of climate, stand structure, and functional groups on blue water flows. We calculated blue water and green water flows and expressed them relative to received precipitation. Relative blue water flow was mainly concentrated in the wetter regions (Atlantic and alpine biomes) of Spain (around 25 %) in comparison with the Mediterranean biomes (10 %–20 %) and during the autumn–winter season. The leaf area index (LAI) of the forest stand is the most important predictor of relative blue water, exhibiting a negative effect until it reaches a plateau at higher levels (around 2.5–3). Deciduous forests showed a greater relative blue water flow than evergreen functional groups (25 %–35 % and 10 %–25 %, respectively) primarily due to leaf fall during the autumn–winter season. This study highlights how green water is decoupled from blue water; namely, blue water depends on winter and autumn precipitation, while green water depends on the spring and summer water demand and how the species' functional traits (deciduous vs. evergreen) can influence blue water production.

Ecohydrological assessment of the water balance of the world's highest elevation tropical forest (Polylepis)

Polylepis trees grow at elevations above the continuous tree line (3000–5000 m a.s.l.) across the Andes. They tolerate extreme environmental conditions, making them sensitive bioindicators of global climate change. Therefore, investigating their ecohydrological role is key to understanding how the water cycle of Andean headwaters could be affected by predicted changes in environmental conditions, as well as ongoing Polylepis reforestation initiatives in the region. We estimate, for the first time, the annual water balance of a mature Polylepis forest (Polylepis reticulata) catchment (3780 m a.s.l.) located in the south Ecuadorian páramo using a unique set of field ecohydrological measurements including gross rainfall, throughfall, streamflow, and xylem sap flow in combination with the characterization of forest and soil features. We also compare the forest water balance with that of a tussock grass (Calamagrostis intermedia) catchment, the dominant páramo vegetation. Annual gross rainfall during the study period (April 2019–March 2020) was 1290.6 mm yr−1. Throughfall in the Polylepis forest represented 61.2 % of annual gross rainfall. Streamflow was the main component of the water balance of the forested site (59.6 %), while its change in soil water storage was negligible (<1 %). Forest evapotranspiration was 54.0 %, with evaporation from canopy interception (38.8 %) more than twice as high as transpiration (15.1 %). The error in the annual water balance of the Polylepis catchment was small (<15 %), providing confidence in the measurements and assumptions used to estimate its components. In comparison, streamflow and evapotranspiration at the grassland site accounted for 63.7 and 36.0 % of the water balance, respectively. Although evapotranspiration was larger in the forest catchment, its water yield was only marginally reduced (<4 %) in relation to the grassland catchment. The substantially higher soil organic matter content in the forest site (47.6 %) compared to the grassland site (31.8 %) suggests that even though Polylepis forests do not impair the hydrological function of high-Andean catchments, their presence contributes to carbon storage in the litter layer of the forest and the underlying soil. These findings provide key insights into the vegetation-water‑carbon nexus in high Andean ecosystems, which can serve as a basis for future ecohydrological studies and improved management of páramo natural resources considering changes in land use and global climate.

Increased tree water use with the development of a dense understory layer in a North American hardwood forest

AbstractThe formation of a single species, recalcitrant understory vegetation layer can limit tree regeneration and, in the long term, modify the composition of forests. Few studies have investigated how recalcitrant vegetation influences competition for water resources although the formation of a dense understory is likely to modify the forest water balance. In eastern North American hardwood forests, the development of a dense understory layer of American beech (Fagus grandifolia) has been observed in stands dominated by sugar maple (Acer saccharum), a phenomenon that shares many characteristics associated with recalcitrant vegetation. Given that water availability is generally negatively correlated with stand density, we hypothesized that the formation of a dense understory beech layer increased competition for water resources, thus leading to reduced water use by sugar maple trees in beech‐dominated stands. Using thermal dissipation sensors, we measured sap flux density (Fd) of two sugar maple trees at three beech‐dominated sites and three control sites. During the growing season, Fd of sugar maple trees was significantly larger at beech‐dominated sites compared to control sites, indicating a greater rate of water use by sugar maples in stands with a dense understory beech layer. We provide two hypotheses to explain our results at the tree scale: (i) reduced cover by forest floor vegetation could limit transpiration by this layer, thus allowing increased water availability to supply transpiration by overstory trees, or (ii) increased tree transpiration rate could be a mechanism to satisfy nutrient requirements in beech‐dominated stands often associated with lower soil fertility.

Effects of forest thinning on sap flow dynamics and transpiration in a Japanese cedar forest

Historically, forest thinning in Japan was conducted to obtain high-quality timber from plantations. Today, in contrast, thinning is also motivated by forest water balance and climate change considerations. It is in this context that the present study examines the effects of thinning on the ecophysiological responses of remaining trees, which are inadequately understood, especially in relation to changes in the magnitude and duration of transpiration. Sap flux densities were measured in both outer and inner sapwood to obtain stand-scale transpiration for two years in the pre-thinning state and three years post-thinning. The effects of thinning on transpiration were quantitatively evaluated based on canopy conductance models. The larger increases in outer sap flux density were found in the first year after the treatment, while those in inner sap flux density were detected in the second and third years. The remaining trees required a few of years to adjust to improved light conditions of the lower crown, resulting in a delayed response of inner sap flux density. As a result of this lag, transpiration was reduced to 71 % of the pre-thinning condition in the first year, but transpiration recovered to the pre-thinning levels in the second and third years due to compensating contributions from inner sap flow. In terms of more accurately chronicling the thinning effect, the distribution of sap flux density with respect to its radial pattern, is necessary. Such measurements are key to more comprehensively examining the ecophysiological response of forest plantations to thinning and, ultimately, its effect on the forest water balance.

Fog interception in spruce-fir and mixed northern hardwood forests of Great Smoky Mountains National Park, Southeast USA

Abstract: Fog interception is a significant component of the water balance of Southern Appalachian spruce-fir forests in the Southeast USA. Here, fog interception rates are quantified for spruce-fir and northern hardwood trees in Great Smoky Mountains National Park (GSMNP), Tennessee and North Carolina, as a precursor to examining how interspecific differences in fog interception could affect catchment water balances if there is widespread vegetation change from spruce-fir to hardwoods. A water-balance approach was implemented, based on paired open-site (rainfall) and beneath-canopy (throughfall) electronically recording tipping-bucket rain gauges that were in place in the spruce-fir zone of GSMNP from May to November 2021. Comparing identified fog interception events to actual conditions captured by a webcam, 90% of verifiable events had conditions that were either clearly or potentially favorable for fog interception. Estimated fog interception gain ranged from averages of 0.24 to 0.69 mm day−1, representing 3 to 8% of rainfall. Results are consistent with the expectation of higher fog interception gain for spruce and fir than for birch, the representative hardwood species. Quantification of fog interception rates provides valuable information about ecohydrological processes in ecologically significant Southern Appalachian spruce-fir forests.

Effect of changes in forest water balance and inferred root reinforcement on landslide occurrence and sediment generation following Pinus radiata harvest on Tertiary terrain, eastern North Island, New Zealand

Background: The frequent occurrence of storm-initiated landslides following harvesting of Pinus radiata D.Don in steep, Tertiary terrain, East Coast region, North Island, New Zealand, is of increasing concern. This paper documents the influence of tree removal and of replacement plantings on the canopy water balance and soil moisture regime when slopes are at their most vulnerable to landslide occurrence. Methods: At a previously established study site, rainfall, throughfall, and soil moisture data were collected before a mature stand of P. radiata was harvested. After harvesting, part of the study site was replanted with P. radiata at 1000 stems ha-1 and part with 500 stems ha-1. Relationships between hydrological changes and landslide occurrence are discussed in relation to planting density, site factors, root system development, silvicultural regimes, and alternative land use options for mitigating erosion in highly erodible hill country. Results: Following harvesting, soil moisture levels remained higher for longer than under a mature forest until rainfall interception and evapotranspiration returned to pre-harvest levels. This coincided with canopy closure, irrespective of planting density. After thinning, interception and evapotranspiration decreased, then regained the equivalent of a closed canopy 2 years later. Landslide occurrence was highest on slopes &gt;25° and with a NE aspect. Sediment generation rates were highest in 2─4-year-old plantings, then decreased markedly with increasing tree age. Conclusions: Irrespective of planting density, P. radiata had little influence on the soil-water regime until canopy interception, evapotranspiration rates, soil-drying and recharge cycles returned to pre-harvest levels, coinciding with canopy closure. During this period, pore-water pressures at times of heavy or prolonged rainfall likely result in soil saturation and an increase in landslides. The progressive loss of root strength of the harvested trees had a secondary influence. The duration of the post-harvest period of heightened slope vulnerability to landslide initiation is a function of the combined influences of site factors on rates of tree growth and survival, and of the planting density regime on the canopy water balance and soil water content until the development of an effective live soil-root reinforcement system. For areas identified as high risk, the targeting of high-value timber species with longer rotation length, including consideration of coppicing species, would minimise the risk of slope failure at harvest. Very high-risk areas unsuited to rotational harvesting will ultimately require transitioning to a permanent indigenous forest cover.

Variation in Hydraulic Properties of Forest Soils in Temperate Climate Zones

The structure of forests in temperate climates has been changing to ensure the resilience of trees. This change affects the local water balance. Knowledge of soil hydraulic properties (SHP) is essential to assess the water cycle in ecosystems. There is little knowledge about the impact of tree species on SHP and the water balance. Based on a compilation of 539 related studies we aimed at identifying the effects of tree species and age on SHP in temperate climates. However, most studies concentrated on soil biogeochemical properties, whereas only 256 studies focused on SHP. The literature presents no standard methods for assessing SHP and there is no knowledge of their variations in forests. We present a systematic overview of the current state of knowledge on variations in SHP based on forest type in temperate climates. We identify the gaps and weaknesses in the literature and the difficulties of evaluating the reviewed studies. More studies following standardised methodologies are needed to create a robust database for each forest type and soil texture. It would improve the assessment of the forest water balance through calibrated plot/site-scale process models. Such a database does not yet exist, but it would greatly improve the management and development of future forest ecosystems.

Characteristics and modelling of sap flow of degraded Populus simonii in areas where the ecology is vulnerable

AbstractPlant transpiration plays a crucial role in the water balance of forest stands and water yield in arid and semiarid ecosystems. However, our understanding of the characteristics and modeling of transpiration or sap flow of degraded plantations in vulnerable ecology areas is still limited. In this study, the sap flow velocity (SF) of Populus simonii under different categories of degradation (LD, lightly degraded; MD, moderately degraded; SD, seriously degraded) during the growing season in the water‐wind erosion crisscross region of the Chinese Loess Plateau was continuously monitored and estimated using the modified Jarvis‐Stewart (MJS), modified process‐based (MBTA) and multiple linear regression (MLR) models. We found that the daily SF of P. simonii under the three categories of degradation was significantly different (p &lt; 0.05), and degradation degree was negatively correlated with SF. Soil water deficit significantly reduced SF (p &lt; 0.05). Soil water conditions and P. simonii degradation change the peak time of SF rather than the startup time. Three models give good performance for simulating the daily SF of degraded P. simonii, especially the MJS model. The stress of vapour pressure deficit (VPD) on SF first decreases and then increases with increasing VPD, and the VPD threshold is LD &lt; MD &lt; SD. The results provide a valuable reference for revealing the causes of degradation and assessing the drought resistance of plantation ecosystems in water‐limited areas.

A low cost, low power sap flux device for distributed and intensive monitoring of tree transpiration

Accurate estimation of transpiration in individual trees is important for understanding plant responses to environmental drivers, closing the water balance in forest stands and catchments, and calibrating earth system models, among other applications. However, the cost and power consumption of commercial systems based on sap flow methods still limit their usage. We developed and tested a cost-effective (<$150), simple to construct, and energy efficient sap flux device based on the heat pulse method. Energy savings were achieved by reducing the voltage of heat pulses and using an internal clock to completely shut down the device between pulses. Device accuracy was confirmed by laboratory estimates of sap flow made on excised branches of Acer saccharum and Tsuga canadensis (adjusted R2 = 0.96). In a 174-d field installation of 12 devices, batteries (eight rechargeable Ni-MH AA) needed to be replaced every 14 days. Sap flux measurements in the field tracked expected variations in vapor pressure deficit and tree phenology. The low cost, compact design, reliability, and power consumption of this device enable sap flux studies to operate with more replication and in more diverse ecological settings than has been practical in the past.

Water Balance Components of Sub-Mediterranean Downy Oak Landscapes of Southeastern Crimea

This article discusses the processes of moisture intake, redistribution, and consumption within the downy oak forest community, along with their interannual and interseasonal water balance dynamics. The study of the water balance components was conducted using a combination of field research methods and geoinformation modeling on the territory of the Karadag landscape and ecological station of the Karadag Nature Reserve for the period from 2010 to 2020. The study of the water balance of downy oak forests located at the furthest extent of their range represents an important problem, whose solution will further scientific understanding by uncovering individual patterns of the internal organization of such systems. The indicators having the most tangible impact on the water balance are the amount of precipitation and evapotranspiration. The average annual precipitation on the territory of Karadag for the analyzed period was 448 mm; in recent years, a decrease in the amount of precipitation has been recorded. The evapotranspiration values within the downy oak forests approximately coincide with the values of this indicator in the Mediterranean region to average 450 mm per year. The influence of stemflow and relief features on the redistribution of moisture within the landscape is described. The analyzed water balance components’ dynamics form conditions conducive to the displacement of steppe communities by forest species.

The effects of land‐use conversion on evapotranspiration and water balance of subtropical forest and managed tea plantation in Taihu Lake Basin, China

AbstractWidespread clearing of natural forests for tea plantations have profoundly altered the local water balance and energy budget. However, studies of comprehensive effects of land‐use conversion on energy and hydrological cycling are still rare. In this study, we used 5 years (2014–2018) of eddy covariance (EC) data, modelled estimates, and hydrological monitoring of different functional layers (canopy, litter and soil layers) to summarize the seasonal and inter‐annual variations and differences of evapotranspiration and water balance components in tea plantations (Camellia sinensis) and natural bamboo forests (Phyllostachysedulis). Results suggested the bamboo forest had higher actual evapotranspiration (ETa) (835 ± 5 mm) than the tea plantation (730 ± 19 mm), owing to differences in canopy conditions and plant physiology. Evident differences in ETa components were found between the tea plantation site and the bamboo forest site. The proportion of transpiration (Etr) was higher at the bamboo site (0.71–0.76) than the tea plantation site (0.60–0.65). Additionally, the soil evaporation ratio (0.10–0.13) was lower at the bamboo forest site than the tea plantation site (0.21–0.24) due to vegetation structure differences altering the radiation energy partitioning between the two sites. Moreover, the tea plantations were more susceptible to drought stress with low water storage in soil and simple rhizome‐root system. The comprehensive water holding capacity of canopy, litter, and soil layers of bamboo forest was 1.0–1.8 times of that in tea plantation. Compared with bamboo forest, pruning and losing of soil with intensive management activities enhanced drainage at the tea plantation site. This study provides insights into land‐use conversion of subtropical forest and managed tea plantation that have important implications on regional energy and water budgets.

Inter-annual variability of catchment water balance in a montane spruce forest

ABSTRACT Climatic conditions in temperate montane forests vary over multiple temporal scales. This study examines the effects of inter-annual climatic variability on the long-term water balance of a small headwater catchment with gradually changing vegetation characteristics. The research is based on direct observations of basic hydrological variables (precipitation, stream discharge), and process-based modelling of difficult-to-measure variables (forest transpiration, wet canopy evaporation, snow sublimation). The catchment water balance is evaluated in terms of the catchment water storage. The uncertainty of the estimated storage changes is expressed as a convolution of partial uncertainties associated with the individual components of catchment water balance. The results suggest a significant decrease of catchment water storage over the study period. The decrease is assumed to be caused by increasing winter temperatures, resulting in a more frequent occurrence of liquid precipitation and less prominent spring snowmelt. Considering vegetation’s conservative water-balance management proved essential for the studied environment.

Effects of long‐term nitrogen addition on water use by Cunninghamia lanceolate in a subtropical plantation

AbstractThe deposition of reactive nitrogen (N) has substantially increased in subtropical regions due to human activities. However, the effects of long‐term N addition on the water‐use efficiency of subtropical forests are poorly understood. Here, we conducted an 11‐year experiment in a subtropical Cunninghamia lanceolate plantation with four N‐addition levels: N0, N1, N2, and N3 (equivalent to 0, 6, 12, and 24 g N m−2 year−1, respectively). A thermal dissipation probe system was used to calculate sap flow, and plant biomass carbon was assessed by field investigation. The whole‐plant water use and water‐use efficiency were estimated. In addition, the δ13C of tree rings was used to indicate the plant intrinsic water‐use efficiency. The results showed that N3 treatment significantly increased the annual sap flow velocity, especially in summer and winter. Annual water use, plant growth, and water‐use efficiency did not significantly differ among the N treatments, but water use tended to be higher in N3 treatment than in N0 treatment. Furthermore, the significant reduction of δ13C in N3 treatment than in N0 treatment supported the inference that N addition could increase water use. We conclude that long‐term addition of high levels (but not of low levels) of N increased whole‐plant water use in C. lanceolate plantations. Our findings indicate that N deposition accompanied by high temperature and drought events may negatively affect water balance in subtropical forests.

Forest water use is increasingly decoupled from water availability even during severe drought

ContextKey to understanding forest water balances is the role of tree species regulating evapotranspiration (ET), but the synergistic impact of forest species composition, topography, and water availability on ET and how this shapes drought sensitivity across the landscape remains unclear.ObjectivesOur aims were to quantify (1) the effect of forest composition and topography including elevation and hillslope gradients on the relationship between ET and water availability, and (2) whether the relationship has changed over time.MethodsWe used remotely sensed Landsat and MODIS ET to quantify forest ET across the Blue Ridge ecoregion of the southeastern USA. Then quantified metrics describing ET responses to water availability and trends in responses over time and assessed how these metrics varied across elevation, hillslope, and forest composition gradients.ResultsWe demonstrated forest ET is becoming less constrained by water availability at the expense of lateral flow. Drought impacts on ET diverged along elevation and hillslope gradients, and that divergence was more pronounced with increasingly severe drought, indicating high elevation and drier, upslope regions tend to maintain ET rates even during extreme drought. We identified a decoupling of ET from water availability over time, and found this process was accelerated at higher elevations and in areas with more diffuse-porous trees.ConclusionsGiven the large proportion of forests on the landscape distributed across high elevation and upslope positions, reductions in downslope water availability could be widespread, amplifying vulnerability of runoff, the health of downslope vegetation, and aquatic biodiversity.

Effects of forest growth in different vegetation communities on forest catchment water balance

Forest ecosystems are critical for adjusting the dynamic balance of the hydrological cycle. This balance is affected by vegetation community types, phenology, and forest density. Previous long-term catchment-scale model studies have focused on changes in forest areas while ignoring the above factors. Since the 1980s, climate change caused by increases in atmospheric CO2 levels has enhanced forest growth. Moreover, amendments to forest management policies, including intermediate cuttings caused by economic factors, have yielded unprecedented changes in forest ecosystems. In this study, we designed a methodology and created a credible model using the Soil and Water Assessment Tool (SWAT) that can precisely reflect water balance variations caused by different ecosystem situations during long-term changes in forest density. We focused on the Yamato River catchment in Western Japan, which includes three planted forests and one primeval forest, each markedly different with respect to vegetation community composition and management policy. In the process, we examined the ratio of coniferous vegetation and broad-leaved vegetation in different forest areas, used remote sensing methods to quantify the maximum and minimum leaf area index (LAI) of each forest region over 40 years, and calibrated the model by comparing the LAI growth curve, evapotranspiration, and streamflow with observed data. Moreover, we separated the decadal canopy evaporation, transpiration, and soil evaporation from the SWAT output results. We found that (1) forest evapotranspiration has increased in recent decades because of the above reasons; (2) in young or well-managed forests, the forest water balance may have changed significantly with forest growth. For long-term studies, it is necessary to distinguish the growth characteristics of different forests during different periods, and a detailed definition of a mixed forest is required. The forest parameters and growth characteristics are critical for understanding forest ecosystems and cannot be ignored at catchment-scale.

Influence of light, water stress and shrub cover on sapling survival and height growth: the case of A. unedo, F. ornus and S. domestica under Mediterranean climate

Resprouter species are of great interest for Mediterranean forest resilience as they can survive fire or drought by activating dormant vegetative buds to produce regrowth. Understanding which factors control the early growth and survival of these species is needed to develop strategies to enhance stand resilience. In this study, three broadleaved resprouter species, two trees—Fraxinus ornus and Sorbus domestica—and a shrub—Arbutus unedo—were planted under a gradient of pine cover. Sapling survival and height growth were monitored for 6 years, 2 years after their plantation, as well as light and shrub cover. We developed a two-strata forest water balance model, which produces a water stress index. We aimed to determine the effect of light, water stress and shrub cover on sapling survival and height growth using a modelling approach. We found that high levels of transmittance had a negative impact on both survival and height growth of F. ornus and S. domestica, while it had a positive effect on A. unedo growth. Water stress was found to negatively affect survival and height growth of all species. Lastly, shrub cover had an overall positive effect on the survival and growth of saplings, indicating a facilitating effect of the shrubs. Our results suggest that shade-tolerant tree species like F. ornus and S. domestica are more suited to moderate and light cover, while photoinhibition-tolerant shrub species like A. unedo are more adapted to open conditions. To maximize sapling survival and growth, we recommend preserving the shrubby understorey when present.

Evapotranspiration from the understory of a tropical dry deciduous forest in Cambodia

The water balance of tropical dry deciduous forests is less well understood than some other forest ecosystems. To help close this knowledge gap, we separately measured the evapotranspiration from the whole ecosystem (ETW), transpiration (TR) and interception loss (IL) from overstory trees, and evapotranspiration from the understory vegetation (ETU) in a tropical dry deciduous forest in Cambodia. It was found that ETW was equivalent to 73.7% of rainfall (P) at the annual scale. In the dry season, ETW corresponded to 120.1% of P, which indicates the utilization of soil water replenished during the wet season. The sum of transpiration estimated by the thermal dissipation (TD) method with the original coefficient (TRG), IL, and ETU was smaller than ETW, except for the middle of the dry season, due to an underestimation of TRG. Although recently reported calibration coefficients can reasonably correct TRG, future calibrations of the TD method are highly recommended for the precise evaluation of single-tree-scale transpiration in tropical dry forests. The annual contribution of the understory vegetation to ETW (ETU/ETW) was 34.6%, leading to the conclusion that the understory vegetation cannot be ignored when trying to gain a comprehensive understanding of the hydrologic cycle in tropical dry forests. The seasonal variations in ETU/ETW were mainly controlled by the leaf area index (LAI) of overstory trees, resulting from the overall stability of ETW and decreasing trend of ETU with increasing LAI in the wet season, with the opposite holding true in the dry season, i.e., decreasing ETW with the decline of LAI and less variations of ETU. Thus, LAI influenced both the seasonality and the annual contribution in ETU/ETW, exerting a notable influence on hydrological cycling in this forest.

North American temperate conifer (Tsuga canadensis) reveals a complex physiological response to climatic and anthropogenic stressors.

Rising atmospheric CO2 (ca) is expected to promote tree growth and lower water loss via changes in leaf gas exchange. However, uncertainties remain if gas-exchange regulation strategies are homeostatic or dynamical in response to increasing ca, as well as evolving climate and pollution inputs. Using a suite of tree ring-based δ13C-derived physiological parameters (Δ13C, ci, iWUE) and tree growth from a mesic, low elevation stand of canopy-dominant Tsuga canadensis in north-eastern USA, we investigated the influence of rising ca, climate and pollution on, and characterised the dynamical regulation strategy of, leaf gas exchange at multidecadal scales. Isotopic and growth time series revealed an evolving physiological response in which the species shifted its leaf gas-exchange strategy dynamically (constant ci; constant ci/ca; constant ca-ci) in response to rising ca, moisture availability and site conditions over 111yr. Tree iWUE plateaued after 1975, driven by greater moisture availability and a changing soil biogeochemistry that may have impaired a stomatal response. Results suggested that trees may exhibit more complex physiological responses to the changing environmental conditions over multidecadal periods, and complicating the parameterisation of Earth system models and the estimation of future carbon sink capacity and water balance in midlatitude forests and elsewhere.

Climatic water balance of mesophilic montane forest in the Huasteca region

We worked in areas of mesophilic mountain forest in the states of Puebla, Hidalgo and Veracruz, located within the Huasteca region. By its nature, the mountain mesophyll forest is a good water catcher. But its forest cover has decreased as a consequence of anthropogenic activities, negatively impacting water catchment. The temporal evolution (1979–2015) of the humidity index of the areas where mountain mesophyll forest exists was associated with the changes in its cover from 1997 to 2016. The results show that from 1979 to 2004, the humidity index decreased as a consequence of more than 29% deforestation. From 2005 to 2016, the deforestation rate did not exceed 1% and the humidity index presented an increasing trend. The conservation of this ecosystem is recommended as a priority to improve the amount of water in the region.

Transpiration of Dominant Tree Species Varies in Response to Projected Changes in Climate: Implications for Composition and Water Balance of Temperate Forest Ecosystems

The climate is changing in many temperate forests with winter snowpack shrinking and an increasing frequency of growing season air temperatures exceeding long-term means. We examined the effects of these changes on growing season rates of transpiration (sap flow) in two snow removal experiments in New Hampshire and Massachusetts, USA. Snow was removed during early winter, resulting in greater depth and duration of soil freezing compared to untreated plots. We examined the dominant tree species at each site, Acer saccharum at Hubbard Brook, NH and Acer rubrum and Quercus rubra at Harvard Forest, MA. Trees responded to a smaller snowpack, but with distinct species-specific responses consistent with ecological traits. Snow removal decreased rates of sap flow per kPa vapor pressure deficit (VPD) in sugar maples in the early growing season and red maples throughout the growing season. In contrast, sap flow rates per kPa VPD increased for red oak with snow removal. Downscaled climate projections from the Coupled Model Intercomparison Project indicate increases in heat stress days at both sites by the end of the century, leading to increased rates of whole-season transpiration across all three tree species, which will be offset in red maples and increased in red oaks with a smaller snowpack and increased frequency of heat stress days. Results of this study demonstrate that the combined effects of changes in climate during the growing season and winter will impact transpiration differently among tree species, with implications for forest water balance and tree species composition in the northeastern USA