Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important ecological, hydrological, and socioeconomic implications. Using a simplified water‐balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into evaporation and transpiration. On the basis of this framework, we suggest that streamflow and evaporation processes are affected by woody plant encroachment in different ways, depending on the degree and seasonality of aridity and the availability of subsurface water. Differences in landscape physiography, climate, and runoff mechanisms mediate the influence of woody plants on hydrological processes. Streamflow is expected to decline as a result of woody plant encroachment in landscapes dominated by subsurface flow regimes. Similarly, encroachment of woody plants can be expected to produce an increase in the fractional contribution of bare soil evaporation to evapotranspiration in semiarid ecosystems, whereas such shifts may be small or negligible in both subhumid and arid ecosystems. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change—such as potential changes in carbon cycling within an ecosystem. In locations where woody plant encroachment results in increased plant transpiration and concurrently the availability of soil water is reduced, increased accumulation of carbon in soils emerges as one prediction. Thus, explicitly considering the ecohydrological linkages associated with vegetation change provides needed information on the consequences of woody plant encroachment on water yield, carbon cycling, and other processes.

Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1-100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks.

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The natural ratio of stable carbon isotopes (δC) was compared to leaf structural and chemical characteristics in evergreen conifers in the north-central Rockies, United States. We sought a general model that would explain variation in δC across altitudinal gradients. Because variation in δC is attributed to the shifts between supply and demand for carbon dioxide within the leaf, we measured structural and chemical variables related to supply and demand. We measured stomatal density, which is related to CO supply to the chloroplasts, and leaf nitrogen content, which is related to CO demand. Leaf mass per area was measured as an intermediate between supply and demand. Models were tested on four evergreen conifers: Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii, and Pinus contorta, which were sampled across 1800 m of altitude. We found significant variation among species in the rate of δC increase with altitude, ranging from 0.91‰ km for A. lasiocarpa to 2.68‰ km for Pinus contorta. Leaf structure and chemistry also varied with altitude: stomatal density decreased, leaf mass per area increased, but leaf nitrogen content (per unit area) was constant. The regressions on altitude were particularly robust in Pinus contorta. Variables were derived to describe the balance between supply and demand; these variables were stomata per gram of nitrogen and stomata per gram of leaf mass. Both derived variables should be positively related to internal CO supply and thus negatively related to δC. As expected, both derived variables were negatively correlated with δC. In fact, the regression on stomatal density per gram was the best fit in the study (r =0.72, P<0.0001); however, the relationships were species specific. The only general relationship observed was between δC and LMA: δC (‰)=-32.972+ 0.0173×LMA (r =0.45, P<0.0001). We conclude that species specificity of the isotopic shift indicates that evergreen conifers demonstrate varying degrees of functional plasticity across environmental gradients, while the observed convergence of δC with LMA suggests that internal resistance may be the key to understanding inter-specific isotopic variation across altitude.

Rapid changes to the biosphere are altering ecological processes worldwide. Developing informed policies for mitigating the impacts of environmental change requires an exponential increase in the quantity, diversity, and resolution of field‐collected data, which, in turn, necessitates greater reliance on innovative technologies to monitor ecological processes across local to global scales. Automated digital time‐lapse cameras – “phenocams” – can monitor vegetation status and environmental changes over long periods of time. Phenocams are ideal for documenting changes in phenology, snow cover, fire frequency, and other disturbance events. However, effective monitoring of global environmental change with phenocams requires adoption of data standards. New continental‐scale ecological research networks, such as the US National Ecological Observatory Network (NEON) and the European Union's Integrated Carbon Observation System (ICOS), can serve as templates for developing rigorous data standards and extending the utility of phenocam data through standardized ground‐truthing. Open‐source tools for analysis, visualization, and collaboration will make phenocam data more widely usable.

[1] Recent studies have illuminated the process of hydraulic redistribution, defined as the translocation of soil moisture via plant root systems, but the long-term ecohydrologic significance of this process is poorly understood. We investigated hydraulic redistribution (HR) by Prosopis velutina Woot. (velvet mesquite) in an upland savanna ecosystem over a two-year period. Our goal was to quantify patterns of HR by mesquite roots and assess how this affects tree water use and productivity. We used the heat ratio method to monitor bi-directional sap flow, an analog of HR, in both lateral and tap roots. Additionally, we monitored soil water content and used the eddy covariance technique to quantify ecosystem carbon dioxide and water exchange. Mesquite roots redistributed large amounts of water throughout the year, even during periods of canopy dormancy. Dormant season precipitation (November-March) was often taken up by shallow lateral roots and transferred downward in the soil profile by deeper lateral and tap roots. Such a transfer was also apparent when the trees were active and moisture from summer rainfall was plant available in the upper soil layers. As the upper soil layers dried, sap flow moving toward the canopy in the lateral roots diminished and water use from deeper soils increased via the taproots. The relationship between root sap flow and above-canopy fluxes suggested that deeper ''stored'' water from HR allowed the trees to transpire more in the spring that followed a winter with significant downward redistribution. Patterns of lateral and tap root sap flow also implied that redistribution may extend the growing season of the trees after summer rains have ended and surface soils are dry, thus allowing the trees to photosynthesize through periods of seasonal drought. The large hydrologic magnitude and the ecological effects of HR we studied, along with mounting evidence of this process occurring in many other ecosystems, indicates that HR should be accounted for in many ecohydrologic modeling efforts.

Anthropogenic climate change is likely to alter the patterns of moisture availability globally. The consequences of these changes on species distributions and ecosystem function are largely unknown, but possibly predictable based on key ecophysiological differences among currently coexisting species. In this study, we examined the environmental and biological controls on transpiration from a piñon-juniper (Pinus edulis-Juniperus osteosperma) woodland in southern Utah, USA. The potential for climate-change-associated shifts in moisture inputs could play a critical role in influencing the relative vulnerabilities of piñons and junipers to drought and affecting management decisions regarding the persistence of this dominant landscape type in the Intermountain West. We aimed to assess the sensitivity of this woodland to seasonal variations in moisture and to mechanistically explain the hydraulic strategies of P. edulis and J. osteosperma through the use of a hydraulic transport model. Transpiration from the woodland was highly sensitive to variations in seasonal moisture inputs. There were two distinct seasonal pulses of transpiration: a reliable spring pulse supplied by winter-derived precipitation, and a highly variable summer pulse supplied by monsoonal precipitation. Transpiration of P. edulis and J. osteosperma was well predicted by a mechanistic hydraulic transport model (R2 = 0.83 and 0.92, respectively). Our hydraulic model indicated that isohydric regulation of water potential in P. edulis minimized xylem cavitation during drought, which facilitated drought recovery (94% of pre-drought water uptake) but came at the cost of cessation of gas exchange for potentially extended periods. In contrast, the anisohydric J. osteosperma was able to maintain gas exchange at lower water potentials than P. edulis but experienced greater cavitation over the drought and showed a lesser degree of post-drought recovery (55% of pre-drought uptake). As a result, these species should be differentially affected by shifts in the frequency, duration, and intensity of drought. Our results highlight the sensitivity of this woodland type to potential climate-change-associated shifts in seasonal moisture patterns and demonstrate the utility of mechanistic hydraulic models in explaining differential responses of coexisting species to drought.

In water-limited environments of the intermountain region of North America, summer precipitation may play a role in the structure and function of aridland communities and ecosystems. This study examined the potential reliance on summer precipitation of two widespread, coexisting woody species in the southwestern United States, Pinus edulis Englmn. (Colorado piñon) and Juniperus osteosperma (Torr) Little (Utah juniper). The current distributions of P. edulis and J. osteosperma are highly suggestive of different dependencies on summer rainfall. We hypothesized that P. edulis was dependent on summer precipitation, utilizing summer precipitation even during extremely dry summers, whereas J. osteosperma was not dependent, using summer precipitation only when amounts were above some minimum threshold. Using sap flux and stable isotopic methods to assess seasonal water sources and water use efficiency, we examined the response of these two species to seasonal variations in moisture at a site located near the northern limits of the North American monsoon. Both sap flux and isotopic results indicated that P. edulis was responsive to summer rain, while J. osteosperma was not. Following summer rain events, sap flux density increased in P. edulis for several days, but not in J. osteosperma. Isotopic evidence indicated that P. edulis took up summer-derived moisture to a greater extent than J. osteosperma. Values of the natural abundance stable isotope ratio of carbon of leaf soluble carbohydrates increased over the summer for P. edulis, indicative of assimilation at higher water use efficiency, but were invariant for J. osteosperma. Our results supported the hypothesis that P. edulis and J. osteosperma are differentially sensitive to summer precipitation and are discussed in the light of potential changes in the seasonality of precipitation associated with climate change.