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Insect pests cost billions of dollars per year globally, negatively impacting food crops and infrastructure, and contributing to the spread of disease. Timely information regarding developmental stages of pests can facilitate early detection and control, increasing efficiency and effectiveness. In 2018, the U.S. National Phenology Network (USA-NPN) released a suite of ‘Pheno Forecast’ map products relevant to science and management. The Pheno Forecasts include real-time maps and short-term forecasts of insect pest activity at management-relevant spatial and temporal resolutions and are based on accumulated temperature thresholds associated with critical life-cycle stages of economically important pests. Pheno Forecasts indicate, for a specified day, the status of the insect’s target life-cycle stage in real time across the contiguous United States. The maps are available for 12 pest species including the invasive emerald ash borer (Agrilus planipennis Fairmaire [Coleoptera: Buprestidae]), hemlock woolly adelgid (Adelges tsugae Annand), and gypsy moth (Lymantria dispar Linnaeus [Lepidoptera: Erebidae]). Preliminary validation based on in-situ observations for hemlock woolly adelgid egg and nymph stages in 2018 indicated the maps to be ≥93% accurate depending on phenophase. Since their release in early 2018, these maps have been adopted by tree care specialists and foresters across the United States. Using a consultative mode of engagement, USA-NPN staff have continuously sought input and critique of the maps and delivery from end users. Based on feedback received, maps have been expanded and modified to include additional species, improved descriptions of the phenophase event of interest, and e-mail-based notifications to support management decisions.

ContextLandscape science relies on foundational concepts of landscape ecology and seeks to understand the physical, biological, and human components of ecosystems to support land management decision-making. Incorporating landscape science into land management decisions, however, remains challenging. Many lands in the western United States are federally owned and managed for multiple uses, including recreation, conservation, and energy development.ObjectiveWe argue for stronger integration of landscape science into the management of these public lands.MethodsWe open by outlining the relevance of landscape science for public land planning, management, and environmental effects analysis, including pertinent laws and policies. We identify challenges to integrating landscape science into public land management, including the multijurisdictional nature and complicated spatial pattern of public lands, the capacity of agencies to identify and fill landscape science needs, and perceptions about the meaning of landscape approaches to management.ResultsWe provide several recent examples related to landscape monitoring, restoration, reclamation, and conservation in which landscape science products were developed specifically to support decision-making.ConclusionsWe close by highlighting three actions—elevating the importance of science-management partnerships dedicated to coproducing actionable landscape science products, identifying where landscape science could foster efficiencies in the land-use planning process, and developing scenario-based landscape models for shrublands—that could improve landscape science support for public land planners and managers.

• Rising atmospheric CO

Citizen science involves a range of practices involving public participation in scientific knowledge production, but outcomes evaluation is complicated by the diversity of the goals and forms of citizen science. Publications and citations are not adequate metrics to describe citizen-science productivity. We address this gap by contributing a science products inventory (SPI) tool, iteratively developed through an expert panel and case studies, intended to support general-purpose planning and evaluation of citizen-science projects with respect to science productivity. The SPI includes a collection of items for tracking the production of science outputs and data practices, which are described and illustrated with examples. Several opportunities for further development of the initial inventory are highlighted, as well as potential for using the inventory as a tool to guide project management, funding, and research on citizen science.

Warming temperatures associated with climate change can have indirect effects on migratory birds that rely on seasonally available food resources and habitats that vary across spatial and temporal scales. We used two heat-based indices of spring onset, the First Leaf Index (FLI) and the First Bloom Index (FBI), as proxies of habitat change for the period 1901 to 2012 at three spatial scales: the US National Wildlife Refuge System; the four major bird migratory flyways in North America; and the seasonal ranges (i.e., breeding and non-breeding grounds) of two migratory bird species, Blue-winged Warbler (Vermivora cyanoptera) and Whooping Crane (Grus americana). Our results show that relative to the historical range of variability, the onset of spring is now earlier in 76% of all wildlife refuges and extremely early (i.e., exceeding 95% of historical conditions) in 49% of refuges. In all flyways but the Pacific, the rate of spring advance is generally greater at higher latitudes than at lower latitudes. This differential rate of advance in spring onset is most pronounced in the Atlantic flyway, presumably because of a "warming hole" in the southeastern US. Both FLI and FBI have advanced markedly in the breeding ranges-but not the non-breeding ranges-of the two selected bird species, albeit with considerable intra-range variation. Differences among species in terms of migratory patterns and the location and extent of seasonal habitats, as well as shifts in habitat conditions over time, may complicate predictions of the vulnerability of migratory birds to climate change effects. This study provides insight into how differential shifts in the phenology of disparate but linked habitats could inform local- to landscape-scale management strategies for the conservation of migratory bird populations.

Citizen science is a growing phenomenon. With millions of people involved and billions of in-kind dollars contributed annually, this broad extent, fine grain approach to data collection should be garnering enthusiastic support in the mainstream science and higher education communities. However, many academic researchers demonstrate distinct biases against the use of citizen science as a source of rigorous information. To engage the public in scientific research, and the research community in the practice of citizen science, a mutual understanding is needed of accepted quality standards in science, and the corresponding specifics of project design and implementation when working with a broad public base. We define a science-based typology focused on the degree to which projects deliver the type(s) and quality of data/work needed to produce valid scientific outcomes directly useful in science and natural resource management. Where project intent includes direct contribution to science and the public is actively involved either virtually or hands-on, we examine the measures of quality assurance (methods to increase data quality during the design and implementation phases of a project) and quality control (post hoc methods to increase the quality of scientific outcomes). We suggest that high quality science can be produced with massive, largely one–off, participation if data collection is simple and quality control includes algorithm voting, statistical pruning, and/or computational modeling. Small to mid-scale projects engaging participants in repeated, often complex, sampling can advance quality through expert-led training and well-designed materials, and through independent verification. Both approaches—simplification at scale and complexity with care—generate more robust science outcomes.

Developing Enterprise Tools and Capacities for Large-Scale Natural Resource Monitoring; Fort Collins, Colorado, 12–13 October 2016

Changes in the timing of plant and animal life cycle events, in response to climate change, are already happening across the globe. The impacts of these changes may affect biodiversity via disruption to mutualisms, trophic mismatches, invasions and population declines. To understand the nature, causes and consequences of changed, varied or static phenologies, new data resources and tools are being developed across the globe. The USA National Phenology Network is developing a long-term, multi-taxa phenological database, together with a customizable infrastructure, to support conservation and management needs. We present current and potential applications of the infrastructure, across scales and user groups. The approaches described here are congruent with recent trends towards multi-agency, large-scale research and action.

Questions Woody plant encroachment into former grasslands currently represents a major physiognomic shift globally. Seedling establishment is a critical demographic bottleneck and is considered to be alleviated by increases in water availability and negatively impacted by interactions with grasses, particularly when water stress increases. However, interactions with grasses that are not actively competing for resources (‘passive interactions’ when grasses are dead) has seldom been considered. Could the transition from a live to a dead grass (litter) canopy favour recruitment of woody seedlings in a semi-arid grassland of the American SW? How does the sign and intensity of grass–seedling interactions change across drastically different summer precipitation regimes with and without passive interactions? Location Sonoran Desert shrub savanna at the Santa Rita Experimental Range, near Tucson, AZ, US. Methods Four cohorts of Prosopis velutina seeds were planted annually (2002–2005) under rainout shelters that intercepted all incoming precipitation on a soil with sandy loam texture. Summer precipitation was manipulated to simulate either a 50% increase or decrease in the long-term mean, and cover was manipulated to simulate a grassland dominated by the C4 bunchgrass Heteropogon contortus or left unvegetated. Emergence and survival of P. velutina was monitored and compared across cover types, along with monitoring of soil water content and light interception. Results Strong active competition was observed with live grasses, under both summer drought and also under ample summer water supply. However, the pattern was reversed and strong passive facilitation of P. velutina was observed over time when grass canopies died and remained in place. This passive facilitation under dry summers was so strong that recruitment under dead grass conditions was comparable to that observed when ample water supply removed the effects of competition on unvegetated plots. Conclusions After significant mortality of standing grass canopies, which typically compete for limited soil moisture resources, rates of recruitment by shrubs may increase even with significant seasonal drought. This work extends our understanding of interactions among co-located organisms and their effects on plant community dynamics, and introduces a new hypothesis on how grass litter facilitates woody plant encroachment during seasonal droughts.

The sensitivity of phenology to environmental drivers can vary across geography and species. As such, models developed to predict phenology are typically site- or taxon-specific. Generation of site- and taxon-specific models is limited by the intensive in-situ phenological monitoring effort required to generate sufficient data to parameterize each model. Where in-situ phenological observations exist, the data are often subject to analytical issues due to the limited duration of any individual monitoring program, spotty site- and species- level coverage, lack of standardized methodology, and infrequent or variable census intervals. Together, these characteristics constrain our ability to make phenological inferences outside of select sites and taxa where long-duration, intensive monitoring has occurred. In this study, we leveraged two national, standardized phenology datasets to develop a multi-species and multi-site state-space survival model of the onset of deciduous tree and shrub spring (leaf out) and fall (leaf-color) events across temperate ecoregions of the United States. We used data from two national-scale phenological databases, a 9-year, broadly distributed dataset from the USA National Phenology Network and a 4-year dataset from the National Ecological Observatory Network, to quantify regional and interspecific variation in sensitivity to environmental drivers for both spring and fall leaf phenophases. Spring leaf out was generally promoted by longer days, spring growing degree day accumulation, overwinter chilling, and was suppressed by frost events, whereas fall leaf color was promoted by shorter days and cold accumulation. The sensitivity to most environmental drivers tended to be more variable among species than among the regions as defined here (EPA ecoregions of North America, excluding desert and tropical areas). The results of this study lay the groundwork for incorporating the growing collection of phenological observations into a generalized framework for predicting the transition states for any species, in any location.

Workshop on Phenology at Scales from Individual Plants to Satellite Pixels; Cambridge, Massachusetts, 21–23 June 2016

Citizen science has advanced science for hundreds of years, contributed to many peer-reviewed articles, and informed land management decisions and policies across the United States. Over the last 10 years, citizen science has grown immensely in the United States and many other countries. Here, we show how citizen science is a powerful tool for tackling many of the challenges faced in the field of conservation biology. We describe the two interwoven paths by which citizen science can improve conservation efforts, natural resource management, and environmental protection. The first path includes building scientific knowledge, while the other path involves informing policy and encouraging public action. We explore how citizen science is currently used and describe the investments needed to create a citizen science program. We find that: 1. Citizen science already contributes substantially to many domains of science, including conservation, natural resource, and environmental science. Citizen science informs natural resource management, environmental protection, and policymaking and fosters public input and engagement. 2. Many types of projects can benefit from citizen science, but one must be careful to match the needs for science and public involvement with the right type of citizen science project and the right method of public participation. 3. Citizen science is a rigorous process of scientific discovery, indistinguishable from conventional science apart from the participation of volunteers. When properly designed, carried out, and evaluated, citizen science can provide sound science, efficiently generate high-quality data, and help solve problems.

NPS landscape dynamics monitoring project, NPScape; NASA-NPS Landscape Climate Change Vulnerability Project (NASA Applied Sciences program) [10-BIOCLIM10-0034]; United States Geological Survey [G14AC00405]

© 2016 Elmendorf et al. Phenology is an integrative science that comprises the study of recurring biological activities or events. In an era of rapidly changing climate, the relationship between the timing of those events and environmental cues such as temperature, snowmelt, water availability, or day length are of particular interest. This article provides an overview of the observer-based plant phenology sampling conducted by the U.S. National Ecological Observatory Network (NEON), the resulting data, and the rationale behind the design. Trained technicians will conduct regular in situ observations of plant phenology at all terrestrial NEON sites for the 30-yr life of the observatory. Standardized and coordinated data across the network of sites can be used to quantify the direction and magnitude of the relationships between phenology and environmental forcings, as well as the degree to which these relationships vary among sites, among species, among phenophases, and through time. Vegetation at NEON sites will also be monitored with tower-based cameras, satellite remote sensing, and annual high-resolution airborne remote sensing. Ground-based measurements can be used to calibrate and improve satellite-derived phenometrics. NEON's phenology monitoring design is complementary to existing phenology research efforts and citizen science initiatives throughout the world and will produce interoperable data. By collocating plant phenology observations with a suite of additional meteorological, biophysical, and ecological measurements (e.g., climate, carbon flux, plant productivity, population dynamics of consumers) at 47 terrestrial sites, the NEON design will enable continental-scale inference about the status, trends, causes, and ecological consequences of phenological change.

Continental-scale monitoring is vital for understanding and adapting to temporal changes in seasonal climate and associated phenological responses. The success of monitoring programs will depend on recruiting, retaining, and managing members of the public to routinely collect phenological observations according to standardized protocols. Here, we trace the development of infrastructure for phenological monitoring in the US, culminating in the USA National Phenology Network, a program that engages scientists and volunteers.

Large increases in the density of woody plants in former grasslands have been documented globally over the past two centuries. Prosopis velutina (velvet mesquite) has recently expanded on over 38 million ha in the grasslands of the American southwest. Mesquite establishment and expansion is potentially influenced by competitive interactions with grasses, particle size distribution (texture) of soil and changes in the amount and/or seasonality of precipitation, amongst other factors. To investigate the relative importance of precipitation seasonality, grass competition, and soil texture on mesquite seedling and establishment, we experimentally manipulated seasonality of precipitation across 72 1.5 m × 1.7 m plots that we planted with two grasses from contrasting functional groups (or left unvegetated) across two highly contrasting soil textures within a semi-desert grassland in Arizona, USA, wherein we followed the demography of 8640 planted mesquite seeds between 2002 and 2006. Up to 70% of the variance in seedling emergence was explained by soil water content, and seedling mortality was more dependent on summer than on winter precipitation. Mortality of less drought tolerant grasses after drought in coarse textured soils created conditions most favorable for seedling establishment, whereas lower rates of seedling establishment were observed in finer textured soils or in plots dominated by more drought tolerant grasses. We conclude that mesquite encroachment is episodic, based on discontinuous patterns of seedling establishment, favored on coarse textured soils and associated with unique conditions of high summer water availability when precipitation is not limiting and when competition from grasses is lowered.

Aims In this study, we examined the effects of Solidago altissima (hereafter Solidago) and two species in the genus Verbesina, Verbesina virginica and Verbesina occidentalis (hereafter Verbesina), on the structure of an old-field plant community and establishment by an invasive plant species, Lespedeza cuneata (hereafter Lespedeza). Materials and Methods We removed Solidago, Verbesina and both Solidago and Verbesina from 4-m 2 plots in an intact old-field community during two growing seasons. We then quantified the effects of these removals on richness, evenness, diversity and composition of the subdominant plant community. We also measured the total aboveground biomass and the aboveground biomass of the subdominant community. To assess how these removals affected establishment by Lespedeza ,w e planted 20 seeds in each plot and tracked seedling emergence and survival for one growing season. Important Findings Subdominant community evenness and Shannon diversity were higher in plots from which Solidago and Verbesina were removed relative to control plots. However, there were no effects of dominant species removal on species richness or composition of the subdominant community. Total aboveground biomass was not affected by dominant species removal, suggesting that the community of subdominant species exhibited compensation. In fact, subdominant community biomass was greater when Solidago, but not Verbesina, was removed. Light availability was also greater in plots where Solidago was removed relative to control plots throughout the growing season. In addition, removal of dominant species, in particular Solidago, indirectly reduced the emergence, but not survival, of Lespedeza seedlings by directly promoting subdominant community biomass. Taken together, our results suggest that dominant old-field plant species affect subdominant community structure and indirectly promote establishment by Lespedeza.

Atmospheric and climatic change can alter plant biomass production and plant community composition. However, we know little about how climate change-induced alterations in biomass production affect plant species composition. To better understand how climate change will alter both individual plant species and community biomass, we manipulated atmospheric [CO2], air temperature, and precipitation in a constructed old-field ecosystem. Specifically, we compared the responses of dominant and subdominant species to our climatic treatments, and explored how changes in plant dominance patterns alter community evenness over 2 years. Our study resulted in four major findings: (1) all treatments, elevated [CO2], warming, and increased precipitation increased plant community biomass and the effects were additive rather than interactive, (2) plant species differed in their response to the treatments, resulting in shifts in the proportional biomass of individual species, which altered the plant community composition; however, the plant community response was largely driven by the positive precipitation response of Lespedeza, the most dominant species in the community, (3) precipitation explained most of the variation in plant community composition among treatments, and (4) changes in precipitation caused a shift in the dominant species proportional biomass that resulted in lower community evenness in the wet relative to dry treatments. Interestingly, compositional and evenness responses of the subdominant community to the treatments did not always follow the responses of the whole plant community. Our data suggest that changes in plant dominance patterns and community evenness are an important part of community responses to climatic change, and generally, that such compositional shifts can alter ecosystem biomass production and nutrient inputs.

Research in community invasibiliy has focused on biotic and abiotic factors that influence the establishment of invasive species and whether such factors vary with spatial scale. Here, we investigate the role of both biotic and abiotic factors associated with the initial establishment of Lespedeza cuneata (L. cuneata) and its abundance at three spatial scales: neighborhoods (9-m2 plots), communities (50-m2 transect) and old fields (5,000–70,000 m2). We asked: (1) Do resource availability and community structure affect the establishment of L. cuneata?, and (2) Are resource availability and community structure associated with patterns of L. cuneata abundance from neighborhood scales to old-field scales? To investigate the first question, we manipulated soil nitrogen (N) availability at three levels in an existing old-field community and tracked emergence and persistence of L. cuneata seedlings, as well as total plant biomass of the community, availability of light, and soil moisture content. To address the second question, we performed surveys in which we estimated L. cuneata foliar cover at community scales (50-m2 belt transects) and old-field scales (total area of 28 ha), and assessed the same biotic and abiotic variables as in the field experiment. The experiment revealed that establishment and persistence by L. cuneata seedlings were 15× and 5× lower in N-added plots than in N-reduced plots. Total plant community biomass was 30% greater in N-added plots than in N-reduced plots. Conversely, light and soil moisture were 60 and 20% lower in N-added plots than in N-reduced plots. Surveys of old fields indicated that community biomass was positively associated with L. cuneata cover at old-field scales likely resulting from greater soil N input from nitrogen fixation in fields with greater L. cuneata cover. In sum, these results indicate that biotic factors associated with establishment of a Rank 1 invasive plant species at the community scale are also related to its distribution at the old-field scale, but the direction of such associations changed across scales.

Rising concentrations of atmospheric carbon dioxide ([CO{sub 2}]) may influence forest successional development and species composition of understory plant communities by altering biomass production of plant species of functional groups. Here, we describe how elevated [CO{sub 2}] (eCO{sub 2}) affects aboveground biomass within the understory community of a temperate deciduous forest at the Oak Ridge National Laboratory sweetgum (Liquidambar styraciflua) free-air carbon dioxide enrichment (FACE) facility in eastern Tennessee, USA. We asked if (i) CO{sub 2} enrichment affected total understory biomass and (ii) whether total biomass responses could be explained by changes in understory species composition or changes in relative abundance of functional groups through time. The FACE experiment started in 1998 with three rings receiving ambient [CO{sub 2}] (aCO{sub 2}) and two rings receiving eCO{sub 2}. From 2001 to 2003, we estimated species-specific, woody versus herbaceous and total aboveground biomass by harvesting four 1 x 0.5-m subplots within the established understory plant community in each FACE plot. In 2008, we estimated herbaceous biomass as previously but used allometric relationships to estimate woody biomass across two 5 x 5-m quadrats in each FACE plot. Across years, aboveground biomass of the understory community was on average 25% greater in eCO{sub 2} more » than in aCO{sub 2} plots. We could not detect differences in plant species composition between aCO{sub 2} and eCO{sub 2} treatments. However, we did observe shifts in the relative abundance of plant functional groups, which reflect important structural changes in the understory community. In 2001-03, little of the understory biomass was in woody species; herbaceous species made up 94% of the total understory biomass across [CO{sub 2}] treatments. Through time, woody species increased in importance, mostly in eCO{sub 2}, and in 2008, the contribution of herbaceous species to total understory biomass was 61% in aCO{sub 2} and only 33% in eCO{sub 2} treatments. Our results suggest that rising atmospheric [CO{sub 2}] could accelerate successional development and have longer term impact on forest dynamics. « less

Aims The direct effects of atmospheric and climatic change factors— atmospheric [CO2], air temperature and changes in precipitation— can shape plant community composition and alter ecosystem function. It is essential to understand how these factors interact to make better predictions about how ecosystems may respond to change. We investigated the direct and interactive effects of [CO2], warming and altered soil moisture in open-top chambers (OTCs) enclosing a constructed old-field community to test how these factors shape plant communities. Materials and methods The experimental facility in Oak Ridge, TN, USA, made use of 4-m diameter OTCs and rain shelters to manipulate [CO2] (ambient, ambient + 300 ppm), air temperature (ambient, ambient + 3.5C) and soil moisture (wet, dry). The plant communities within the chambers comprised seven common old-field species, including grasses, forbs and legumes. We tracked foliar cover for each species and calculated community richness, evenness and diversity from 2003 to 2005. Important findings This work resulted in three main findings: (1) warming had speciesspecific effects on foliar cover that varied through time and were altered by soil moisture treatments; (2) [CO2] had little effect on individual species or the community; (3) diversity, evenness and richness were influenced most by soil moisture, primarily reflecting the response of one dominant species. We conclude that individualistic species responses to atmospheric and climatic change can alter community composition and that plant populations and communities should be considered as part of analyses of terrestrial ecosystem response to climate change. However, prediction of plant community responses may be difficult given interactions between factors and changes in response through time.

Summary • The hypothesis that drought intensity constrains the recovery of photosynthesis from drought was tested in the C3 woody legume Prosopis velutina, and the mechanisms underlying this constraint examined. • Hydraulic status and gas exchange were measured the day before a 39 mm precipitation pulse, and up to 7 d afterwards. The experiment was conducted under rainout shelters, established on contrasting soil textures and with different vegetation cover at the Santa Rita Experimental Range in southeastern Arizona, USA. • Rates of photosynthesis and stomatal conductance after re-watering, as well as the number of days necessary for photosynthesis to recover after re-watering, were negatively correlated with predawn water potential, a measure of drought intensity (R2 = 0.83, 0.64 and 0.92, respectively). • Photosynthetic recovery was incomplete when the vascular capacity for water transport had been severely impaired (percentage loss of hydraulic conductance > 80%) during the drought, which largely increased stomatal limitations. However, changes in biochemical capacity or in mesophyll conductance did not explain the observed pattern of photosynthesis recovery. Although the control that hydraulic limitations impose on photosynthesis recovery had been previously inferred, the first empirical test of this concept is reported here.

Climate change predictions for the desert southwestern U.S. are for shifts in precipitation patterns. The impacts of climate change may be significant, because desert soil processes are strongly controlled by precipitation inputs (“pulses”) via their effect on soil water availability. This study examined the response of soil respiration—an important biological process that affects soil carbon (C) storage—to variation in pulses representative of climate change scenarios for the Sonoran Desert. Because deserts are mosaics of different plant cover types and soil textures—which create patchiness in soil respiration—we examined how these landscape characteristics interact to affect the response of soil respiration to pulses. Pulses were applied to experimental plots of bare and vegetated soil on contrasting soil textures typical of Sonoran Desert grasslands. The data were analyzed within a Bayesian framework to: (1) determine pulse size and antecedent moisture (soil moisture prior to the pulse) effects on soil respiration, (2) quantify soil texture (coarse vs. fine) and cover type (bare vs. vegetated) effects on the response of soil respiration and its components (plant vs. microbial) to pulses, and (3) explore the relationship between long-term variation in pulse regimes and seasonal soil respiration. Regarding objective (1), larger pulses resulted in higher respiration rates, particularly from vegetated fine-textured soil, and dry antecedent conditions amplified respiration responses to pulses (wet antecedent conditions dampened the pulse response). Regarding (2), autotrophic (plant) activity was a significant source (∼60%) of respiration and was more sensitive to pulses on coarse- versus fine-textured soils. The sensitivity of heterotrophic (microbial) respiration to pulses was highly dependent on antecedent soil water. Regarding (3), seasonal soil respiration was predicted to increase with both growing season precipitation and mean pulse size (but only for pulses between 7 and 25 mm). Thus, the heterogeneity of the desert landscape and the timing or the number of medium-sized pulses is expected to significantly impact desert soil C loss with climate change.

Summary 1. A major research focus in population and community ecology is to establish a mechanistic understanding of plant interactions and demographic responses. The first step towards this mechanistic approach relies on understanding the differences in stress caused by different environmental conditions. Leaf-level photosynthetic rate ( A ) within and among plant populations provides important insight into population and community processes, but is difficult to acquire with sufficient replication under field conditions. Instead, chlorophyll fluorescence ( F v / F m ) and predawn water potential ( Ψ pd ) are often used in arid and semi-arid ecosystems. 2. F v / F m reflects the photoactivation status of photosystem II (PSII), whereas Ψ pd indicates water availability in the rhizosphere. Here we compare these indices with A in two perennial C 4 grasses (native Heteropogon contortus and invasive Eragrostis lehmanniana ) and in seedlings of the C 3 shrub Prosopis velutina growing on highly contrasting sandy loam and loamy clay soils in experimental plots. Measurements were made the day prior to and up to 7 days following a 39-mm rainfall pulse after 2 months of drought. 3. A was more sensitive across a broad range of environmental conditions, whereas F v / F m and Ψ pd only responded to periods of protracted drought. The use of these measures was further complicated because their values varied daily and we observed different time-lags in their response to precipitation pulses. 4. We suggest sampling schemes and a priori measurements to capture the value that is representative for the question of interest, and that match the pulsed biological activity in these ecosystems. Finally, we suggest the use of these measures in combination with measurements providing integration over longer time periods, such as δ 13 C, δ 18 O and N concentration in bulk leaf tissue.