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Key Message Tropical forests play a disproportion- ately large role in the global climate system, yet the extent to which nutrients limit the potential for tropical trees to increase carbon gain as atmospheric carbon dioxide rises is unknown. Abstract This review focuses on what is known about tropical tree responses to experimental nutrient addition and how such information is critical for developing a more complete picture of the ability of tropical forest to respond to a changing world. Most of our knowledge of nutrient limitation of eco-physiological processes in tropical trees is derived from stand-scale nutrient addition experiments, in which physiological or growth responses signify limitation by that element. Our knowledge is further supplemented by fertilization studies of individual plants in pots. There is emerging evidence that fine root biomass decreases and maximum photosynthetic rates, water transport capacity and plant growth in tropical trees increase with nutrient addition, but the magnitude of response depends upon the successional status of the species, the size of the individual, light availability and the element in question. The sheer variation in responses of tropical trees to nutrient addition calls for a more complete evaluation across tropical environments.

Abstract Background and Aims Pollinators provide critical ecosystem services, maintaining biodiversity and benefitting global food production. However, plants, pollinators and their mutualistic interactions can be affected by drought, which has increased in severity and frequency under climate change. Using two annual, insect-pollinated wildflowers (Phacelia campanularia and Nemophila menziesii), we asked how drought impacts floral traits and foraging preferences of a solitary bee (Osmia lignaria) and explored potential implications for plant reproduction. Methods In greenhouses, we subjected plants experimentally to drought to induce water stress, as verified by leaf water potential. To assess the impact of drought on floral traits, we measured flower size, floral display size, nectar volume and nectar sugar concentration. To explore how drought-induced effects on floral traits affected bee foraging preferences, we performed choice trials. Individual female bees were placed into foraging arenas with two conspecific plants, one droughted and one non-droughted, and were allowed to forage freely. Key Results We determined that P. campanularia is more drought tolerant than N. menziesii, based on measures of turgor loss point, and confirmed that droughted plants were more drought stressed than non-droughted plants. For droughted plants of both species, the floral display size was reduced and the flowers were smaller and produced less, more-concentrated nectar. We found that bees preferred non-droughted flowers of N. menziesii. However, bee preference for non-droughted P. campanularia flowers depended on the time of day and was detected only in the afternoon. Conclusions Our findings indicate that bees prefer to visit non-droughted flowers, probably reducing pollination success for drought-stressed plants. Lack of preference for non-droughted P. campanularia flowers in the morning might reflect the higher drought tolerance of this species. This work highlights the potentially intersecting, short-term physiological and pollinator behavioural responses to drought and suggests that such responses might reshape plant–pollinator interactions, ultimately reducing reproductive output for less drought-tolerant wildflowers.

Abstract California faces many threats to food security, ranging from water limitations resulting from long-term drought to invasive pests and diseases. Major tree crops, such as citrus and avocado, are threatened by Citrus Greening and Fusarium Dieback, respectively, posing significant economic losses to growers and farm sustainability. Pomegranate (Punica granatum L.) was previously a minor tree crop in California, but has become an important specialty crop, with planted area increased by 10-fold during the last twenty years, and is currently a $200 million annual industry. Pomegranate is not threatened so far by any pest or disease and is a drought- and salt-tolerant crop that can be cultivated on marginal land, which makes it an attractive alternative crop for the growers facing water and disease issues. For this investigation, two pomegranate field trials were initiated and followed over four years to evaluate site effects on establishment, precocity, photosynthesis and water relations to assist in determining appropriate cultivars for coastal versus inland climates. Traits measured included orchard establishment, photosynthesis, water potential, and flowering and yield traits. There were significant site and cultivar effects on many traits as well as site-cultivar interactions. The coastal trial grew significantly faster than the semi-arid inland site, however, the inland site was more productive than the coastal site for the first three years. Production during year four of establishment was similar at both sites.

Photosynthetic capacity, determined by light harvesting and carboxylation reactions, is a key plant trait that determines the rate of photosynthesis; however, in Earth System Models (ESMs) at a reference temperature, it is either a fixed value for a given plant functional type or derived from a linear function of leaf nitrogen content. In this study, we conducted a comprehensive analysis that considered correlations of environmental factors with photosynthetic capacity as determined by maximum carboxylation (Vc,m) rate scaled to 25°C (i.e.,Vc,25; μmol CO2·m−2·s−1) and maximum electron transport rate (Jmax) scaled to 25°C (i.e.,J25; μmol electron·m−2·s−1) at the global scale. Our results showed that the percentage of variation in observedVc,25andJ25explained jointly by the environmental factors (i.e., day length, radiation, temperature, and humidity) were 2–2.5 times and 6–9 times of that explained by area‐based leaf nitrogen content, respectively. Environmental factors influenced photosynthetic capacity mainly through photosynthetic nitrogen use efficiency, rather than through leaf nitrogen content. The combination of leaf nitrogen content and environmental factors was able to explain ~56% and ~66% of the variation inVc,25andJ25at the global scale, respectively. Our analyses suggest that model projections of plant photosynthetic capacity and hence land–atmosphere exchange under changing climatic conditions could be substantially improved if environmental factors are incorporated into algorithms used to parameterize photosynthetic capacity in ESMs.

AbstractPlant species of concern often occupy narrow habitat ranges, making climate change an outsized potential threat to their conservation and restoration. Understanding the physiological status of a species during stress has the potential to elucidate current risk and provide an outlook on population maintenance. However, the physiological status of a plant can be difficult to interpret without a reference point, such as the capacity to tolerate stress before loss of function, or mortality. We address the application of plant physiology to conservation biology by distinguishing between two physiological approaches that together determine plant status in relation to environmental conditions and evaluate the capacity to avoid stress-induced loss of function. Plant physiological status indices, such as instantaneous rates of photosynthetic gas exchange, describe the level of physiological activity in the plant and are indicative of physiological health. When such measurements are combined with a reference point that reflects the maximum value or environmental limits of a parameter, such as the temperature at which photosynthesis begins to decline due to high temperature stress, we can better diagnose the proximity to potentially damaging thresholds. Here, we review a collection of useful plant status and reference point measurements related to photosynthesis, water relations and mineral nutrition, which can contribute to plant conservation physiology. We propose that these measurements can serve as important additional information to more commonly used phenological and morphological parameters, as the proposed parameters will reveal early warning signals before they are visible. We discuss their implications in the context of changing temperature, water and nutrient supply.

SummaryDrought‐induced mortality and regional dieback of woody vegetation are reported from numerous locations around the world. Yet within any one site, predicting which species are most likely to survive global change‐type drought is a challenge.We studied the diversity of drought survival traits of a community of 15 woody plant species in a desert‐chaparral ecotone. The vegetation was a mix of chaparral and desert shrubs, as well as endemic species that only occur along this margin. This vegetation boundary has large potential for drought‐induced mortality because nearly all species are at the edge of their range.Drought survival traits studied were vulnerability to drought‐induced xylem cavitation, sapwood capacitance, deciduousness, photosynthetic stems, deep roots, photosynthetic responses to leaf water potential and hydraulic architecture. Drought survival strategies were evaluated as combinations of traits that could be effective in dealing with drought.The large variation in seasonal predawn water potential of leaves and stem xylem ranged from −6·82 to −0·29MPa and −6·92 to −0·27MPa, respectively. The water potential at which photosynthesis ceases ranged from −9·42 to −3·44MPa. Architecture was a determinant of hydraulic traits, with species supporting large leaf area per sapwood area exhibiting high rates of water transport, but also xylem that is vulnerable to drought‐induced cavitation. Species with more negative midday leaf water potential during the growing season also showed access to deeper water sources based on hydrogen isotope analysis.Drought survival mechanisms comprised of drought deciduousness, photosynthetic stems, tolerance of low minimum seasonal tissue water potential and vulnerability to drought‐induced xylem cavitation thus varied orthogonally among species, and promote a diverse array of drought survival strategies in an arid ecosystem of considerable floristic complexity.

SignificanceThe disruption of Classic Maya society coincided with extended droughts, as suggested by numerous paleoclimatic studies. However, the role of drought in civil upheaval and demographic decline is complicated by the difficulty of linking relatively coarse estimates of meteorological drought with fine-scale plant processes that underpin agriculture. Our analysis of drought resistance across the historically documented, indigenous food plants of ethnographic Maya groups shows a broad range of foods gradually dwindling through droughts of increasing severity. This finding implies that short to moderate droughts could have caused agricultural disruption but not subsistence collapse. However, multiyear extreme drought is consistent with agricultural collapse and the specter of starvation, unless mitigated by food storage or trade from areas less affected by drought.