• Energy Research
• 6. Clean water close

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.

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.

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.

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.

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.

AbstractCurrent models used for predicting vegetation responses to climate change are often guided by the dichotomous needs to resolve either (i) internal plant water status as a proxy for physiological vulnerability or (ii) external water and carbon fluxes and atmospheric feedbacks. Yet, accurate representation of fluxes does not always equate to accurate predictions of vulnerability. We resolve this discrepancy using a hydrodynamic framework that simultaneously tracks plant water status and water uptake. We couple a minimalist plant hydraulics model with a soil moisture model and, for the first time, translate rainfall variability at multiple timescales – with explicit descriptions at daily, seasonal, and interannual timescales – into a physiologically meaningful metric for the risk of hydraulic failure. The model, parameterized with measured traits from chaparral species native to Southern California, shows that apparently similar transpiration patterns throughout the dry season can emerge from disparate plant water potential trajectories, and vice versa. The parsimonious set of parameters that captures the role of many traits across the soil–plant–atmosphere continuum is then used to establish differences in species sensitivities to shifts in seasonal rainfall statistics, showing that co‐occurring species may diverge in their risk of hydraulic failure despite minimal changes to their seasonal water use. The results suggest potential shifts in species composition in this region due to species‐specific changes in hydraulic risk. Our process‐based approach offers a quantitative framework for understanding species sensitivity across multiple timescales of rainfall variability and provides a promising avenue toward incorporating interactions of temporal variability and physiological mechanisms into drought response models.