Herbarium specimens reveal century-long trait shifts in poison ivy due to anthropogenic CO2 emissions

PREMISE

Previous experimental studies have shown that poison ivy (Toxicodendron radicans; Anacardicaceae) responds to elevated CO2 with increased leaf production, water-use efficiency, and toxicity (allergenic urushiol). However, long-term field data suggest no increase in poison ivy abundance over time. Using herbarium specimens, we examined whether poison ivy and other species shifted leaf traits under natural conditions with increasing atmospheric CO2 (pCO2 ) over the past century.

METHODS

We measured stomatal density, leaf area, leaf N, leaf C:N, leaf carbon isotope discrimination (Δleaf ), and intrinsic water-use efficiency (iWUE) from 327 specimens collected from 1838 to 2020 across Pennsylvania. We compared poison ivy's responses to two evolutionarily related tree species, Toxicodendron vernix and Rhus typhina (Anacardiacae) and one ecological analog, Parthenocissus quinquefolia (Vitaceae), a common co-occurring liana.

RESULTS

Stomatal density significantly decreased (P < 0.05) in poison ivy and the ecologically similar liana P. quinquefolia over the past century, but did not change in the related trees T. vernix and R. typhina. None of these species showed significant trends in changes in leaf N or C:N. Surprisingly, in poison ivy, but not the other species, Δleaf increased with increased pCO2 , corresponding to significant declines in iWUE over time.

CONCLUSIONS

In contrast to the results of short-term experimental studies, iWUE decreased in poison ivy over the last century. Trait responses to pCO2 varied by species. Herbarium specimens suggest that realized long-term plant physiological responses to increased CO2 may not be reflected in short-term experimental growth studies, highlighting the value of collections.

Water Metabolism of Lonicera japonica and Parthenocissus quinquefolia in Response to Heterogeneous Simulated Rock Outcrop Habitats

The karst carbon sink caused by rock outcrops results in enrichment of the bicarbonate in soil, affecting the physiological process of plants in an all-round way. Water is the basis of plant growth and metabolic activities. In heterogeneous rock outcrop habitats, the impact of bicarbonate enrichment on the intracellular water metabolism of plant leaf is still unclear, which needs to be revealed. In this paper, the Lonicera japonica and Parthenocissus quinquefolia plants were selected as experimental materials, and electrophysiological indices were used to study their water holding, transfer and use efficiency under three simulated rock outcrop habitats, i.e., rock/soil ratio as 1, 1/4 and 0. By synchronously determining and analyzing the leaf water content, photosynthetic and chlorophyll fluorescence parameters, the response characteristics of water metabolism within leaf cells to the heterogeneous rock outcrop habitats were revealed. The results showed that the soil bicarbonate content in rock outcrop habitats increased with increasing rock/soil ratio. Under the treatment of a higher concentration of bicarbonate, the leaf intra- and intercellular water acquisition and transfer efficiency as well as the photosynthetic utilization capacity of P. quinquefolia decreased, the leaf water content was lower, and those plants had low bicarbonate utilization efficiency, which greatly weakened their drought resistance. However, the Lonicera japonica had a high bicarbonate use capacity when facing the enrichment of bicarbonate within cells, the above-mentioned capacity could significantly improve the water status of the leaves, and the water content and intracellular water-holding capacity of plant leaves in large rock outcrop habitats were significantly better than in non-rock outcrop habitats. In addition, the higher intracellular water-holding capacity was likely to maintain the stability of the intra- and intercellular water environment, thus ensuring the full development of its photosynthetic metabolic capacity, and the stable intracellular water-use efficiency also made itself more vigorous under karstic drought. Taken together, the results suggested that the water metabolic traits of Lonicera japonica made it more adaptable to karst environments.