Nitrogen resorption efficiency in autumn leaves correlated with chlorophyll resorption, not with anthocyanin production

A prominent hypothesis for the adaptive value of anthocyanin production in the autumn leaves of some species of trees is that anthocyanins protect leaves from photooxidative stress at low temperatures, allowing a better resorption of nutrients, in particular, nitrogen, before leaf fall. While there is evidence that anthocyanins enable photoprotection, it is not clear whether this translates to improved nitrogen resorption and how this can explain inter-specific variation in autumn colours. A recent comparative analysis showed no correlation between temperature and anthocyanin production across species but did not analyse nitrogen content and nitrogen resorption efficiency. Here we provide this comparison by comparing the nitrogen content of mature and senescent leaves and their autumn colours in 55 species of trees. We find no correlation between the presence of anthocyanins and the efficiency of nitrogen resorption. We find, instead, that nitrogen resorption is more efficient in species with yellow autumn colours, pointing to chlorophyll resorption, rather than anthocyanin synthesis, as the main determinant of nitrogen resorption efficiency. Hence our results do not corroborate the photoprotection hypothesis as an explanation for the evolution of autumn colours.

The C:N:P Stoichiometry of Planted and Natural Larix principis-rupprechtii Stands along Altitudinal Gradients on the Loess Plateau, China

Carbon:nitrogen:phosphorus (C:N:P) stoichiometry plays a critical role in nutrient cycling, biodiversity, and ecosystem functionality. However, our understanding of the responses of C:N:P stoichiometry to elevation and forest management remains elusive. Here we sampled 18 Larix principis-rupprechtii sites along altitudinal gradients (1700-2300 m) on Guandishan Mountain in the Loess Plateau, China. We determined the leaf, litter, and soil C N P contents and C:N:P stoichiometric ratios, as well as nutrient resorption efficiency (NuRE), and diameter at breast height (DBH) increments in both planted and natural stands, and then tested the impacts of elevation and stand origin on these parameters’ management. We found different C:N:P stoichiometry between natural and planted forests. The results revealed that: soil C, N, and N:P ratios, litter C:P and N:P ratios, leaf C:N and N:P ratios increased significantly; however, soil C:N ratios, litter P, leaf N and P, nitrogen resorption efficiency (NRE), and DBH increments decreased significantly with elevation in the planted forests. Soil C,N and N:P ratios, litter C, as well as C:N and C:P ratios increased significantly with elevation in natural forests. The soil N, P and N:P ratios, litter C:P and N:P ratios, leaf C, C:P and N:P ratios, nitrogen resorption efficiency (NRE), phosphorus resorption efficiency (PRE), and DBH increments were, on average, higher in the planted, rather than natural forests. Our results indicated that there was an enhancing P-limitation in both the planted and natural forests, and the plantations were more restricted by P. Moreover, compared to natural forests, plantations converged toward a higher conservative N- and P-use strategy by enhancing resorption efficiencies of internal nutrient cycling and a higher annual growth rate.

Drought reduces tree growing season length but increases nitrogen resorption efficiency in a Mediterranean ecosystem

Mediterranean ecosystems are hotspots for climate change, as the highest impacts are forecasted for the Mediterranean region, mainly by more frequent and intense severe droughts. Plant phenology is a good indicator of species' responses to climate change. In this study, we compared the spring phenology of cork oak trees (Quercus suber), an evergreen species, over 2 contrasting years, a mild year (2004) and a dry year (2005), which was the most severe drought since records exist. We evaluated the timing of occurrence, duration, and intensity of bud development, budburst, shoot elongation, trunk growth, and leaf senescence (phenophases) and assessed the nitrogen resorption efficiency from senescent to green leaves. The temperature was the main driver of budburst. Nevertheless, water had the main role of constraining all the other phenophases by strongly reducing the growing season length (−48 %) and consequently the tree growth. Basal area increment was the most affected growth variable (−36 %), although it occurred at a similar rate in the 2 years. Shoot elongation was also reduced (−21 %), yet elongation occurred at a higher rate in the dry year compared to the mild year. Leaf senescence during the bulk period was higher in the dry year, in which leaves were shed at the same rate over a longer period. Nitrogen concentrations in green and senescent leaves were affected by drought and nitrogen resorption efficiency increased remarkably (+22 %). Our results highlight the importance of studying different phenological metrics to improve our understanding of the ecosystem's responses to climate change. The faster dynamics observed in shoot elongation, while all other phenophases developed at the same rate, indicate that leaf area development is privileged in cork oak. Water availability was the main driver of spring growth in this Mediterranean ecosystem; however, growth may be affected by complex interplays between precipitation and temperature, such as higher temperatures during dry winters or heatwaves during spring, that are likely to result in water stress. Longer studies are needed to disentangle those interplays. Finally, a higher nitrogen resorption efficiency in response to drought appears to be an adaptive trait that mitigates the limitation in nitrogen uptake by the roots during drought and contributes to improving tree fitness in the short term but will probably exert a negative feedback on the nitrogen cycle in the long term, which might affect the ecosystem functioning under the forecasted droughts.

Does Plant Size Influence Leaf Elements in an Arborescent Cycad?

Plant size influences the leaf nutrient relations of many species, but no cycad species has been studied in this regard. We used the arborescent Cycas micronesica K.D. Hill to quantify leaf nutrient concentrations of trees with stems up to 5.5-m in height to determine if height influenced leaf nutrients. Green leaves were sampled in a karst, alkaline habitat in Rota and a schist, acid habitat in Yap. Additionally, senesced leaves were collected from the trees in Yap. Minerals and metals were quantified in the leaf samples and regressed onto stem height. Green leaf nitrogen, calcium, manganese, and iron decreased linearly with increased stem height. Senesced leaf carbon, iron, and copper decreased and senesced leaf nitrogen increased with stem height. Nitrogen resorption efficiency decreased with stem height. Phosphorus and potassium resorption efficiencies were not influenced by plant size, but were greater than expected based on available published information. The results indicate leaf nutrient concentrations of this cycad species are directly influenced by plant size, and illuminate the need for adding more cycad species to this research agenda. Plant size should be measured and reported in all cycad reports that include measurements of leaf behavior.