Herbaria Reveal Herbivory and Pathogen Increases and Shifts in Senescence for Northeastern United States Maples Over 150 Years

Recent studies suggest climate-related delays in the timing of leaf coloration and abscission in maple trees but lack baseline data prior to the late 20th century. To better understand how autumn foliar phenology and late-season damage risks have changed for this genus over the past century, we evaluated 2,972 digitized herbaria specimens of red and sugar maple collected between 1826 and 2016 for the presence of leaves, autumn leaf coloration, and pathogen or herbivore damage. We found that the onset (first appearance) of colored leaves has shifted 0.26 days later each year, leading to a delay of more than a month in autumn phenology since 1880. We find that these shifts are related to precipitation regimes in both the fall and summer seasons and that more severe droughts are associated with higher probabilities of colored leaves. Moreover, we found that the probability of both herbivory and pathogen damage has increased significantly over the study period. In particular, we find a strong association between increasing summer drought conditions and increased probability of herbivory. Furthermore, the presence of foliar damage increased the probability of leaf coloration on herbaria specimens. However, the end-of-season abscission date (last appearance of leaves) was strongly associated with herbivory and climate in a contrary direction: Increasing yearly drought, higher fall temperatures, and the presence of herbivory were associated with earlier abscission. In fact, the last leaf dates for specimens with herbivory were nearly 2 weeks earlier than specimens without herbivore damage. Our study documents significant changes in maple senescence over the last 150 years and suggests that incorporating herbivory into models may improve our ability to predict forest responses to climate shifts.

Nitrate fertilization may delay autumn leaf senescence, while amino acid treatments do not

Fertilization with nitrogen (N)-rich compounds leads to increased growth, but may compromise phenology and winter survival of trees in boreal regions. During autumn, N is remobilized from senescing leaves and stored in other parts of the tree to be used in the next growing season. However, the mechanism behind the N fertilization effect on winter survival is not well understood and it is unclear how N levels or forms modulate autumn senescence. We performed fertilization experiments and showed that treating Populus saplings with high or low levels of inorganic nitrogen resulted in a delay in senescence. In addition, by using precise delivery of solutes into the xylem stream of Populus trees in their natural environment, we found that delay of autumn senescence was dependent on the form of N administered: inorganic N (NO3-1) delayed senescence but amino acids (Arg, Glu, Gln, and Leu) did not. Metabolite profiling of leaves showed that the levels of tricarboxylic acids (TCA), arginine catabolites (ammonium, ornithine), glycine, glycine-serine ratio and overall carbon-to-nitrogen (C/N) ratio were affected differently by the way of applying NO3-1 and Arg treatments. In addition, the onset of senescence did not coincide with soluble sugar accumulation in any of the treatments. Taken together, metabolomic rearrangement under different N forms or experimental setups could modulate senescence process, but not initiation and progression in Populus. We propose that the different regulation of C and N status through direct molecular signaling of NO3-1 could account for the contrasting effects of NO3-1 and Arg on senescence.

Does drought advance the onset of autumn leaf senescence in temperate deciduous forest trees?

Severe droughts are expected to become more frequent and persistent. However, their effect on autumn leaf senescence, a key process for deciduous trees and ecosystem functioning, is currently unclear. We hypothesized that (I) severe drought advances the onset of autumn leaf senescence in temperate deciduous trees and (II) tree species show different dynamics of autumn leaf senescence under drought. We tested these hypotheses using a manipulative experiment on beech saplings and 3 years of monitoring mature beech, birch and oak trees in Belgium. The autumn leaf senescence was derived from the seasonal pattern of the chlorophyll content index and the loss of canopy greenness using generalized additive models and piecewise linear regressions. Drought and associated heat stress and increased atmospheric aridity did not affect the onset of autumn leaf senescence in both saplings and mature trees, even if the saplings showed a high mortality and the mature trees an advanced loss of canopy greenness. We did not observe major differences among species. To synthesize, the timing of autumn leaf senescence appears conservative across years and species and even independent of drought, heat and increased atmospheric aridity. Therefore, to study autumn senescence and avoid confusion among studies, seasonal chlorophyll dynamics and loss of canopy greenness should be considered separately.

“I Was Like an Autumn Leaf That Looks Pretty From the Outside, but Would Break Once You Touched It”: A Case Study of the Lived Experience of Breast Cancer Survival

In this hermeneutic phenomenological case study, we explored the lived experiences of one Saudi Arabian woman, Sahara, living with breast cancer and after, identifying her culture’s impact on the “meaning-making” process. We derived the data from a semi-structured interview and analyzed using interpretive phenomenological analysis (IPA). The themes were: (1) “discourse”: being a breast cancer patient; (2) “sociality”: the complex sense of living with visibility and invisibility; and (3) “selfhood”: regaining the sense of being normal. The study benefits healthcare providers, who need to understand women’s life-world, the impact of culture when designing a program of survival care, and the response to their needs.

Response to Comment on “Increased growing-season productivity drives earlier autumn leaf senescence in temperate trees”

Our study showed that increases in seasonal productivity drive earlier autumn senescence of temperate trees. Norby argues that this finding is contradicted by observations from free-air CO2 enrichment (FACE) experiments, where elevated CO2 has been found to delay senescence in some cases. We provide a detailed answer showing that the results from FACE studies are in agreement with our conclusions.

Comment on “Increased growing-season productivity drives earlier autumn leaf senescence in temperate trees”

Zani et al. (Research Articles, 27 November 2020, p. 1066) propose that enhancement of deciduous tree photosynthesis in a CO2-enriched atmosphere will advance autumn leaf senescence. This premise is not supported by consistent observations from free-air CO2 enrichment (FACE) experiments. In most FACE experiments, leaf senescence or abscission was not altered or was delayed in trees exposed to elevated CO2.

The effect of growing-season productivity on autumn phenology in temperate trees

<p><strong>Phenological shifts in plants greatly affect biotic interactions and lead to multiple feedbacks to the climate system</strong><strong>. Increases in growing-season length under warmer climates are expected to drive changes in water, nutrient, and energy fluxes as well as enhancing ecosystem carbon uptake</strong><strong>. Yet, future trajectories of growing-season lengths remain highly uncertain because the intrinsic and extrinsic factors triggering autumn leaf senescence, including lagged effects of spring and summer productivity</strong><strong>, are poorly understood. Here, we use 434,226 spring leaf-out and autumn leaf senescence observations of temperate trees from Central Europe between 1948 and 2015 to test the effect of seasonal photosynthetic activity on leaf senescence, thereby exploring the extent to which growing-season lengths are internally regulated by constraints on productivity. We found that spring and summer productivity was a critical driver of autumn phenology, with earlier leaf senescence in years with high seasonal photosynthetic activity. Our new process-based model, incorporating information on growing-season photosynthesis, increased the accuracy of existing autumn phenology models by 22–61%. Furthermore, the physiological constraint of growing-season photosynthesis reversed the predictions of autumn phenology over the rest of the century. </strong><strong>While current phenology models predict that leaf senescence will occur 7–19 days later </strong><strong>by the end of the 21<sup>st</sup> century</strong><strong>, </strong><strong>we estimate that leaf senescence will, in fact, advance by 3–6 days</strong><strong>.</strong><strong> </strong><strong>Our results reveal important constraints on future growing-season lengths and the carbon uptake potential of temperate trees and enhance our capacity to forecast long-term changes in ecosystem functioning, which is critical to improve our understanding of Earth System dynamics in response to climate change.</strong></p>