Written by Bernardita Sallato, Lee Kalcsits, Tory Schmidt and Matthew Whiting, February 2023.
What happens in a normal fall?
Leaf senescence occurs naturally and gradually in response to key environmental cues (chiefly daylength and temperature). During the process, trees remobilize carbohydrates and nutrients from leaves to the perennial parts of the tree where they are stored for the following spring (Marschner, 2002). These reserves help them withstand freezing conditions and, most importantly, are the resources that support flowering, early fruit growth, and canopy development in the following spring. Leaf senescence is a complex process regulated by growth hormones, nutrient sensing, and stress response networks (Havé et al., 2017), initiated by near or slight freezing conditions in tree fruit. Thus, tree vigor (source-sink demand), likely influence the initiation of leaf senescence.
Nutrient and carbohydrate remobilization is initiated long before the initiation of senescence (Loescher et al. 1990). Davidson et al. (2021) reported similar patterns for almond and walnuts in California. Studies of carbohydrate reserves in the trunk show they are highest just prior to leaf abscission and then gradually decrease until reaching their minimum following flowering in the spring. Similarly, nitrogen reserves are lowest during flowering in the spring, and gradually increase throughout the year, until reaching their maximum levels just prior to leaf abscission. Remobilization during leaf senescence contributes to final storage reserves but the amount contributed is relatively minor compared to the total carbohydrate already stored in the perennial parts of trees. In contrast, nitrogen from leaves contribute to the largest proportion (up to 80%) of the stored N in roots and trunks. Previous studies have shown that N remobilization starts with early leaf senescence in peaches (Gomez et al. 2020) and sweet cherries (Brown, 2023), reaching a maximum at leaf senescence.
Why didn’t leaves drop in 2022?
With a historically cold, wet spring in 2022, the season got a late start, however, it was prolonged due to an unusually warm autumn. In much of the Yakima valley, for example, we observed active growth in apple trees in the second half of October (Figure 2). This suggests that the normal signals for initiating senescence and leaf abscission had not yet been perceived by trees and that levels of natural growth hormones (e.g., auxins, and gibberellins) were high. The relatively warm period of October and early November was followed by a sudden hard freeze in the middle of November. It was this rapid change in weather that froze leaves on the tree before they initiated the abscission processes.
What happens when leaves don’t drop?
The freeze events in November would have cut short remobilization of reserves from leaves to the perennial tree parts for storage. In normal conditions, leaves would export about half of the total N back into the tree (Raouf Malik et al., 2018). In recently collected frozen ‘WA 38’ and ‘Honeycrisp’ leaves by Sallato’s lab, only 13% of the N had been remobilized from ‘WA 38’, and 21% from ‘Honeycrisp’. Thus, reserves would be slightly depleted. However, we anticipate that this may have only a minor impact on nutrient and carbohydrate supply since remobilization would have already started prior to that stage. Thus, the warm conditions in October may have reduced nutrient and carbohydrate remobilization, however large impacts to tree health are not expected. In fact, we have seen similar scenarios in previous years (2014) where warm October conditions were followed by rapid freezing in the first week of November and delayed senescence with little consequence to tree health and performance. We also often see these types of conditions in the tree nursery where leaf defoliation treatments are applied to stimulate leaf senescence prior to digging with little consequence to storage reserves, survival, and tree health after planting the next year.
Will this affect winter survival, fruit set, and growth the following year?
The sudden freeze and consequently lower remobilization, should resemble premature leaf removal. In WA, we evaluated the impact of early defoliation in sweet cherries, and there was no negative impact on reserves, yield, and fruit quality the following season. Only after two consecutive years of early defoliation and with no additional N application, were carbohydrate and nutrient levels impacted. For more information, review Early Fall Defoliation in Sweet Cherry article.
In addition, the late and prolonged shoot growth observed across the state in 2022, would have increased nutrient uptake and carbohydrate accumulation. Even though tree reserves typically peak at leaf senescence, the relative losses due to disrupted leaf abscission should be marginal given the high levels of reserves that had already accumulated in September and October of 2022.
- Maintain your normal delay dormant spray with boron and zinc, which are generally low in Eastern Washington.
- If you had a high crop load in 2022 and are concerned about tree vigor, support N with ground application during bloom. Nitrogen uptake will start when N reserves become exhausted. If limited, uptake will likely start earlier than the benchmark of ca. 40 days after full bloom.
- Monitor nutrient levels by sampling leaves in June to determine if your trees are within adequate levels, deficient or excessive. For more information look up leaf tissue analyses. Note that N levels in leaves should always be contrasted with tree vigor.
Davidson, A.M., Le, S.T., Cooper, K.B. et al. 2021. No time to rest: seasonal dynamics of non-structural carbohydrates in twigs of three Mediterranean tree species suggest year-round activity. Sci Rep 11, 5181. https://doi.org/10.1038/s41598-021-83935-1
Gomez, L., Vercambre, G., & Jordan, M. O. 2020. Spatial-temporal management of nitrogen and carbon on the peach tree (Prunus persicae L. Batsch.). Scientia Horticulturae, 273, 109613.
Loescher, W. H., McCamant, T., & Keller, J. D. 1990. Carbohydrate reserves, translocation, and storage in woody plant roots. HortScience, 25(3), 274-281.
Raouf Malik, A., M. Feza Ahmad, RHS Raja, S. A. Bangroo, S. Kirmani and R. Mushtaq. 2018. Seasonality of nutrients in the leaves of apple trees cultivars Red Delicious and Benoni. Journal of Pharmacognosy and Phytochemistry 2018; 7(4): 3185-3188.
Marschner H. 2002. Mineral Nutrition of Higher Plants. 3rd edition. Academic Press, London, U.K.
Havé, M., Marmagne, A., Chardon, F., Masclaux-Daubresse, C. 2017. Nitrogen remobilization during leaf senescence: lessons from Arabidopsis to crops, Journal of Experimental Botany, Volume 68, (10): 2513–2529, https://doi.org/10.1093/jxb/erw365
Sallato, B. and Whiting, M.D. 2022. Early fall defoliation reduces yield and bud nutrient concentration in ‘Selah’ sweet cherry. IX International Symposium on Mineral Nutrition of Fruit Crops, June 28-30, 2021, Israel. Acta Horticulturae. 1333, 195-202. DOI: 10.17660/ActaHortic.2022.1333.25.