Xia Zhixiang how to improve the survival rate
With the rapid development of urban areas, planting trees during the hot and dry months of July and August has become a common practice. However, in recent years, there has been an unusual trend in the survival rates of Toona sinensis (Chinese mahogany) planted during these seasons. This article explores the reasons behind this phenomenon and discusses effective strategies to improve tree survival.
In 2001, I was involved in the construction of the Shihuifang Landscape Project in Suzhou. In 2002 and 2003, I managed the greening projects for Shihu West Road and Shannon Apartment. During these projects, the survival rate of Toona sinensis saplings with a diameter at breast height (DBH) of 8–10 cm (referred to as Xiao Xiangyu) was relatively low. According to general tree growth patterns, younger seedlings usually have higher survival rates than older ones. So why did this happen?
One major factor is the extreme weather conditions in July and August. In Suzhou, temperatures often range from 29°C to 30°C, with daily minimums between 25°C and 27°C, and maximums reaching up to 36°C or even 37°C. Although it's the rainy season, long dry spells—sometimes lasting 10 to 20 days—can occur. Plants naturally develop stress resistance when facing adverse conditions. However, when metabolic activity is high, which typically happens when moisture levels are high, stress resistance tends to be lower.
Most of the Toona sinensis planted in summer were pruned significantly. The stems of mature trees have mottled surfaces and deep cracks, are highly lignified, and have low water content, making them more resistant to heat and drought. In contrast, young saplings have green, tender branches with high moisture and strong metabolism, but poor drought tolerance.
Another issue is the imbalance between water supply and demand. Newly planted seedlings often lose their leaves during pruning, forcing the plant to rely on stem surface transpiration for water distribution. Mature Toona sinensis stems are well-lignified and can protect against sunburn and retain moisture. However, young saplings lack this protection and must continuously evaporate water to maintain metabolic activity, leading to quick dehydration under intense sunlight.
Additionally, the root systems of newly planted trees are often severely damaged. The mud balls around the roots are typically 8–10 times the DBH, meaning only a small portion of the root system remains intact. Most of the roots are cut off, leaving little absorption capacity. Over time, the tree must rely solely on the limited root system in the mud ball, which struggles to meet its water needs. Without sufficient moisture, the tree quickly loses balance and begins to wilt.
When symptoms like leaf blackening and wilting appear, measures such as soil loosening, radial trenching, and applying watering agents can help. These methods are effective for larger seedlings, but not for the smaller ones. Once the small saplings show signs of decline, they rarely recover. Within 5 to 7 days, the leaves, small branches, and even the entire plant may die.
To address these issues, several measures can be taken. First, avoid planting seedlings during the hottest and driest periods, especially for species like Toona sinensis. Second, implement shading and cooling techniques, such as wrapping the trunk and branches with straw rope, plastic film, and regular misting. Third, prune the above-ground parts to reduce evaporation and water loss. Fourth, ensure that the root ball is properly shaped and that as many functional roots as possible are preserved during digging. Fifth, strengthen post-planting care, including timely watering and soil management. Finally, consider using biological agents like root stimulants or vitality enhancers to promote faster root development and improve survival chances.
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