The introduction should not include subheadings. Urban trees have garnered considerable attention for their ecological benefits in carbon sequestration and microclimate moderation [1–3], which contribute to the migitation of climate risks in urban settlements[4]. However, the exacerbation of global warming has intensified the occurrence of extreme climate events, particularly heatwaves and droughts, posing unprecedented threats to the growth and adaptation of urban trees [5, 6].
In recent years, heatwaves have often been accompanied by lower relative humidity, a phenomenon termed "dry heatwaves" [7, 8]. Compared to the period before the 1970s, the frequency of simultaneous occurrence of summer heatwaves and droughts in China has increased tenfold in the 21st century [9]. And the historic dry heatwaves attacking the middle and lower reaches of the Yangtze River during the summer of 2022 has led to extensive forest mortality in Central China [10], which is anticipated to suffer even more from dry heatwaves in the future[11]. When trees experience heatwaves in conjection with drought conditions, additional challenges arise [12]. Dry heatwaves can exacerbate trees’ energy load and water deficienty, rendering them more susceptible to environmental stresses [13, 14].
Urban trees exhibit substantial interspecific variability in their tolerance to dry heatwaves, referred to as “dry-heat tolerance” in abbreviation [15]. With the increasing prevalence of dry heatwave occurrences, the idenification of tree species possessing high dry-heat tolerance is crucial for enhancing the climate resilience of urban vegetation. Methods for assessing the responsive dry-heat tolerance of trees primarily involved physiological indicator measurement and visual tree assessment. Physiological measurement evaluates the dry-heat tolerance of tree individuals mainly based on parameters indicating photosynthetic and water use efficiency, conducted in controlled experiments or field investiations [16–19]. Visual tree assessment (VTA) evaluates tree vitality under climate stress by visually assessing the degree of morphological damage or abnormality for tree individuals, with higher vitality indicating greater tree tolerance [20–24]. Crown and leaf damage are considered important indicators for visual tree assessment, given that leaf damage is one of the most sensitive symptoms of plants in response to environmental hydrothermal stresses [25, 26]. By scoring the percentage of leaf wiltering, scorch, discoloration, and defoliation, the morphorlogical tolerance of various urban tree species to heatwave or drought threats has been evaluated in Japan [27], China [28], Australia [19, 29], and Central Europe [25]. Based on the long-term monitoring of crown dynamics in size, density, and epicormic growth, Callow evaluated the response characteristics of Holly Oak trees (Quercus ilex) to prolonged drought [26]. In addition to crown damage, damage symptoms on branches, stems, and other parts of the tree were also utilized in the tolerance assessment [30–32]. Visual tree assessment, as a straightforward and efficient method to evaluate trees’ health condition and morphological response to environmental stresses, can reflect the direct impacts of climate threats on the landscape of urban forests.
Urban habitats act as predisposing factors influencing tree responses to climate change [23]. The adverse conditions prevalent in urban settings, such as shading, limited growing spaces, and conflicts with surrounding artificial infrastructure, can diminish tree vitality and further exacerbate the vulnerability of urban trees to climate change [33–36]. On the other hand, specific aspects of urban habitats may also provide refuge for certain tree species, potentially mitigating the adverse effects of climate change [37], and even offering essential resources for the conservation of threatened species [38].
Previous studies have examined the impacts of specific habitat charactersitics on the growth and adaptability of urban trees. Notable investigations have focused on variables such as the type of urban green spaces [39–41], as well as the impermeability of underlying surfaces Some scholars further explored how urban habitats influence trees’ responses to extreme climates [42]. For instance, Savi illustrated the correlation between the drought tolerance of Holm oak trees (Quercus ilex) and the proportion of impermeable surface at planting sites [43]. Rahman observed divergent transpiration water loss in small-leaved Lime trees (Tilia cordata) growing in different types of squares with varying degree of building enclosure under the summer heat stress in Munich, Germany [44].
The aforementioned studies mainly explored the impact of the variation in single habitat factor on the growth and acclimation of urban trees, however, the comprehensive impact mechanism of multiple habitat factors as a integrity remains unclear for typical urban settlements. And furthre investigation into the multifaceted comprehensive influence is imperative for illustrating how heterogeneous urban environment is shaping trees’ tolerance to climate threats. Among the diverse habitat types in urban built-up areas, campus habitats are distinguished by the integration of fragmented green-blue spaces, impervious surfaces and buildings, reflecting the typical characteristics of high-density habitat types such as residential and commercial areas [45]. Therefore, campus environments can offer ideal settings for exploring the influence of habiat features on tree heat tolerance in densely pupulated and constructed urban settlements.
This study was conducted in a university campus green space in Wuhan, where heat risks are prevalent and escalating. Employing visual tree assessment method, we quantitatively measured the dry-heat tolerance of common urban tree species in response to a recent historic dry heatwave event, and explored the multifactorial impact mechanism of habitat characteristics on tree heat tolerance. Our objective is to elucidate the varying degrees of dry-heat tolerance among different tree species, reveal the key habitat factors influencing the dry-heat tolerance, and identify the spatial scales of habitat with the most prominent effects. The findings are expected to provide strategic insights for enhancing the climate resilience of urban vegetation in central Chinese cities.