Adaptation Responses of Dioecious and Hermaphroditic Tree Species to Abiotic Stress

1. Introduction

As sessile organisms, plants must cope with an ever-changing environment, including frequent abiotic stresses that are detrimental to plant growth and development, e.g., drought, high temperature, chilling, salt, ultraviolet radiation, nutrient deficiency, and toxic metal toxicity in soil [1]. With the onset of global warming, the negative effects of abiotic stress on tree physiology and the metabolism are increasing, directly limiting tree growth and survival [2,3]. In addition, as a key taxon of forest ecosystems, dioecious tree species usually dominate the maintenance and dynamics of terrestrial ecosystems. To promote the sustainability of forest ecosystems, efforts are needed to improve our knowledge of the molecular, physiological, and ecological adaptive responses of trees that may play a key role in tree resilience.

Abiotic stress alters many physiological processes, including photosynthesis, respiration, and transpiration, as well as chlorophyll content, phytohormone signaling, enzyme activity, and the expression of stress resistance genes [4]. On the one hand, abiotic stress can induce non-adaptive responses in trees, such as the disruption of membrane fluidity, protein structure, and molecular interactions. On the other hand, trees can enhance stress resistance through adaptive responses, including the repair of stress damage, balancing of cellular homeostasis, and physiological regulation to levels appropriate for the particular stress condition [5]. The present collection of several excellent papers highlights the mechanisms and ecological processes behind tree responses to anthropogenic climate change and pollutants.

1.1. Adaptation of Trees to Abiotic Stress Caused by Climate Change

Light is an important environmental factor in photosynthesis, but the too low or too high light intensity will reduce the photosynthetic efficiency of trees. In order to adapt to dynamic changes in external light intensity, trees have developed unique physiological mechanisms to maintain efficient photosynthesis as much as possible [6]. Since insufficient light may be the main limiting factor for the regeneration and reproductive growth of Camellia oleifera (Abel) in Pinus massoniana understory plantations, it is necessary to improve our understanding of the adaptation of C. oleifera under different light regimes. Zhang et al. [7] presented a comprehensive analysis of the morphological characteristics, biomass allocation, and physiological traits of C. oleifera leaves and twigs under different light regimes, as well as their plasticity indices. This study highlighted that C. oleifera requires at least 75% light regimes to complete its normal growth development and fruit set. The different response and adaptation strategies of C. oleifera to changing light regimes were elucidated, which is valuable for formulating appropriate management measures for C. oleifera.

In addition to the light intensity, seasonal variation also triggers adaptive responses in trees. Li et al. [8] determined the seasonal variation in the antioxidant enzyme activity, osmoregulatory substrate content, pigment content, and photosynthetic efficiency of four evergreen broadleaf Rhododendron spp. (Rhododendrons), indicating that Rhododendron aganniphum had a higher stress resistance and light energy use efficiency in high-altitude habitats. During the warm season, the Rhododendrons adapted to subalpine habitats through heat dissipation to avoid damage from excessive radiation. Meanwhile, during the cold season, Rhododendrons scavenged reactive oxygen species (ROS) and increased an intracellular fluid concentration to avoid the damage caused by chilling temperatures. These findings can provide a reference for the conservation and application of these valuable ornamental evergreen broadleaf Rhododendrons, and enrich the theory of plant ecophysiology in the high altitudes of the Qinghai-Tibetan Plateau.

Drought is one of the major constraints limiting the growth and productivity of woody plants worldwide. Climate change is causing a global redistribution of precipitation, which has led to increased attention on the effects of changes in the precipitation intensity and frequency on the seedlings of woody plants in warm temperate forests. Li et al. [9] discussed the effects of variability in both the intensity and frequency of the water supply on the physiological traits, biomass, and growth of an important plantation timber species, Robinia pseudoacacia. They found that R. pseudoacacia seedlings were more sensitive to the rainfall frequency than rainfall intensity due to the shallow root system. Therefore, increasing the frequency of irrigation would have a more favorable effect on the survival and growth of R. pseudoacacia and eventually would promote the process of vegetation restoration in future climate scenarios.

1.2. Adaptation of Trees to Herbicides

Weeds in forest crops are mostly controlled by herbicides, but long-term residues of herbicides can affect the seed bank, preventing germination and causing toxicity to trees [10]. Barroso et al. [11] declared that foliar fertilization contributed to better growth and development of eucalyptus and reduced the effects of herbicides on eucalyptus. They found that eucalyptus clones had different responses to foliar fertilization and herbicide application. Eucalyptus seedlings were more sensitive to indaziflam and glyphosate + S-metolachlor herbicides, and foliar fertilization reduced the indaziflam toxicity in clone AEC 056. Therefore, foliar fertilization can reduce the toxicity of herbicides to trees and can be used as a tree protection agent in the future, playing a beneficial role in weed management in forested areas.

1.3. Differences in Response to Abiotic Stress in Dioecious Trees

Dioecious plants comprise about 6% of angiosperm species and many dioecious trees are pioneers in terrestrial ecosystems [12]. Relatively fewer studies have been conducted on dioecious tree species than hermaphrodite in terms of the response to abiotic stress [13]. Studies have shown that the self-protection ability of the photosynthetic system, osmotic regulation ability, and ROS scavenging ability of Populus cathayana males were stronger than females under drought stress [14]. Cai et al. [15] investigated the phenotypic and physiological changes in Salix myrtillacea females and males under 60 d drought and wet nitrogen (N) deposition on leaves. The results showed that female cuttings exhibited a better growth and drought resistance due to their increased water acquisition capacity and instantaneous water use efficiency, higher foliar abscisic acid and auxin levels, and greater antioxidant activities. It is noteworthy that although the leaf-intercepted N deposition improved the aerial growth of drought-stressed male cuttings, drought-stressed female cuttings grew better than males under N deposition. Therefore, S. myrtillacea females showed a better growth and drought resistance than males under both drought and N deposition and are more suitable for forestation. Although this study cannot fully demonstrate the adaptive mechanism of adult S. myrtillacea trees, it supports the selection of the dominant sex in the afforestation of unisexual S. myrtillacea in drought and high N deposition regions.

Furthermore, the stress resistance of male and female P. cathayana to the radioactive element uranium (U) was investigated by Xia et al. [16]. U stress mainly affected the non-stomatal factors and photosystem Ⅱ during photosynthesis. Under U stress, males would scavenge ROS and protect the integrity of the cell membrane and accumulated a higher U concentration in the roots, while the transport of U from the roots to the leaves was lower than that of females. Therefore, male trees had a higher tolerance capacity and a greater ability to remediate U-polluted soil than females. This study provides new insights into the sex-specific accumulation and physiological adaptation related to U tolerance in dioecious trees. It is of great importance for future afforestation and phytoremediation by woody plants.

2. Conclusions

In summary, this Special Issue discusses a wide range of topics on tree adaptation to abiotic stress and provides a new step in understanding the relationship between tree adaptation to stress and anthropogenic environmental change, and studies of trees’ resistance to abiotic stress at the sex level can guide future afforestation and ecological restoration. However, there is still room for improvement considering multiple stressors or stress memories, and in integrating experimental techniques with practical applications at the physiological, ecological, and molecular levels. Finally, we would like to thank all the authors involved in this Special Issue for their high-quality contributions. We are also grateful to the reviewers for their time and expertise, and to the Editors-in-Chief of Forests for the honor of publishing this Special Issue.