Elsevier

Flora

Volume 279, June 2021, 151812
Flora

Ring-forming plants in the Egyptian deserts

https://doi.org/10.1016/j.flora.2021.151812Get rights and content

Highlights

  • We studied the anatomical structure of 300 plant species from Egyptian deserts.

  • 94 species had distinct ring boundaries.

  • Precipitation is the main limiting factor for ring formation.

  • Further studies on wood xylogenesis of these species are required.

Abstract

The formation of annual growth rings is an imperative requirement for studying the dendroecological potential of plant species. Little is known about the distinctness of growth rings in the wood of desert plants in Africa, particularly in southern Mediterranean regions. To fill this knowledge gap, we collected and anatomically analyzed about 300 perennial desert plants from different areas in Egypt. Out of the 300 studied plant species, 94 species were identified as growth-ring forming plants. They belong to 78 genera and 33 families. Trees, shrubs/subshrubs, and perennial or short-lived herbs are represented by 7, 55, and 32 species, respectively. Trees and shrubs represent 7.4% and 58.5% of the total, respectively. The distinctness of growth rings in these species was indicated either by the type of wood porosity, fibers, marginal parenchyma layers or by a mixture of these characteristics. Ring porosity, fiber layer, and parenchyma were the main indicators for growth-ring distinctness in 77.7%, 60.6%, and 33.0% of growth-ring forming plants, respectively. We conclude that some of the desert plants form distinct growth rings depending on site conditions, while precipitation is the main driver for its formation. Further studies on the annual rhythm of xylem formation are required to confirm the annual nature of growth rings and potential dendroecological use of these plants.

Introduction

Plant anatomy as a branch of botany, is focusing on the microscopic structure of plant tissues and organs (Crang et al., 2019). An accurate description and knowledge of the anatomical structure of the main plant organs (i.e., stems, roots, and branches) is indispensable for accurate conclusions regarding internal physiological processes and the effects of environmental factors on them. Trees and shrubs are subjected to a wide spectrum of environmental conditions and stresses; hence, they developed mechanisms to adapt their xylogenesis to sudden or continuous changes of these conditions to guarantee their survival (De Micco et al., 2019, Matisons et al., 2019). Variations in growth-ring width and related cross-dating techniques are used for precise dating of annual growth rings in trees, shrubs, and herbs (Schweingruber & Börner, 2018). The increased application of wood anatomy in growth-ring sciences improve our understanding of plants' ecological performance.

The International Association of Wood Anatomists IAWA (IAWA Commitee, 1989) defines distinct growth-ring boundaries by an abrupt structural change at the end of the ring, typically involving a change in fiber wall thickness and radial fiber diameter. Furthermore, six other growth-ring markers co-occurring with changes in fiber morphology are listed (Tarelkin et al., 2016). Regarding the formation of distinct growth rings in tropical and sub-tropical (i.e., southern Mediterranean) plants, it is known that seasonal fluctuations of climatic factors are the main driving factors (Liphschitz and Lev-Yadun 1986; Worbes 1999; Touchan et al., 2008; Castagneri et al., 2017). In these regions, precipitation represents the main limiting factor for tree growth (Rozendaal and Zuidema, 2011). This is because precipitation is either very low or absent during the dry season, leading to leaf shedding or to the formation of small summer leaves (Fahmy et al., 1990; Hegazy and Elhag, 2006) and cambial dormancy (de Lara and Marcati, 2016; Liphschitz and Lev-Yadun, 1986). Consequently, this induces the formation of distinct growth-ring boundaries (Tarelkin et al., 2016). Many studies have concluded that distinct growth-rings could not be formed in all plants in tropical or Mediterranean regions due to a less distinct climatic seasonality (Balzano et al., 2019; Fahn et al., 1986; Tarelkin et al., 2016). Moreover, false rings and intra-annual density fluctuations are formed in the xylem caused by climatic fluctuations within the same growing season (Balzano et al., 2019; Worbes and Nin, 2002). Hence, to answer many ecological questions about the nature of tree growth in arid environments, there is a need for more environmental and growth data. One of the ways to achieve this objective is to study growth-ring forming plants in these harsh environments. This will surely improve our understanding of climate-growth relationships in arid and semi-arid environments like that of the southern Mediterranean basin.

There are many studies carried out on the anatomical structure of plant species in the region. These studies investigated the general anatomical features of species (e.g., Abd Elhalim et al. 2016), its responses to water deficiency and drought conditions in the desert (e.g., Saadeddin and Doddema 1986; Danin 1991; Fahmy 1997; Al-Khalifah et al., 2006; Hegazy et al., 2006; Gorai et al., 2015). Some other valuable studies were published on ring formation in desert trees and shrubs (e.g., Fahn 1955; Fahn 1959) or using the anatomical features to identify tree and shrub species in the region (Fahn et al., 1986).

Recently, in southern Mediterranean countries, some studies were carried out in more detail to investigate the relationship between the annual growth and wood characteristics of tree species and climate variables (e.g., Castagneri et al., 2017; Farahat and Gärtner 2019). In arid and semi-arid habitats desert trees, shrubs, and perennial herbs represent key species in these harsh environments. Information regarding the population growth dynamics, annual productivity, and age of such species are essential to conservation and sustainable management plans. In southern Mediterranean countries, only a few studies have focused on the relationship between tree growth and climate variables for few tree species such as Moringa peregrina, Juniperus phoenicea, Pinus halepensis, Quercus ithaburensis, and Q. boissieri (e.g., Touchan et al., 1999, Touchan et al., 2008; Castagneri et al., 2017; Farahat and Gärtner 2019). Neumann et al. (2000) published a significant study for the plants' wood anatomy in the Sahara and the Sahel of Africa. When examining the anatomical structure of species growing in deserts in databases as "Inside wood" (IAWA Committee 2004; Wheeler 2011), for many of them the ring boundaries are indicated as "distinct, indistinct or absent". This wide description is given, for instance, for Acacia seyal Delile and Calotropis procera (Aiton), two typical desert species. So, all structures seem to be possible for a certain species, but there is no indication for potential reasons and descriptive details of the respective ring boundaries are frequently missing. This uncertainty in the distinctness of growth-ring boundaries is most likely related to the site's environmental conditions from which the specimens were collected. In warm and dry regions, precipitation is the most important factor that limits growth and determine the distinctness of the growth-ring boundaries in plant species (Schweingruber, 2007). There is only limited knowledge about the distinctness of growth-ring boundaries in desert plants in Southern Mediterranean countries. This gap can be better illuminated by exploring the anatomical structure of desert plants sampled at specific desert sites with known environmental conditions. This may open a new window for further studies (e.g., xylogenesis and dendroecological research) in this region that could improve our understanding about the ecological performance of the desert species under drought conditions.

Therefore, the main objective of this study was to explore the anatomical structure of some trees, shrubs, and perennial herbs in Egyptian deserts with a special focus on their growth-ring formation. We hypothesized that perennial desert plants, including trees, shrubs, and herbs, could form true growth rings when they become sensitive to the seasonal fluctuations in precipitation and have an annual dormant growth stage.

Section snippets

Study area

Egypt is part of the Sahara and occupies Africa's northeastern corner and the Sinai Peninsula (Said, 1962), with about 95% of land as a desert. Egypt is divided into eight phytogeographical regions (Fig. 1, Wickens, 1977) including three deserts (the Eastern and Western desert, and the Sinai Peninsula), the Nile valley with delta and Fayoum depression, Oasis, the Mediterranean coastal strip from Libyan borders eastwards to Rafah on the Egyptian-Palestinian border, the Red Sea coastal lands, and

Results and discussion

Out of about 300 species that have been collected, 94 species were identified as ring-forming plants, and we studied them in detail. On the site along the Mediterranean coastal strip we found 34 species forming distinct growth-rings, on the site near Cairo 11, in St. Catherine 36, in Hurghada 5, and in Aswan site 8. The high number of ring-forming species in the Mediterranean coastal strip and St. Catherine may be ascribed to the high number of collected samples from both sites. We think that

Conclusions

This study concludes that not all plants in the Egyptian desert can form growth rings, but some species do it regularly. According to this study and other relevant literature, the growth rings in the same plant species may be distinct or indistinct depending on the environmental site conditions. Nevertheless, precipitation is the key driver for growth-ring formation in the Egyptian deserts. Re-sprouting growth after the dry summer is the main feature in many growth-ring forming plants in this

CRediT authorship contribution statement

Emad A. Farahat: Conceptualization, Data curtion, Writing – original draft, Writing – review & editing. Holger Gärtner: Conceptualization, Writing – review & editing, Data curtion.

Declaration of Competing Interest

Conflict of interest: the authors declare that they have no conflict of interest

Acknowledgments

This study was supported partially by the Egyptian Science and Technology Development Fund (STDF) to E. Farahat (grant nr. 25341) and the Swiss National Science Foundation (SNSF, grant nr. IZSEZ0_192144 / 1).

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