Plant Cuttings

Oak origins: From acorns to species and the tree of life by Andrew L Hipp, 2024. The University of Chicago Press.

The world seems awash with so-called ‘origin stories’ (Kyle Deguzman), e.g., for the seemingly never-ending characters from the Marvel universe. As a counter to those more animalistic and fictional tales, it’s a pleasure to give a shout-out for Andrew Hipp’s Oak origins [which book is here appraised], something that is altogether much more factual (and botanical).

Not just oaks…

Before getting to the core of this review, it’s reasonable to point out that Oak origins isn’t just the origin story of oaks. Although, oaks feature most prominently in the book, Hipp also takes the opportunity to use these specific tree species as a way to present information about the origins and development of species more generally. In so doing he provides his readers with a useful and important introduction to the bigger topic of evolution and the interconnectedness of living things. To do so, Hipp uses examples of plants other than oaks, and, along the way, also has quite a lot to say about the evolution of human beings. So, there’s much more than oaks here*.

Overview

Oak origins‘ 202 pages of main text [Introduction, seven numbered chapters, and an Epilogue] are complemented with c. 120 pages of supporting material – Appendix**, Notes, Literature cited, and Index. That split – roughly two-thirds text to one-third other material – is a very good ratio, and one that’s appropriate to what is undeniably a scholarly text. However, although Oak origins is academic – many of its chapters are rather technical and require effort on the part of the reader if s/he is to get the best/most of it – the book is readable [Ed. – but do not try and read it all in one sitting.]

Text: Generally, the book is very well-written, with an engaging style, great imagery, and often appears to have been penned with the soul of a poet (or, at least, a careful observer of nature). For instance, the first paragraph of Chapter 7 (p. 175) is an example of great ‘nature writing’. And, some of the book’s best writing generally is within the first few paragraphs of that chapter. But, as readable as they are, all chapters can be quite challenging technically (and technical terms are usually explained on first mention). Hipp definitely writes for an intelligent audience.

Illustrations: Oak origins is abundantly illustrated throughout, predominantly with black-and-white drawings by Rachel Davis (the author’s wife per p. 205). Whilst Davis’ drawings have a charming quality about them, they also accurately depict features of many of the plant species discussed in the book (and whose value is enhanced by explanatory captions rather than just titles). However, the one illustration that stood out to me as being a little odd is on p. 201. Captioned “Fox squirrel in snow, retrieving acorns”, I couldn’t see any acorns. Additionally, and underlining the book’s scientific credentials, there is a graph – “Clouds of molecular (DNA) variation, eastern North American White Oak Group (Q. sect. Quercus)” (p. 62), an “Oak distribution map” (pp. 126/7), a diagram of “Five interbreeding eastern North American White Oak Section (Q. sect. Quercus) species” (p. 76) [illustrated with their leaves]; and two phylogenetic trees, “Fagales tree of life” (pp. 100/1), and “Oak tree of life” (pp. 124/5).

What can you expect?

Clues to the book’s content are found at the start, in the Foreword. Written by Béatrice Chassé (Béatrice Chassé, 2017) [who, as Former President of the International Oak Society [IOS], Editor of the IOS’s journal, International Oaks, and Cofounder of the Arboretum des Pouyouleix (France) (Béatrice Chassé, 2011 (per p. xi), is someone who knows more than a bit about oaks], we are told that “Oak Origins presents the story of what has been achieved (with oaks as the focal point) in understanding evolution ever since Darwin’s genial idea, and as such, it is not one story but three. The first is the story of evolution, the story of the Tree of Life— specifically, the oak tree of life. The second is the story of how our understanding of this tree of life has evolved into what it is today. The third, which is at once the sum and more than the sum of those two stories, is Andrew Hipp’s story.” From which you may – rightly – infer that the book will have both factual material and a more personal style reflecting the author’s experiences with oaks.

By way of emphasising the academic nature of the book, Chassé continues, “It is a story that plunges you into the history of the species concept, the tectonic and climatic transformations of the planet Earth, the ecology of population dynamics, plant sexuality and developmental biology, the plasticity of genotype and phenotype, and ecology as a key factor in maintaining the integrity of species while they benefit from hybridization and introgression, which, in turn, provide the raw material of natural selection and adaptability” (p. x). Although that may sound a little too serious, Chassé does add that “You will find poetry, suspense, and humor in Andrew’s science, and in his writing. You can enter his story in many different ways, but once you are in it you will be captivated…” (p. xi). Which, in this reviewer’s view, is a pretty good summary of how the book does what it does – and which should help to allay any concerns from non-specialist readers.

Why was the book written?

Having had the Foreword-writer’s perspective, what does the author have to tell us about his book? In the Introduction Hipp tells us that he “wrote Oak Origins in part because it is a book I would like to have read when I was working as a naturalist in my twenties” (p. 2). But “This is also a book I would enjoy reading today, as a researcher who spends a lot of his time working on oaks. This is the book I want to be able to pull off the shelf to contextualize the things I see in my research and my walks in the woods” (p. 3). Accordingly, “Oak Origins is a portrait of oaks from several angles, bringing together historic and current research to tell the history of our evolving understanding of these magnificent plants” (p. 3). In doing so, Hipp “wrote this book not only for myself, however, but also for you, whether you are a naturalist, scientist, teacher, gardener, or simply interested. Oak Origins is a collection of stories braided through the forests and other plant communities where oaks thrive” (p. 3). Which sounds about right.

Some more about the contents

Chapter summaries: Chapter 1 Flowers and acorns is a good account of oak biology, with a bias towards reproduction and lots of insights into oak seed science. Chap 2 Variation is quite a technical chapter looking at variation within oak species – environment-driven plasticity, and heritable genetic variation. Chap. 3 Species and their hybrids is an interesting – and quite fascinating – biological tale with focus on oaks (Quercus spp.). Here, Hipp takes us on a journey that embraces 56 million years of oak evolution and development that covers plant geography, adaptivity/adaptability, reproduction strategies, ecology, and a plant genetics primer. With discussion of such topics as the species concept, hybridisation, backcrossing, introgressive hybridisation/introgression (p. 73) [“Numerous studies of genetic diversity in co-occurring oak species show that closely related oak species often swap genes in the wild” (p. 67); “Oaks have earned— perhaps justly— a reputation for promiscuity” (p. 67)]. It’s another quite challenging read: Hipp certainly keeps the reader on his/her toes. Chap. 4 Origins discusses the origins of the Fagaceae, the plant family to which the oak genus, Quercus, belongs, and provides a good introduction to taxonomy and phylogeny – and the notion of clades. Chap. 4 also tells us about the evolution of the acorn.

Chap. 5 Radiation is the story of ‘how modern oaks became’, in which features known as ‘land bridges’ have an important part to play as pathways for dispersal of oak species, the Bering land bridge, and the North Atlantic land bridge, in particular. Chap. 6 “Pharaoh’s dance” begins with a page and a half about the jazz piece Pharaoh’s Dance by Miles Davis. Initially it was not at all clear to this reviewer what that added to a book about oaks. Eventually, Hipp reveals that the way that particular musical composition was put together is a very good analogy for the way in which genes are combined and moved between species during the evolutionary history of the oak. Accordingly, there was lots here about linkage maps. So, quite a clever analogy, which helped understanding of this very technically-challenging chapter. Chap. 7 Oak communities*** was probably my favourite chapter; its focus on oak was skilfully used to provide insights into bigger ecological ideas, e.g. how the nutrient status of leaves of different oak species at time of fall has follow-up consequences for their decomposition and the associated soil microflora assemblage; the fascinating story of Temnothorax ants and their use of acorns; and how vessel characteristics and tylosis in members of the white oak group differ from those in the red oak group thereby contributing to the concept of phylogenetic niche conservatism. This chapter also displays the author’s humility in admitting to an identification error of an oak species – a result of convergence, some cases of which “have produced oaks so similar that even the most seasoned botanists have found them difficult to tell them apart” (p. 180). With its holistic view of oak biology and ecology, Chap. 7 is a nice one on which to end the book.

Is the oak story ended..?

The final section of the book is the Epilogue. Here, and in light of the stated fact that “The latest conservation assessment of oaks suggests that 31% of the world’s oaks are threatened with extinction” (p. 198), Hipp muses on oaks’ future, and ponders the question, how long will they survive? You will need to read the book to find the author’s answer to that question. But, whilst Hipp’s recounting of the origin of oaks may now be over, the oak’s story is ongoing. In several places in Oak origins we are told that there are currently approx. 425 named species of Quercus. To that number must be added at least one more oak species – Quercus zhekuni – named as new to science by Min Deng et al. (2024). No doubt there are more Quercus species waiting to be discovered and named. The oak story is probably far from over.

Are all of these facts sourced?

Oak origins is jam-packed with factual statements. Which is good to see. But, are those statements evidence-based? Short answer: Probably…

Longer answer: read on. The text of Oak origins is devoid of any indications of sources for the statements made. To uncover such evidence the book provides a Notes section and a list of Literature cited. The layout and purpose of the approx. 31 pages of Notes are described by Hipp: “References and some auxiliary information are organized by chapter [Ed. – and for the Epilogue], then by section within each chapter. Sections separated by white space in the text are indicated here in the notes by page number and the first sentence of the section. Within each section, references are provided mostly [my emphasis] in the order in which they appear in the main text, with main topics indicated in boldface and subtopics in some cases indicated by underlines. Only key references are provided [my emphasis], but they should suffice to provide the interested reader with access to some of the most relevant literature for essentially all topics [my emphasis] covered in the book” (p. 212). The consequence of such an interesting approach is that one has to search through the Notes section to find a potential source [indicated by author(s) name(s) and year of publication – with fuller citation details in the Literature cited section]. Even then, there is no guarantee that the sources(s) will be found per Hipp’s statement that not all topics will necessarily be sourced.

Because the specific wording of the fact stated in a chapter isn’t always provided in the Notes, it’s not easy to know if one has tracked down the relevant source or not. Here is just one example. “Oaks provide an estimated $22 billion in net value to the United States each year through their effects on cooling the climate and improving air quality” (p. 194). Does Hipp provide the source for such a verifiable fact? From the Notes I found “Net monetary value of oaks: Cavender-Bares et al. (2022)” (p. 243). Checking the cited source we find this statement “Benefits provided by trees in the US are distributed across the tree of life (Fig 1B), yet two major lineages—the pines (Pinus) and the oaks (Quercus)—respectively generated $25.4 B [Ed – presumably this is an abbreviation for ‘billions’, although it’s not one I’ve seen before] and $22.3 B in net benefit annually between 2010 and 2012 and are by far the most valuable genera in the contiguous US (Table 1)”. For this monetary example, Hipp’s cited source does deliver the goods. However, and although I found no direct mention of ‘cooling the climate’ amongst the regulating services (John Wilson & Richard Primack) that trees might provide in the cited source [but, Cavender-Bares et al. (2022) do talk about climate regulation benefits via carbon storage in tree biomass], there was plenty about trees and their effects on air quality. Overall, in this instance, Hipp’s stated source seems to support his statement.

I did not check in detail any other statements for disclosure of sources in the book, but, my impression is that Hipp has been quite diligent in stating sources. But that’s only a feeling, sceptical and critical readers will need to satisfy themselves that all facts in which they have an interest are suitably sourced.

Literature Cited

Oak origins has a most impressive listing of literature cited – which provides fuller citation details of the sources provided in the Notes section to support statements made in-text. Presented in alphabetical order of first-named author, and occupying approx. 64 pages, it primarily consists of scientific articles [again emphasising that this is an academic text] (and usually with their DOIs/URLs). Amongst the hundreds of papers are approx. 200 publications dated post-2019 (and 35 published in 2023) – which probably means that Oak origins is as up-to-date as one can expect for a book published in 2024. Several of the listed publications have Hipp as an author or co-author. And why not? If you’ve published relevant material, it deserves to be cited. Plus, knowing how much the book’s author has contributed to the topic gives the tome that important added dose of ‘legitimacy’, and justification for Hipp being its author. With that publication record I’m suitably assured that Hipp knows what he is talking about.

Summary

The challenging nature of Oak origins by Andrew Hipp requires patience and concentration on the part of the readers. But that commitment and perseverance will be well rewarded with a fascinating tale of the evolutionary origins and development of oak trees and the important role they play in the colonising and greening of the land by plants, and the ecology of the planet. Along the way you will learn a lot about oak biology – reproductive strategies in particular – and receive a very good grounding in the intricacies of plant genetics and an understanding of ‘how species come to be’. Although Oak origins is not a plants-and-people book, it provides a lot of information about a tree that has iconic status for humanity.

* Although Oak origins is about more than oaks, one thing it is not really about is the importance of oaks to people. That is despite the book’s press release and web site telling us, “Oaks are familiar to almost everyone, and beloved. They are embedded in our mythology. They have fed us, housed us, provided wood for our ships and wine barrels and homes and halls, planked our roads, and kept us warm.” However, Hipp isn’t entirely silent on the matter of human uses of oak and oak products. For instance, he mentions oak galls as a “central ingredient of ink used from the Middle Ages through the nineteenth century” (p. 192); the use of oak wood as staves for wine barrels, as a source of lumber for shipbuilding; for the manufacture of charcoal; and for acorn-eating. Hipp also states that “Holm oak shaped Mediterranean economy and culture through the modern age only because its ancestors arrived in the Mediterranean from East Asian tropical origins” (p. 135). But those are very brief passages amongst the overwhelmingly biological-evolutionary bulk of the book. For more on the value of oak to humanity, I suggest readers look at Oak: Frame of Civilization by William Bryant Logan – which book is cited by Hipp as a source for some of his facts about human uses of oak. Or Peter Young’s Oak.

** To help readers get to grips with the naming of oaks, an Appendix provides two lists of oak names, presumably for species referred to in the book. First, Scientific names to common names – in scientific name order by Genus, with common English names and name of family to which they belong (the section within the Quercus genus is also shown for Quercus spp.). Common names of the 10 Quercus sections are also shown. Second, Common names to scientific names, is the same information but in alphabetical order of the common name used in the first list. NB, both common and scientific names are also listed in the Index (per p. 5). Of the approx. 425 species Hipp tells us are within the Quercus genus, 54 are named in these lists. Somewhat curiously, 73 Quercus spp. are listed in the Index.

For more information on Quercus species (and other genera), the International Dendrology Society has a “modern, web-based encyclopaedia of woody plants hardy in the temperate parts of the world”. The site, Trees and shrubs online, has many pictures of the trees as well as information about them, and looks like an invaluable resource for your tree- and shrub-based browsing pleasure.

*** Chapter 7 also has quite a lot to say about oaks and mycorrhizal fungi. Interestingly, although the concept of the wood-wide web [WWW] (Katie Field & Emily Magkourilou; Vincent Merckx et al., 2024; Sophie Yeo) is mentioned by Hipp that is confined to the book’s Notes section. And, even then, Hipp is suitably cautious, saying, “However, the data are still inconclusive regarding the extent and importance of common mycorrhizal connections (the “wood-wide web”) (Karst, Jones, and Hoeksema 2023)” (p. 242). [Ed. – That is also a very good example of making a statement and citing the source. But, that’s from the Notes. If only sources for the facts within chapters and the epilogue were shown in such a straightforward way…]

REFERENCES

Jeanine Cavender-Bares et al., 2022. The Hidden Value of Trees: Quantifying the Ecosystem Services of Tree Lineages and Their Major Threats across the Contiguous US. PLOS Sustainability and Transformation 1(4): e0000010; https://doi.org/10.1371/journal.pstr.0000010

Béatrice Chassé, 2011. A Botanic Oak Collection Recruited for Science. International Oak Journal No. 22: 23-26.

Béatrice Chassé, 2017. A Tour of the Oaks of the Arboretum des Pouyouleix. Arnoldia 74(3): 29–39; https://doi.org/10.5962/p.287340

Min Deng et al., 2024. Quercus zhekunii M Deng & J Huang, a new sclerophyllous oak from Guangxi, southwest China. Nordic Journal of Botany; doi: 10.1111/njb.04512

Justine Karst et al., 2023. Positive citation bias and overinterpreted results lead to misinformation on common mycorrhizal networks in forests. Nat Ecol Evol 7: 501–511; https://doi.org/10.1038/s41559-023-01986-1 [NB the version shown in Hipp has different page numbers to those shown in this citation, which is taken from the journal’s website]

Vincent Merckx et al., 2024. Mycoheterotrophy in the wood-wide web. Nat. Plants 10: 710–718; https://doi.org/10.1038/s41477-024-01677-0