Hiroshima's Survivor Trees Still Lean Toward Ground Zero: What 81 Years of Scar Tissue Tells a Tree Doctor
Hiroshima's Survivor Trees Still Lean
Toward Ground Zero: What 81 Years of Scar Tissue Tells a Tree Doctor
Isaki Kasahara
Isaki Kasahara
Japanese Arborist & Certified Tree
Doctor (Jumoku-i) | Bridging Japan’s Tree Care, Urban Forestry & Landscape
Culture with the World | Heritage Trees · Tree Risk Assessment · Niwaki
2026年8月15日
Introduction
Hiroshima's survivor trees — hibakujumoku (被爆樹木), the trees that
lived through the atomic bombing of 6 August 1945 — still lean toward
the hypocenter. That is not a metaphor. When researchers went out and measured
them, the majority of undisturbed survivors were found tilting toward ground
zero, carrying burn scars up the face that met the blast and abnormal swelling
on the face that did not. The City of Hiroshima registers 159 such trees,
standing between 370 and 2,160 metres from the hypocenter.
Almost everything written about
hibakujumoku in English stops at endurance. The endurance is real and deserves
the telling. But for anyone who works on trees, these are also something rarer:
a population that took a single catastrophic, directional injury on a known
date, at a known distance, and then lived eight more decades under observation.
Arboriculture almost never gets a dataset like that. Read as wood rather than
as symbol, these trees say a great deal about how any scarred veteran should be
assessed — in Hiroshima, or in a city park anywhere.
What Hibakujumoku Actually Are
A registry, not a census
Hiroshima established its registration
system in 1993. To be listed, a tree must be documented as standing within
roughly two kilometres of the hypocenter on the day of the bombing, must still
be alive, and must have an owner willing to preserve it. The current list holds
159 trees.
That distinction matters, because English
accounts often blur it. Far more than 159 trees survived the bombing. The
registry is not a count of survivors — it is a count of survivors we have
documentation and a custodian for. Many stand on private temple and shrine
land, and entered the record only because a priest or a family kept the story
alive.
The two closest survivors
The nearest registered tree is a weeping
willow 370 metres from ground zero. The blast knocked it down, but new shoots
pushed up from the roots. The second nearest, a kurogane holly at 410 metres,
lost everything above ground and stood as a burned stump until 1949, when it
finally sprouted.
Both point at the same mechanism, and it is
the first thing a tree doctor notices here: survival was not survival of the
trunk. It was survival of the root system and the dormant buds within it.
Camphor, ginkgo, kurogane holly and willow — the species dominating the
registry — all share strong latent-bud and basal-sprouting capacity. Species
without it never came back, so they are not in the list to be counted. The
registry is partly a record of which regeneration strategies work when
everything above ground is gone.
This is the quiet link to Japan's other old
trees. As I have written on why Japan's ancient trees live for a thousand
years, longevity here rarely means an unbroken original trunk. It means a
lineage of replacement stems from a root system that never died.
The Trees Still Point at Ground Zero
文章內容
What the measurements show
This is the finding that changed how I read
these trees. A study presented to the Japanese Institute of Landscape
Architecture measured 56 hibakujumoku in the field, sorting them into
single-stemmed trees whose injuries are still visible and which have never been
transplanted (29), trees moved at some point (22), and trees that regenerated
after losing the entire above-ground stem (5).
Among the 29 undisturbed, visibly injured
trees:
62% lean toward the hypocenter within ±15
degrees; 79% within ±30 degrees.
A Rayleigh test rejected uniform
distribution of orientation decisively (r = 0.611, n = 29, p < 0.001).
75% carry vertical burn scarring and bark
fissures on the hypocenter-facing side.
59% show pronounced eccentric swelling on
the side away from the hypocenter.
The transplanted group showed no such
pattern (r = 0.356, p > 0.05). Move the tree, and the record is erased. What
survives in the undisturbed trees is not sentiment but geometry: eight decades
on, they are still oriented by an event that lasted a fraction of a second.
Why a burned tree leans toward the blast,
not away from it
The instinctive reading is backwards, and
the correction is where the arboriculture lives. A tree pushed by a blast wave
should fall away from it. But this lean was not made in 1945. It was made
slowly, afterwards, by growth.
The thermal flash killed the cambium on the
hypocenter-facing side, and dead cambium lays down no new wood, ever. The
surviving cambium opposite must compensate for the strength the trunk lost, and
it responds by thickening hard — precisely the eccentric growth recorded in 59%
of the sample. Run that forward eighty years: one flank expands annually while
the other is frozen, and the stem axis is progressively levered toward the side
that stopped growing — toward ground zero. Root extension was likewise sparse
on the blast side and vigorous on the sheltered side, so the anchorage went
asymmetric too, settling the tree further the same way.
So the lean is not the damage. The lean is
the tree's own eight-decade calculation of what happened to it, expressed in
wood. A hibakujumoku is a recording instrument and a compass at once — and, as
far as I can find, no one has said so in English before.
Reading a Trunk Whose Other Half Is Dead
Why the usual rule of thumb fails here
Tree risk assessment often leans on the
ratio of residual wall thickness to stem radius — the familiar t/R shorthand.
For a stem that approximates a hollow cylinder, it is a serviceable first
approximation.
For a hibakujumoku it is close to
meaningless. The cross-section is not a circle; it is a fused pairing of a
frozen, fire-killed flank and a hypertrophied living one, differing in
geometry, density and moisture. Averaging across them produces a number describing
no real part of the tree. Anyone assessing fire-scarred, lightning-struck or
one-sided veterans meets the same trap.
A field note: dead wood reads hard
文章內容
Let me be careful about what is mine and
what is Hiroshima's. I have not put a needle into a hibakujumoku. What I can
offer is what I find repeatedly in my own diagnostic work in Japan, which bears
directly on how such trees should be handled.
When I use a resistograph — an IML RESI
PowerDrill — to measure penetration resistance inside a trunk, fully dead wood
often reads as high resistance simply because it has dried and hardened. Apply
the interpretive habits you developed on living wood, and you can read a dead
flank as sound. On a tree whose entire hypocenter-facing side is necrotic, that
error is not academic; it is the difference between a defensible assessment and
a dangerous one. (I go into the instrument's behaviour in detail in my piece on
how a resistograph works.)
Which is why the decision that matters most
is where you drill at all. I do not scan a tree. I let the external examination
choose the points — abnormal sound under the mallet, fruiting bodies, bark
anomalies — and drill only where failure risk concentrates. The resistograph
gives a measured value along the line the needle travelled: the window is
narrow, but what it shows is real. Combined with external findings it is
powerful; used alone it is a guess with a graph attached. Much of that external
reading needs no instrument at all, as I set out in reading tree decay without
technology. In Japan the sequence is formalised: inspection, visual diagnosis,
instruments, integrated judgement — the working method of a certified jumoku-i,
Japan's tree doctor qualification.
On a fire-scarred veteran, the first thing
I look at is not the burn. It is how much sound wood remains on the living side
— because that is what is holding the tree up.
What is actually killing them now
The symbolic framing implies these trees
are safe. They are not.
A survey of the registered survivor trees
in fiscal 2016 found problems — lifting bark, insect damage — in roughly 40% of
them. The city has been treating 39 trees whose condition deteriorated.
Of 58 survivor trees held by 23 private
temples, shrines and organisations, at least 14 are of active concern for age
or disease.
The tabunoki at Honkyō-ji has a hollowing
trunk with fungal fruiting bodies — an unambiguous signal of advancing internal
decay.
The Somei-Yoshino cherry at Ikari Shrine is
declining from light starvation caused by changes in what has been built around
it.
Decay and shade. Those are the same two
agents grinding down street trees and heritage trees across the world. What
threatens Hiroshima's survivor trees in 2026 is not radiation. It is
urbanisation and time.
Keeping Them Alive: The Discipline of Not
Doing Too Much
Thirty years of charts
Chikara Horiguchi, a tree doctor based in
Hiroshima, has worked on these trees since 1992 and has built condition records
for 161 of them. Two of his stated principles deserve to be read by anyone
responsible for old trees anywhere:
"I don't carry out procedures that
wound a tree in order to make it look tidy." Pruning is held to the
minimum required to maintain vitality — not to shape.
"Too much water and fertiliser fails
every time." Well-meaning over-treatment weakens far more veterans than
neglect does.
Western arboriculture reached much the same
place in its veteran tree management doctrine. What is notable is that in Japan
the conclusion was arrived at independently, through practice, on trees no
textbook covered.
Moving the building instead of the tree
文章內容
写真：Hachigamine / Wikimedia Commons / CC BY-SA 4.0
The ginkgo at Anraku-ji stands 2,160 metres
from the hypocenter — the farthest tree on the register. It burned, sprouted
the following year, and now stands about 20 metres tall with a girth of 4.55
metres. The burn scarring on its hypocenter-facing side is still visible. And
when the tree's growth met the temple gate, the gate's roof was cut through to
let it pass.
That decision is culturally telling. Rather
than reduce the tree to suit the structure, the structure was altered to suit
the tree. The same instinct shows in Japan's heavy use of props to hold an old
canopy in place instead of shortening it — the approach behind veterans like
the 2,000-year-old camphor at Kinomiya Shrine. Western practice more often
reaches for crown reduction and retrenchment: keep the tree by making it
smaller. Japanese practice frequently chooses the other option: keep the form,
and build the support around it. Neither is universally right — but outside
Japan the profession is often unaware there are two options on the table.
The seeds have already left Japan
There is a part of this story readers can
join. Green Legacy Hiroshima, run under the UN Institute for Training and
Research, distributes second-generation seeds and seedlings from hibakujumoku
worldwide, working with 165 partners across 42 countries. Ginkgo and camphor
dominate the openly published lists; requests go through the organisation
directly.
Which means these trees are no longer only
monuments. They are a genetic lineage being distributed and backed up across
the planet. It is the same principle we work by on any irreplaceable tree: the
most reliable way to preserve one is to stop trying to preserve only that one.
Frequently Asked Questions
Q: Are Hiroshima's survivor trees damaged
by radiation? A: The damage visible today is overwhelmingly thermal and blast
injury from 1945, plus eight decades of ordinary decay and environmental
change. The problems recorded now — hollowing trunks, lifting bark, insect
damage, light starvation — are the same ones affecting urban veterans that were
never near a bomb.
Q: How did trees regenerate after the
entire above-ground part burned? A: Because living tissue and dormant buds
survived at the root collar and in the roots. Species with strong sprouting
capacity — camphor, ginkgo, kurogane holly, willow — rebuild a stem from the
base, as the kurogane holly 410 metres from the hypocenter did in 1949. Note,
though, that a stem regrown this way attaches to the old base differently from
an original trunk, and that union can become a long-term structural weak point.
Q: How should a badly scarred veteran tree
be assessed? A: In sequence. Japanese practice runs inspection → visual diagnosis → instrumented
diagnosis → integrated judgement. Let external findings — sound under the
mallet, fruiting bodies, bark anomalies —
select where instruments go, rather than scanning the whole
tree. Never rely on one instrument, and remember that dead wood can dry and
read as hard, flattering a necrotic flank.
Q: Can anyone obtain seeds from the
A-bombed trees? A: Green Legacy Hiroshima (UNITAR) distributes
second-generation seeds and saplings and publishes lists of what is available,
working through 165 partners in 42 countries. The correct route is to approach
the organisation directly with your planting purpose and your plan for
long-term care.
Conclusion
It is easy to write about hibakujumoku as
miracles. Doing so misses the technical questions they are still putting to us.
How do you assess a stem whose cambium died
on one side eighty years ago? How do instruments misread burned wood? How do
you hold back from over-treating a tree that looks like it needs help? How far
will we alter a building, a plan, a street so a tree can keep its shape? None
of these are questions about Hiroshima alone. They apply to every fire-scarred
veteran, every lightning-struck giant, every declining large tree hemmed in by
a city.
And the trees that have been answering them
for 81 years are still standing, still leaning toward the place where it began.
Field notes from Japan's trees arrive a few
times each month — subscribe via the form in the footer, or follow the work on
LinkedIn.
**〈廣島的倖存樹仍朝向爆心傾斜：81年疤痕告訴樹醫師的事〉
繁體中文摘要**
廣島的「被爆樹木」（hibakujumoku）是 1945 年 8 月 6 日原子彈爆炸後仍存活的樹。研究者實地測量後發現，多數未被移植、仍保留原始傷痕的樹木
至今仍向爆心方向傾斜，並在面向爆心的一側留下垂直燒灼痕與樹皮裂縫，而背向爆心的一側則出現異常膨大。
文章指出，這些樹不只是象徵，更是罕見的科學資料：
它們在 確定的時間、確定的方向
受到一次巨大傷害，並在之後 存活 80 年以上，讓樹藝師能觀察長期的生長反應。
一、被爆樹木是「登錄」而非「存活數量」
廣島市自 1993 年建立登錄制度，條件包括：
- 樹木在爆心 2 公里內
- 1945 年當天已存在
- 至今仍存活
- 有願意保存的所有者
目前登錄 159 棵，但實際存活的樹遠超過此數。許多樹因寺廟或家族保存故事而被記錄。
最靠近爆心的兩棵樹：
- 370 公尺：垂柳，被吹倒後從根部重新萌芽
- 410 公尺：黑鐵柿，地上部分全毀，1949 年才從根部再生
這顯示倖存不是樹幹倖存，而是
根系與休眠芽倖存。 能強力萌芽的樹種（樟樹、銀杏、黑鐵柿、柳樹）才得以重生。
二、樹木為何「向爆心傾斜」？
研究測量 56 棵樹，其中 29 棵未移植、傷痕仍可見：
- 62% 在 ±15° 內朝向爆心傾斜
- 79% 在 ±30° 內朝向爆心
- 75% 爆心側有垂直燒灼與裂皮
- 59% 背向爆心側有明顯膨大
移植過的樹則完全沒有這種方向性。
傾斜不是爆炸造成的，而是 80 年的生長造成的
文章引用一句原文：
“The lean is not the damage. The lean is
the tree's own eight-decade calculation of what happened to it.”
原因如下：
- 爆心側的形成層被熱閃殺死 → 永遠不再生長
- 背向爆心側的形成層為補強而加速增厚 → 造成偏心木
- 根系也呈非對稱生長 → 爆心側弱、背向側強
- 80 年累積結果 → 樹幹軸線被慢慢「推向」不再生長的一側，也就是爆心方向
因此，樹木本身成了「羅盤」，以生長記錄爆炸方向。
三、為何一般的樹幹厚度比率（t/R）在此失效？
被爆樹木的橫切面不是圓形，而是：
- 一側完全死亡、乾硬
- 一側極度肥厚、含水量高
兩側密度與結構差異巨大，平均值毫無意義。
樹醫師提醒： 死木因乾燥會在鑽阻儀上呈現「高阻力」，容易被誤判為健康木材。 因此診斷時必須依外部徵象選擇鑽點，而非掃描整棵樹。
四、被爆樹木真正的威脅不是輻射，而是城市化與老化
調查顯示：
- 約 40% 樹木有樹皮剝離、蟲害等問題
- 多棵樹因真菌腐朽或光照不足而衰退
- 這些問題與全球都市老樹完全相同
五、日本樹醫師的管理原則
樹醫師堀口力的兩項原則值得全球採用：
- 「不為了整齊而傷害樹木。」 修剪只為維持生命力，不為美觀。
- 「過度澆水與施肥永遠會失敗。」 過度照顧比忽略更容易害死老樹。
日本的老樹管理偏向「保留原貌、以支撐結構輔助」，
例如安樂寺的銀杏長到碰到寺門時，是 切建築物的屋頂讓樹通過，而不是修樹。
六、被爆樹的種子已在全球擴散
UNITAR 的「Green Legacy Hiroshima」計畫已將第二代種子與苗木送往 42 國、165 個合作夥伴。 銀杏與樟樹為主要品種。
這些樹不再只是紀念物，而是全球性的基因保存行動。
七、文章的核心問題
作者指出，被爆樹木提出的問題適用於所有受傷老樹：
- 如何評估一側形成層已死去 80 年的樹幹？
- 儀器如何誤讀燒灼木？
- 如何避免「過度治療」？
- 我們願意為了保留一棵老樹改變多少建築與城市規劃？
這些問題不只屬於廣島，而是所有城市老樹的共同課題。
0 意見:
張貼留言