California’s Tallest and Most Iconic Trees Are Getting Heatstroke

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Save this storySave this storyThis story originally appeared on Vox and is part of the Climate Desk collaboration. A grove of coast redwood trees is a forest of forests. Individual trees can live for thousands of years, and as they grow, their canopies develop mats of soils and ferns, creating something like a second forest floor hundreds of feet above the ground.

保存此报道。此报道最初发表于 Vox，是 Climate Desk 合作项目的一部分。一片海岸红杉林就像是森林中的森林。单棵树木可以存活数千年，随着它们的生长，树冠上会形成土壤和蕨类植物垫，在离地面数百英尺的高处创造出类似第二个森林地表的环境。

These elevated ecosystems are home to creatures like the wandering salamander, which can spend its entire life cycle on top of a single tree, and the marbled murrelet, a seabird whose nesting habits were a mystery until one day in 1974, when a tree-trimmer stumbled upon a nest 150 feet above the ground. Sometimes a stray seed—a sitka spruce or oak or even another redwood—finds its way up a massive redwood and grows into another tree entirely, nestled among the branches of a giant.

这些高处的生态系统是漫游蝾螈等生物的家园，它们可以在一棵树上度过整个生命周期；这里也是斑海雀的栖息地，这种海鸟的筑巢习性曾是个谜，直到 1974 年的一天，一名修剪树木的工人偶然在离地 150 英尺高处发现了一个鸟巢。有时，一颗流浪的种子——无论是西加云杉、橡树还是另一棵红杉——会落入巨大的红杉树上，并在巨树的枝桠间长成另一棵树。

For millennia, redwoods have been the anchors of this incredibly stable ecosystem; they can even survive wildfires that devastate other tree species. They’ve also been the source of plenty of human inspiration and drive millions of dollars to California’s tourist economy. But like other living things, they aren’t immune to warming temperatures, and it turns out these trees have a particular weak spot.

几千年来，红杉一直是这个极其稳定的生态系统的支柱；它们甚至能在摧毁其他树种的野火中幸存下来。它们也是许多人类灵感的源泉，并为加州的旅游经济带来了数百万美元的收入。但像其他生物一样，它们无法对变暖的气温免疫，事实证明，这些树木有一个特定的弱点。

A new study from researchers at the University of California, Davis, using measurements taken twice a month for two years, found that coast redwood leaves essentially dial down their photosynthesis during periods of extreme heat. This means trees are slowing their “breathing” and “eating” because they can’t cool down fast enough when temperatures rise. It’s a problem that’s only going to get worse as climate change continues to drive global temperatures up, and it raises urgent questions about the future of these iconic trees and the layers of life that live on and around their shadow-dappled lengths.

加州大学戴维斯分校研究人员的一项新研究，通过两年来每月两次的测量发现，海岸红杉的叶子在极端高温期间会基本停止光合作用。这意味着树木正在减缓它们的“呼吸”和“进食”，因为当气温升高时，它们无法足够快地降温。随着气候变化持续推高全球气温，这个问题只会变得更糟，这也引发了关于这些标志性树木及其周围生命层未来的紧迫问题。

To understand what’s happening to the redwoods—and what that may mean for the species that rely on them—first we need to take a quick foray into tree anatomy. Trees use the pores in their leaves, stomata, to do two things: take carbon dioxide in, and let water out. Open stomata mean the tree is “breathing” in carbon dioxide and photosynthesizing, but probably also letting water out. Closed stomata mean the tree is conserving water but also slowing down its photosynthesis, since it isn’t taking in any carbon dioxide.

为了了解红杉正在发生什么，以及这对依赖它们的物种意味着什么，我们首先需要快速了解一下树木的解剖结构。树木利用叶片上的气孔来做两件事：吸收二氧化碳和排出水分。气孔张开意味着树木正在“呼吸”二氧化碳并进行光合作用，但也可能正在流失水分。气孔关闭意味着树木在节约用水，但也减缓了光合作用，因为它没有吸收任何二氧化碳。

“Because leaves are a physical, light-absorbing object, they heat up just like asphalt would in the sun,” said Lily Klinek, a PhD student at UC Davis who was the lead author on the study. “They’re heating up faster than the air around them,” she said. “And one way that they can cope with this is by opening their stomata and allowing water to transpire out of the leaf, which is kind of like the way that humans sweat. When water evaporates off our skin, it cools us down, and it’s the same for leaves. But they can only really do that if they have enough water to lose.”

“因为叶子是物理性的吸光物体，它们会像阳光下的沥青一样变热，”加州大学戴维斯分校的博士生、该研究的主要作者莉莉·克莱内克（Lily Klinek）说。“它们的升温速度比周围空气快，”她说。“它们应对这种情况的一种方法是打开气孔，让水分从叶片中蒸腾出来，这有点像人类出汗的方式。当水分从我们的皮肤上蒸发时，会让我们降温，叶子也是如此。但它们只有在有足够水分流失的情况下才能真正做到这一点。”

Klinek and her colleagues studied coast redwoods in Mendocino County, near the southern part of their range, where the researchers used specialized instruments to measure whether or not individual leaves were photosynthesizing. During heat waves, they found, coast redwoods hit a point where they can’t cool down fast enough, and they just keep their stomata closed. While that saves water, it also means the tree is not taking in more carbon—which means it’s not “eating” or building up energy stores it can use to keep growing.

克莱内克和她的同事们研究了位于门多西诺县（Mendocino County）海岸红杉分布区南部的红杉，研究人员使用专门的仪器来测量单片叶子是否在进行光合作用。他们发现，在热浪期间，海岸红杉会达到一个无法足够快地降温的临界点，于是它们只能保持气孔关闭。虽然这节省了水分，但也意味着树木无法吸收更多的碳——这意味着它没有在“进食”或积累可用于持续生长的能量储备。

This may sound like a technicality of tree physiology. But it could have a big impact on these forests, creating a vicious cycle where trees don’t have enough carbon stored to replenish their leaves in the future, which means they won’t be able to photosynthesize as efficiently, which will starve them even further, repeating until trees eventually die. And while this sort of behavior has been observed in tropical trees, it’s the first time anyone has studied how coast redwoods respond to the same heat stress.

这听起来可能只是树木生理学上的一个技术细节。但它可能会对这些森林产生巨大影响，形成一个恶性循环：树木没有储存足够的碳来在未来补充叶片，这意味着它们无法高效地进行光合作用，这将使它们进一步饥饿，并不断重复，直到树木最终死亡。虽然这种行为在热带树木中已被观察到，但这是首次有人研究海岸红杉如何应对同样的热应激。

“The term people use is carbon starvation,” said Chris Still, a forest ecologist at Oregon State University who didn’t participate in the study. “Leaves can be expensive to build for a plant.” Most of the redwoods in California and the Pacific Northwest nowadays are new-growth forests planted in the last couple of centuries. That means the trees are relatively hungry, like growing adolescents, compared to their old-growth cousins, so they need to suck in lots of carbon dioxide to grow taller.

“人们使用的术语是‘碳饥饿’，”未参与该研究的俄勒冈州立大学森林生态学家克里斯·斯蒂尔（Chris Still）说。“对于植物来说，制造叶片的成本很高。”如今加州和太平洋西北地区的大多数红杉都是过去几个世纪种植的次生林。这意味着与原始森林中的同类相比，这些树木相对“饥饿”，就像正在成长的青少年一样，因此它们需要吸收大量的二氧化碳才能长得更高。

Redwoods are also incredibly good at storing carbon. Thanks to their bulk and long lives, they store more carbon dioxide per acre than any other forest in the world. “Not only is it negative feedback on the climate system itself, with less carbon sequestration from these trees and forests, but the trees themselves are going to obviously not grow as big,” said Anthony Ambrose, a plant physiological ecologist and co-founder of the Ancient Forest Society. So even if the trees survive, they might not have as much space for fern and soil mats, which means less habitat for the creatures that live in them, and they may not grow as tall as they once used to, which could hurt them in another way.

红杉在储存碳方面也非常出色。由于其巨大的体积和漫长的寿命，它们每英亩储存的二氧化碳比世界上任何其他森林都多。“这不仅是对气候系统本身的负面反馈，因为这些树木和森林的碳封存量减少了，而且树木本身显然也不会长得那么大，”植物生理生态学家、古森林协会（Ancient Forest Society）联合创始人安东尼·安布罗斯（Anthony Ambrose）说。因此，即使树木能够存活，它们可能也没有足够的空间来容纳蕨类植物和土壤垫，这意味着生活在其中的生物栖息地减少了，而且它们可能无法长到曾经那样高，这可能会以另一种方式伤害它们。

A central threat to these trees, Klinek explained, is that in addition to the heat stress, they may also lose a crucial source of hydration and protection: Coast redwoods have an intricate relationship with the fogs of their native Northern California and the Pacific Northwest. The trees are so tall that their canopies reach high into fog layers, where they can sip water directly from the sky. The fog also shades the trees from the sun and helps coast redwoods create their own microclimate, which is why these forests are noticeably cooler than the areas adjacent to them. But warmer oceans and drier air are making fog more sporadic, which means the trees are missing a crucial source of relief from heat that is only getting more intense. “It’s kind of a double whammy,” said Todd Dawson, a plant ecophysio

克莱内克解释说，这些树木面临的一个核心威胁是，除了热应激之外，它们还可能失去一个关键的水分和保护来源：海岸红杉与它们原产地北加州和太平洋西北地区的雾气有着复杂的关系。这些树木非常高，树冠可以伸入高处的雾层，直接从天空中汲取水分。雾气还能为树木遮挡阳光，帮助海岸红杉创造自己的微气候，这就是为什么这些森林明显比邻近地区凉爽的原因。但海洋变暖和空气干燥使得雾气变得更加稀少，这意味着树木失去了一个缓解日益严重的高温的关键来源。“这简直是双重打击，”植物生态生理学家托德·道森（Todd Dawson）说。