题图：根据20年的卫星图像估算的地球陆地生态系统的平均季节性生长周期。Terasaki Hart等人/《自然》
（《澳纽网》综合编译报道） – 春夏秋冬——这个我们习以为常的年度节律，实际上可能远比我们想象的要复杂得多。澳大利亚联邦科学与工业研究组织（CSIRO）的一项最新研究，通过一种全新的方法，利用卫星图像以前所未有的细节，描绘了地球陆地生态系统的季节性生长周期。这项发表在顶尖期刊《自然》（Nature）上的研究，不仅为我们呈现了一幅令人惊叹的地球“呼吸”动态图，还揭示了全球多处季节性“异步”的热点地区——在这些地区，相距不远的两个地方，其季节节律可能完全不同步。
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从太空俯瞰季节变迁
季节设定了生命的节奏。从古至今，人类通过观察物候（phenology）来适应自然。如今，借助长达数十年的卫星图像档案，科学家们可以从太空的宏观视角，来观察全球植被的生长周期。
然而，以往的方法大多基于一个简单的假设：季节是清晰分明的。这在拥有严酷冬季的欧洲和北美等高纬度地区行之有效，但在热带和干旱地区，植被的生长变化更为微妙，没有明确的“生长季”，传统方法便会失灵。
通过对20年的卫星图像应用一种新的分析方法，研究团队绘制出了一张更精确、更详尽的全球植被生长周期时序图。
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意外的发现：“双高峰”模式与“异步热点”
这张全新的动态地图，除了展示了如高纬度地区春天延迟等预期模式外，还揭示了更多令人惊讶的规律。
其中一个最引人注目的模式，出现在全球五个地中海气候区（包括加利福尼亚、智利、南非、澳大利亚南部和地中海地区本身）。这些地区都共享一种“双高峰”的季节性生长模式，森林的生长周期高峰，通常比其他生态系统晚大约两个月。
这一发现解释了该研究的另一个核心成果：地中海气候区及其邻近的干旱地区，是全球季节性活动“异步”（out-of-sync）的主要热点。换言之，在这些区域内，地理位置相近的地方，其季节周期的时间可能存在巨大差异。
其他全球性的“异步热点”主要出现在热带山区。研究人员推测，这可能与山脉对气流的复杂影响有关，从而决定了局部的季节性降雨和云量模式。
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季节性与生物多样性之间有何关联？
研究团队最初的动机，就是为了找出这些季节性“异步”的地区。而他们发现，这些“异步热点”与地球上许多生物多样性热点（拥有大量动植物物种的地区）高度重叠，这可能并非巧合。
科学家们提出了一个深刻的生态和进化假说： 在这些“异步”地区，由于植被的生长周期（即可利用的资源）在邻近地区之间是不同步的，许多物种的季节性繁殖周期也可能会变得不同步。
这种繁殖周期的“错位”，将减少不同种群之间杂交的可能性。久而久之，这些种群预计会产生遗传分化，甚至最终可能分裂成不同的物T种。如果这个过程持续发生，从长远来看，这些地区将孕育出极其丰富的生物多样性。
从太空到地面：现实世界的影响
这不仅仅是一个理论模型。研究团队发现，这张卫星地图能够准确预测地面上植物开花时间的显著差异，以及邻近种群之间遗传相关性的差异。
一个极具说服力的例子是哥伦比亚的咖啡收成。在这里，跨过一座山脉、仅有一天车程的两个咖啡种植园，其咖啡树的开花结果周期，可能会像它们分别位于地球的南北半球一样，完全不同步。
理解季节性模式在空间和时间上的复杂性，不仅对进化生物学至关重要，对于理解动物迁徙、气候变化对生态系统的影响，甚至对农业和人类活动地理分布的规划，都具有根本性的意义。
读者可以通过这个交互式在线地图，更详细地探索这项研究的成果。
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附原文：
See Earth’s seasons in all their complexity in a new animated map
The annual clock of the seasons – winter, spring, summer, autumn – is often taken as a given. But our new study in Nature, using a new approach for observing seasonal growth cycles from satellites, shows that this notion is far too simple.
We present an unprecedented and intimate portrait of the seasonal cycles of Earth’s land-based ecosystems. This reveals “hotspots” of seasonal asynchrony around the world – regions where the timing of seasonal cycles can be out of sync between nearby locations.
We then show these differences in timing can have surprising ecological, evolutionary, and even economic consequences.
Watching the seasons from space
The seasons set the rhythm of life. Living things, including humans, adjust the timing of their annual activities to exploit resources and conditions that fluctuate through the year.
The study of this timing, known as “phenology”, is an age-old form of human observation of nature. But today, we can also watch phenology from space.
With decades-long archives of satellite imagery, we can use computing to better understand seasonal cycles of plant growth. However, methods for doing this are often based on the assumption of simple seasonal cycles and distinct growing seasons.
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This works well in much of Europe, North America and other high-latitude places with strong winters. However, this method can struggle in the tropics and in arid regions. Here, satellite-based estimates of plant growth can vary subtly throughout the year, without clear-cut growing seasons.
Surprising patterns
By applying a new analysis to 20 years of satellite imagery, we made a better map of the timing of plant growth cycles around the globe. Alongside expected patterns, such as delayed spring at higher latitudes and altitudes, we saw more surprising ones too.
One surprising pattern happens across Earth’s five Mediterranean climate regions, where winters are mild and wet and summers are hot and dry. These include California, Chile, South Africa, southern Australia, and the Mediterranean itself.
These regions all share a “double peak” seasonal pattern, previously documented in California, because forest growth cycles tend to peak roughly two months later than other ecosystems. They also show stark differences in the timing of plant growth from their neighbouring drylands, where summer precipitation is more common.
Spotting hotspots
This complex mix of seasonal activity patterns explains one major finding of our work: the Mediterranean climates and their neighbouring drylands are hotspots of out-of-sync seasonal activity. In other words, they are regions where the seasonal cycles of nearby places can have dramatically different timing.
Consider, for example, the marked difference between Phoenix, Arizona (which has similar amounts of winter and summer rainfall) and Tucson only 160 km away (where most rainfall comes from the summer monsoon).
Other global hotspots occur mostly in tropical mountains. The intricate patterns of out-of-sync seasons we observe there may relate to the complex ways in which mountains can influence airflow, dictating local patterns of seasonal rainfall and cloud. These phenomena are still poorly understood, but may be fundamental to the distribution of species in these regions of exceptional biodiversity.
Seasonality and biodiversity
Identifying global regions where seasonal patterns are out of sync was the original motivation for our work. And our finding that they overlap with many of Earth’s biodiversity hotspots – places with large numbers of plant and animal species – may not be a coincidence.
In these regions, because seasonal cycles of plant growth can be out of sync between nearby places, the seasonal availability of resources may be out of sync, too. This would affect the seasonal reproductive cycles of many species, and the ecological and evolutionary consequences could be profound.
One such consequence is that populations with out-of-sync reproductive cycles would be less likely to interbreed. As a result, these populations would be expected to diverge genetically, and perhaps eventually even split into different species.
If this happened to even a small percentage of species at any given time, then over the long haul these regions would produce large amounts of biodiversity.
Back down to Earth
We don’t yet know whether this has really been happening. But our work takes the first steps towards finding out.
We show that, for a wide range of plant and animal species, our satellite-based map predicts stark on-ground differences in the timing of plant flowering and in genetic relatedness between nearby populations.
Our map even predicts the complex geography of coffee harvests in Colombia. Here, coffee farms separated by a day’s drive over the mountains can have reproductive cycles as out of sync as if they were a hemisphere apart.
Understanding seasonal patterns in space and time isn’t just important for evolutionary biology. It is also fundamental to understanding the ecology of animal movement, the consequences of climate change for species and ecosystems, and even the geography of agriculture and other forms of human activity.
Want to know more? You can explore our results in more detail with this interactive online map, which we also include below.
Drew Terasaki Hart, Ecologist, CSIRO
This article is republished from The Conversation under a Creative Commons license. Read the original
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