Forests are often described as the lungs of the planet. They absorb carbon dioxide from the atmosphere, release oxygen, and store carbon in trunks, branches, roots, leaves, and soils. Because of this, forests occupy a central place in climate-change discussions. Afforestation, reforestation, forest conservation, and carbon-credit programmes all rely on one basic assumption: if trees take up more carbon dioxide, they will store more carbon in long-lived biomass, especially wood.

A recent study published in Science Advances challenges this apparently simple assumption. The paper, titled “Decoupled Carbon Assimilation and Growth Responses to Aridity in Temperate Deciduous Oaks,” shows that trees may continue photosynthesizing even after they have largely stopped growing. In other words, the carbon entering a tree through photosynthesis does not always become new wood.

This distinction may sound technical, but it has profound implications for climate science. If forests keep absorbing carbon dioxide but convert a smaller fraction of it into woody biomass under warmer and drier conditions, then their long-term capacity to store carbon may be lower than expected.

Photosynthesis Is Not the Same as Carbon Storage

At school level, we learn that plants use sunlight, water, and carbon dioxide to produce sugars through photosynthesis. This is correct, but it is only the beginning of the story. Once carbon enters a tree, it can follow many pathways. Some of it becomes cellulose and lignin in wood. Some is stored as starch or soluble sugars. Some is sent to roots. Some supports flowers, fruits, seeds, and defensive chemicals. A large fraction is also consumed by respiration, the metabolic process that keeps living cells alive.

Thus, photosynthesis measures carbon entry, not necessarily carbon storage. Long-term carbon sequestration depends on where the captured carbon finally goes. A forest may remain green and photosynthetically active, yet still store less carbon in durable woody tissues.

The new oak-forest study makes this point very clearly. The researchers examined temperate deciduous oak forests across North America and found that photosynthesis and aboveground wood growth are not perfectly synchronized. Across 137 tree-ring sites, annual growth became relatively insensitive to climate variability after midsummer, even though 26–36% of annual gross primary productivity still occurred during that later part of the season. In simpler words, a substantial fraction of yearly carbon uptake happened after stem growth had already slowed or stopped.

Why Wood Growth Stops Earlier

To understand this finding, we need to look inside the tree.

Wood is produced by a thin living layer beneath the bark called the vascular cambium. This cambium generates new xylem cells inward, which eventually become wood. But wood formation is not just a matter of having enough sugar. Cells must divide, expand, deposit thick secondary walls, and undergo lignification. These processes require not only carbon but also water, nutrients, favourable temperature, hormonal signals, and adequate cellular pressure.

Water is especially important. Expanding cells need turgor pressure, which depends on hydration. Under atmospheric dryness, even when leaves can still photosynthesize, cambial cell expansion may become difficult. The tree may continue taking in carbon, but it may no longer be physiologically safe or efficient to invest that carbon in new stem growth.

This is where vapor pressure deficit, or VPD, becomes important. VPD is a measure of atmospheric dryness—how strongly the air “pulls” water from leaves. Warm, dry air increases VPD. As VPD rises, trees lose water more rapidly. To protect themselves, they may reduce stomatal opening, slow growth, and shift internal priorities from expansion toward survival. The Science Advances study found that the decoupling between photosynthesis and growth intensified with interannual variability in VPD.

Trees Save, Spend, and Redirect Carbon

A useful way to understand a tree is to imagine it as a living economy. Photosynthesis is income. Growth is only one possible expenditure. A tree must also pay for maintenance, repair, defence, reproduction, storage, and underground partnerships with fungi and microbes.

When conditions are favourable, a larger portion of carbon can be invested in new wood. When conditions become stressful, the tree may choose survival over expansion. It may store carbon as non-structural carbohydrates such as starch and sugars. It may send more carbon below ground to support roots or mycorrhizal fungi. It may invest in defensive chemicals to resist insects and pathogens. It may simply burn more carbon through respiration, especially as temperature rises.

This is why modern plant ecology increasingly distinguishes between source limitation and sink limitation. The older view emphasized source limitation: growth depends mainly on how much carbon the leaves supply. The newer view recognizes sink limitation: growth also depends on whether tissues such as cambium, roots, and developing organs can actually use that carbon to build new biomass.

Christian Körner famously argued that plant growth is often controlled not by carbon supply alone, but by the capacity of tissues to convert resources into biomass. Carbon can be abundant, yet growth may still be limited by water, temperature, nutrients, or developmental constraints.

Why This Matters for Climate Models

Earth system models are essential tools for predicting future climate. They estimate how much carbon will remain in the atmosphere, how much will enter oceans, and how much will be stored by land ecosystems. Forests are a major part of these calculations.

Many models represent photosynthesis relatively well because it can be linked to leaf physiology, climate variables, and increasingly satellite-based observations. However, representing carbon allocation—how trees distribute carbon among wood, roots, storage, respiration, and other sinks—is much harder.

If models assume a tight relationship between photosynthesis and woody growth, they may overestimate forest carbon storage under future climates. This is the central warning of the oak study. Forests may remain active carbon absorbers, but not all absorbed carbon becomes long-lived biomass. If drought, heat, and atmospheric dryness increasingly push trees toward maintenance and survival rather than growth, then forests may provide a weaker climate-buffering service than expected.

This does not mean forests are unimportant. On the contrary, it shows why forest conservation is even more important. Old and mature forests store enormous amounts of carbon accumulated over decades or centuries. Disturbing them can release carbon that cannot be quickly replaced. But it does mean that planting trees alone cannot be treated as a simple substitute for reducing fossil-fuel emissions.

Lessons for India and the Himalaya

Although the study focused on North American oak forests, its message is highly relevant to India. Many Indian ecosystems are already exposed to increasing heat stress, irregular rainfall, severe drought spells, and altered seasonal patterns. Himalayan forests, including oak-dominated systems, are ecologically valuable and climatically sensitive. Rising temperatures and changes in moisture regimes could alter not only forest composition but also the way trees allocate carbon internally.

For regions such as Uttarakhand, where forests also support water regulation, biodiversity, livelihoods, and slope stability, this research offers an important reminder: forest health cannot be judged only by greenness. A canopy may look active while stem growth, carbon storage, regeneration, or hydraulic safety are under stress.

Future forest monitoring in India should therefore combine multiple approaches: tree-ring studies, phenology observations, eddy covariance towers, satellite remote sensing, soil moisture records, and long-term ecological plots. Such integrated monitoring would help answer a crucial question: are Indian forests merely remaining green, or are they continuing to store carbon in long-lived biomass?

The Bigger Scientific Message

The deeper message of this research is that trees are not passive carbon machines. They are living organisms that balance growth, survival, reproduction, storage, and defence. Climate change does not merely change how much carbon trees absorb; it changes what trees do with that carbon.

This insight should reshape how we discuss forests in climate policy. The question is not simply, “How much CO₂ do trees take up?” The more important question is, “How much of that carbon remains stored, for how long, and in what form?”

Forests remain among the most important natural allies against climate change. But their role is complex, dynamic, and vulnerable. A warming and drying atmosphere may allow trees to keep photosynthesizing while quietly reducing their investment in wood. That hidden shift could influence the future carbon balance of the planet.

The new study on temperate oaks therefore offers a timely warning: green leaves do not always mean growing forests, and carbon uptake does not always mean carbon storage.

Suggested SEO excerpt: A recent Science Advances study shows that trees can continue photosynthesizing after wood growth has slowed or stopped, challenging the assumption that carbon uptake always becomes long-term forest carbon storage.