Stem photosynthesis is coordinated with seasonal growth activity in two temperate tree species

This study demonstrates that in two temperate tree species, stem photosynthesis is strongly coordinated with seasonal growth activity and water status rather than chlorophyll content, peaking in May to partially refix respiratory CO2 before declining as the growing season progresses.

The Gist

Imagine a tree not just as a giant leafy umbrella, but as a bustling city. Usually, we think of the leaves as the "factories" that make food (sugar) using sunlight, while the trunk and branches are just the "roads" and "storage warehouses" that transport and hold that food.

But this new research reveals a secret: The trunk and branches have their own tiny factories too.

Here is the story of what the scientists discovered, explained simply:

1. The "Skin" Factories

Most people think only leaves can photosynthesize (turn sunlight into energy). But in this study, scientists looked at the bark of two common trees: the Norway Maple and the Wild Cherry. They found that the bark is actually green and full of tiny chlorophyll factories, just like leaves.

Think of the tree's bark as a solar-powered skin. Even though it's covered by a tough outer layer (like a coat), the skin underneath is still catching sunlight and making energy.

2. The Busy Spring Rush

The researchers tracked these trees all year long. They found that the bark's "factories" work hardest in May and June.

• The Analogy: Imagine a construction site. In the spring, the tree is building new wood and growing leaves. It needs a massive amount of energy. The bark factories kick into high gear to help pay for this construction.
• The Finding: The trees were breathing out carbon dioxide (respiration) because they were working so hard. But the bark was also sucking some of that CO2 back in to make new sugar. It was a busy recycling loop!

3. The "Net Loss" Reality

Here is the twist: Even though the bark was making energy, it wasn't making enough to cover all the costs of the tree's daily life.

• The Analogy: Think of the tree's trunk as a small business. The bark factory is a side hustle that earns some money. But the cost of running the business (keeping cells alive, growing wood) is so high that the side hustle doesn't quite cover the bills.
• The Result: The tree still loses a little bit of carbon overall through its trunk. However, the bark's side hustle saves the tree from losing as much as it would otherwise. In the Wild Cherry trees, the bark managed to recycle about 39% of the carbon the tree breathed out! That's a huge savings.

4. Water is the Fuel Gauge

The scientists also checked how "thirsty" the trees were. They found a surprising link: The happier and more hydrated the tree was, the better its bark factories worked.

• The Analogy: You might think that when a plant is dry, it tries harder to survive. But for these trees, it's the opposite. When the tree is well-watered, the bark factories run smoothly. When the tree is dry, the factories slow down. It's like a car engine that runs best with high-quality fuel; if the fuel (water) is low, the engine sputters.

5. The "Green" Secret

You might think that if the bark has more green pigment (chlorophyll), it makes more energy. But the study found something interesting: It's not about how much green paint you have; it's about how the tree is growing.

• The Analogy: Imagine two cars. One has a shiny new engine (lots of green pigment), but it's sitting in a garage. The other has an older engine but is being driven hard on the highway. The one being driven hard (the growing tree) uses more energy.
• The Finding: The amount of green pigment in the bark didn't change much throughout the year. Instead, the bark's energy production was tied to how fast the tree was growing and how much water it had. The bark seems to know exactly when to work hard based on the tree's needs, not just based on how much "green paint" it has.

The Big Picture

This study teaches us that trees are more complex than we thought. The trunk isn't just a passive pole; it's an active participant in the tree's energy economy.

• Why it matters: As climate change alters how much water trees get and how fast they grow, understanding that tree trunks have their own "solar panels" helps us predict how much carbon trees can store. It turns out, the whole tree—from the leaves to the roots—is working together to keep the forest alive.

In short: Trees have solar-powered skin that works overtime in the spring to help build new wood, but they still need their leaves to do the heavy lifting. And just like us, they work best when they are well-hydrated!

Technical Summary

1. Problem Statement

Woody stems are not merely structural supports; they possess photosynthetic capabilities that allow for the refixation of CO2 released by stem respiration. This process enhances carbon-use efficiency and provides local non-structural carbohydrates (NSCs) crucial for cambial growth and xylem hydraulic function (e.g., embolism reversal). While the importance of stem photosynthesis is recognized, there is a significant gap in understanding its seasonal dynamics in temperate tree species. Specifically, it remains unclear how stem photosynthesis and respiration fluctuate throughout the growing season, how these processes coordinate with cambial activity and water status, and whether chlorophyll content is the primary driver of seasonal variability in stem gas exchange.

2. Methodology

The study employed a high-resolution seasonal monitoring approach on two temperate tree species: Norway maple (Acer platanoides L.) and Sweet cherry (Prunus avium L.).

• Site & Subjects: Experiments were conducted at the Kamenný vrch site in Brno, Czech Republic. Eight mature, sun-exposed individuals of each species were selected.
• Sampling Design: Branches were sampled seven times between March and December 2024, covering key phenological phases from bud swelling to dormancy.
• Measurements:
  • Gas Exchange: Net photosynthesis ($P_n$) and dark respiration ($R_d$) were measured on 10-cm stem segments using an IRGA system under controlled conditions (2000 $\mu$mol m$^{-2}$ s$^{-1}$ irradiance, 25°C). Gross photosynthesis ($P_g$) was calculated as $P_n + R_d$. Relative CO$_2$ refixation was also calculated.
  • Anatomy: Stem cross-sections were analyzed to determine the thickness of the cambial zone, xylem, phloem, cortex, and phellem (periderm) to assess secondary growth.
  • Pigments: Chlorophyll a, chlorophyll b, and carotenoids were extracted from bark and xylem tissues to determine total pigment content and the Chl a/b ratio.
  • Water Status: Branch relative water content (RWC) and water potential ($\Psi_B$) were measured.
  • Environmental Data: Air temperature, vapor pressure deficit (VPD), and soil water potential were monitored continuously.
• Statistical Analysis: Linear mixed-effect models were used to evaluate seasonal trends and relationships between gas exchange, anatomical traits, and physiological parameters, accounting for repeated measures on individual trees.

3. Key Contributions

• Seasonal Coordination: The study provides evidence that stem photosynthesis and respiration are tightly coordinated throughout the season, peaking simultaneously during periods of active growth.
• Decoupling of Pigments and Function: It challenges the assumption that total chlorophyll content is the primary driver of seasonal photosynthetic rates in stems, showing instead that water status and growth activity are more critical.
• Hydraulic Link: It investigates the link between stem photosynthesis and water status, testing the hypothesis that photosynthesis is upregulated during drought to aid hydraulic recovery, finding instead a positive correlation with water availability.
• Species Comparison: It offers a comparative analysis between two distinct temperate species, highlighting differences in magnitude and seasonal patterns of stem gas exchange.

4. Key Results

• Seasonal Patterns of Gas Exchange:
  • Both $P_g$ and $R_d$ followed a similar seasonal trajectory, reaching maximum values in May and June (coinciding with rapid leaf expansion and peak secondary growth) and declining toward the end of the season.
  • Net Photosynthesis ($P_n$) was negative throughout the year for both species, indicating that stem respiration consistently exceeded photosynthesis.
  • CO$_2$ Refixation: Despite net efflux, stems refixed between 3% and 59% of respired CO$_2$. Refixation efficiency was highest in early spring (before leaf flush) and decreased as the season progressed. P. avium showed significantly higher refixation rates (up to 59%) compared to A. platanoides (up to 36%).
• Drivers of Photosynthesis:
  • Water Status: $P_g$ was positively correlated with stem water potential ($\Psi_B$). Contrary to the hypothesis that photosynthesis increases under water stress to aid embolism repair, higher photosynthetic rates were observed when stems were better hydrated.
  • Chlorophyll Content: Total chlorophyll content showed weak seasonal variation and did not significantly correlate with $P_g$. However, the Chl a/b ratio decreased significantly during the peak growing season (May–July), suggesting an acclimation to lower light penetration through thickening bark (phellem).
  • Growth Correlation: $P_g$ and $R_d$ were strongly correlated with the thickness of living tissues (cambial zone, phloem, cortex), linking metabolic activity directly to growth demands.
• Anatomical Changes: The cambial zone was thickest in May/June. Phellem thickness increased sharply in P. avium from May to June, correlating with the shift in the Chl a/b ratio.

5. Significance

• Carbon Budgeting: The findings suggest that models of tree carbon budgets must account for the strong seasonality of stem gas exchange. Ignoring the peak activity in spring/early summer could lead to underestimations of stem carbon cycling.
• Physiological Mechanism: The tight coordination between respiration and photosynthesis suggests a regulatory loop where high respiration (providing CO$_2$) stimulates photosynthesis, which in turn provides carbohydrates for respiration and growth. This interplay helps maintain internal CO$_2$/O$_2$ balance and prevents hypoxia in metabolically active tissues like the cambium.
• Hydraulic Function: The positive relationship between water status and photosynthesis implies that stem photosynthesis is most active when water is available, potentially supporting growth and maintenance rather than acting primarily as a drought-recovery mechanism in these temperate species.
• Ecological Adaptation: The high refixation rates in early spring highlight the ecological importance of stem photosynthesis in temperate trees, particularly for species like P. avium that flower before leaf flush, providing a local carbon source when foliar photosynthesis is not yet fully operational.

In conclusion, the study demonstrates that stem photosynthesis in temperate trees is a dynamic, seasonally regulated process driven by growth activity and water status rather than static pigment content, playing a vital role in the seasonal carbon economy and hydraulic maintenance of the tree.