Phenology of 50 tree species across 9 years in a South Asian tropical rainforest indicates complex influence of climate, traits, and phylogeny

A nine-year study of 50 tree species in a South Asian tropical rainforest reveals that while leaf flushing and flowering are primarily driven by proximate climatic cues like daylength and precipitation, fruiting phenology is more strongly constrained by phylogenetic history and functional traits, suggesting lineage-specific vulnerabilities to climate change.

The Gist

Imagine a massive, ancient library where the books are trees. In most libraries, everyone opens their pages at the same time every year (like in temperate forests where spring triggers everything). But in this specific library—the tropical rainforests of India's Western Ghats—the rules are much more chaotic. It rains almost all year, the sun is often hidden behind clouds, and there isn't a single "start button" for the trees to follow.

This paper is like a 9-year detective story where researchers watched 50 different species of trees (representing hundreds of individual trees) to figure out: What makes these trees decide when to grow new leaves, bloom flowers, or drop fruit?

Here is the story of their findings, broken down into simple concepts:

1. The Three Acts of the Tree's Life

The researchers tracked three main "acts" in the tree's life cycle:

• The Haircut (Leaf Flushing): When trees grow new leaves.
• The Party (Flowering): When trees bloom.
• The Gift (Fruiting): When trees produce fruit.

2. The Weather vs. The Family Tree

The study asked a big question: Do trees follow the weather (like a clock set by the sun and rain), or do they follow their family history (like a recipe passed down from their ancestors)?

The Haircut and The Party: The Weather Wins

For growing new leaves and blooming flowers, the trees are like sun-worshippers who hate the rain.

• The Analogy: Imagine you are trying to paint a fence. You can't do it when it's pouring rain or when the sky is grey and cloudy. You wait for a sunny, dry day.
• The Finding: These trees mostly wait for a brief window when the clouds break and the sun shines bright, and the rain stops. They don't care much about their family tree; a pine tree and a mango tree might both decide to "party" at the exact same time because the weather finally looked good.
• The Result: These events happen in short, synchronized bursts. When the sun comes out, everyone does it at once.

The Gift: The Family Tree Wins

For fruiting, the story changes completely.

• The Analogy: Imagine a family of bakers. Even if the weather is perfect, a grandmother's family might always bake bread on Tuesdays, while a cousin's family always bakes on Fridays. They do it because that's "how we've always done it," not because the oven is hotter on those days.
• The Finding: Fruiting is less about the immediate weather and more about evolutionary habits. Closely related trees (like cousins in the same family) tend to drop their fruit at the same time of year, even if the weather is a bit different.
• The Result: Fruiting is spread out over a longer time. It's not a synchronized party; it's a slow, steady stream of gifts throughout the year.

3. Why Does This Matter? (The Climate Change Warning)

The researchers are worried about what happens when the weather becomes unpredictable due to climate change.

• The Flexible Ones: The trees that grow leaves and flowers based on the weather are like chameleons. If the weather gets weird, they might change their schedule. They might flush leaves or bloom at strange times. This is risky, but they have some flexibility.
• The Stubborn Ones: The trees that fruit based on their family history are like pigeons with a fixed route. They are programmed to drop fruit in April or May, no matter what.
  • The Danger: If climate change makes April too dry or too hot, these trees might still drop their fruit then. But if the fruit drops when the rain isn't there, the seeds won't grow. Worse, the animals (birds and monkeys) that eat the fruit might have already moved on or changed their own schedules.
  • The Mismatch: This creates a "broken date." The tree offers fruit, but the animal isn't there to eat it and spread the seeds. The tree's lineage could eventually die out because it can't change its ancient schedule fast enough.

The Bottom Line

In these tropical rainforests:

1. Leaves and Flowers are like solar-powered devices: They turn on when the sun is out and the rain stops. They are flexible and react quickly to the weather.
2. Fruit is like a heirloom watch: It runs on a schedule set by ancestors. It is rigid and doesn't change easily.

The Takeaway: As the climate gets more chaotic, the "solar-powered" trees might survive by adapting, but the "heirloom watch" trees (especially certain families like Lauraceae and Euphorbiaceae) are in trouble. They might keep dropping fruit at the wrong time, leading to a silent crisis where forests stop regenerating because the timing of life has fallen out of sync.

Technical Summary

1. Problem Statement and Context

Tropical rainforests, particularly those in relatively aseasonal regions near the equator, exhibit high phenological complexity and low temporal synchrony compared to temperate ecosystems. While abiotic climatic cues (irradiance, precipitation, temperature) are known to drive phenology, the relative contributions of proximate climatic factors versus evolutionary constraints (functional traits and phylogenetic relatedness) remain unclear.

Specifically, there is a gap in understanding:

• How short-duration seasonal pulses of irradiance and water deficit influence leafing, flowering, and fruiting in aseasonal forests.
• Whether interspecific phenological differences are driven by functional trait variation or phylogenetic conservatism.
• How these mechanisms might interact under shifting climate regimes, potentially leading to lineage-specific vulnerabilities.

2. Methodology

Study Site and Design:

• Location: Anamalai Tiger Reserve and Valparai Plateau, Western Ghats, India (mid-elevation tropical wet evergreen rainforest, 700–1405 m asl).
• Duration: 9 years (March 2017 – February 2026).
• Subjects: 50 tree species (representing ~70% of individuals in surveyed transects), totaling 920–1,077 individuals (10–42 trees/species).
• Sampling: Monthly monitoring of seven phenology trails (2.9 km total). Phenophases (leaf flush, flower open, ripe fruit) were scored on a 0–4 intensity scale based on canopy coverage.

Data Collection:

• Climate Data: Recorded via a local HOBO weather station (2014–2025) including rainfall, temperature, irradiance, relative humidity, and photoperiod.
• Traits: Functional traits (wood density, seed size, maximum height) were sourced from the Zenodo database.
• Phylogeny: A resolved species phylogeny was generated using the V.PhyloMaker2 R package based on the GBOTB megatree.

Analytical Approach:

• Signal Processing: Phenological time-series were analyzed using R (pracma package) to identify peaks, calculating frequency (peaks/year), amplitude (peak intensity), duration, and peak month.
• Climate Correlations: Monthly time-series cross-correlation and cross-covariance analyses were performed at a 1-month lag to link climatic variables to phenophase expression.
• Mantel Tests: Used to assess correlations between dissimilarity matrices of phenology, functional traits, and phylogenetic distance.
• Phylogenetic Signal: Pagel's $\lambda$ was estimated using the phytools R package to quantify the degree of phylogenetic dependence for phenological traits (frequency, amplitude, duration, peak month).

3. Key Results

Phenological Patterns:

• Leaf Flushing & Flowering: Exhibited distinct seasonality with high synchrony. Leaf flushing peaked in Feb–May; flowering in March–June. Both showed high amplitude (0.87 and 0.5, respectively) and shorter durations.
• Fruiting: Showed greater temporal spread, lower synchrony, and a broader peak window (May–September). Fruiting duration was highly variable (mean 4.8 months).

Climate Drivers:

• Leaf Flushing & Flowering: Strongly negatively associated with precipitation and daylength in the preceding month. Leaf flushing also showed positive associations with irradiance. This suggests these phases are triggered by dry, high-light periods (reduced cloud cover).
• Fruiting: Showed weak or non-significant correlations with climatic variables for the majority of species, indicating a decoupling from immediate abiotic cues.

Phylogeny and Traits:

• Leafing/Flowering: No significant phylogenetic signal or correlation with functional trait dissimilarity. These traits appear evolutionarily labile and driven by environmental convergence.
• Fruiting:
  • Mantel Test: A weak but significant positive correlation between phenological dissimilarity and phylogenetic distance ($r = 0.13, p = 0.01$).
  • Pagel's $\lambda$: Peak fruiting month showed a very strong phylogenetic signal ($\lambda = 0.96, p = 0.03$). Fruiting duration showed a moderate signal ($\lambda = 0.75, p = 0.04$).
  • Family Patterns: Specific families (e.g., Lauraceae, Sapotaceae) consistently peaked in pre-monsoon months, while others (e.g., Meliaceae) peaked in wet seasons, reflecting lineage-specific strategies.

4. Key Contributions

1. Decoupling of Phenophases: The study provides empirical evidence that in aseasonal tropical forests, vegetative (leafing) and reproductive (flowering) phenologies are primarily driven by proximate climatic cues (irradiance and precipitation deficits), whereas fruiting phenology is constrained by phylogenetic history.
2. Quantification of Phylogenetic Signal: It demonstrates that while leafing and flowering are evolutionarily plastic, the timing of fruit ripening is highly conserved ($\lambda \approx 1$), suggesting that reproductive timing is an intrinsic, lineage-specific trait rather than a direct response to current climate.
3. Methodological Rigor: The application of signal processing techniques to 9-year monthly datasets allows for the precise characterization of phenological frequency and amplitude in complex, aseasonal systems where traditional seasonal averages fail.

5. Significance and Implications

• Climate Change Vulnerability:
  • Leafing/Flowering: Likely to become more variable and potentially desynchronized as climate becomes more erratic (stochastic dry periods), but species may retain plasticity to track these shifts.
  • Fruiting: The strong phylogenetic signal implies low evolutionary flexibility. Lineages with conserved fruiting times (e.g., pre-monsoon fruiters like Lauraceae) may face temporal mismatches if climate change alters precipitation patterns or frugivore availability. They cannot easily shift their fruiting windows to match new climatic optima.
• Ecosystem Resilience: The study suggests that the future stability of tropical forest regeneration depends less on climate shifts alone and more on the resilience of specific phylogenetic lineages to maintain coupling with dispersal agents.
• Conservation Priority: Families with phylogenetically conserved fruiting strategies that are projected to face moisture deficits (e.g., pre-monsoon fruiters in the Western Ghats) should be prioritized for conservation to prevent regeneration failure.

In conclusion, the paper argues that while climate dictates the when of leafing and flowering in aseasonal forests, evolutionary history dictates the when of fruiting, creating a potential bottleneck for forest regeneration under rapid climate change.