Tropical Plants Are Flowering Weeks Earlier and Later Than Two Centuries Ago

The assumption has been persistent: tropical plants, living in climates where temperatures vary little across seasons, should be buffered against the climate-driven shifts in flowering timing documented in temperate and boreal species worldwide. A study using 230 years of herbarium specimens puts that assumption under serious pressure.

Skylar Graves and Erin Manzitto-Tripp at the University of Colorado-Boulder compiled collection dates for more than 8,000 flowers representing 33 tropical plant species gathered between 1794 and 2024. Comparing when those specimens were collected across time revealed that the annual flowering windows of these species have shifted, on average, by two days per decade - a rate comparable to what has been reported for temperate and boreal plants responding to rising temperatures.

The Evidence Embedded in Museum Collections

Herbarium specimens - pressed, dried plant samples mounted on archival paper - carry a collection date and location alongside the plant itself. For species with distinct, predictable annual flowering periods, those dates function as phenological records stretching back centuries, well before any systematic ecological monitoring began.

The 8,000+ specimens represent 33 tropical species with clear annual flowering windows. By plotting collection dates against the decade in which specimens were gathered, the researchers could track how flowering timing has moved over time. The average two-day-per-decade shift compounds to weeks or months across the span of the dataset - and this study captures more than two centuries.

The most extreme examples illustrate how large those shifts can become. Ghanian rattlepod shrubs flowered 17 days earlier in the 1990s than they did in the 1950s - a nearly three-week change across four decades. Brazilian amaranth trees now flower 80 days later than they did in the 1950s, representing a fundamental reorganization of when that plant is reproductively active.

Why This Challenges the Tropical Assumption

Tropical flowering timing has generally been thought to respond primarily to rainfall seasonality, photoperiod, and other non-temperature cues, since absolute temperature variation across months is far smaller at low latitudes. This study cannot determine from herbarium data alone whether changing temperatures, altered rainfall patterns, or some combination is driving the changes.

What it establishes clearly is that shifts of comparable magnitude to those seen in temperate species are occurring in tropical species. The causal mechanism may differ, but the phenological outcome is similar. That finding matters because the relationships between plants, pollinators, and seed-dispersing animals in tropical ecosystems have evolved around particular flowering schedules.

"We have found tropical plants are not insulated from the impacts of climate change," said Graves.

Ecological Consequences: When Timing Breaks Down

Flowering time determines when pollinators have access to nectar and pollen, when frugivores can access ripe fruit, and when herbivores encounter vulnerable young leaves. If a tree shifts its flowering by 80 days but its primary pollinator's activity peaks around the original schedule, pollination fails. The ecological downstream effects of phenological mismatch are difficult to model from herbarium data alone - but the potential for disruption in already-stressed tropical ecosystems is real.

The study's limitation is inherent to its design: specimens are collected opportunistically, not systematically. Collection pressure in a given year can introduce biases into apparent flowering peaks. The researchers controlled for this where possible. No specific funding was received for this work.

"Herbarium specimens make up a massive source of data, far greater in both geographic and temporal scale than any one researcher can hope to achieve in their lifetime," Graves noted. "I hope studies like mine can be persuasive for increased funding of herbaria and their digitization worldwide."