Research report: Lessons from a tropical deciduous shrub species: leaf fall can play a more important role than rain in leaf budding

Manual removal of old leaves increased leaf-bud production in the tropical deciduous shrub Peixotoa tomentosa during a rainless dry season, according to a field experiment in Brazil. Shrubs whose remaining leaves were removed at the end of June produced more buds on average than untouched plants and plants receiving repeated irrigation. The study involved one shrub species at one site, so its result does not establish that leaf loss generally replaces rainfall as a trigger for budding across the Cerrado or other tropical ecosystems.

The experiment took place in a roughly 10-hectare area of Cerrado Típico vegetation in Congonhas, Minas Gerais, in southeastern Brazil. The site lies at about 950 metres above sea level and has deep red latosol soil. Its climate is classified as subtropical at altitude, with dry winters from May through September and mild summers from October through April.

P. tomentosa, a member of the Malpighiaceae family, is a shrub that can reach about 3 metres in height. It is classified as brevideciduous and can lose all its leaves during the dry season for a period of up to 21 days. Before treatment, the researchers marked 45 mature shrubs averaging about 1.5 metres in height and separated them by at least 10 metres.

The plants were divided equally among three groups: 15 controls, 15 irrigation recipients and 15 plants assigned to early leaf removal. At the end of June, when the shrubs were beginning to shed their leaves, researchers manually removed all remaining foliage from the removal group. This intervention was distinct from natural leaf fall; natural leaf loss was not tested as an independent treatment, so the experiment cannot establish how manual defoliation compares with the full sequence of natural senescence and abscission.

Plants in the irrigation group were watered with approximately 2.5 litres per plant at five-day intervals, beginning on July 4 and ending on August 23. Watering started after the plants had lost their leaves and before the first recorded leaf buds appeared. The modest volume was intended to avoid saturating the soil, restricting root oxygenation and causing root rot.

Leaf budding was then monitored twice weekly from July 4 through August 25, with all buds counted on each of 12 observation dates. Daily precipitation data from an automatic station approximately 8 kilometres away recorded no rainfall between July 1 and August 25. Leaf buds were first observed on July 18 in all three groups.

Bud numbers subsequently declined through August as the buds developed into young leaves, and by August 25 no buds remained in any group. The shrubs had then shifted rapidly and synchronously from young foliage to mature, hairy, green leaves. On July 21, the removal group reached its highest observed mean, 9.67 buds per plant with a standard error of 0.80, compared with 6.07 ± 0.33 in controls and 6.47 ± 0.26 in irrigated plants.

Across the monitored period, the removal group averaged 4.98 ± 0.29 leaf buds, compared with 3.62 ± 0.16 in the control group and 3.85 ± 0.11 in the irrigation group. Generalized linear mixed models accounted for repeated observations by treating treatment as a fixed factor and individual plants as random factors. The overall difference among groups was statistically significant, with a model statistic of 14.995 and a probability value of 0.0001.

Tukey post hoc comparisons showed that the removal treatment differed from both the control group and the irrigation treatment, each with a probability value of 0.0001. The control and irrigation groups did not differ significantly from one another, with a probability value of 0.671. In the measured population, adding water did not measurably increase bud production, whereas manually removing the remaining old leaves did.

Soil sampling showed that irrigation did change the immediate moisture conditions around the treated plants. On August 12, four days after that week's watering, the researchers collected 15 soil samples near roots of irrigated plants and 15 near roots of control plants, each extending about 20 centimetres into the soil. Relative water content was calculated from wet and dry weights.

Samples associated with irrigated plants averaged 4.16% ± 0.40 relative water content, compared with 3.16% ± 0.23 for control samples, a statistically significant difference with a probability value of 0.01. The absence of a budding response therefore did not mean that the watering treatment failed to alter soil moisture; the extra moisture was not associated with significantly more buds. In a plant with no leaves, the surface available for water loss shrinks, which can help leafless branches rehydrate and may reduce the risks of xylem embolism and desiccation.

The study’s background identifies leaf fall as a water-conservation response and notes that plants can adjust water demand and supply through changes in leaf area, transpiration, rooting depth and internal water storage. The authors also suggest that P. tomentosa’s extensive root system may reach deeper moisture and buffer the shrub against seasonal water shortage. Neither water movement nor root access was measured directly in this experiment, however, so these are proposed mechanisms rather than individually demonstrated causes of the treatment effect.

Nutrient relocation was another proposed factor. The researchers suggested that removing all remaining leaves at the same time sharply reduced transpiration without preventing nutrients from being transferred to developing young leaves. The experiment measured bud counts and soil moisture, not nutrient movement, carbon exchange or plant water status, so it could not distinguish the transpiration pathway from nutrient effects or determine how much each contributed to the response.

What can be stated directly is that manual leaf removal preceded greater bud production under the specific dry-season conditions of the study. The result does not show that artificial defoliation would be beneficial in field management, nor does it test whether the number, timing or persistence of naturally shed leaves has the same effect. The findings offer a species-specific clue to how seasonal plant development may respond to environmental change.

In the Cerrado, phenological events follow a sequence in which many deciduous plants shed their foliage and then produce new leaves during the dry season. Changes in rainfall and temperature can alter that sequence, and the timing of leaf senescence and budding can affect carbon uptake and nutrient cycling. Warming and drought may influence senescence in opposite directions, with effects varying among species and regions.

Against that wider context, the experiment identifies leaf removal rather than added water as the factor associated with increased budding in P. tomentosa. Its central lesson is narrower than a general claim about rain and leaf fall: during a completely rainless seven-week monitoring period in this Brazilian population, early manual defoliation increased leaf-bud production, while irrigation alone did not.

Subject of Research: Field experiment on manual defoliation, irrigation and dry-season leaf budding in Peixotoa tomentosa

Article Title: Lessons from a tropical deciduous shrub species: leaf fall can play a more important role than rain in leaf budding

Article References: Henriques, N. R., & Pereira, C. C. (2022). Lessons from a tropical deciduous shrub species: leaf fall can play a more important role than rain in leaf budding. Neotropical Biology and Conservation, 17(4), 239-251. https://doi.org/10.3897/neotropical.17.e93846

Image Credits: AI Generated

DOI: 10.3897/neotropical.17.e93846

Keywords: Peixotoa tomentosa, Cerrado, leaf budding, manual defoliation, irrigation, plant phenology, seasonal drought, transpiration, tropical savanna, Brazil, Lessons, tropical