Responses of Tropical Tree Seedlings to Nutrient Addition: A Meta-Analysis to Understand Future Changes in Tropical Forest Dynamics

Academic Background

Tropical forests are critical components of global biodiversity and ecosystem functions. However, due to the impacts of human activities and climate change, tropical forests are facing unprecedented pressures. In particular, increased nutrient leaching and deposition may significantly alter the productivity, structure, and function of tropical vegetation. To better understand how nutrient deposition affects the regeneration capacity of tropical ecosystems, researchers conducted a systematic review and meta-analysis of experiments involving nitrogen (N), phosphorus (P), nitrogen-phosphorus (NP), or nitrogen-phosphorus-potassium (NPK) additions to seedlings of tropical tree and shrub species.

Nutrient limitation in tropical regions often leads to multiple resource constraints. Previous studies have suggested that tropical forest productivity may be co-limited by nitrogen and phosphorus, but the applicability of this conclusion across different regions and ecosystems remains unclear. Additionally, the early growth stages of seedlings are particularly sensitive to changes in resource availability. Nutrient addition may affect seedling recruitment, growth, and survival rates, thereby influencing species community composition and spatial distribution. Therefore, understanding the effects of nutrient addition on tropical seedlings is crucial for predicting future changes in tropical forest dynamics.

Source of the Paper

This paper was co-authored by Daisy Cárate Tandalla, Jürgen Homeier, and Péter Batáry, affiliated with RPTU Kaiserslautern-Landau in Germany, the University of Göttingen, and the Institute of Ecology and Botany in Hungary, respectively. The study was accepted on November 15, 2024, and published in the journal Current Forestry Reports, with the DOI 10.1007/s40725-024-00240-6.

Research Process

1. Literature Search and Data Extraction

The researchers collected experimental data on the responses of tropical and subtropical tree and shrub seedlings to nutrient addition through systematic review and meta-analysis. The literature search was conducted primarily in the Web of Science and Scopus databases using the PICO (Population, Intervention, Comparator, Outcome) search strategy. A total of 59 relevant publications were selected, covering 264 observations of growth rates, 149 observations of shoot biomass, and 179 observations of root biomass from 160 species.

2. Data Standardization and Classification

To ensure data consistency, the researchers standardized the classification of species’ growth strategies and climatic conditions. Species’ growth strategies were categorized based on wood specific gravity (WSG), while climate data, including mean annual temperature (MAT), mean annual precipitation (MAP), and precipitation seasonality, were extracted from the CHELSA climate model.

3. Effect Size Calculation and Statistical Analysis

The researchers used the response ratio as the effect size indicator to calculate the differences between experimental and control groups. To account for heterogeneity among studies, hierarchical mixed-effects meta-analysis models were employed, incorporating multiple moderators such as experiment type, nutrient type, climate conditions, and light levels.

Main Findings

1. Effects of Nutrient Addition on Seedling Growth Rates

Overall, nutrient addition significantly increased the growth rates of tropical seedlings by an average of 14%. Among the nutrient treatments, NPK addition showed the most significant effect, increasing growth rates by 27%, while P addition alone had a weaker effect, increasing growth rates by only 12%. Additionally, seedling growth rates increased by 20% in transplantation experiments, whereas in-situ addition experiments showed no significant effect.

2. Effects of Nutrient Addition on Shoot Biomass

Nutrient addition significantly increased seedling shoot biomass by an average of 26%. The most significant effects were observed with N and NP additions, increasing shoot biomass by 38% and 48%, respectively, while P addition alone had a weaker effect, increasing shoot biomass by only 16%. Furthermore, shoot biomass increased by 30% in pot experiments.

3. Effects of Nutrient Addition on Root Biomass

In contrast to shoot biomass, nutrient addition had a smaller effect on root biomass. The only significant effect was observed with P addition alone, which increased root biomass by 17%.

4. Moderating Effects of Climate and Light

Climate variables, such as mean annual temperature and precipitation, did not significantly affect seedling responses. However, precipitation seasonality showed some moderating effects. Seedlings from seasonally dry sites exhibited the most significant increases in growth rates and shoot biomass, with increases of 38% and 70%, respectively. Additionally, seedling growth rates and shoot biomass significantly increased under high light conditions.

Conclusions and Significance

This study, through meta-analysis, revealed the response patterns of tropical seedlings to nutrient addition, indicating that co-limitation by nitrogen and phosphorus is prevalent in tropical ecosystems. The study also found that different experimental methods and climatic conditions significantly affect seedling responses, particularly in seasonally dry regions and under high light conditions, where the effects of nutrient addition were more pronounced.

The scientific value of this research lies in providing new evidence for understanding nutrient limitation mechanisms in tropical forests and offering important insights for predicting the impacts of future climate change and human activities on tropical forests. Additionally, the study emphasizes the importance of combining in-situ observations and experimental methods in future research to gain a more comprehensive understanding of species’ true responses.

Research Highlights

1. Discovery of Multiple Nutrient Co-Limitation: The study confirmed that tropical seedling growth is co-limited by nitrogen and phosphorus, challenging the previous notion that tropical forests are primarily limited by phosphorus.
2. Moderating Effects of Climate and Light: The study systematically analyzed the effects of climate and light on seedling responses to nutrients for the first time, revealing significant effects in seasonally dry and high-light conditions.
3. Comparison of Experimental Methods: The study compared the effects of pot, transplantation, and in-situ addition experiments, providing important references for future experimental designs.

Other Valuable Information

The study also pointed out that future research should focus on the effects of forest succession status, soil type, and topography on nutrient addition outcomes to gain a more comprehensive understanding of tropical forest nutrient dynamics. Furthermore, the study recommended tracking additional parameters, such as forest succession status, to further elucidate the impacts of nutrient deposition on tropical soils and vegetation.