Severe impact of past changes in climate and nutrient availability on a lowland tropical rainforest
A new study headed by Assistant Professor Ana Prohaska from the Centre of GeoGenetics uncovers long-term ecological responses of a Southeast Asian lowland forest to marked changes in climate changes and nutrient availability by analysing ancient sediments from a Philippine lake. The findings are published in New Phytologist.
Lowland tropical forests cover nearly 10% of the planet’s land area and play a pivotal role in regulating the Earth's climate. Understanding the long-term impact of future climate change on lowland tropical rainforests is therefore critical, but, at the same time, challenging because these forests are dominated by large, long-lived trees whose responses only become apparent on the scale of decades to centuries. Using fossil plant records, it is possible to “wind the clock back” and study these forests as they were hundreds to thousands of years ago, providing a valuable perspective on this problem.
A study conducted by an international team of researchers reveals that past climate change, along with varying nutrient availability, had significant impacts on the dominant forest type of Southeast Asia – lowland dipterocarp forest. The researchers analysed a series of proxies from sediment cores retrieved from Bulusan lake in the north-eastern Philippine spanning the past 1,400 years. They found evidence of a negative effect of reduced rainfall and limited phosphors availability on several tree taxa, including the ecologically dominant dipterocarp trees. These findings underscore the importance of understanding long-term ecological effects of climate changes and nutrient availability for predicting future dynamics of lowland tropical rainforests in a warming world.
Lowland tropical rainforest of Southeast Asia
Lowland tropical rainforests in Southeast Asia are among the world’s largest tracts of tropical rainforests. While these forests once occupied much of peninsular Malaysia, Sumatra, Java, Borneo, and the Philippines, they are presently reduced to only 30% of their original range due to unprecedented rates of deforestation. These forests are ecologically unique in that they are overwhelmingly dominated by a single tree family, Dipterocarpaceae, which accounts for 10% of the total tree diversity, encompasses over 50% of the trunk cross-sectional area, and comprises nearly 80% of all the tallest trees in the forest. Hence, the lowland rainforests of Southeast Asia are commonly termed as ‘lowland dipterocarp forests.’ Another unique feature of lowland dipterocarp forests is that they do not flower every year. Instead, they engage in synchronized large-scale flowering every 2-7 years. These ‘mass flowering’ events are believed to be triggered by drought conditions caused by El Niño.
Dual impact of El Niño events on dipterocarp forests
El Niño is a climate phenomenon that originates in the tropical Pacific and causes changes to weather patterns around the globe. El Niño events bring markedly drier conditions to large parts of Southeast Asia. With ongoing global warming, El Niño events are projected to become more severe and frequent. Given the evidence from previous studies suggesting that El Niño-induced droughts trigger mass flowering in lowland dipterocarp forests, more severe and frequent El Niño events may potentially boost forest reproduction. However, such conditions may also increase the mortality of trees in these forests, with large trees such as dipterocarps being especially susceptible. These two seemingly opposing effects of El Niño on lowland dipterocarp forests, coupled with the long life span of the large trees, makes it challenging to predict the long-term impact of enhanced drought conditions based on observational studies alone.
Sediment cores provide a window into past ecological responses
To examine the long-term impact of enhanced drought conditions on lowland dipterocarp forests, the team of researchers carried out an analysis of sediment cores retrieved in 2013 from Lake Bulusan, located in the north-eastern Philippines. The collected sediments offer a window into the vegetation responses to climate changes over the past 1,400 years, including a prolonged period of El Niño-like conditions during the Little Ice Age, as identified in another recent study. To reconstruct past climatic and ecological dynamics at Bulusan Lake, the team used a series of paleoproxies including fossil pollen, charcoal, stable isotopes, and chemical element concentrations.
Drier conditions and limited phosphorus availability decrease dipterocarp abundance
The researchers found that as conditions at the Bulusan Lake became drier in the past, there was a notable decrease in dipterocarp tree abundance, signaling the sensitivity of these trees to changes in water availability. This was also the case for several other tree taxa. These results suggest that any positive effect of El-Nino induced drought conditions on the reproduction of dipterocarp trees are overtaken by the negative effects on their growth and survival.
The team also found a positive relationship between phosphorus availability and the abundance of several tree taxa on the landscape, including dipterocarp trees. Phosphorus has been long considered to be the main limiting nutrient in lowland rainforests based on evidence from short observational studies. The results from the new study further support this hypothesis by providing the first evidence of the impact of phosphorus limitation on the productivity of lowland dipterocarp forests on longer time scales.
Implications for conservation and management
With predicted rise in the frequency and severity of El Niño events over the course of this century, the study supports a mounting concern that these forests might undergo transformative changes under enhanced drought regime through the reduction in the number of dipterocarp trees. As giant, old-growth trees with a very complex canopy, dipterocarp trees embody the concept of ‘tropical mega-trees’. Such trees play several distinctive roles in forest ecosystems, such as offering unique habitats, food sources and nesting sites, while also underpinning much of forest productivity and carbon storage. Additionally, dipterocarp trees profoundly shape pollination and animal migration patterns in lowland dipterocarp forests through mass flowering and seeding behavior. Lead researcher Ana Prohaska emphasizes the significance of this study, noting: “While it has been previously postulated that drought may boost dipterocarp reproduction, our study indicates that the overall effect of drought is negative, likely due to increased tree mortality. Given the heightened risk of future droughts and the endangered status of most dipterocarp species, especially in the Philippines where only 3% of their original forests remain, sustainable practices will be crucial for the survival of these exceptional trees.”
Read more about the publication here.
New Phytologist is a leading international journal focusing on high quality, original research across the broad spectrum of plant sciences, from intracellular processes through to global environmental change. The journal is owned by the New Phytologist Foundation, a not-for-profit organisation dedicated to the promotion of plant science. https://www.newphytologist.org/.