Amazon Tree Mortality

Figure 1. Examples of dead and alive trees monitored in the Central Amazon.

Amazonian rainforest tree mortality driven by climate and functional traits

Izabela Aleixo, Darren Norris, Lia Hemeric, Antenor Barbosa, Eduardo Prata, Flávia Costa, & Lourens Poorter

The Problem: Climate scientists are constantly learning and sharing new details about climate change and its possible effects in the future. However, many of the impacts of climate change have already surfaced and revealed the fragility of our ecosystems. Recently, scientists have observed increasing tree mortality in tropical forests, which are some of the most biodiverse and ecologically important places in the world. Could tree mortality be another consequence of climate change–one that’s happening right now? This study by Aleixo and others (2019) explores this possible connection between modern climate change and downfall of tropical forests.

What data were used? This study uses monthly climate and tree mortality records along with about 50 years of observational data in the Amazon rainforest. Climate data include precipitation, temperature, and humidity. Tree mortality data is categorized by specific traits such as wood density (soft or hard), successional position (when a species colonizes a new area), and leaf phenology (deciduous or evergreen).

Figure 2. a–d, Variation in tree mortality (a), precipitation (b), temperature (c) and humidity (d). When analysing the variation in mortality within years, we found that 19% of all deaths occurred in January (analysis of variance, d.f. = 11; P < 0.001). Interestingly, January is one of the wettest months of the year, suggesting that waterlogged soils and storms may enhance mortality. Monthly values (circles), averages (black lines) and 95% confidence intervals (dashed grey lines) over the study period (1965–2016) are shown.

Methods: Aleixo and others (2019) tracked global climate and tree mortality in an area of the Amazon rainforest monthly for one year. They looked for increased tree mortality that aligned with variations in the climate data. They also examined tree mortality of different species traits during significant climate events in the past 50 years. These events include climate anomalies like El Niño or La Niña (click here to learn more about these).

Results: This study found that Amazon tree mortality is driven by climate, but the relationship is complex. For example, droughts can lead to immediate or slow tree death, depending on the mechanisms at play. Additionally, if a tree has harder wood, it is less likely to die during a drought. Aleixo and others (2019) also found that weather events like low rainfall or high temperatures can either immediately enhance tree mortality or cause increased mortality up to two years later. Similar outcomes are associated with years where El Niño or La Niña are particularly extreme. Various species traits may protect trees from dying under certain weather or climate events, but no single Amazon species is completely safe from the effects of climate change.

Why is this study important? As our climate continues to change and weather events become more extreme, the future of our forests remains uncertain. Even the most biodiverse and ecologically robust regions in the world are susceptible to the effects of climate change. This study provides a modern framework for us to understand those effects. From this, scientists can refine dynamic global vegetation models that predict how forests will respond to climate variability in the future.

Citation: Aleixo, I., D. Norris, L. Hemerik, A. Barbosa, E. Prata, F. Costa, and L. Poorter (2019), Amazonian rainforest tree mortality driven by climate and functional traits, Nature Climate Change, 9(5), 384-388, doi:10.1038/s41558-019-0458-0.

Figure 3. Comparisons of the ratios of annual mortality for different functional groups of species, calculated using two classes of wood density (that is, the mortality of soft-wooded species (0.30–0.69 g cm−3) divided by the mortality of hard-wooded species (0.70–1.10 g cm−3)), successional position (that is, the mortality of pioneer species divided by the mortality of late species) and deciduousness (that is, the mortality of evergreen species divided by the mortality of deciduous species) over 52 years and during 5 years of highest peak mortality (the 1982, 1992 and 2016 El Niño droughts, the 1999 La Niña wet year and the 2005 NAO drought). The black line shows where the ratio is equal to 1 (that is, the mortality rate of the two classes is the same). The results of a Pearson’s chi-squared test are shown. Asterisks indicate a significant result (P ≤ 0.05). Annual mortality rates were higher for pioneers compared with late-successional species, for soft compared with hardwood species, and for evergreen compared with deciduous species. When the mortality rates of the functional groups were compared between normal and extreme years, pioneers experienced much higher mortality rates than climax species in the two El Niño and La Niña years. Soft-wooded species experienced much higher mortality rates than hard-wooded species in the El Niño 1982 year. Evergreens experienced much higher mortality rates than deciduous species in the NAO year.

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