The Paradoxical Illusion of a “Plumper Forest” on a Warming Planet?
It's very simple, it's simple laws of physics, it's well-established laws of physics. More greenhouse gases, more warming, as highlighted by our science via Professor John Mitchell. Rewording, the current context of global warming in the Anthropocene has been a decisive driver of significant changes in various ecosystems on Earth. With a more practical example over decades, one largest remaining tropical forests in the Earth system — the Amazon — was considered a system in equilibrium in the 1980s by Salati & Vose (1984).
However, in the 1990s, this system showed an increase in its dynamics under an atmosphere richer in carbon dioxide. The Amazon continued as a vigorous sink until the 2005 drought, which revealed the sensitivity of that ecosystem to extreme drought events such as the 2005-2010 drought. Later, in the 2010s and 2020s, independent studies revealed that the Southern portion acted as a source rather than a net sink.
Working within Amazon during the 2005-2010 droughts, we monitored tree diameter with aluminium dendrometric bands with a calliper. That work led us to think on the seasonal water mass in the tree biomass. Later, it aligned with findings on the increasing tree size, despite rising climate extremes. Within a more carbon-rich atmosphere, and with the "Increasing Turnover Through Time in Tropical Forests".
Rewording, it would not be surprising to expect that in the presence of water and other ecological factors such as radiation and nutrients under a given optimal condition, such photosynthetic terrestrial organisms could be amplifying its role as a net sink. Both the "Increasing Turnover Through Time in Tropical Forests" and the "Increasing tree size across Amazonia" denote clear signs of a vigorous change within the forests.
Vitally, during that extreme anomaly in 2015-16, temperature and water stress undermined the capacity of ecosystems to function as sinks rather than sources. That 2015-16 El Niño implied an indistinct sink of zero. Meanwhile, the 2024 extreme event remains one unknown issue and unquantified concern to the net balance in the Amazon ecosystem over decades.
This communication addresses the issue by contextualising some research memory findings, highlighting the "Increasing tree size across Amazonia", which provides us with a clearer picture. Large trees grow thicker than those in the sub-canopy, where trees with the same probability of occurrence might not be growing as much as one can expect via a simple positive relationship between carbon dioxide concentration and growth.
1. Measuring the Trees’ Pulse: Aluminium Dendrometer Chronicles and Seasonal Turgor
During the 2005–2010 drought–extreme events, I was part of that interim as a research volunteer in a curious project. A colleague, as the main researcher of that project, used a calliper to measure the distance between two holes in an aluminium dendrometric tape, which encircled the entire thickness of trees with specific diameters at research sites. We discuss the seasonal mass in terms of water from tree biomass as the tree-biological-seasonal water mass.
The intriguing issue is related to seasonal turgor. However, one question intrigued us: within the net balance of that wet-season turgor, where was that singular growth going? There, while we were discussing with a colleague from Zoology, we coined that phenomenon simply as the mass in water from tree biomass. In other words, regarding that liquid turgor within the wet season, causing a dendrometric growth to explode, and even so to collapse at diverse forests, when the Amazon ecosystem was indistinguishable from zero in the 2015-16 event.
The dendrometric band was extended by a metal spring — A spring connecting the two ends of the dendrometric tape, with elasticity allowing the tree's growth to be evaluated in terms of thickness, measured by the researcher as the distance between one hole and another at its extremities. Eventually, we had to replace or repair the aluminium dendrometric tapes. Just as we had to replace the numbered plates and aluminium nails, because some would come loose from the trees, or the trees would even swallow them whole.
2. Water as Mass in Memory: The Trees’ Invisible-Seasonal Growth Enigma
We measured the diameter increase or decrease in millimetres. The measurements were taken at bimonthly intervals throughout the year. At the peaks of the dry season or even the wet season, it was noticeable how some tree species became more turgid than others. In particular, the drought-tolerant and water-affiliated Jacaratia spinosa (Aubl.) A.DC. In contrast, others seemed not to grow as much — visually speaking—, millimetre-wise, at each season.
Although an increasing net sink, to be precise, before the 2005 and 2010 extreme droughts, those forests ended each annual cycle growing at around three tons of Carbon per hectare. And even so, during one LBA program conference, we presented the dynamics of such forests as one of the highest in the Amazon. We attributed this unique dynamic to singular abiotic and biotic factors, including the notorious role of Guadua spp. (Gramineae) in the southwestern Amazon's forest succession as a component in this equation as a whole.
3. Amazonian Tree Gigantism: Resistance and Resilience Under a Warming Planet.
Later, and more recently, with the "Increasing tree size across Amazonia" published in Nature Plants, we can reaffirm some underlying assumptions within the "Increasing Turnover in Tropical Forests", as well as noting an indistinct zero sink during that 2015-16 extreme anomaly in terms of temperature and water stress. We have observed the vigour translated into resilience and resistance of a larger sink in the terrestrial system with the Amazonian forest gigantism.
To highlight, a consistent trend was revealed through key phytosociological metrics. We noted the increase in tree size and quantity in the Amazon over the decades. Remarkably, when the basal area — a type of horizontal spatial area occupied on the forest floor — was investigated regarding the relative annual trend. The large trees basal area increased twice as much per decade compared with the smaller ones.
It was highlighted a per-decade increase in one order of 3.3% in terms of basal area within a richer atmosphere with carbon dioxide. Within one equivalent-order between one and two trees per hectare per decade in Amazonia, the quantity of large trees at the stand level increased at the rate of 6.6% per decade.
Fig. 2: Spatial trends of mean tree size and the scale parameter across Amazonian forests. Distribution of annual trends of mean tree BA (a) and scale parameter (b) per inventory plot across Amazonia. Arrows show the magnitude and direction of trends at each plot location, with blue arrows showing increasing trends and red arrows showing declining trends. Original source with details via the Increasing tree size across Amazonia.
To end, before turning the page, after this initial special communication about the "Increasing tree size across Amazonia" in the story context, with a memory on the water mass in terms of tree biomass. One lesson learned along the way is that although many species are more or less resilient and resistant than others. The frequency and intensity of extreme climate events — translated in terms of temperature increase and water stress during extreme drought — have culminated (and will culminate) in grave threats to the gigantism of a large proportion of tropical trees on Earth.