Old trees matter

Past disturbance and reforestation efforts have led to forests with a younger age structure. Such change in age structure could shape future drought responses in forest ecosystems and associated ecosystem services.
Published in Earth & Environment
Old trees matter
Like

Deforestation and forest harvesting across large areas, followed by local reforestation efforts, have led to less mature forests that are dominated by trees with a younger age structure. Indeed, we have been cutting down a lot of mature trees, particularly the biggest and oldest ones storing a lot of carbon, including atmospheric carbon dioxide captured more than hundreds to thousands years ago. At the same time, we tried to make up for our mistakes by planting more and more saplings along with natural succession. It means that our terrestrial ecosystems are gradually shifting to a younger forest age structure storing atmospheric carbon dioxide from more recent times. Land-use changes, chiefly converting afforested lands to agricultural lands, compounded with wood harvest, have expanded the area of young forests (< 140 years old) from 4.8 million km2 in 1900 to 12.5 million km2 recently. Such a global change in the age structure of trees in forests implies important consequences for ecosystem functioning and carbon cycling, especially so in response to ongoing climate change.

Indeed, in the meantime, the increasing frequency of droughts, sometimes amplified by more frequent heatwaves, such as during this summer 2022 in Europe, poses a daunting threat on the hydraulic functioning of trees with the potential to lead to massive forest dieback. But are all individuals from a given tree species equal in their responses to droughts? Given this context and the increasing frequency of young individuals in the forest age structure, key questions are: can younger trees better cope with drought than their older relatives? Do younger trees have higher resistance than older trees during drought? Do younger or older trees recover more quickly from drought?

Our study, recently published in Nature Climate Change, does provide a clear answer to these timely questions: old trees tend to be more resistant but less resilient than young adult trees. But first, how old is a young tree compare to its eldest? Let's be clear here that we mainly focus on those individuals that have already reached the upper canopy layer. Hence, by young trees we here refer to established adult trees reaching the top of the canopy layer of mature forests, thus excluding seedlings, sapling and juveniles from the understory layer.

A plantation of young adult trees of Liriodendron tulipifera,
North Carolina, USA. Photo credit: Tsun Fung Au 

But then, how do we accurately assess the age of mature trees? Although tree size (e.g., tree diameter or tree height) is a nice and easy proxy to estimate tree age, especially so when trees are still very young (e.g., seedling and sapling), such age-size relationship tend to be blurred when trees get more mature and reach the upper canopy layer. That is, there is a lot of variation in tree age even though they share the same size. Trees with the same size could differ by 9 times in age (!) while the biggest tree is not necessary the oldest one for a given species and vice versa. For example, the oldest European beech (Fagus sylvatica) is 623-year-old with tree trunk diameter of 65cm but the largest individual of the same species (110cm in diameter) is only 210-year-old. Similarly, the oldest and the largest Heldreich’s pine (Pinus heldreichii) can differ by over 600 years in age where the oldest individual is 1230-year-old (160cm in diameter) but the largest one is just 550-year-old (180cm in diameter).

So now, where can we get an accurate information on tree age for canopy-dominant trees? Many of us have already learned during primary school that counting the number of tree-rings is the best way to infer the age of an individual tree. Fortunately, tree-ring data are now publicly available thanks to multiple scientists who used to collect tree-ring for paleoclimate reconstruction purposes and who agreed on sharing this priceless information. Let’s take a minute to say a big thank you to all tree-ring data contributors who generously shared their priceless data! This means, from the perspective of paleoclimate scientists, that several thousands of mature tree individuals were drilled to the centre of the tree trunk (i.e., the pith) to maximize the time span of climate reconstruction. Hence, the estimation of tree age from these databases is very reliable! Of course, sampling errors may still exist but these errors should be randomly distributed, thus only adding noise to the analyses. Overall, we retrieved more than 20,000 individuals of trees from over hundred drought-sensitive species across five continents.

Accurate tree age can be informed by counting the number of tree-rings.
Photo credit: Tsun Fung Au

Now we have absolute tree age for numerous tree individuals of multiple tree species. Let's run some analyses! Wait a minute... Is the absolute tree age the best way for our analyses when we have multiple species? Probably not, but why? Do you think it is fair to compare a 30-years-old Homo sapien to a 30-years-old Felis catus. Of course not! A 30-year-old Felis catus is so much older than the same absolute age of Homo sapien. Similarly, a 200-year-old broadleaf tree can be very old, but a conifer tree with the same age is still quite young relative to the longevity of some conifer species that can be up to a couple thousand years (e.g., Bristlecone pine).

We used a relative age classification to account for different longevity across multiple species so that age-dependent responses can be compared across species. Although this ranking approach is quite unusual in the field of dendrochronology, it allowed us to classify individuals from different species as “relatively young” or “relatively old” in a comparable and fair manner. For each drought-sensitive species, we classified the youngest and oldest 25% of the population as the young and old canopy-dominant cohorts while the interquartile (the remaining 50%) was classified as the intermediate cohort. This relative ranking allows us to compare tree individuals from different species that have very distinct life history.

Finally! We are good to run some analyses: we want to look at how much tree growth is reduced (i.e., sensitivity and resistance) during drought and how these trees recover after drought (i.e., resilience). During mild droughts, young hardwoods from the upper canopy layer had a 28% growth reduction, compared to a 21% reduction for their oldest relatives. Young conifers are also less resistant to drought (27% growth reduction) than the eldest and the difference between young and old conifers is 2.5%. Some of you may wonder: Why should we bother? The difference is so minimal! I mean just 2.5% or 7% depending on the tree type (deciduous vs. coniferous). Well, a notorious drought happened in Europe during summer 2003, which caused 30% reduction in gross primary productivity and released 0.5 petagram (1015) of carbon per year into the atmosphere (equivalent to the carbon footprint of ~380,000,000 round trips flying between Los Angeles and New York). This amount of carbon release is just the European contribution. When we consider the amount of carbon being released due to drought at global scale, such difference is not trivial at all. In addition, during extreme droughts, such difference could grow to 17% between young and old hardwoods. In reality, hardwood trees are dominant in most temperate biomes. That means we need to conserve a sufficient amount of old hardwood trees to minimize the short-term effects of droughts.

An old tree is more resistant to drought stress than its younger relatives. 
An old cypress tree (Fokienia hodginsii) in Vietnam. Photo credit: Tsun Fung Au 

How about the recovery ability from drought? Our study shows that young trees have higher ability to recover from droughts than old trees. This suggests, in the long run, that the higher resilience of young trees may benefit to carbon sequestration. Yet, older trees remains unbeatable in terms of the residence time of carbon storage. The latest Intergovernmental Panel on Climate Change (IPCC) report identified reforestation as a nature-based solution to mitigate climate change. Therefore, consideration of forest age could be an important aspect when dealing with climate change, such that a good mixture of old and young trees is necessary to cope with the short-term and long-term effects of droughts.

A native species (Phyllanthus emblica) recently planted
during a reforestation project in Hong Kong.
Photo credit: Tsun Fung Au 

Using Hong Kong, China, as an example, Acacia confusa (Taiwan acacia) was widely planted in the countryside within the vicinity of Hong Kong city after WWII, chiefly as a mean to control for soil erosion and landslide following clear-cuts of the native forests by the soldiers (for strategic purposes and fuelwood). Not only is A. confusa a non-native species (you can tell from its common name), but also A. confusa is a very short-lived species, roughly living for about 50-60 years. This means that A. confusa did not contribute too much to Hong Kong's biodiversity and long-term residence time of carbon storage while releasing a lot of the carbon stored post WWII when dying. Thus, the Hong Kong government decided to replace A. confusa with more native and long-lived species to restore the habitat and to ensure the native trees could live long enough to mitigate climate change impacts.

As of today’s climate change context, a top priority is to conserve the existing old trees that still contribute to the long-term residence time of atmospheric carbon dioxide captured several hundreds to thousands of years ago, while being also more tolerant to climate-induced droughts as we have demonstrated in our study. While it takes time for young trees to reach maturity and be resistant to climate-induced droughts, reforestation is still beneficial and should be conducted with caution to consider its impacts on local community. Having a diverse composition of tree species and also a diverse age structure within a forest is the key for the ecosystems to withstand future climate change.

Diverse forest composition in terms of tree species and structure in terms of tree age could help withstand climate change and extreme events.
Photo credit: Tsun Fung Au

Please sign in or register for FREE

If you are a registered user on Research Communities by Springer Nature, please sign in