“Senescence” is defined as “deterioriation with age”, and, in biology, usually refers not to an accumulation of external injuries over one’s life, but to an inherent process of going downhill physically and physiologically, as many of us are experiencing now. In humans, things start going wrong, you get creaky, or your mind might go and diseases of age will occur. The evolutionary reason why animals aren’t immortal are not completely clear, but there are evolutionary theories. (One of them is that genes that make us reproduce early, but have the side effect of hurting us as we’re older, will be subject to positive natural selection.)
But what about plants? Do they senesce, too? Well clearly some plants are genetically programmed to live only a year or so, but the paper at hand is concerned with trees. Do they senesce, too, or do they have a limited life span simply because, over time, bugs, fires. lightning, climate change, and so on eventually cause them to die?
This new paper, which has sixty-one authors (!) says “yes”, but answering a bit more limited question: do larger trees have reduced fecundity (i.e., seed production)? Since we don’t know the age of a tree without counting its rings, the authors use size as a surrogate of age, though in many species the correlation between tree size and age is not tremendously strong. Another issue is that even in single trees, much less species, seed production varies tremendously from year to year, being huge in so-called “mast years.” Every squirrel knows this. So you can’t just look at seed production in one tree in one year, or even in an entire species in one year, to find out if it goes down as a tree ages. (“Size”, by the way, is estimated as the diameter of the trunk.)
We’d like to know this for several reasons: ecological prediction, use of trees to produce fruit or nuts (do they need to be replaced at a given time? and, if so, when?), and for studies of what truncates life spans in various organisms.
Up to now the assumption has been that log of fecundity goes up with the log of a tree’s diameter, but the data from various species has been conflicting. This new paper in Proc. Nat. Acad. Sci. USA uses data from 597 species of trees, with measurements taken from 585,670 individual trees and 10,542,239 tree-years (this explains why there are so many authors). The conclusion? Yes, in general fecundity declines with tree size. (Fecundity is measured via standardized methods of seed sampling.
Click on the screenshot below to read the paper, or get the pdf here; the full reference is at the bottom.
The results are simple, and can be shown in one graph (below). Of all the tree species tested, 63% showed a decline in fecundity (relative to trunk diameter) as they age (actually, as they get bigger), while another 17% show an increase in seed production that slows down as the tree ages. The conclusion, then, is that “80% of the 597 species tested here show declining rates of increase in fecundity with diameter. . . and 63% of the total actually decrease.” They consider this “empirical evidence for declining fecundity with size”, ergo with age. In other words, the reproductive effort of trees, like that of many animals, slows down as the organism ages. Trees get old and less functional.
Here are some figures showing that decline. Subfigures A-C are for temperate regions, and D-F are tropical regions. Each plot shows the standardized (by diameter) fecundity versus standardized diameter (see paper for how these were calculated), and each line represents one species of tree.
Plots A and D show a pattern of declining relative fecundity with diameter (age surrogate), and these have most of the data for both regions. Standardized fecundity is taken to be fecundity relative to maximum fecundity, which is why in A and D, it peaks at 1.0.
Plots B and E show a pattern whereby fecundity first increases with diameter and then, as the tree gets bigger, the rate of fecundity increase begins to level off (a “sigmoid” graph), showing that the increase in reproductive effort slows down as trees get bigger (and older).
Finally, plots C and F show a pattern in which standardized fecundity continually increases as the tree gets bigger. (It’s possible that if they kept measuring or found the very largest trees, the increase might slow down.)
Clearly, A and D represent most of the trees surveyed.
I’ve put the journal’s caption below the figure; click to enlarge it.
So if anyone asks you if trees get old, you can tentatively answer: “Well, they appear to, at least insofar as older trees reduce their relative investment into seeds.”
Qiu, T., M.-C. Aravena, R. Andrus, D. Ascoli, Y. Bergeron, R. Berretti, M. Bogdziewicz, T. Boivin, R. Bonal, T. Caignard, R. Calama, J. Julio Camarero, C. J. Clark, B. Courbaud, S. Delzon, S. Donoso Calderon, W. Farfan-Rios, C. A. Gehring, G. S. Gilbert, C. H. Greenberg, Q. Guo, J. Hille Ris Lambers, K. Hoshizaki, I. Ibanez, V. Journé, C. L. Kilner, R. K. Kobe, W. D. Koenig, G. Kunstler, J. M. LaMontagne, M. Ledwon, J. A. Lutz, R. Motta, J. A. Myers, T. A. Nagel, C. L. Nuñez, I. S. Pearse, Ł. Piechnik, J. R. Poulsen, R. Poulton-Kamakura, M. D. Redmond, C. D. Reid, K. C. Rodman, C. L. Scher, H. Schmidt Van Marle, B. Seget, S. Sharma, M. Silman, J. J. Swenson, M. Swift, M. Uriarte, G. Vacchiano, T. T. Veblen, A. V. Whipple, T. G. Whitham, A. P. Wion, S. J. Wright, K. Zhu, J. K. Zimmerman, M. Żywiec, and J. S. Clark. 2021. Is there tree senescence? The fecundity evidence. Proceedings of the National Academy of Sciences 118:e2106130118.