Is there tree senescence? The fecundity evidence

Abstract

Despite its importance for forest regeneration, food webs, and human economies, changes in tree fecundity with tree size and age remain largely unknown. The allometric increase with tree diameter assumed in ecological models would substantially overestimate seed contributions from large trees if fecundity eventually declines with size. Current estimates are dominated by overrepresentation of small trees in regression models. We combined global fecundity data, including a substantial representation of large trees. We compared size-fecundity relationships against traditional allometric scaling with diameter and two models based on crown architecture. All allometric models fail to describe the declining rate of increase in fecundity with diameter found for 80% of 597 species in our analysis. The strong evidence of declining fecundity, beyond what can be explained by crown architectural change, is consistent with physiological decline. A downward revision of projected fecundity of large trees can improve the next generation of forest dynamic models.

Keywords: allometric scaling; crown architecture; tree fecundity; tree life history; tree senescence.

Fig. 1.

The relationship between fecundity and diameter for species in temperate (A–C) and tropical (D–F) regions, where diameter and fecundity are scaled as $D/D_{opt}$ and $f\left(D\right)/f\left(D_{opt}\right)$, respectively. A and D exhibit type A species (fecundity eventually declines); B and E show type B species (sigmoid increase in fecundity); C and F represent type C species (continuous increase in fecundity). Line transparency is proportional to the 90% credible interval width across the diameter ranges, such that confident predictions are opaque, and vice versa. The percentages of species for each type of fecundity–diameter relationship are summarized in Table 1. $D_{opt}$ is the diameter when maximum fecundity occurs.

Fig. 2.

Reconstructed evolution history of $D_{opt}$ (the diameter when maximum fecundity occurs) using continuous character mapping. Note that the histogram is based on $D_{opt}$ from all type A species (378 species in total) in Fig. 1 while 281 ($\sim$ 74.3%) of them have phylogenetic trees. Phylogenetic signal was estimated using Pagel’s $\lambda$.

Fig. 3.

Neither allometric model can generate fecundity decline with size for realistic parameter ranges. (A) Crown surface area evaluated from images of six species (SI Appendix, Fig. S1). (B) Crown surface area evaluated from 110 species in North America using the ideal tree distribution (ITD) model (91). Colors indicate genera (ordered alphabetically for 13 main genera). Crown surface area $C_{SA}$ continues to increase with diameter from observed and simulated data with $C_{SA}$ for each species displayed as a proportion of the maximum $C_{SA}$ value.

Fig. 4.

In open-grown trees CSA describes the allometric relationship between diameter and the crown exposed to high sunlight where reproductive effort is often concentrated. The CSA model combines crown architecture with shading from neighbors to evaluate the cumulative exposure with depth into the crown, as shown with two examples here (details in SI Appendix, section S2). Photo credits: James S. Clark.