Limits to reproduction and seed size-number trade-offs that shape forest dominance and future recovery

The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential.

Author Correction: Continent-wide tree fecundity driven by indirect climate effects

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-22025-2

Continent-wide tree fecundity driven by indirect climate effects

Indirect climate effects on tree fecundity that come through variation in size and growth (climate-condition interactions) are not currently part of models used to predict future forests. Trends in species abundances predicted from meta-analyses and species distribution models will be misleading if they depend on the conditions of individuals. Here we find from a synthesis of tree species in North America that climate-condition interactions dominate responses through two pathways, i) effects of growth that depend on climate, and ii) effects of climate that depend on tree size. Because tree fecundity first increases and then declines with size, climate change that stimulates growth promotes a shift of small trees to more fecund sizes, but the opposite can be true for large sizes. Change the depresses growth also affects fecundity. We find a biogeographic divide, with these interactions reducing fecundity in the West and increasing it in the East. Continental-scale responses of these forests are thus driven largely by indirect effects, recommending management for climate change that considers multiple demographic rates.

RESPONSE OF SUGAR MAPLE TO CALCIUM ADDITION TO NORTHERN HARDWOOD FOREST

Watershed budget studies at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, have demonstrated high calcium depletion of soil during the 20th century due, in part, to acid deposition. Over the past 25 years, tree growth (especially for sugar maple) has declined on the experimental watersheds at the HBEF. In October 1999, 0.85 Mg Ca/ha was added to Watershed 1 (W1) at the HBEF in the form of wollastonite (CaSiO3), a treatment that, by summer 2002, had raised the pH in the Oie horizon from 3.8 to 5.0 and, in the Oa horizon, from 3.9 to 4.2. We measured the response of sugar maple to the calcium fertilization treatment on W1. Foliar calcium concentration of canopy sugar maples in W1 increased markedly beginning the second year after treatment, and foliar manganese declined in years four and five. By 2005, the crown condition of sugar maple was much healthier in the treated watershed as compared with the untreated reference watershed (W6). Following high seed production in 2000 and 2002, the density of sugar maple seedlings increased significantly on W1 in comparison with W6 in 2001 and 2003. Survivorship of the 2003 cohort through July 2005 was much higher on W1 (36.6%) than W6 (10.2%). In 2003, sugar maple germinants on W1 were approximately 50% larger than those in reference plots, and foliar chlorophyll concentrations were significantly greater (0.27 g/m2 vs. 0.23 g/m2 leaf area). Foliage and fine-root calcium concentrations were roughly twice as high, and manganese concentrations twice as low in the treated than the reference seedlings in 2003 and 2004. Mycorrhizal colonization of seedlings was also much greater in the treated (22.4% of root length) than the reference sites (4.4%). A similar, though less dramatic, difference was observed for mycorrhizal colonization of mature sugar maples (56% vs. 35%). These results reinforce and extend other regional observations that sugar maple decline in the northeastern United States and southern Canada is caused in part by anthropogenic effects on soil calcium status, but the causal interactions among inorganic nutrition, physiological stress, mycorrhizal colonization, and seedling growth and health remain to be established.

Cupric oxide oxidation products of northern peat and peat-forming plants

Alkaline cupric oxide oxidation and proximate analysis were used to investigate the sources and diagenetic state of organic matter in six Sphagnum-dominated peatlands located between Alberta, Canada, and Ohio, U.S.A. Cupric oxide oxidation was also used to characterize vascular and nonvascular wetland plant species to provide a specific biological fingerprint of these plant tissues. Oxidation of 15 species of Sphagnum moss released large quantities of unsubstituted p-hydroxyl phenolic compounds as well as the species specific sphagnum acid (p-hydroxy-β-[carboxymethyl]-cinnamic acid). By contrast, vascular plant tissues released large amounts of lignin oxidation products. Cupric oxide oxidation of Sphagnum peat from more northerly sites produced mainly p-hydroxyl phenolic monomers with lesser amounts of vascular lignin derived phenols. In contrast, southern sites and those dominated by woody vegetation produced oxidation products characteristic of vascular plant lignin. A distinct relationship exists between the amount of acid-insoluble Klason lignin and both the diagenetically sensitive phenolic acid to aldehyde ratios as well as the total yield of vanillyl phenolic oxidation products. We found evidence of selective decay of phenolic lignin precursors. These relationships indicate the lignin component in surficial layers of Sphagnum-dominated peat is influenced by both Sphagnum and vascular plant lignin, and the structure of lignin appears to undergo diagenetic changes in these layers. Application of an end-member mixing model revealed that lignin oxidation products poorly predicted vegetational composition of the lignin in more decomposed peat, probably as a result of selective decay of lignin structural phenols. Key words: lignin, organic soil, proximate analysis, Sphagnum moss, wetland.