JAPANESE JOURNAL OF ECOLOGY

Ontogenic color changes in fully turgid flowers ("floral color change") occur in many angiosperms and constitute a good opportunity for studying the role of biotic interactions in shaping organisms using an integrated approach. Two evolutionary hypotheses explain this peculiar trait, in which plants retain reproductively non-functioning, rewardless flowers that have changed in color: 1) the retention of old flowers enhances cross-pollination because it increases pollinator attraction, while the color difference helps pollinators to detect young flowers; and 2) rewardless flowers discourage pollinators from staying on a plant and reduce self-pollination. These hypotheses have not been tested in terms of realized cross-pollination and fail to explain several observed patterns: the inconsistent effects of old flowers on pollinator attraction; the prevalence of species without floral color changes; the variation in floral parts that undergo color changes; and the difference in proximate factors triggering color changes. Future research should examine this diversity and develop an integrated approach, including comparisons across related species of the color and reward changes, floral longevity, flowering schedule, and floral design; an understanding of diverse pollinator responses to color change in relation to their cognitive abilities; and a theoretical exploration of the evolution of these traits.

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Anthropogenic emissions of reactive nitrogen (N) have increased since the industrial revolution, increasing atmospheric N deposition. This paper reviews the effects of atmospheric N deposition on forest ecosystems. During continuous N deposition, forest ecosystems can reach a point of "N saturation," when the amount of deposited N exceeds the biological N demand of the ecosystem. Stream-water nitrate (NO₃^-) concentrations increase with the annual N deposition, and thus the concentrations and fluctuations of stream-water NO₃^- can serve as an index of N saturation. However, many factors control N saturation at the watershed scale, including vegetation, soil, and land use history. Wide variation in N status at the watershed scale is caused by N loss due to natural disturbances, which contribute to the maintenance of N limitation, i.e., N limitation theory. The N limitation theory predicts that natural disturbances synergistically contribute to delays in the onset of N saturation in temperate ecosystems. Consequently, quantitative studies of the relevant N processes and mechanisms should include not only NO₃^-, but also dissolved organic N and denitrification. The concept of a biological critical load was developed as an index using species composition or specific species. The biological critical load is more sensitive than the biogeochemical critical load. With the levels of N deposition increasing globally, we recommend the organization of a widespread monitoring network that includes forested areas and considers the biological critical load. The responses to N deposition vary among tree species and regions, whereas the net primary production in most temperate and tropical areas is likely enhanced by N deposition via interactions with increasing atmospheric CO₂. Deposition of N also increases soil carbon storage and decreases soil C/N, suggesting the existence of abiotic N fixing in soil. Stable isotope analysis, which can distinguish deposited N from the soil N stock, is useful for understanding the mechanisms driving the abiotic fixing of deposited N in soil.

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