Asbjornsen H., Campbell J.L., Jennings K.A., Vadeboncoeur M.A., McIntire C., Templer P.H., Phillips R.P., Bauerle T.L., Dietze M.C., Frey S.D., Groffman P.M., Guerrieri R., Hanson P.J., Kelsey E.P., Knapp A.K., McDowell N.G., Meir P., Novick K.A., Ollinger S.V., Pockman W.T., Schaberg P.G., Wullschleger S.D., Smith M.D., Rustad L.E. (2018) Guidelines and considerations for designing field experiments simulating precipitation extremes in forest ecosystems. Methods in Ecology and Evolution. 9: 2310-2325.LinkDoi: 10.1111/2041-210X.13094
Precipitation regimes are changing in response to climate change, yet understanding of how forest ecosystems respond to extreme droughts and pluvials remains incomplete. As future precipitation extremes will likely fall outside the range of historical variability, precipitation manipulation experiments (PMEs) are critical to advancing knowledge about potential ecosystem responses. However, few PMEs have been conducted in forests compared to short-statured ecosystems, and forest PMEs have unique design requirements and constraints. Moreover, past forest PMEs have lacked coordination, limiting cross-site comparisons. Here, we review and synthesize approaches, challenges, and opportunities for conducting PMEs in forests, with the goal of guiding design decisions, while maximizing the potential for coordination. We reviewed 63 forest PMEs at 70 sites world-wide. Workshops, meetings, and communications with experimentalists were used to generate and build consensus around approaches for addressing the key challenges and enhancing coordination. Past forest PMEs employed a variety of study designs related to treatment level, replication, plot and infrastructure characteristics, and measurement approaches. Important considerations for establishing new forest PMEs include: selecting appropriate treatment levels to reach ecological thresholds; balancing cost, logistical complexity, and effectiveness in infrastructure design; and preventing unintended water subsidies. Response variables in forest PMEs were organized into three broad tiers reflecting increasing complexity and resource intensiveness, with the first tier representing a recommended core set of common measurements. Differences in site conditions combined with unique research questions of experimentalists necessitate careful adaptation of guidelines for forest PMEs to balance local objectives with coordination among experiments. We advocate adoption of a common framework for coordinating forest PME design to enhance cross-site comparability and advance fundamental knowledge about the response and sensitivity of diverse forest ecosystems to precipitation extremes. © 2018 The Authors. Methods in Ecology and Evolution © 2018 British Ecological Society
Craine J.M., Elmore A.J., Wang L., Aranibar J., Bauters M., Boeckx P., Crowley B.E., Dawes M.A., Delzon S., Fajardo A., Fang Y., Fujiyoshi L., Gray A., Guerrieri R., Gundale M.J., Hawke D.J., Hietz P., Jonard M., Kearsley E., Kenzo T., Makarov M., Marañón-Jiménez S., McGlynn T.P., McNeil B.E., Mosher S.G., Nelson D.M., Peri P.L., Roggy J.C., Sanders-DeMott R., Song M., Szpak P., Templer P.H., Van der Colff D., Werner C., Xu X., Yang Y., Yu G., Zmudczyńska-Skarbek K. (2018) Isotopic evidence for oligotrophication of terrestrial ecosystems. Nature ecology & evolution. 2: 1735-1744.LinkDoi: 10.1038/s41559-018-0694-0
Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (δ15N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ15N declined by 0.6-1.6‰. Examining patterns across different climate spaces, foliar δ15N declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ15N of tree rings and leaves from herbarium samples over the past 75-150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores.
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