Back to the Future: The Responses of Alpine Treelines to Climate Warming are Constrained by the Current Ecotone Structure
Alpine treeline ecotones are considered early-warning monitors of the effects of climate change on terrestrial ecosystems, but it is still unclear how accurately treeline dynamics may track the expected temperature rises. Site-specific abiotic constraints, such as topography and demographic trends may make treelines less responsive to environmental fluctuations. A better understanding on how local processes modulate treelines’ response to warming is thus required. We developed a model of treeline dynamics based on individual data of growth, mortality and reproduction. Specifically, we modeled growth patterns, mortality rates and reproductive size thresholds as a function of temperature and stand structure to evaluate the influence of climate- and stand-related processes on treeline dynamics. In this study, we analyze the dynamics of four Pyrenean mountain pine treeline sites with contrasting stand structures, and subjected to differing rates of climate warming. Our models indicate that Pyrenean treelines could reach basal areas and reproductive potentials similar to those currently observed in high-elevation subalpine forest by the mid twenty-first century. The fastest paces of treeline densification are forecasted by the late twenty-first century and are associated with higher warming rates. We found a common densification response of Pyrenean treelines to climate warming, but contrasting paces arise due to current size structures. Treelines characterized by a multistratified stand structure and subjected to lower mean annual temperatures were the most responsive to climate warming. In monostratified stands, tree growth was less sensitive to temperature than in multistratified stands and trees reached their reproductive size threshold later. Therefore, our simulations highlight that stand structure is paramount in modulating treeline responsiveness to ongoing climate warming. Synthesis. Treeline densification over the twenty-first century is likely to occur at different rates contingent on current stand structure and its effects on individual-level tree growth responses to warming. Accurate projections of future treeline dynamics must thus incorporate site-specific factors other than climate, specifically those related to stand structure and its influence on tree growth.
Keywordsclimate warming mountain pine plant–climate interactions Pyrenees Pinus uncinata reproductive size threshold stand structure tree growth treeline shift
The authors thank several people for their help in site selection and field sampling. The authors thank the Spanish Ministry of Research who funded this research through projects AMB95–0160 and REN2002–04268-C02. J.C. Linares’ contribution was partly supported by the European Union FEDER 0087 TRANSHABITAT and the “Retos” Project CGL2013-48843-C2-2R (Spanish Ministry of Economy and Competitiveness). Enric Batllori acknowledges the support of a Marie Curie IIF grant (Marie Curie IIF, PIFF-GA-2103-625547). The autors sincerely thank the useful comments provided by two anonymous reviewers and the subject editor.
Conflict of Interest
The authors have no conflicts of interest to declare.
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