Climate Change and Tree Demography

Measuring temperature in Mt. Rainier National Park, USA
Two lab members retrieving temperature sensors from trees in permanent forest stands located in Mt. Rainier National Park (Washington State, USA)

How will climate change influence the distribution and performance of plant species? To answer this question, we need to know what controls range limits, and how those controls differ by species. Climate is thought to determine where species can survive given their physiological tolerances, with biotic interactions such as competition acting to further restrict those distributions. Despite the long acceptance of these ideas by ecologists, we still lack a complete understanding of the forces that constrain species’ distributions. First, few studies have quantified how both climate and competitive interactions affect species performance across ranges – key for assessing the relative importance of fundamental vs. realized niches. Second, most range limit studies ignore the possibility of historical legacies – when the distributions of long-​​lived and/or locally-​​dispersing species reflect past rather than current conditions. Finally, we generally lack the data and analytical approaches to estimate how rapidly species distributions will respond to changing conditions. Addressing these shortcomings is essential, given rapid rates of climate change that have already resulted in the upwards or polewards movement of many species, with continued range shifts expected.

To explore these questions, we are combining observational monitoring, common garden experiments and statistical modeling to assess the controls over current and future altitudinal range limits of tree species on Mt. Rainier, Washington. Because Mt. Rainier covers large climatic gradients, it is an ideal natural laboratory for exploring how climate change will affect forest and meadow communities. Using field observations, experiments and statistical modeling, we have addressed questions like

  1. Which climatic factors limit performance at range limits, and how do these vary among species (e.g. Ettinger et al. 2011, Kroiss & Hille Ris Lambers 2015, Legendre-​​Fixx et al 2018)?; and
  2. What is the relative importance of climate relative to other factors (e.g. competitive interactions, edaphic conditions) for demographic performance (e.g. Hille Ris Lambers et al 2013, Theobald et al 2016, Hille Ris Lambers et al 2016, Ettinger & Hille Ris Lambers 2013 & 2017, Ford et al 2017, Ford et al 2020)?

    Ongoing data collection includes monitoring of tree growth and survival (every 5-6 years, >40 years data), seed production, seedling regeneration and microclimate (temperature, snow depth, soil moisture, date of snowmelt - 10+ years data) in 18 stands located across Mt. Rainier National Park. New Project on Masting: We are just starting to use long-​​term seed production data, paired with manipulative experiments and camera traps to explore the drivers of masting (temporally variable seed production) and implications of masting for seed predator population dynamics (and the predators of those predators). New Project on Tree Regeneration Niches: In collaboration with Rubén D. Manzanedo, we are setting up parallel monitoring efforts of seed production and seedling densities in Swiss montane forests, paired with measurements of seedling traits in the greenhouse. Data from both USA and Swiss forest sites will allow us to explore the climatic constraints on tree seedling regeneration, relevant to understanding how rapidly trees will be able to colonize newly suitable habitats with warming climates.
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