[MCN] With fire, warming and drought, Yellowstone forests could be grassland by mid-century

Tue Oct 22 18:55:41 EDT 2019

With fire, warming and drought, Yellowstone forests could be grassland by mid-century <https://news.google.com/articles/CBMiamh0dHBzOi8vbmV3cy53aXNjLmVkdS93aXRoLWZpcmUtd2FybWluZy1hbmQtZHJvdWdodC15ZWxsb3dzdG9uZS1mb3Jlc3RzLWNvdWxkLWJlLWdyYXNzbGFuZC1ieS1taWQtY2VudHVyeS_SAQA?hl=en-US&gl=US&ceid=US%3Aen>
For a study looking at the ability of trees in Yellowstone National Park to recover from fire under climate conditions that are warmer and drier than they have been ...
University of Wisconsin-Madison

Full news report : https://news.wisc.edu/with-fire-warming-and-drought-yellowstone-forests-could-be-grassland-by-mid-century/ <https://news.wisc.edu/with-fire-warming-and-drought-yellowstone-forests-could-be-grassland-by-mid-century/>

Ecological Applications Article
https://doi.org/10.1002/ecm.1340 <https://doi.org/10.1002/ecm.1340>   
Origins of abrupt change? Postfire subalpine conifer regeneration declines nonlinearly with warming and drying
Winslow D. Hansen <https://esajournals.onlinelibrary.wiley.com/action/doSearch?ContribAuthorStored=Hansen%2C+Winslow+D>  Monica G. Turner <https://esajournals.onlinelibrary.wiley.com/action/doSearch?ContribAuthorStored=Turner%2C+Monica+G>
First published: 17 January 2019 
 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#>
PDF <https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecm.1340>
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Abstract
Robust tree regeneration following high‐severity wildfire is key to the resilience of subalpine and boreal forests, and 21st century climate could initiate abrupt change in forests if postfire temperature and soil moisture become less suitable for tree seedling establishment. Using two widespread conifer species, lodgepole pine (Pinus contorta var. latifolia) and Douglas‐fir (Pseudotsuga menziesii var. glauca), we conducted complementary experiments to ask (1) How will projected early‐ to mid‐21st‐century warming and drying affect postfire tree seedling establishment and mortality? (2) How does early seedling growth differ between species and vary with warming and drying? With a four‐year in situ seed‐planting experiment and a one growing season controlled‐environment experiment, we explored effects of climate on tree seedling establishment, growth, and survival and identified nonlinear responses to temperature and soil moisture. In our field experiment, warmer and drier conditions, consistent with mid‐21st‐century projections, led to a 92% and 76% reduction in establishment of lodgepole pine and Douglas‐fir. Within three years, all seedlings that established under warmer conditions died, as might be expected at lower elevations and lower latitudes of species’ ranges. Seedling establishment and mortality also varied with aspect; approximately 1.7 times more seedlings established on mesic vs. xeric aspects, and fewer seedlings died. In the controlled‐environment experiment, soil temperatures were 2.0°–5.5°C cooler than the field experiment, and warming led to increased tree seedling establishment, as might be expected at upper treeline or higher latitudes. Lodgepole pine grew taller than Douglas‐fir and produced more needles with warming. Douglas‐fir grew longer roots relative to shoots, compared with lodgepole pine, particularly in dry soils. Differences in early growth between species may mediate climate change effects on competitive interactions, successional trajectories, and species distributions. This study demonstrates that climate following high‐severity fire exerts strong control over postfire tree regeneration in subalpine conifer forests. Climate change experiments, such as those reported here, hold great potential for identifying mechanisms that could underpin fundamental ecological change in 21st‐century ecosystems.

Introduction

The resilience of forests may erode with warming and increased natural disturbance during this century, which could cause them to change fundamentally (Gauthier et al. 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0034>, Ghazoul et al. 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0036>, Reyer et al. 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0090>, Trumbore et al. 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0112>, Johnstone et al. 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0055>, Ghazoul and Chazdon 2017 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0035>). Resilience is the capacity of a system to absorb disturbances while retaining function, structure, feedbacks, and thus, identity (Walker et al. 2006 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0124>), and if resilience is lost, transitions to alternate states can occur (e.g., conversion from forest to non‐forest; Scheffer 2009 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0098>, Ratajczak et al. 2018 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0087>). 

There is tremendous interest in determining how and why regional forests may change because of the consequences for carbon storage (Bonan 2008 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0007>, Seidl et al. 2014 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0100>), climate regulation (Thom et al. 2017a <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0106>), biodiversity (Thom et al. 2017b <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0107>), and provision of ecosystem services (Turner et al. 2013 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0116>, Seidl et al. 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0101>). However, changes in regional forests will likely emerge from aggregate effects of drivers acting on local processes, such as reproduction, seedling establishment, tree growth, and mortality (Allen and Starr 1982 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0002>, Filotas et al. 2014 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0032>, Messier et al. 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0073>, Rose et al. 2017 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0095>). For example, early tree regeneration is critical to ensure forest resilience following large high‐severity (stand‐replacing) disturbances. Thus, research identifying and characterizing mechanisms that could initiate transitions in forests to alternate states with changing climate and disturbance is essential.

Following a high‐severity disturbance, transition to an alternate state first requires an origin mechanism, or an ecosystem process capable of producing fundamental change in a system when it is acted upon by a forcing driver (e.g., climate; Petraities and Latham 1999 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0082>, Jackson 2006 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0050>). Once the origin mechanism has initiated transition to the alternate state, positive feedbacks must stabilize it (Connell and Slatyer 1976 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0015>, Connell and Sousa 1983 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0016>). Identifying origin mechanisms is critical, as positive feedbacks are irrelevant if transitions are not initiated. State changes in forests are often difficult to identify because mature trees are long lived and tolerate a wide range of environmental conditions (Lloret et al. 2012 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0068>). However, large severe disturbances can catalyze rapid reorganization (Crausbay et al. 2017 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0018>, Hansen et al. 2018 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0042>). In the Alaskan boreal forest, for example, increased severity of large stand‐replacing fires (i.e., where all trees are killed) has caused regional transitions in postfire tree species composition from spruce to deciduous dominance (Johnstone et al. 2010 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0054>, Mann et al. 2012 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0071>).

The mechanisms underpinning tree seedling establishment following large severe wildfires are especially important in subalpine and boreal conifer forests of western North America. These systems are characterized by infrequent high‐severity fires (Turner and Romme 1994 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0135>) and are dominated by conifers, which must re‐establish from seed. Early postfire tree seedling establishment shapes stand structure and species composition for decades (Turner et al. 1997 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0114>, 2004 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0115>, Kashian et al. 2005 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0056>), making seedling establishment a critical determinant of postfire resilience (Donato et al. 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0025>, Hansen et al. 2018 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0042>). Historically, subalpine forests followed an adaptive cycle (sensu Gunderson and Holling 2003 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0039>) where forests burned, creating favorable conditions for seedling establishment, and succession led back to structurally and functionally similar mature forest (Romme 1982 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0093>, Turner et al. 1994 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0113>, Holling 2001 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0045>, Johnstone et al. 2004 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0053>, Allen et al. 2014 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0003>; Fig. 1 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-fig-0001>A). However, tree seedlings are very sensitive to temperature and soil moisture (Lotan 1964 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0070>, Cochran and Berntsen 1973 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0013>, Rochefort et al. 1994 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0092>, Walck et al. 2011 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0123>, Kueppers et al. 2017 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0061>) and warmer and drier conditions following fires could initiate abrupt change in subalpine forests as conditions become less suitable for tree regeneration (Johnstone et al. 2010 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0054>, Landhäusser et al. 2010 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0063>, Harvey et al. 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0043>, Martínez‐Vilalta and Lloret 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0072>, Liang et al. 2017 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0066>; Fig. 1 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-fig-0001>B,C).

 <https://wol-prod-cdn.literatumonline.com/cms/attachment/08cb9b1f-eb76-4fa1-afcc-00e571fa1a04/ecm1340-fig-0001-m.jpg>Figure 1
Open in figure viewer <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#>PowerPoint <https://esajournals.onlinelibrary.wiley.com/action/downloadFigures?id=ecm1340-fig-0001&doi=10.1002%2Fecm.1340>
(A) Subalpine forests of western North America generally move through four phases of the adaptive cycle (Gunderson and Holling 2003 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0039>): release (occurrence of severe wildfire), reorganization (initiation of successional trajectory), exploitation (succession back to mature forest), and conservation (mature forest). (B) During the reorganization phase, tree seedlings are sensitive to temperature and soil moisture and warming, drying conditions could initiate regeneration failure, causing transitions to alternate states (pictured, experimental pots of soil in which drought treatments have been implemented). (C) However, warming can also release seedlings from cold temperature limitation and enhance establishment (pictured, tree seedlings experiencing experimental warming). (D) If seedlings establish, interspecific variation in seedling growth may determine which individuals survive and thrive under warming and drying conditions (pictured, lodgepole pine seedling).
If tree seedlings establish, interspecific variation in early growth can determine which individuals survive and thrive (Richter et al. 2012 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0091>; Fig. 1 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-fig-0001>D). Differences in growth patterns among species are often consistent with broader life history strategies and generally involve tradeoffs (e.g., prioritizing belowground growth over aboveground growth) that may confer advantage to some environmental conditions, while disadvantaging seedlings in other conditions (Eskelinen and Harrison 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0029>). These differences will likely mediate climate change effects on competitive interactions (Kunstler et al. 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0062>), successional trajectories, and tree species distributions (Salguero‐Gómez et al. 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0096>).

Experiments are a valuable tool for understanding how postfire tree regeneration may respond to climate change because they are designed to reveal mechanisms and attribute causation (Carpenter 1998 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0010>, Jentsch et al. 2007 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0051>, Thompson et al. 2014 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0109>, Nooten and Hughes 2017 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0080>). Experiments are also useful for finding thresholds where incremental changes in environmental drivers cause nonlinear system responses (Groffman et al. 2006 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0038>, Kreyling et al. 2013 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0059>). However, designing climate change experiments can be challenging because multiple climate variables are projected to change simultaneously, making them difficult to untangle (Kreyling and Beier 2013 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0058>, De Boeck et al. 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0023>). Further, there are inherent tradeoffs in experimental design between treatment realism and controlling for confounding abiotic and biotic factors. Finally, climate change effects can take years to manifest (Tilman 1989 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0110>) and can play out over large spatial domains (Petraities and Latham 1999 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0082>). Here, we present two seed planting experiments that were designed with these challenges in mind. The experiments encompass both in situ and controlled conditions and explore the multi‐year effects of projected 21st‐century warming and drying on postfire regeneration of two widespread conifers, lodgepole pine (Pinus contorta var. latifolia) and Douglas‐fir (Pseudotsuga menziesii var. glauca) in Yellowstone National Park (Wyoming, USA).

Yellowstone National Park is primarily an extensive central subalpine plateau, where lodgepole pine forests have experienced large stand‐replacing fires every 100 to 300 yr during the Holocene (Romme 1982 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0093>, Millspaugh et al. 2000 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0074>, Power et al. 2011 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0085>). Historically, dense carpets of lodgepole pine seedlings established soon after fires and trees grew rapidly during early succession (Turner et al. 2004 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0115>, 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0117>). Lower montane forests of Yellowstone are composed of drought‐tolerant Douglas‐fir trees, which regenerate more slowly after fire (Donato et al. 2016 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0025>), trading rapid aboveground growth for investment in deep root systems (Burns and Honkala 1990 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0009>). Douglas‐fir trees may be well suited to shift upslope with climate change (Hansen and Phillips 2015 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0040>). Spring–summer temperature in Yellowstone could warm 4.5–5.5°C by the end of this century, while precipitation amount is not projected to change (Westerling et al. 2011 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0125>). As a result, moisture deficit is widely expected to increase, fostering greater fire activity (Westerling et al. 2011 <https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1340#ecm1340-bib-0125>). Thus, the importance of postfire tree seedling establishment to continued subalpine forest resilience is likely to only grow.

Our complementary experiments addressed two questions: (1) How will projected early‐ to mid‐21st century warming and drying affect postfire tree seedling establishment and mortality? We hypothesized that projected warming and drying would reduce postfire lodgepole pine seedling establishment and enhance establishment of Douglas‐fir seedlings. (2) How does initial seedling growth differ between species and vary with warming and drying? We expected lodgepole pine to grow taller and produce more needles with warming and we expected Douglas‐fir to grow longer roots in response to drying.

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