Global Precipitation Part 2: Ecosystem & Management Implications

By Jennifer Hushaw

This bulletin is the second in a two part piece on changes in global precipitation. In it, we discuss how water availability shapes forests and recent observations of forest decline linked to drought and heat stress. We then turn our attention to the issue of drought, including the factors that contribute to drought risk and the management options for mitigating it. While there will be a mix of wetter and drier conditions in the future, depending on the region, the background trend of warming temperatures will exacerbate drying – making more frequent and severe drought one of the more obvious climate-related risks.

(click here to download a pdf of this article or a one-page synopsis)

Changes in Global Precipitation: A Recap

Predicting how global climate change will affect future precipitation is one of the most challenging and uncertain areas of climate modeling research. However, there are some consistent patterns that have emerged from model projections, as described in part one of this bulletin. These general rules-of-thumb are summarized here:

  • Increase in total global precipitation
  • Regional differences, i.e. changes will not be uniform – increases in mid and high latitudes; decreases in subtropics
  • General pattern of wet-get-wetter, dry-get-drier
  • Increased contrast between wet and dry seasons
  • Increased winter precipitation in mid and high latitudes
  • Increased frequency of heavy precipitation events
  • A few areas of model agreement in terms of drying (soil moisture), including northeast and southwest South America and southwestern U.S.

Why Water Availability Matters    

Water availability is likely to change in the future, due to the combination of increasing average temperatures and changes in the total, timing, and intensity of precipitation events. Water availability, in terms of soil moisture, is a particularly important metric for forest management, since it can drive changes in forest structure and species composition over time. Water availability affects tree mortality, seedling recruitment, and resource allocation within individual trees (e.g. root-shoot ratio), and these factors ultimately influence competition between species.

Tree Species Migration

Severe or long-term decreases in water availability can predispose forest areas to large-scale die-off that opens the door for colonization of new species. While climate-induced tree species movement is usually a gradual process, it can happen more rapidly when sporadic mortality events eliminate competition from established species – arguably the biggest immediate barrier to species migration. We have seen some examples of this mortality-facilitating-colonization pattern in places like the Green Mountains of Vermont, where there is evidence that the upslope migration of hardwood species was likely accelerated by canopy turnover after red spruce experienced dieback from acid rain in the 1960’s and 70’s (Beckage et al. 2008) .

Tree Resource Allocation

Water availability, including the amount and timing of rainfall, is critical to forest structure because it changes how individual trees allocate their resources between the above and belowground portions of the stem. Under drier conditions, trees will generally respond by decreasing inputs to foliage and aboveground woody biomass while increasing fine roots, which improves their ability to draw on limited soil water resources. This is a helpful adaptation, but it reduces growth in the merchantable part of the tree. Likewise, in wetter areas, trees will put more resources into foliage and increase growth rate, with fewer fine roots. This capitalizes on growth potential and increases competitiveness, but it can also result in a shallower root system that increases risk of blow down and vulnerability to future drought conditions (Farrior et al. 2013; McDowell et al. 2008).

Different species have different amounts of plasticity in the degree and speed with which they can shift resources in response to changing conditions and, as with any adaptation, there are tradeoffs, e.g. it has been shown that this flexibility, while beneficial in terms of adaptability, can be detrimental in stressful environments (Richter et al. 2012).

Water Stress and Forest Mortality

Recently, researchers have documented widespread tree mortality on a global scale that is at least partly attributed to drought and heat stress (see this map from Hartmann et al. 2015, which shows locations of substantial drought- and heat-induced tree mortality around the globe since 1970). The impacts were observed in wet areas, as well as semi-arid regions, which indicates that increasing temperatures may be playing a significant role – by increasing water loss through transpiration, reducing tree vigor, and accelerating insect development and reproduction (Allen et al. 2010; Hartmann et al. 2015).

There are many examples of forest impacts in the U.S. that have been linked to water stress, including aspen decline in the west (Worrall et al. 2013), increased mortality of pine and oak species in the Central Coast and Southern Sierra Ranges of California (USFS 2015), loss of big trees (>2 ft dbh) throughout California (McIntyre et al. 2015), and regional-scale die-off of piñon pine (Breshears et al. 2008). The ultimate consequences of forest die-off driven by drought and heat stress are unclear and researchers have highlighted the importance of investigating these implications (Anderegg, Kane, and Anderegg 2013).

 

At this time, we don’t have sufficient data to know whether forest mortality is increasing globally. Although, these observations have sparked a huge body of research on the physiological mechanisms that influence how plants avoid, tolerate, and/or recover from drought stress. A better understanding of exactly how and why certain trees succumb to drought will improve predictions of global-scale forest impacts from climate change.

Presently, the understanding is that drought-related mortality happens via three interrelated pathways – carbon starvation, hydraulic failure, and biotic attack (Figure 1). Carbon starvation occurs when photosynthesis is reduced and trees are forced to use up their carbon reserves – a consequence of closing leaf stomata, which reduces water loss but does not allow CO2 uptake. Hydraulic failure occurs when plants dehydrate past the point of no return. Insects and pathogens can amplify or be amplified by both of these processes (McDowell et al. 2008), e.g. carbon starvation will reduce resin production and make it difficult for trees to pitch out an attacking beetle.

Evaluating Drought Risk

Evidence suggests we will experience more frequent and severe drought due to climate change, but this risk is not universal and it varies with site characteristics and forest type.  Determining whether it is a significant risk for your forestland involves considering all the factors that influence intensity, exposure, and vulnerability to drought.

Intensity

Higher temperatures increase the intensity of individual drought events by water loss through direct evaporation and forest transpiration (collectively known as evapotranspiration). Additionally, as conditions dry, there is a feedback that exaggerates this process – less soil moisture means less cooling from transpiration and temperatures go up even further. This is similar to the way human sweat helps reduce body temperature – if you lose your ability to sweat when you are hot, your body temperature will begin to increase rapidly.

Including evapotranspiration in model simulations (rather than precipitation alone) increases the percentage of global land area that is projected to experience moderate drying by the end of this century (from 12 to 30%). Importantly, researchers found that this effect will even make relatively wet areas more drought prone: “Increased PET [potential evapotranspiration] not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone” (Cook et al. 2014). This interaction with temperature has also been implicated in the severity of the recent California drought, where researchers have found that the occurrence of drought years has increased primarily because of the increased probability of warm-dry conditions, rather than a substantial change in the probability of a precipitation deficit (Diffenbaugh, Swain, and Touma 2015). The bright side is that these drying trends will be beneficial in some areas where conditions have historically been excessively wet – this will help alleviate issues of reduced productivity and limited access in these locations.

Exposure

Site characteristics, including soil texture, depth to water table, and topography, have a big influence on the amount of drought exposure on a given site (i.e. the likelihood that a given location will experience drought conditions). These factors influence soil water holding capacity, run off, and evaporation rates, which all mediate the direct effects of precipitation change.

 

Vulnerability

Vulnerability is primarily determined by the tree species mix on site, specifically the level of drought tolerance exhibited by each species. The variability in forest drought tolerance from region to region creates some unique advantages and disadvantages in terms of vulnerability. For example, the southeastern U.S. has a species mix with a relatively high drought tolerance compared to the northeast, which reduces the risk of negative drought impacts in that region. However, there is a much higher diversity in terms of the mix of drought tolerant and intolerant species in the Appalachian region and the northeast, which might be beneficial if future conditions are highly variable.

Note: We recommend referencing Russell et al. 2014 for maps of average drought tolerance and diversity of drought tolerance classes among tree species in the eastern U.S. 

 

Overall Risk

Taken together, these factors tell us that sites that are projected to have large increases in temperature and decreases in precipitation, with low soil water holding capacity, and a drought-intolerant species mix will have the highest levels of drought risk (in terms of intensity, exposure, and vulnerability).  In contrast, an area may have a high likelihood of intense drought in the future, but the risk may be mitigated by a drought tolerant species mix and better site conditions. The regions of greatest concern going forward will be places where all these factors overlap.

Reducing Drought Risk Through Management

From the perspective of an individual forest manager, there is not much that can be done to reduce the intensity of future drought conditions, but the following areas offer opportunities to reduce risk by reducing exposure and/or vulnerability:

  • Land base
    • Focus resources on sites with soil and topographic characteristics that generally retain moisture
  • Species mix
    • Use silvicultural techniques that favor regeneration of drought tolerant species
    • Plant genotypes from warmer and dryer areas of a species range
  • Reduce stocking
    • A number of studies conducted in different forest types throughout the U.S. and Europe (primarily pine-dominated) have highlighted the utility of thinning for moderating drought impacts on growth, increasing drought resistance, and improving the speed of recovery after drought events (D’Amato et al. 2013; Kerhoulas et al. 2013; Kohler et al. 2010; Slodicak, Novak, and Dusek 2011).
    • Although it is worth noting that thinning has been shown to have negligible effects on drought tolerance in sparse forest canopies (B. Law, personal communication, May 6, 2015), such as some dry western forests where self-thinning has naturally taken place.

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Sources

Allen, Craig D., Alison K. Macalady, Haroun Chenchouni, Dominique Bachelet, Nate McDowell, Michel Vennetier, Thomas Kitzberger, et al. 2010. “A Global Overview of Drought and Heat-Induced Tree Mortality Reveals Emerging Climate Change Risks for Forests.” Forest Ecology and Management 259 (4): 660–84. doi:http://dx.doi.org/10.1016/j.foreco.2009.09.001.

Anderegg, William R. L., Jeffrey M. Kane, and Leander D. L. Anderegg. 2013. “Consequences of Widespread Tree Mortality Triggered by Drought and Temperature Stress.” Nature Clim. Change 3 (1): 30–36. doi:10.1038/nclimate1635.

Breshears, David D, Orrin B Myers, Clifton W Meyer, Fairley J Barnes, Chris B Zou, Craig D Allen, Nathan G McDowell, and William T Pockman. 2008. “Tree Die-off in Response to Global Change-Type Drought: Mortality Insights from a Decade of Plant Water Potential Measurements.” Frontiers in Ecology and the Environment 7 (4): 185–89. doi:10.1890/080016.

Cook, BenjaminI., JasonE. Smerdon, Richard Seager, and Sloan Coats. 2014. “Global Warming and 21st Century Drying.” Climate Dynamics 43 (9-10): 2607–27. doi:10.1007/s00382-014-2075-y.

D’Amato, Anthony W., John B. Bradford, Shawn Fraver, and Brian J. Palik. 2013. “Effects of Thinning on Drought Vulnerability and Climate Response in North Temperate Forest Ecosystems.” Ecological Applications 23 (8): 1735–42. doi:10.1890/13-0677.1.

Diffenbaugh, Noah S., Daniel L. Swain, and Danielle Touma. 2015. “Anthropogenic Warming Has Increased Drought Risk in California.” Proceedings of the National Academy of Sciences 112 (13): 3931–36. doi:10.1073/pnas.1422385112.

Farrior, Caroline, E., Ray Dybzinski, Simon A. Levin, and Stephen W. Pacala. 2013. “Competition for Water and Light in Closed-Canopy Forests: A Tractable Model of Carbon Allocation with Implications for Carbon Sinks.” The American Naturalist 181 (3): 314–30.

Hartmann, Henrik, Henry D. Adams, William R. L. Anderegg, Steven Jansen, and Melanie J. B. Zeppel. 2015. “Research Frontiers in Drought-Induced Tree Mortality: Crossing Scales and Disciplines.” New Phytologist 205 (3): 965–69. doi:10.1111/nph.13246.

Kerhoulas, Lucy P., Thomas E. Kolb, Matthew D. Hurteau, and George W. Koch. 2013. “Managing Climate Change Adaptation in Forests: A Case Study from the U.S. Southwest.” Journal of Applied Ecology 50 (6): 1311–20. doi:10.1111/1365-2664.12139.

Kohler, Martin, Julia Sohn, Gregor Nägele, and Jürgen Bauhus. 2010. “Can Drought Tolerance of Norway Spruce (Picea Abies (L.) Karst.) Be Increased through Thinning?” European Journal of Forest Research 129 (6): 1109–18. doi:10.1007/s10342-010-0397-9.

McDowell, Nate, William T. Pockman, Craig D. Allen, David D. Breshears, Neil Cobb, Thomas Kolb, Jennifer Plaut, et al. 2008. “Mechanisms of Plant Survival and Mortality during Drought: Why Do Some Plants Survive While Others Succumb to Drought?” New Phytologist 178 (4): 719–39. doi:10.1111/j.1469-8137.2008.02436.x.

McIntyre, Patrick J., James H. Thorne, Christopher R. Dolanc, Alan L. Flint, Lorraine E. Flint, Maggi Kelly, and David D. Ackerly. 2015. “Twentieth-Century Shifts in Forest Structure in California: Denser Forests, Smaller Trees, and Increased Dominance of Oaks.” Proceedings of the National Academy of Sciences 112 (5): 1458–63. doi:10.1073/pnas.1410186112.

Richter, Sarah, Tabea Kipfer, Thomas Wohlgemuth, Carlos Calderón Guerrero, Jaboury Ghazoul, and Barbara Moser. 2012. “Phenotypic Plasticity Facilitates Resistance to Climate Change in a Highly Variable Environment.” Oecologia 169 (1): 269–79. doi:10.1007/s00442-011-2191-x.

Slodicak, Marian, Jiri Novak, and David Dusek. 2011. “Canopy Reduction as a Possible Measure for Adaptation of Young Scots Pine Stand to Insufficient Precipitation in Central Europe.” Forest Ecology and Management 262 (10): 1913–18. doi:http://dx.doi.org/10.1016/j.foreco.2011.02.016.

USDA Forest Service. 2015. Forest Health Protection Survey: Aerial Detection Survey April 15th-17th, 2015. Accessed online at: http://www.sierranevada.ca.gov/our-work/docs/southsierrasdroughtsurveyapr2015.pdf

Worrall, James J., Gerald E. Rehfeldt, Andreas Hamann, Edward H. Hogg, Suzanne B. Marchetti, Michael Michaelian, and Laura K. Gray. 2013. “Recent Declines of Populus Tremuloides in North America Linked to Climate.” Forest Ecology and Management 299 (0): 35–51. doi:http://dx.doi.org/10.1016/j.foreco.2012.12.033.

 

 

Global Precipitation Part 1: Trends & Projections

By Jennifer Hushaw

The discussion of global precipitation will be covered in two parts. This bulletin (part one) provides an overview of the precipitation trends observed over the last century, as well as projections of future change. Part two will cover a range of forest ecosystem and management implications of changing precipitation patterns and drought stress.

(click here to download a PDF of this article or a one-page synopsis)

Observations of Global Precipitation

Observations suggest that there has been an increase in globally averaged precipitation on land over the last century.  This is based on several global datasets that all show a statistically significant increase since 1900 (Figure 1). However, the datasets do not agree on how much it has increased. This primarily stems from a lack of sufficient data across the globe, especially during the early 1900’s.  The upward trend is also much clearer in the early part of the record than in the years after 1950. As a result, there is only low confidence in the observed global change prior to 1950 and medium confidence in the trends since that time.


Figure1-Global-Precip_Trend

The data also reveal notable variability in precipitation trends at different latitudes, but problems with data quality, coverage, and agreement have resulted in low confidence in most regional observations. However, there is at least one area of the world where clear trends have emerged – namely, the mid-latitudes of the Northern Hemisphere (30⁰N to 60⁰N). Here, the data show a likely overall increase in precipitation, with medium confidence prior to 1950 and high confidence afterward (Figure 2).

Figure2-NHemisphere_Precip_Trend

Future Global Precipitation

There are several metrics to consider when thinking about changes in future precipitation, these include changes in the total annual amount, shifts in timing, and changes in the intensity of individual precipitation events – all of which have important ramifications for soil moisture dynamics and water availability for forests.

Total

On average, models project a gradual increase in global precipitation over the 21st century. This is because warmer temperatures drive more evaporation and the increase in water vapor leads to more precipitation. While we expect an increase in the total global amount, this does not mean every individual region will receive more precipitation. Instead, some areas will actually become much drier because we also expect existing regional precipitation profiles to be amplified – a wet-get-wetter and dry-get-drier pattern. In particular, it is very likely we will see an increase in precipitation at high and some mid-latitudes, whereas in the subtropics an overall decrease is more likely than not (Figure 3).

Figure3-Global_Precip_Projections

Note: In the near-term, these projected changes are likely to be fairly small compared to natural variability from circulation patterns like the El Nino-Southern Oscillation, for example. This will also be true for some regions even in the long-term under a low emissions scenario (see extensive hatching in low emissions map, above). This dominance of internal variability at certain scales is an important recurrent theme.

Timing

Shifts in the timing and/or amount of precipitation from one season to another can significantly affect ecosystem processes and the hydrologic cycle. Models suggest that we will see these types of shifts and the largest changes will be at high latitudes. In particular, the contrast between the wet and dry season is likely to increase in most places as temperatures warm. Although, there will be some notable regional exceptions along the equator and poleward edges of the subtropical dry zone, where changes in atmospheric circulation will cause precipitation patterns to shift completely. In both mid- and high-latitude regions the projected increase is larger for winter than summer (Figure 4). However, this increase in average winter precipitation will occur at the same time that increasing temperatures contract the length of the snow season.

Figure4-Seasonal_Precip_Change

Intensity

Warmer temperatures allow the air to hold more water vapor when it is saturated, so when it rains or snows in a warmer world there is simply more water coming down, which will increase the intensity of individual precipitation events. As a result, the probability of heavy precipitation is projected to increase, and the effect will be stronger with increased warming (see Fischer and Knutti 2015, Figure 3 (a), (b), and (c) for maps showing projected change in the probability of heavy precipitation events under different levels of warming across the globe; units are probability ratios, i.e. the ratio of current or future probability to the pre-industrial probability). As a greater fraction of total rainfall comes in the form of these heavy downpours it will also affect soil moisture because more precipitation will be lost as runoff, rather than being absorbed by the soil or going to replenish ground water supplies. For these reasons, intense rainfall events will also increase the likelihood of flash floods.

Uncertainty of Precipitation Projections

We have a high level of confidence in estimates of future temperature because those projections are based on basic physical principles and significant model consensus, but precipitation is much more uncertain. And, as with temperature, the uncertainty grows as we scale down – from estimates of change in the global average to regional projections. There is less agreement among global climate models regarding the direction and magnitude of precipitation change, especially in certain regions.

There are a number of reasons for this uncertainty, including:

  1. The small spatial scales on which precipitation dynamics take place (e.g. cloud microphysics)

The scale of these processes is much smaller than the resolution of global climate models, so those dynamics cannot be simulated from first principles, instead they must be described using parameterization, which introduces some room for error.

  1. A large degree of spatial variability from one location to the next

More data coverage is necessary to accurately capture precipitation trends, in contrast with temperature, which is highly autocorrelated over long spatial scales (i.e. the temperature at one location is closely related to the temperature nearby, so just a few data points can be used to calculate a large area).

  1. The indirect relationship between greenhouse gases and precipitation change

At a very basic level, greenhouse gases influence the climate by changing the amount of heat, which changes atmospheric pressure, which leads to changes in rainfall patterns. Because the influence of greenhouse gases is several steps removed, it is more challenging and complex to simulate how changes in those concentrations will affect precipitation. However, in recent years scientists have improved their understanding of the mechanics underlying projected changes in precipitation and, as a result, we have a much higher degree of confidence in some of the patterns that have emerged from global climate models (e.g. wet-get-wetter, dry-get-drier).

 

Drought: A Closer Look

Changes in the length and severity of drought are important metrics of precipitation change. Generally, drought implies a moisture deficit relative to some previous norm. More specifically there are three types of drought: meteorological, hydrological, and soil moisture (Figure 5). Of these, soil moisture drought is perhaps the most relevant for forests because it is closely tied to plant water availability.

Soil moisture drought is driven by reduced precipitation and/or increased evapotranspiration. In fact, at seasonal or longer time scales, an increase in evapotranspiration (which is indirectly driven by temperature) can lead to more frequent and intense periods of soil moisture drought. Soil characteristics, rooting depth, vegetation, an on-going lack of precipitation, and previous soil moisture and groundwater conditions also play an important role in determining this type of drought risk.

The 2013 Intergovernmental Panel on Climate Change improved previous work on soil moisture projections by using a consistent soil depth across all climate models in their report. The results yielded high confidence in regions where surface soils are expected to dry, but little-to-no confidence in locations where surface soils are projected to be wetter. The models also consistently projected drying conditions in certain regions, specifically the Mediterranean, northeast and southwest South America, southern Africa, and the southwestern U.S. (Figure 6). Although, they disagreed on the direction of change (wetter or drier) in other large regions, such as central Asia or the high northern latitudes. Consequently, there are only a handful of locations where we can currently be confident about how soil moisture will change in the future and those locations are all projected to become drier.

Part two of the global precipitation bulletin will delve into how and where these projected changes in precipitation, drought, and soil moisture are likely to impact forests. It will also include a discussion of the optimal species characteristics, silvicultural decisions, and infrastructure design for minimizing risk from these climate variables and capitalizing on growth potential.

 

Sources:

Collins, M., R. Knutti, J. Arblaster, J.-L. Dufresne, T. Fichefet, P. Friedlingstein, X. Gao, W.J. Gutowski, T. Johns, G. Krinner, M. Shongwe, C. Tebaldi, A.J. Weaver and M. Wehner, 2013: Long-term Climate Change: Projections,

Commitments and Irreversibility. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Fischer, E.M. and Knutti, R. 2015. Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes. Nature Climate Change. Advance online publication: http://dx.doi.org/10.1038/nclimate2617.

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