The climatic water balance in an ecological context

Transcription

1 Western Mountain Initiative The climatic water balance in an ecological context Nathan L. Stephenson U.S. Department of the Interior U.S. Geological Survey

2 Thesis: Consideration of the climatic water balance improves our ability to understand and forecast the effects of climatic changes on the biological world. (1) The superiority of the climatic water balance in ecological studies. (2) In a changing climate, increasing water availability will NOT counteract the effects of increasing evaporative demand. And why soil moisture may be less important than you think! (3) Don t forget AET!

3 Thesis: Consideration of the climatic water balance improves our ability to understand and forecast the effects of climatic changes on the biological world. (1) The superiority of the climatic water balance in ecological studies. (2) In a changing climate, increasing water availability will NOT counteract the effects of increasing evaporative demand. And why soil moisture may be less important than you think! (3) Don t forget AET!

4 Mean annual temperature (C) Mean annual precipitation (cm) Whittaker 1975 Traditional view: At broad scales, vegetation type is a function of some measure of annual energy supply, annual water supply, or their ratios or differences.

5 Mean annual temperature (C) Temperature Potential evapotranspiration (PET) Net radiation Growing degree-days Precipitation (P) P/PET (or PET/P) PET-P (or P-PET) Mean annual precipitation (cm) Whittaker 1975 Traditional view: At broad scales, vegetation type is a function of some measure of annual energy supply, annual water supply, or their ratios or differences.

6 The Big Problem: The traditional view does not consider the seasonal interactions of energy and water. E.g., with nearly identical mean annual temperature and precipitation: 18 Deciduous broadleaved forest Erie, Pennsylvania 18 Evergreen needleleaved forest Portland, Oregon Precipitation (mm) Precipitation Temperature Precipitation (mm) Temperature (C) Precipitation Temperature Temperature (C) J F M A M J J A S O N D Month J F M A M J J A S O N D Month

7 (1) By explicitly incorporating the seasonal interactions of energy and water, the climatic water balance is biologically more meaningful than traditional climatic metrics. mm Water Water supply (rain plus snow melt water) Potential evapotranspiration 1J F 2 M 3 4A M 5 J 6 7J A 8 S 9 1 O 11 N 12 D Month

8 Actual evapotranspiration (AET) reflects the simultaneous availability of biologically usable energy and water, and is therefore an index of site potential for productivity. mm Water Water supply (rain plus snow melt water) S AET Potential evapotranspiration D 1J F 2 M 3 4A M 5 J 6 7J A 8 S 9 1 O 11 N 12 D Month

9 Climatic water deficit (D) is evaporative demand that is not met by available water, and is thus an index of potential effects of drought stress on plants. (Don t confuse with soil water deficit!) mm Water Water supply (rain plus snow melt water) S AET Potential evapotranspiration D 1J F 2 M 3 4A M 5 J 6 7J A 8 S 9 1 O 11 N 12 D Month

10 Surplus is an index of biologically unusable water excess water that leaves a site, through runoff or subsurface flow, without being evaporated or transpired. 15 mm Water 1 5 S AET D 1J F 2 M 3 4A M 5 J 6 7J A 8 S 9 1 O 11 N 12 D Month

11 (2) The climatic water balance (AET and D) is better correlated with the distribution of vegetation types. Example: North American coniferous and deciduous forest distributions. Mean annual precipitation (mm) Deciduous forest Coniferous forest Mean annual temperature (C)

12 (2) The climatic water balance (AET and D) is better correlated with the distribution of vegetation types. Example: North American coniferous and deciduous forest distributions. Mean annual precipitation (mm) Deciduous forest Coniferous forest Annual actual evapotranspiration (mm) Deciduous forest Coniferous forest Mean annual temperature (C) Annual climatic water deficit (mm) Stephenson 1998, J. Biogeogr.

13 (3) The climatic water balance exhibits these properties across more than ten orders of magnitude of spatial scale. North America Stephenson 199, Am. Nat. Stephenson 1998, J. Biogeogr.

14 (3) The climatic water balance exhibits these properties across more than ten orders of magnitude of spatial scale. Sequoia Nat l Park Stephenson 1998, J. Biogeogr.

15 Summary -- the superiority of the climatic water balance in ecological studies: More biologically meaningful. Better correlated with biological processes and vegetation and species distributions. Exhibits these properties across >1 orders of magnitude of spatial scale.

16 Thesis: Consideration of the climatic water balance improves our ability to understand and forecast the effects of climatic changes on the biological world. (1) The superiority of the climatic water balance in ecological studies. (2) In a changing climate, increasing water availability will NOT counteract the effects of increasing evaporative demand. And why soil moisture may be less important than you think! (3) Don t forget AET!

17 Mean annual temperature (C) At continental and global scales, most authors have treated metrics of energy and water supplies as orthogonal axes. That is, they recognize that water does NOT counteract the effects of energy. Mean annual precipitation (cm) Whittaker 1975

18 Elevation Yet at local to regional scales, some of those same authors additively combine metrics of energy and water supplies into a single moisture scalar, thus assuming that the effects of water and energy counteract one another. E.g., Whittaker s moisture scalar combines metrics related to evaporative demand (slope aspect and exposure) with metrics related to water availability (slope position and proximity to water). Topographic moisture gradient Whittaker & Niering 1965, Ecology

19 For understanding and forecasting plant distributions, we care less about soil moisture than the dynamics that gave us that soil moisture. Normal conditions Plant response Wet Dry Dynamics of evap. demand and water supply Water level

20 For understanding and forecasting plant distributions, we care less about soil moisture than the dynamics that gave us that soil moisture. Drought (low precipitation) Wet Dry

21 For understanding and forecasting plant distributions, we care less about soil moisture than the dynamics that gave us that soil moisture. Add water Wet Dry

22 For understanding and forecasting plant distributions, we care less about soil moisture than the dynamics that gave us that soil moisture. Normal precip, but high evaporative demand Wet Dry

23 For understanding and forecasting plant distributions, we care less about soil moisture than the dynamics that gave us that soil moisture. Add water Wet Dry

24 Consider these scenarios in terms of annual AET and D. The colored dots represent species climatic niches; dotted diagonal are isolines of constant PET. 2 Annual AET (mm) Annual Deficit (mm)

25 Because PET = AET + D, drought (low precipitation) can only result in increases in D that are precisely offset by identical declines in AET. 2 Annual AET (mm) Annual Deficit (mm)

26 Adding the right amount of water can put the site back to exactly where it started in climate space. 2 Annual AET (mm) Annual Deficit (mm)

27 In contrast, because PET = AET + D, increasing PET must increase AET, D, or both. That is, the effects of changing PET are nearly orthogonal those of changing water availability 2 Annual AET (mm) Annual Deficit (mm)

28 so that adding water CANNOT put the site back to exactly where it started in climate space... 2 Annual AET (mm) Annual Deficit (mm)

29 and big enough increases in PET and water availability will move the site to another species climatic niche, even though soil moisture hasn t changed. 2 Annual AET (mm) Annual Deficit (mm)

30 What are some implications for the effects of climatic changes on mountain ecosystems? Example: Unlike temperature changes, precipitation changes should not be expected to cause coordinated directional shifts in species elevations.

31 PET declines sharply with increasing elevation. Evaporative rate (cc/day) Santa Catalina Mtns, AZ, summer of 1911 South-facing slopes North-facing slopes Elevation (m) Data from Shreve 1915 A spherical porous cup atmometer Livingston 1935, Ecology

32 PET declines sharply with increasing elevation. Annual AET (mm) 2 1 High Elevation Low Annual Deficit (mm)

33 Thus, temperature-induced increases in PET will alter a site s water balance to support species currently found at lower elevations. This is why we expect temperature-induced upslope migrations. 2 Annual AET (mm) Annual Deficit (mm)

34 At a given elevation, local water availability depends on things like soil depth, proximity to water, etc. High 2 (e.g. deep soils) Local water availability 1 Low Annual AET (mm) (e.g. shallow soils) Annual Deficit (mm)

35 Declining precipitation will alter a site s water balance to support species currently found at sites with lower water availability but at the same elevation 2 Annual AET (mm) Annual Deficit (mm)

36 and increasing precipitation will alter a site s water balance to support species currently found at sites with higher water availability but at the same elevation. This is why we DON T expect precip-induced elevational changes. 2 Annual AET (mm) Annual Deficit (mm)

37 These expectations are empirically supported by observations of species distributions along natural climatic gradients. Stephenson 1998, J. Biogeogr., Fites-Kaufman et al. 27, TVC

38 Thesis: Consideration of the climatic water balance improves our ability to understand and forecast the effects of climatic changes on the biological world. (1) The superiority of the climatic water balance in ecological studies. (2) In a changing climate, increasing water availability will NOT counteract the effects of increasing evaporative demand. And why soil moisture may be less important than you think! (3) Don t forget AET!

39 Parting thoughts: Moisture availability is a slippery term of limited value, and can only be understood in two dimensions. E.g., does an increase in both AET and D yield a wetter or a drier environment? Arguably, both! In many (most?) cases it s better to drop the concept of moisture availability in favor of explicit consideration of AET and D.

40 Thank you for your attention!