Temporal and spatial variation in how vegetation alters the soil moisture response to climate manipulation
Pierre Liancourt1, Anarmaa Sharkhuu2, Lkhagva Ariuntsetseg3,4, Bazartseren Boldgiv4, Brent R. Helliker1, Alain F. Plante2, Peter S. Petraitis1, Brenda B. Casper1
1Department of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018, USA
2Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104-6316, USA
3Department of Botany, University of Wyoming, Laramie, WY 82071, USA
4Department of Ecology, National University of Mongolia, Ulaanbaatar 210646, Mongolia
Aims Soil water balance, key for ecosystem processes, is determined by multiple factors, including precipitation, temperature, slope and vegetation. How these interact with climate change and the relevant time scale of the interactions are poorly understood. We investigated the interplay among climate change, local abiotic conditions (slope) and biotic factors (vegetation or not) on soil water balance in a steppe grassland on the south exposure of a northern Mongolia valley. Methods We manipulated climate using passive warming open top chambers (OTCs), similar to those used in other systems. Areas of bare ground were created inside the OTCs to explicitly evaluate the effect of vegetation on soil moisture and its dynamics. The experiment was set up at two topographic locations, a steep upper slope and a gentle lower slope. Volumetric soil moisture content was measured throughout each growing season in a small area where vegetation had been removed and where it was left intact both inside OTCs and in control plots. To account for OTCs intercepting some precipitation, we also examined treatment effects on soil drying rates. Results Vegetation and climate manipulation reduced soil moisture more strongly in the wetter of the two years and just after rains. Similarly, treatment effects were more pronounced on the wetter lower slope. Averaged across the growing season, climate manipulation did not affect soil water differentially in vegetated and unvegetated areas, but seasonal variation in the strengths of treatment effects and interactions between climate and vegetation reflected plant developmental phenology. Soil drying rate was faster on the drier upper slope or with vegetation and faster overall in the drier year. In the dry year 2010, soil drying was slower in OTCs, likely because of wind interception. Conclusions Monthly or seasonal averages of soil moisture would have provided poor information about the interplay among factors affecting soil water balance in this system. Our study illustrates the utility of experimentally examining the interaction between biotic and abiotic factors and considering relevant time scales when investigating the complex effects of climate change on ecosystem processes.
Keyword: OTC,Removal experiment,Slope,Steppe grassland,Time scale,Vegetation,Volumetric soil moisture content
Soil moisture is an essential driver of most ecosystem processes. It influences the nitrogen cycle, via microbial activity (Stark and Firestone 1995; Barnard et al. 2006) and carbon storage and cycling (Tate and Ross 1997; Flanagan and Johnson 2005; Garten et al. 2009). Soil moisture is an essential resource affecting productivity and hence plant and microbial community composition and diversity (e.g., Clark et al. 2009; Castro et al. 2010). Limited soil water selects for aridity tolerance and drives many biotic interactions (Novoplansky and Goldberg 2001; Pugnaire et al. 2004; Michalet 2006). Finally, soil moisture has consequences for climate itself, by affecting dewpoint temperature and surface thermodynamics with feedbacks to precipitation (Myoung and Nielsen-Gammon 2010). Climate change is projected to involve not only an increase in average temperatures (2.5°C over this century) but also altered precipitation patterns (Easterling et al. 2000; IPCC 2007). The less frequent but stronger precipitation events forecast for many regions by most models should dramatically affect ecosystem processes and plant community structure and composition (Castro et al. 2010; Dermody et al. 2007; Knapp et al. 2008; Weltzin et al. 2003; Yang et al. 2011). However, the vulnerability and responses of ecosystems to climate change drivers appear contingent on the local conditions and dominant vegetation (Grime et al. 2000; Scholze et al. 2006; Liancourt et al. 2009); responses may differ both in direction and intensity even at small spatial scales (Harte et al. 1995; Klein et al. 2004). Ecosystem water balance is determined by inputs in the form of precipitation and by outputs through evaporation, transpiration, leaching, and surface runoff. Local inputs and outputs vary substantially both temporally and spatially due to storms, wind, slope, aspect, and vegetation cover and phenology, but little is understood about the interactions of these factors, especially in the context of climate warming (Flanagan and Johnson 2005; Dermody et al. 2007; Engel et al. 2009). The effect of vegetation on ecosystem water balance, in particular, is complex and may either mitigate or exacerbate the effects of climate change. On one hand, vegetation can increase soil drying through transpiration, but on the other, it can bring water up from deeper soil layers via hydraulic lift (Horton and Hart 1998), lessen runoff on steep slopes (Dunne et al. 1991), and reduce soil evaporation due to shading, as happens in European calcareous (Liancourt et al. 2005) and subalpine grasslands (Gross et al. 2008). Zavaleta et al. (2003) have reported that at the end of the growing season in annual-dominated Mediterranean grasslands, vegetation can offset the effect of climate warming on soil moisture. We study the interaction between vegetation cover and climate manipulation in the Eurasian steppe, the largest grassland in the world and one that provides crucial services for local herder populations. Specifically, our experiment is set in northern Mongolia, where a substantial rise in temperatures has been observed over the last 40 years (+1.7°C in the Hцvsgцl area, Namkhaijanstan 2006). The intensification of grazing observed in the area, because of the decrease in nomadic habits of the local people, potentially makes the system more vulnerable to climate change. The danger is that stress from overgrazing and altered climate may lead semi-arid grasslands to shift catastrophically to desert. To our knowledge, no empirical data based on experimental manipulation of climate are available for the cold and dry northern Mongolian steppe, although some recent studies set in Inner Mongolia, China have considered soil moisture together with climate manipulation (Niu et al. 2008; Liu et al. 2009; Xia et al. 2010). Our main goal is to evaluate how experimental summer climate manipulation affects soil moisture while taking into account spatial heterogeneity due to topography and vegetation cover and temporal variation in precipitation. We manipulated climate using passive warming open top chambers (OTC), similar to those used in other systems (Marion et al. 1997; Klein et al. 2004) and created areas of bare ground inside the OTCs to evaluate explicitly the effect of vegetation on soil moisture and its dynamics. Because passive warming chambers alter precipitation and air advection and convection in addition to temperature (Marion et al. 1997), we recognize them as climate manipulators, not solely warming devices (see Dabros et al. 2010). As response variables, we examine standing soil moisture and the rate of soil drying, an approach that compensates for the interception of some precipitation by the climate manipulators. Soil moisture loss is of particular interest because it serves as a key indicator of how abiotic and biotic factors affect the distribution of water within the ecosystem, providing basic information for understanding how climate change will alter the ecology of the region. Due to the elevated air temperatures achieved, we hypothesized that (i) climate manipulation would significantly increase soil drying rates after precipitation events. Because we expected transpiration to outweigh other effects of plants on soil water, we hypothesized that (ii) the presence of vegetation would also increase the drying rate. We had no a priori hypothesis regarding how climate manipulation and vegetation might interact with each other to affect soil drying or how such an interaction might depend on incipient moisture levels. Our daily measures of soil moisture, however, allowed us to assess how interactions among experimental factors varied temporally, both within growing seasons and between 2009 and the drier year of 2010. Finally, we anticipated that (iii) climate manipulation and vegetation would each reduce soil moisture more strongly in locations where soils are inherently wetter and plant productivity is greater, which in our system is lower on the slope.
Material and Methods
The experiment was conducted during two consecutive summers, from June to August 2009 and 2010, on a south-facing slope of steppe grasslands in the Dalbay river valley, on the eastern shore of Lake Hцvsgцl (51° 01.405′ N, 100° 45.600′ E; ranging from 1670 m to 1800 m in elevation), northern Mongolia. The average annual air temperature is −4.5°C, with the coldest average monthly temperature of −21°C (January) and warmest of 12°C (July) (Nandintsetseg et al. 2007). The average annual precipitation measured for the last 40 years, averaged for three weather stations south (Hatgal), west (Renchinlhumbe) and north (Hankh) of Lake Hцvsgцl, was 265 mm (Namkhaijanstan 2006). In 2008, we installed a year-round meteorological station approximately a third of the way up the south-facing slope in order to measure precipitation, temperature, wind speed and insolation. Overall, Jan.–Dec. 2009 was wetter (270 mm) than Jan.–Dec. 2010 (246 mm). Summer rainfall (June to Aug.) was also greater in 2009 (201 mm) than in 2010 (178 mm), when most of it fell between Aug. 11 and 17; 42 mm occurred in a single storm on Aug. 12, 2010. Permafrost is not present on the south-facing slope but is found in a nearby riparian zone and on northfacing slopes under the taiga forest, which consists of Larix sibirica and Pinus sibirica. The steppe vegetation is a mixture of sedges (e.g., Carex pediformis, Carex dichroa), grasses (e.g., Festuca lenensis, Koeleria macrantha, Agropyron cristatum, Helictotrichon schellianum, Stipa krylovii) and short forbs (e.g., Aster alpinus, Potentilla acaulis, Artemisia commutata, Thymus gobicus). The vegetation differs both in composition and in total cover between the upper and lower slope, the two locations of experimental climate manipulation chambers. In 2009, the lower slope (elevation 1670 ma.s.l.) had a total vascular plant cover of 78% and the upper slope (elevation 1800 ma. s.l.) 64%. Carex pediformis is the most common species on the lower slope and Potentilla acaulis on the upper. Lichen cover is also greater on the upper slope. The soil is sandy loam texture, of alluvial origin, and classified as a non-carbonated Dark Kastanozem (Aridic Boroll or Typic Ustolls). Bedrock consists of Cenozoic volcanic deposit (Batkhishig 2006).
The experiment and statistical analysis
Fifteen 9Ч9 m experimental blocks were established on the south-facing slope and fenced to exclude grazing livestock. Eight were located lower on the slope (gentle to flat slope) and seven on the upper part of the slope (incline ~20°). Each block enclosed plots in which summer climate is manipulated in a factorial design with precipitation addition treatments on the upper slope and exposure to grazing on the lower. Here we report data and analyses from control plots without added precipitation or grazing. The climate manipulation was accomplished with hexagonal passive, open-top chambers (OTC) measuring 1.0 m wide at the top, 1.5 m at the bottom and 40 cm tall, and made of Sun-Lite® HP fiberglass glazing (Marion et al. 1997). Each of the six side panels was mounted on a clear Lexan frame and attached to the next with hinges. An area of the same footprint and dimensions was delineated in each block as a control. Both OTCs and controls were placed in the same locations both summers. Detailed monitoring of the climate manipulation performance of the OTCs found that over the course of the 2009 and 2010 summer seasons, mean daytime air temperatures at 15 cm height were 1.5°C warmer in the chambers, maximum daytime air temperatures were 3.3°C warmer, and mean nighttime air temperatures were 0.3°C cooler (unpublished data). Differences in soil temperatures at 10 cm depth were not found to be statistically different between control plots and OTCs, but volumetric soil moisture content was 1.6 to 4.1 percentage points less in OTCs compared to control plots (unpublished data). As a check, photosynthetically active radiation (PAR) measurements were made horizontal to the soil, at the ground surface either under the vegetation or in the non-vegetated plots between 11:00 a.m. and 12:00 p.m. (solar noon) on a cloudless day. For the control treatment, PAR was 1140±65 mmol m−2 s−1 in the vegetated areas and 1630±26 mmol m−2 s−1 in the unvegetated areas. In the OTCs, PAR was 963± 88 mmol m−2 s−1 in the vegetated areas, and 1452± 75 mmol m−2 s−1 in the unvegetated areas. Measures of PAR differed significantly between vegetated and nonvegetated areas (P<0.05; OTCs and controls combined).
The results suggest that vegetation, and to a less extent OTCs, intercept light and could therefore decrease evaporation and drying rates. In order to understand the effect of climate treatment and its interaction with vegetation (i.e., transpirational water loss) on volumetric soil moisture content (θ), we created a triangular area of 0.55 m2 of bare soil within each plot by removing the aboveground vegetation in one corner formed by two sides of the chamber and in an area of the same dimension in the control plot. Two sides of the hexagon were oriented N-S. We removed the vegetation from the west-side triangle of half the chambers or control plots selected at random, and from the east-side triangle in the other half of the chamber or control plots. We used a serrated knife (20 cm long) to remove vegetation and trench around the area in order to exclude roots from surrounding vegetation, and we kept these areas vegetation-free by weekly hand weeding. The 1x0.5 marea in the center of the hexagon was left undisturbed for annual vegetation censuses, and so was not part of this study. Preliminary measurements of soil moisture at the onset of the vegetation removal showed no significant differences between the three areas within each plot (data not shown). Volumetric soil moisture content (θ) of the surface soil was measured between 10:00 a.m. and 11:00 a.m. on almost every rainless day in 2009 and daily in 2010, regardless of precipitation events. Two measurements were taken randomly and pooled within each triangular area where the vegetation was removed and in the corresponding vegetated area on the opposite side of the hexagon, using a portable probe (WET-2 sensor, Delta-T Devices Ltd) at a depth of approximately 6 cm. Volumetric soil moisture content data was analyzed for each year separately using univariate repeatedmeasures ANOVA. The fixed factor of slope (location), climate, and vegetation were fully crossed, and time was the repeated factor. Block was a random factor nested within slope; the analyses included interactions between block and climate, vegetation and time. We also performed Tukey’s tests to evaluate the fixed effects and their interactions on θ on particular dates. Because the chambers themselves reduce the amount of precipitation reaching the plots (unpublished data), the rate of soil moisture loss is likely to be a stronger indicator of how climate alteration within OTCs affected soil moisture. Thus, we exam< ined how slope, climate, vegetation and their interactions affected the dynamics of soil moisture after each rainfall event, calculated as the soil drying rate. For each experimental and control plot and for each year, we modeled variation in θ over rainless days following each rainfall event as an exponential decay:
Volumetric soil moisture content (θ)
Both slope and climate treatment had consistent significant effects on θ in both years (Table 1), where θ was 42% drier at the upper slope location than at the lower slope in 2009 and 40% drier in 2010 (Fig. 1).
Soil in the OTC was 17% drier than in the control in 2009 and 33% drier in 2010. Vegetation, as a main effect, was significant in 2009 (7% drier in vegetated areas) but not in 2010, when vegetation reduced θ at the lower slope only (7% drier in vegetated area; significant slope X vegetation interaction; Table 1, Fig. 1b). The climate X vegetation interaction and the climate X vegetation X slope interaction were not significant either year, indicating that over the whole summer, the OTCs did not differentially affect moisture loss from vegetated vs. bare soil at either slope location. The interaction climate Ч slope was significant just in 2010 (Table 1), when the OTC reduced soil moisture by 35% at the lower slope and 30% at the upper slope.
Slope, vegetation and climate treatment showed complex interactions with time in affecting θ both years. Temporal soil moisture dynamics were, not surprisingly, driven by the timing of rainfall events and the duration of intervening rainless periods but were also affected by our experimental manipulations (Fig. 2 and 3). The effects of climate treatment and vegetation on θ were smallest during rainless periods, and this results in significant climate X time and vegetation X time interactions each year. The significant slope X time interaction each year reflects a faster change in θ values during rainfall events and intervening rainless periods at the upper slope (see also Δ soil moisture below).
Soil drying rate (Δ soil moisture)
The soil drying rate was overall greater on the upper slope both years, but effects of climate manipulation and vegetation were not consistent between years. The soil drying rate did not differ between climate treatments in 2009, but it was slower in the OTCs than in control plots in 2010. Soil drying rate increased with vegetation in 2009 but showed no effect of vegetation in 2010 (Table 2, Fig. 5). The soil drying rate varied significantly among the time intervals in each of the two years, but no seasonal trends or consistent patterns were identified. In 2009, there were significant interactions for vegetation X time and climate X vegetation X time indicating variation in how vegetation and its interaction with climate affected the drying rate of soil over time (Table 2).
Our experiment produced several key results. First, if there is an effect of vegetation on soil moisture (2009 only), then it is negative. This result is consistent with our expectation that moisture loss through transpiration or evaporation directly from foliage must more than balance any positive effect of vegetation on soil moisture relative to controls, as might occur from shading or hydraulic lift (Liancourt et al. 2005; Pugnaire et al. 2004). While our climate treatment also reduced soil moisture, on average over the whole season, combined effects of climate treatment and vegetation (2009 only) are additive; there is no interaction between the two. Third, OTCs (2010 only) and vegetation had opposite effects on the soil drying rate, with OTCs reducing it and vegetation increasing it (2009 only). Fourth, important experimental effects differed between years, such as the effect of the vegetation or whether slope interacted significantly with vegetation or climate manipulation. Finally, within a growing season, interactions among slope, climate treatment, and vegetation changed in complex ways over time to affect soil moisture. Identifying the relevant time scale when climate change, especially warming, interacts with other environmental factors is of primary importance for understanding its impact on ecosystem processes (Shaver et al. 2000). We identified two such time scales: (1) yearly differences, probably due primarily to differences in precipitation between the two summers, and (2) seasonal variation, which seems to reflect both the developmental phenology of the plants (see Nord and Lynch 2009) and variation in baseline soil moisture attributable to the timing and amount of precipitation events (Noy-Meir 1973). These within-season changes in interactions among experimental treatments would have been obscured if soil moisture had only been evaluated as monthly or seasonal averages, rather than daily measurements. Climate treatment and vegetation did interact to affect soil moisture at some time points during both growing seasons. In general, vegetation further decreased soil moisture in OTCs during the middle of the season (July) but mitigated or even counterbalanced OTC effects toward the end of the season.
We acknowledge Philippe Choler for discussion and valuable comments on the early version of the manuscript, A. Mac-Rear Crerar and D. Song for comments and help in the field, R. Weeks and B. Nomin-Erdene for help for data collection. We are thankful to S. Undrakhbold and all the camp staff, the American and Mongolian undergrads who spent their summer in Dalbay. This research was done as part of the PIRE-Mongolia project and was supported by the U.S. National Science Foundation under Grant No. 0729786.
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