Effects of heat and drought on carbon and water dynamics in a regenerating semi-arid pine forest: a combined experimental and modeling approach

Abstract. Increasing summer temperatures and a reduction in precipitation will enhance drought stress in Mediterranean and semi-arid ecosystems. Predicting the net effects on forests' carbon and water balance will depend on our ability to disentangle the sensitivity of component fluxes responding to increasing soil and atmospheric drought. Here we studied carbon and water dynamics in a semi-arid regenerating ponderosa pine forest using field observations and process based modeling. Field observations of two summer dry seasons were used to calibrate a soil-plant-atmosphere (SPA) model. In addition, the ecosystem's response to reduced soil drought was quantified based on a field watering experiment and evaluated with the model. Further, the SPA model was used to estimate the relative effects of increasing soil and atmospheric drought over time, by simulating temperature and precipitation scenarios for 2040 and 2080. The seasonality and drought response of ecosystem fluxes was well captured by the calibrated SPA model. Dramatic increases in summer water availability during seasonal drought had a small effect on pine physiology in both the watering experiment and the model. This clearly demonstrates that atmospheric drought induced a strong limitation on carbon uptake in young ponderosa pine due to tight regulation of stomatal conductance. Moreover, simulations showed that net ecosystem exchange (NEE) and gross primary productivity (GPP) were about three times more affected by summer heat and increased evaporative demand than by reductions in summer precipitation. Annual NEE decreased by 38% in response to extreme summer conditions as predicted to occur in 2080 (June–August: +4.5 °C), because of a strong decline in GPP (−17%) while heterotrophic respiration was relatively unaffected (−1%). Considering warming trends across all seasons (September–May: +3 °C and June–August: +4.5 °C), the negative drought effects were largely compensated by an earlier initiation of favorable growing conditions and bud break, enhancing early season GPP and needle biomass. An adverse effect, triggered by changes in early season allocation patterns, was the decline of wood and root biomass. This imbalance may increase water stress over the long-term to a threshold at which ponderosa pine may not survive, and highlights the need for an integrated process understanding of the combined effects of trends and extremes.

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Effects of heat and drought on carbon and water dynamics in a regenerating semi-arid pine forest: a combined experimental and modeling approach

Biogeosciences ◽

10.5194/bg-11-4139-2014 ◽

2014 ◽

Vol 11 (15) ◽

pp. 4139-4156 ◽

Cited By ~ 11

Author(s):

N. K. Ruehr ◽

B. E. Law ◽

D. Quandt ◽

M. Williams

Keyword(s):

Ponderosa Pine ◽

Model Simulation ◽

Dominant Role ◽

Summer Precipitation ◽

Ecosystem Processes ◽

Pine Forest ◽

Water Dynamics ◽

Early Season ◽

Temperature And Precipitation ◽

Semi Arid

Abstract. Predicting the net effects on the carbon and water balance of semi-arid forests under future conditions depends on ecosystem processes responding to changes in soil and atmospheric drought. Here we apply a combination of field observations and soil–plant–atmosphere modeling (SPA) to study carbon and water dynamics in a regenerating ponderosa pine forest. The effects of soil and atmospheric drought were quantified based on a field irrigation experiment combined with model simulations. To assess future effects of intensifying drought on ecosystem processes, the SPA model was run using temperature and precipitation scenarios for 2040 and 2080. Experimentally increased summer water availability clearly affected tree hydraulics and enhanced C uptake in both the observations and the model. Simulation results showed that irrigation was sufficient to eliminate soil water limitation and maintaining transpiration rates, but gross primary productivity (GPP) continued to decrease. Observations of stomatal conductance indicated a dominant role of vapor pressure deficit (VPD) in limiting C uptake. This was confirmed by running the simulation under reduced atmospheric drought (VPD of 1 kPa), which largely maintained GPP rates at pre-drought conditions. The importance of VPD as a dominant driver was underlined by simulations of extreme summer conditions. We found GPP to be affected more by summer temperatures and VPD as predicted for 2080 (−17%) than by reductions in summer precipitation (−9%). Because heterotrophic respiration responded less to heat (−1%) than to reductions in precipitation (−10%), net ecosystem C uptake declined strongest under hotter (−38%) compared to drier summer conditions (−8%). Considering warming trends across all seasons (September–May: +3 °C and June–August: +4.5 °C), the negative drought effects were largely compensated by an earlier initiation of favorable growing conditions and bud break, enhancing early season GPP and needle biomass. An adverse effect, triggered by changes in early season allocation patterns, was the decline of wood and root biomass. This imbalance may increase water stress over the long term to a threshold at which ponderosa pine may not survive, and highlights the need for an integrated process understanding of the combined effects of trends and extremes.

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Environmental Control on Transpiration: A Case Study of a Desert Ecosystem in Northwest China

Water ◽

10.3390/w12041211 ◽

2020 ◽

Vol 12 (4) ◽

pp. 1211 ◽

Cited By ~ 1

Author(s):

Shiqin Xu ◽

Zhongbo Yu

Keyword(s):

Sap Flow ◽

Environmental Control ◽

Arid Ecosystems ◽

Desert Ecosystem ◽

Time Lags ◽

Evaporative Demand ◽

Threshold Control ◽

Stand Transpiration ◽

Semi Arid

Arid and semi-arid ecosystems represent a crucial but poorly understood component of the global water cycle. Taking a desert ecosystem as a case study, we measured sap flow in three dominant shrub species and concurrent environmental variables over two mean growing seasons. Commercially available gauges (Flow32 meters) based on the constant power stem heat balance (SHB) method were used. Stem-level sap flow rates were scaled up to stand level to estimate stand transpiration using the species-specific frequency distribution of stem diameter. We found that variations in stand transpiration were closely related to changes in solar radiation (Rs), air temperature (T), and vapor pressure deficit (VPD) at the hourly scale. Three factors together explained 84% and 77% variations in hourly stand transpiration in 2014 and 2015, respectively, with Rs being the primary driving force. We observed a threshold control of VPD (~2 kPa) on stand transpiration in two-year study periods, suggesting a strong stomatal regulation of transpiration under high evaporative demand conditions. Clockwise hysteresis loops between diurnal transpiration and T and VPD were observed and exhibited seasonal variations. Both the time lags and refill and release of stem water storage from nocturnal sap flow were possible causes for the hysteresis. These findings improve the understanding of environmental control on water flux of the arid and semi-arid ecosystems and have important implications for diurnal hydrology modelling.

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The Future of Semi-Arid Regions: A Weak Fabric Unravels

Climate ◽

10.3390/cli8030043 ◽

2020 ◽

Vol 8 (3) ◽

pp. 43 ◽

Cited By ~ 1

Author(s):

Robert J Scholes

Keyword(s):

Land Surface ◽

Arid Regions ◽

Arid Ecosystems ◽

Plant Production ◽

Climate Trends ◽

Evaporative Demand ◽

Social Ecological ◽

Global Atmospheric Circulation ◽

Global Mean ◽

Semi Arid

The regions of the world where average precipitation is between one fifth and half of the potential plant water demand are termed ‘semi-arid’. They make up 15.2% of the global land surface, and the approximately 1.1 billion people who live there are among the world’s poorest. The inter-annual variability of rainfall in semi-arid regions is exceptionally high, due to intrinsic features of the global atmospheric circulation. The observed and projected climate trends for most semi-arid regions indicate warming at rates above the global mean rate over land, increasing evaporative demand, and reduced and more variable rainfall. Historically, the ecosystems and people coped with the challenges of semi-arid climates using a range of strategies that are now less viable. Semi-arid ecosystems are by definition water limited, generally only suitable for extensive pastoralism and opportunistic cropping, unless irrigation supplementation is available. The characteristics of dryland plant production in semi-arid ecosystems, as they interact with climate change and human systems, provide a conceptual framework for why land degradation is so conspicuous in semi-arid regions. The coupled social-ecological failures are contagious, both within the landscape and at regional and global scales. Thus, semi-arid lands are a likely flashpoint for Earth system changes in the 21st century.

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