Abstract
Climate change is expected to cause droughts that are reminiscent of the dust bowl. While all ecosystems are negatively affected by drought to some degree, grasslands are among those most sensitive. Accurate forecasting of which grasslands are most sensitive to drought is imperative to conserving the many economically and aesthetically valuable services these ecosystems provide. This dissertation utilizes both observational and experimental data, coupled with a systematic literature review, to assess the mechanisms of differential grassland sensitivity to drought. Long-term records of precipitation and aboveground net primary production (ANPP), a key metric of ecosystem function, suggest that xeric grasslands are more sensitive to drought than mesic grasslands. I provide further support for this trend using recent observations of the response and recovery of ANPP following a short-term natural drought in six grassland sites. Predicting the ecological consequences of long-term extreme drought, however, requires a mechanistic understanding of drought sensitivity beyond its climatic determinants, especially considering two sites with similar climatic means can differ dramatically in their sensitivity to climate extremes. Plant traits, which act as proxies for more complex physiological functions, can be scaled through the community (i.e. weighted by species relative abundance) to explain and forecast ecosystem responses to environmental change. Few studies, however, measure community-weighted traits in the context of altered water availability. Following a systematic review of >500 manuscripts, I identify clear knowledge gaps in the field of plant traits research and provide guidelines for using plant traits to understand ecosystem sensitivity to PPT. Specifically, plant trait surveys could be improved by a selection of traits that reflect physiological functions directly related to plant water use with traits weighted by species relative abundance. Informed by these guidelines, I test and validate a high throughput method for assessing leaf turgor loss point, a key metric of drought tolerance, using an osmometer. The osmometer method paves the way for rapid community-scale surveys of drought tolerance across functional types. Finally, I employ a coordinated, long-term rainfall exclusion experiment to assess the drought sensitivity of ANPP and community functional composition (i.e. community-weighted trait means and trait diversity) across six grassland sites. Four years of experimental drought (i.e. 66% removal of growing season rainfall) led to reduced ANPP across all six grasslands, with the sensitivity of ANPP being highly correlated with community functional composition. Specifically, functionally diverse plant communities, as well as those with a high abundance of species with conservative resource use strategies, experienced smaller relative reductions in ANPP following drought. Additionally, drought treatments led to increased functional diversity and decreased community scale drought tolerance, largely due to species re-ordering following dominant species mortality. Increased functional diversity may stabilize ecosystem functioning in response to future drought. However, the shifts in community-scale drought strategies may increase ecosystem drought sensitivity, depending on the nature and timing of recurrent drought. The role these two mechanisms will play in determining ecosystem recovery from and response to future drought will be fascinating to assess. Overall, my research demonstrates the importance of plant traits in understanding differential ecosystem sensitivity to extreme drought, especially when the appropriate traits are measured and weighted by species relative abundance.
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Claims 14
Claim · model-written summary
Xeric grasslands were more drought-sensitive than mesic grasslands in long-term production records.
Verbatim
e0“Long-term records of precipitation and aboveground net primary production (ANPP), a key metric of ecosystem function, suggest that xeric grasslands are more sensitive to drought than mesic grasslands”
Claim · model-written summary
Recent observations at six grassland sites supported greater xeric grassland sensitivity during natural drought.
Verbatim
e0“I provide further support for this trend using recent observations of the response and recovery of ANPP following a short-term natural drought in six grassland sites”
Claim · model-written summary
The literature review identified knowledge gaps in plant trait research.
Verbatim
e0“Following a systematic review of >500 manuscripts, I identify clear knowledge gaps in the field of plant traits research”
Claim · model-written summary
The authors recommend water-use traits weighted by relative species abundance for surveys.
Verbatim
e0“plant trait surveys could be improved by a selection of traits that reflect physiological functions directly related to plant water use with traits weighted by species relative abundance”
Claim · model-written summary
An osmometer method was validated to assess leaf turgor loss point at high throughput.
Not reportedComparatorValue
Verbatim
e0“I test and validate a high throughput method for assessing leaf turgor loss point, a key metric of drought tolerance, using an osmometer”
Claim · model-written summary
Four years of 66% growing-season rainfall removal reduced ANPP in all six grasslands.
- Taxon
- grasslandse0
- Treatment
- 66% removal of growing season rainfalle0
- Outcome
- ANPPe0
- Direction
- reducede0
- Value
- 66%e0
- Qualifier
- Four yearse0
Not reportedComparator
Verbatim
e0“Four years of experimental drought (i.e. 66% removal of growing season rainfall) led to reduced ANPP across all six grasslands”
Claim · model-written summary
Drought sensitivity of ANPP was highly correlated with community functional composition.
- Taxon
- grasslandst0
- Material
- community functional compositione0
- Outcome
- sensitivity of ANPPe0
- Qualifier
- community functional compositione0
Not reportedDirection
Verbatim
e0“the sensitivity of ANPP being highly correlated with community functional composition”
t0“grasslands”
Claim · model-written summary
Functionally diverse communities had smaller relative ANPP reductions after drought.
Verbatim
e0“functionally diverse plant communities, as well as those with a high abundance of species with conservative resource use strategies, experienced smaller relative reductions in ANPP following drought”
Claim · model-written summary
Communities rich in conservative resource-use species had smaller ANPP reductions after drought.
Verbatim
e0“functionally diverse plant communities, as well as those with a high abundance of species with conservative resource use strategies, experienced smaller relative reductions in ANPP following drought”
Claim · model-written summary
Drought increased functional diversity.
Not reportedComparator
Verbatim
e0“drought treatments led to increased functional diversity and decreased community scale drought tolerance”
Claim · model-written summary
Drought decreased community-scale drought tolerance.
- Taxon
- communitye0
- Treatment
- drought treatmentse0
- Outcome
- community scale drought tolerancee0
- Direction
- decreasede0
Not reportedComparator
Verbatim
e0“drought treatments led to increased functional diversity and decreased community scale drought tolerance”
Claim · model-written summary
Dominant species mortality and subsequent re-ordering largely explained community trait changes.
Verbatim
e0“largely due to species re-ordering following dominant species mortality”
Claim · model-written summary
Increased functional diversity may stabilize ecosystem functioning under future drought.
Verbatim
e0“Increased functional diversity may stabilize ecosystem functioning in response to future drought”
Claim · model-written summary
Shifts in community drought strategies may increase future ecosystem drought sensitivity.
Verbatim
e0“the shifts in community-scale drought strategies may increase ecosystem drought sensitivity, depending on the nature and timing of recurrent drought”