Grassland ecosystems comprise a large proportion of the terrestrial biosphere, and are composed of 11,000 different types of grass species. Due to the increasing frequency and severity of global droughts and climate change, there will be implications for plant productivity, geographic species distribution, and widespread plant mortality, especially for the grass species of grassland ecosystems. Disturbances such as distribution, productivity, and productivity can change species composition of grassland ecosystems. Mortality of some species could affect the overall function and resilience of these grassland ecosystems. This study investigates physiological drought tolerance of grass species and its implications across phylogenies and climates in the context of the resilience and function of grassland ecosystems, during and after periods of drought (Craine et al., 2013). An ecosystem’s functions can remain unchanged by drought, if native drought tolerant grass species are already there, and can increase in relative abundance. If not, non-local drought-tolerant grass species may invade, and change the ecosystem functionality, for example by having higher rates of water and carbon dioxide exchange than before.—Hilary Haskell
Joseph, Craine M., Troy W. Ocheltree, Jesse B. Nippert, Gene Towne, Adam M. Skibbe, Steven W. Kembel, and Joseph E. Fargione. “Global Diversity of Drought Tolerance and Grassland Climate-change Resilience.” Nature Climate Change 3 (2013): 63-67.
Craine et al. researched the physiological drought tolerance and leaf functional traits for 426 grass species. The grass species used in this study originated from six different continents, and were grown from seeds either provided by the United States Department of Agriculture or hand-collected in New Zealand. The grasses were grown in a Conviron Growth Chamber, where temperatures were maintained at 25 °C for sixteen hours and 20°C for eight to simulate night and day conditions. Plants received water on a daily basis and fertilizer biweekly. The main parameter for this study was critical leaf water potential, which the authors defined as the level at which stomata conductance of gases such as water vapor, carbon dioxide, and oxygen flow through the stomata of leaves falls below an ecological threshold for continued function and plant vitality. To find critical leaf water potential, five weeks after germination or planting of the grass species studied, the authors stopped watering one sample of each species to simulate a drought. Then, the authors used a steady-state diffusion porometer to measure leaf conductance daily until stomatal closure occurred. After closure, Craine et al. measured the hydrostatic pressure potential, the energy per unit volume of water exerted by the pressure of overlying water, using a Scholander pressure bomb. The leaf water potential represents the tension on the water column of the plant, with lower values indicating greater physiological tolerance of plants to dry soils. From a specie’s dry soil tolerance, the authors inferred that a species was also tolerant of drought. The leaf water potential of stomatal closure is the critical leaf water potential, Ψcrit, for the species.
The diversity of drought tolerant species across global climates is unknown. However, grasslands with a diversity of local drought tolerant grass species are able to adapt to drought without altering overall ecosystem function more so than grasslands with less local drought tolerant grass species diversity. The functions of these grassland ecosystems include carbon uptake, productivity, soil retention, and provision of forage to grazers. These functions could change if there is a lack of diversity across the types of drought tolerant grasses that support these functions, and a subsequent die off of these local drought intolerant species. However, for non-diverse drought tolerant ecosystems, if non-local drought tolerant grass species migrate to the ecosystem during a drought, these functions could be maintained, albeit ecosystem function may be altered post-drought due to the presence of the non-local grass species. Non-local drought tolerant grass species may have different functional traits than the local drought intolerant grass species initially present, thus changing the functional composition of the ecosystem.
Based on some predictions for the rate of climate change in coming years, grass species, and thus ecosystem functions, will not be able to adapt quickly enough to projected variations in temperature and precipitation. After periods of drought in grasslands with low drought tolerant grass species diversity, if non-local drought tolerant grass species migrate to these grassland ecosystems, this could alter the functional composition of ecosystems and the processes that these ecosystems carry out. Craine et al., note that currently, there is not enough data to predict the impacts of drought on the functional composition of grasslands.
Grass species’ drought tolerance varied across the range of mean annual precipitation (MAP) for grasslands (250-1,500mm). This variance is important to the ecosystem’s functional composition and resilience, due to increasingly frequent and severe drought patterns. The highest and lowest Ψcrit for species within 50 mm precipitation intervals of MAP both increased significantly with increasing MAP. The median Ψcrit was (leaf water potential), −4.1 MPa and ranged between −1.4 MPa to −14MPa. Climate envelopes, areas with prevailing meteorological conditions such as precipitation and temperature, were created for 52% of the species studied. For the species in climate envelopes, Craine et al. were able to test the grass’s MPa and the upper and lower bounds of Ψcrit. Ψcritmaximum and minimums only shifted by 1.0MPa, even though wetter regions had more drought intolerant species than drought tolerant species. There were no shifts in the minimum and maximum Ψcrit for the 10% and 90% ranges of MPa, and the inner quartile were only 0.5 MPa or less from MAP 250−1,500mm of precipitation.
Based on these findings, only humid ecosystems with very high levels of precipitation (MAP>1,500mm) would experience changes in functional ecosystem composition and thus functional responses, due to the lack of diversity of drought tolerant species initially local to the ecosystems. There is a negative correlation between maximum drought tolerance and increasing MAP for ecosystems with MAP greater than 1,913 mm. Furthermore, for the ecosystems with higher levels of precipitation, maximum drought tolerance declined with increasing MAP at a faster rate than ecosystems that received less rain. Although these findings are significant, Craine et al., acknowledges that there were relatively few species tested (only about 10% of the species sampled could be used to create climate envelopes). Furthermore, temperature gradients did not have a significant effect on drought tolerance of grassland species. This study suggests that ecosystems with high levels of precipitation might require species migration of non-local drought tolerant species to maintain ecosystem function, due to the lack of diverse drought tolerant species originally local to the ecosystem.
Craine et al. also examined the variation in physiological drought tolerance for grass species in a single grassland, the Konza Native Tallgrass Prairie in northeastern Kansas. The data from this grassland was used for comparison to global data on physiological drought tolerance of grassland species. On the Prairie, mean average temperature is 13 °C and average monthly and temperatures ranged from -3 °C to 27°C. Annual rainfall averaged 833 mm over the years 1983−2009. Fifty-two of the 426 species used in this experiment were collected from the Konza Prairie. The authors concluded that local variation of physiological drought tolerance of grass species in grassland ecosystems is high. Craine et al. compared the Ψcrit distribution from the Konza species with global distribution to find that the global range of physiological drought tolerance for grass species was present at this site alone.
In addition to comparing geographic drought tolerance of grass species, Craine et al. also compared phylogentic variations in drought tolerance using taxonomic data for species from the uniprot database. The authors hypothesized that if certain clades of the same ancestor were to die off, the ecosystem should still maintain its functional diversity of physiological drought tolerance. By studying sixty-five grass species classified into various phylogenies, the authors found that physiological drought tolerance had evolved numerous times, and was widespread in the grass phylogeny. According to the Ψcritbetween clades, there was no significant difference (BEP versus PACMAD; P=.016) or the four subfamilies sampled. Furthermore, C4 species were not on average more or less physiologically drought tolerant than C3species.
Drought tolerant grass species (Ψcrit <−4.1 MPa) have unique traits in comparison to drought intolerant grass species (Ψcrit>−4.1 MPa). Craine et al. used recently expanded leaves and a Li 6400 infrared gas analyzer with red/blue emitting diode light source and CO2 injector to find that drought tolerant species had higher photosynthetic rates (17.0 ± 0.5 versus 15.3±0.5 μmol m−2 s−1, P = 0.01), and higher stomatal conductance (0.18 5± 0.008 versus 0.152 ± 0.008 mol m−2 s−1, P = 0.002) compared with physiologically drought-intolerant grass species (Ψcrit>−4.1 MPa). This finding suggests that evolution of grass species caused by droughts could result in the increased abundance of species that are able to re-adapt to higher levels of precipitation through increased primary productivity post-drought. Higher gas-exchange rates per-unit leaf area is one of the main functional traits that contribute to this finding. There seemed to be a tradeoff between leaf widths and drought tolerance for the 426 grass species examined. The tradeoffs created a triangular model for the functional and structural traits of grasses: wide leaved and drought intolerant, narrow-leaved drought intolerant, and narrow-leaved drought-tolerant. Interestingly, there were no drought-tolerant species with wide leaves. According to Craine et al., drought adaptations tend to be structural rather than morphological. In addition, of twenty species studied, drought-tolerant grasses had narrower xylem, the supporting tissue of vascular plants that conducts water through tracheids and vessels.
Craine et al. conclude that physiological drought tolerance in the grass species of grassland ecosystems is a widely, commonly evolved trait that is distributed both phylogenetically and geographically. Because of this broad distribution of drought tolerance in a diversity of grassland bioclimates, species local to the ecosystem can support natural ecosystem function when drought occurs, without the migration of non-local grass species to the ecosystem. In the face of decreased precipitation, this study suggests that grasslands with high functional diversity for drought tolerance will be resilient to drought without immigration of non-local drought tolerant grass species to maintain ecosystem function. To study ecological survival strategies for drought intolerant species, the authors used the triangle relationship of physiological drought tolerance versus leaf width. This triangular relationship of structural and morphological function includes drought tolerance and leaf width; however, it still leaves out other ecological factors such as shade tolerance and nutrient availability. Furthermore, grass species in areas with low precipitation had significantly (P
The effects of climate change and drought on grassland communities still requires more research. However, according to Craine et al., grassland ecosystems across bioclimatic gradients with grass species that are functionally diverse in their drought tolerance should be able to adapt to drought in order to retain ecosystem function in the face of climate change. The diversity of the drought tolerant grass species local to a grassland ecosystem is imperative to the ecosystem in adapting to more frequent, severe droughts that are predicted to increase with climate change in the future. With the findings from this study, the authors are better able to promote resilient ecosystems and model the effects of droughts on grassland ecosystems.