Contrasting physiological responsiveness of establishing trees and a C₄ grass to rainfall events, intensified summer drought, and warming in oak savanna

Abstract

Climate warming and drought may alter tree establishment in savannas through differential responses of tree seedlings and grass to intermittent rainfall events. We investigated leaf gas exchange responses of dominant post oak savanna tree (Quercus stellata and Juniperus virginiana) and grass (Schizachyrium scoparium, C 4 grass) species to summer rainfall events under an ambient and intensified summer drought scenario in factorial combination with warming (ambient, 11.5 1C) in both monoculture and tree-grass mixtures. The three species differed in drought resistance and response of leaf gas exchange to rainfall events throughout the summer. S. scoparium experienced the greatest decrease in A area (À56% and À66% under normal and intensified drought, respectively) over the summer, followed by Q. stellata (À44%, À64%), while J. virginiana showed increased A area under normal drought (113%) and a small decrease in A area when exposed to intensified summer drought (À10%). Following individual rainfall events, mean increases in A area were 90% for S. scoparium, 26% for J. virginiana and 22% for Q. stellata. The responsiveness of A area of S. scoparium to rainfall events initially increased with the onset of drought, but decreased dramatically as summer drought progressed. For Q. stellata, A area recovery decreased as drought progressed and with warming. In contrast, J. virginiana showed minimal fluctuations in A area following rainfall events, in spite of declining water potential, and warming enhanced recovery. J. virginiana will likely gain an advantage over Q. stellata during establishment under future climatic scenarios. Additionally, the competitive advantage of C 4 grasses may be reduced relative to trees, because grasses will likely exist below a critical water stress threshold more often in a warmer, drier climate. Recognition of unique species responses to critical global change drivers in the presence of competition will improve predictions of grass-tree interactions and tree establishment in savannas in response to climate change.

Impacts of extreme precipitation and seasonal changes in precipitation on plants

“…However, redistributed precipitation had no effect on mortality. Results suggest that species composition of these oak savannas will alter in a climate where precipitation shifts from summer to spring and autumn (Volder et al, 2010). …”

“…Impacts of seasonal changes in precipitation have been reported in boreal forests (Gaul et al, 2008), arctic tundra (Schimel et al, 2004), tropical rainforests in the Amazon (Nepstad et al, 2002;Brando et al, 2006;Brando et al, 2008), oak savannas (Volder et al, 2010;Volder et al, 2013) and Mediterranean regions, such as Spain (Miranda et al, 2009b). Although grasslands, savannas and Mediterranean forests differ markedly in the timing of ambient precipitation and the length of the growing season, experiments suggest patterns can be found in responses to seasonal changes in precipitation, such as strong changes in water stress and productivity in warm or dry seasons and not in cool or wet seasons.…”

“…Increasingly large fluctuations in SWC following redistributed precipitation also drive physiological responses, community structure and the distribution of plant functional types (Volder et al, 2013). Extreme precipitation may influence plant water relations, hydraulic architecture and ANPP (Reyer et al, 2012), whereas changing seasonal precipitation may also influence phenology, including fruit, leaf and early/late wood development, as well as tiller development of grasses (Volder et al, 2010;Misson et al, 2011).…”

“…Further, current mean annual precipitation (MAP), which influences whether the site is xeric, mesic or hydric, will be a key factor in determining whether plant growth increases or declines Heisler-White et al, 2009). Added effects of heat stress, rising mean temperatures, elevated CO 2 , vapour pressure deficit (D) and the current season when most precipitation falls, which influence snowfall and snowmelt, are also likely to influence plant responses to altered precipitation (Volder et al, 2010). Plant functional type could also affect the response to changes in timing.…”

How do variations in the temporal distribution of rainfall events affect ecosystem fluxes in seasonally water-limited Northern Hemisphere shrublands and forests?

“…a shift towards fewer and larger rainfall events, have received less attention; results have so far been inconclusive (Jentsch et al, 2007). Almost all experimental field studies manipulating rainfall amount or distribution have been conducted on annual plant communities or perennial grasslands Miranda et al, 2009), because of the difficulty of performing manipulation experiments on taller canopies (exceptions include Volder et al, 2010;Limousin et al, 2010;Misson et al, 2010). Despite the potential impact of changes in temporal patterns of rainfall distribution on shrublands and forests, data for these ecosystems is sparse.…”