The effect of rainfall amount and timing on annual transpiration in a grazed savanna grassland

The role of precipitation (P) variability on evapotranspiration (ET) and its two components, transpiration (T) and evaporation (E) from savannas, continues to draw significant research interest given its relevance to a number of eco-hydrological applications. Our study reports on six years of measured ET and estimated T and E from a grazed savanna grassland in Welgegund, South Africa. Annual P varied significantly in amount (508 to 672 mm yr−1), with dry years characterized by infrequent early-season rainfall. T was determined using annual water-use efficiency and gross primary production estimates derived from eddy covariance measurements of latent heat flux and net ecosystem CO2 exchange rates. The computed annual T was nearly constant, 331 ± 11 mm yr−1 (T/ET = 0.52), for the four wet years with frequent early wet-season rainfall, whereas annual T was 268 and 175 mm yr−1 during the dry years. Annual T/ET was linearly related to the early wet-season storm frequency. The constancy of annual T during wet years is explained by the moderate water stress of C4 grass and constant annual tree transpiration covering 15 % of the landscape. However, grass transpiration declines during dry spells. Moreover, grasses respond to water availability with a dieback-regrowth pattern, reducing leaf area and transpiration during drought. These changes lead to an anomalous monthly T/ET relation to leaf-area index (LAI). The results highlight the role of the C4 grass layer in the hydrological balance and suggest that the grass response to dry spells and drought is reasonably described by precipitation timing.

Pathways of forest savannization in a mesic African savanna-forest mosaic

Typically, savannas experience frequent fires, which limit tree cover and promote flammable grass accumulation, whereas forests form dense canopies that exclude fires by reducing C4-grass fuel loads and creating a humid microclimate. However, extreme fires occasionally burn into forests. Although these are known to kill forest trees and can make repeat fires more likely, the long-term consequences of an extreme fire event for forest structure and potential forest savannization remain largely unknown. Here, we analysed whether an extreme fire event could, alone, alter forest species composition, vegetation structure, and fire regimes, or whether successive fires after an initial extreme fire event were necessary to trigger a biome transition. We found that forests that only burned once recovered, whereas those that burned again following an initial extreme fire event underwent a transition from closed-canopy forests to open, grassy systems. Our results suggest that, while extreme fires set the stage for a biome transition, subsequent fires are necessary for flammable grasses to colonise and establish, ultimately resulting in a savanna fire regime.

Can improved canopy light transmission ameliorate loss of photosynthetic efficiency in the shade? An investigation of natural variation in Sorghum bicolor.

Previous studies have found that maximum quantum yield of CO2 assimilation (ΦCO2,max,app) declines in lower canopies of maize and Miscanthus, a maladaptive response to self-shading. These observations were limited to single genotypes, leaving it unclear that the maladaptive shade response is a general property of this C4 grass tribe, the Andropogoneae. We explored the generality of this maladaptation by testing the hypothesis that erect leaf forms (erectophiles), which allow more light into the lower canopy, suffer less of a decline in photosynthetic efficiency than drooping leaf (planophile) forms. On average, ΦCO2,max,app declined 27% in lower canopy leaves across 35 accessions, but the decline was over twice as great in planophiles than in erectophiles. The loss of photosynthetic efficiency involved a decoupling between electron transport and assimilation. This was not associated with increased bundle sheath leakage, based on 13C measurements. In both planophiles and erectophiles, shaded leaves had greater leaf absorptivity and lower activities of key C4 enzymes than sun leaves. The erectophile form is considered more productive because it allows a more effective distribution of light through the canopy to support photosynthesis. We show that in sorghum, it provides a second benefit, maintenance of higher ΦCO2,max,app to support efficient use of that light resource.

Ischaemum rugosum (saramollagrass).

I. rugosum is a C4 grass species widely cultivated and naturalized in moist, tropical habitats around the world (Clayton et al., 2015; USDA-ARS, 2015). It is an opportunistic and effective colonizer of open and disturbed areas, swamps, and along roadsides. It is a serious weed in many crops, especially in paddy fields and sugarcane plantations (Holm et al., 1977; Baki and Manidool, 1992). I. rugosum is a highly invasive grass which can produce up to 4,000 seeds per plant and has the potential to grow even in shaded areas (Holm et al., 1977; PROTA, 2015). I. rugosum is listed as invasive in Costa Rica, Cuba, the Dominican Republic and Fiji (Kairo et al., 2003; Chacon and Saborio, 2012; Oviedo Prieto et al., 2012; PIER, 2015). In the USA, the Department of Agriculture considers it a noxious weed; plants found growing within the continental USA should be promptly reported to that agency (Barkworth et al., 2003).

Reliability of phytoliths for reconstructing vegetation dynamics in northeast China

<p>  Phytolith provides a new preconstruction and interpretation of palaeovegetation in either forest or grassland regions. In particular, the phytolith reliability records in both vegetation types should be assessed when they are employed on palaeovegetation reconstruction in north temperate region. Yet this issue has not been clearly investigated. Being two vegetation types (including forest and grassland) in northeast China (NE China) where it is an integrated physical geography unit, they provide some crucial references regarding the phytolith reliability. Thus, we firstly focused the study site of NE China to collect 108 topsoil samples from five dominant community types in forest region and 154 topsoil samples from four dominant community types in grassland region, respectively, to their phytolith assemblages. This study was to establish the reference databases of modern soil phytolith to demonstrate their record reliability. These phytolith data thus better serve palaeovegetation reconstruction in sedimentary sequences using their corresponding vegetation types in two selected regions.</p><p>    Analytical results showed that topsoil phytolith assemblages and their phytolith indices (Iw, Ic and W/G) varied substantially with different vegetation types in NE China; phytolith indices were also variations aligned with vegetation compositions. These finding suggest that phytolith is a reliable proxy using reconstructing palaeovegetation.</p><p>  The palaeovegetation reconstruction based on these phytolith reference databases indicated that NE China had experienced substantial vegetation changes since the late-glacial period. Community types in forest region may have experienced a succession sequence from the open Larix mixed forest to the open woodland, then turning to the closed broadleaf forest, and finally to the closed Pinus koraiensis mixed forest. In particular, we found that vegetation types in grassland region was dominated by a flourish C3 grass steppe since late-glacial period, with a total coverage higher than 50%. The coverage of C3 grass and C4 grass were higher than 25% and 16%, respectively.</p><p>  The palaeovegetation interpretation using these phytolith reference databases since the late-glacial period were consistent with that reconstructed using pollen assemblages in the same stratigraphic profile, confirming the phytolith reliability for reconstructing vegetation type and community type in the NE China. Phytolith record analysis also provided some detailed vegetation information such as the vegetation composition of the understory and Larix abundance in forest region, and the proportion of C3/C4 grass, their biomass and community coverage in grassland region.</p><p>  Thus, this study demonstrates the phytolith reliability to provide new perspectives on palaeovegetation reconstruction in northern temperate regions. Furthermore, this finding acts as a potential reference for exploring the relationship between phytolith and (palaeo)vegetation in other temperate regions.</p><p>(Supported by the National Natural Science Foundation of China (Grant No.41971100,41771214 )</p>