Disturbance as a factor in breaking dormancy and enhancing invasiveness of African grasses in a Neotropical Savanna

Abstract

The Cerrado is threatened by wildfires and invasive species. We aimed to evaluate in laboratory conditions whether temperature fluctuation at the soil surface, resulting from the absence of vegetation due to fire, can affect the germination of Urochloa decumbens and U. brizantha, two invasive African grasses. Seeds of both species were submitted to simulations: 1) temperature during fire at 1cm belowground (F); 2) temperature fluctuation at 1cm belowground without vegetation cover for a month (TF); 3) (F) + (TF); 4) control at 25ºC. After treatments, seeds were put to germinate at 25ºC for 40 days. We had four replicates per treatment and three temporal replicates. We compared germination percentage and the mean germination time among treatments using ANOVA. The treatments TF and F+TF had the highest germination values for both species. The results showed that fire per se could not stimulate seed germination, however, they suggest that a disturbance that produces a pattern of temperature fluctuation is able to break dormancy and enhance seed germination and, consequently, increase the invasiveness of the study species. Vegetation gaps resulting from disturbance may become new sites of invasion. This information is important for making management decisions regarding the control of these species.

Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands

“…underground storage organs) to survive repeated aboveground disturbances, apparently at the cost of colonisation potential (Silcock & Scattini, ; Veldman et al ., ; Silveira et al ., ), grasslands on former agricultural land fail to recover their characteristic plant communities even after many decades (Kirkman et al ., ; Brudvig et al ., ; Vieira et al ., ). Whereas recolonisation of characteristic old‐growth grassland species is extremely limited in secondary grasslands (Silcock & Scattini, ; Ilunga wa Ilunga et al ., ; Veldman et al ., ; Silveira et al ., ), many non‐native invasive and native weedy species are rapid colonisers of post‐agricultural grasslands due to their higher germination and better ability to establish in disturbed environments (Brudvig et al ., ; Gorgone‐Barbosa et al ., ). These weedy species can limit the establishment of desired old‐growth grassland species (Zaloumis, ; Gorgone‐Barbosa, ; Le Stradic et al ., ), posing a huge hurdle to restoration (see also Section II.6).…”

“…Urochloa brizantha ) can reduce ecosystem flammability relative to native‐dominated grasslands, but this can depend on the fire season (Gorgone‐Barbosa et al ., ). Although fire can be effective at controlling fire‐sensitive invasive species (Stevens & Beckage, ; te Beest et al ., ), fire can also promote invasive species that evolved with frequent fires ( Imperata cylindrica ; Lippincott, ) or indirectly promote invasive species by opening recruitment sites, suitable for germination ( Urochloa brizantha and Urochloa decumbens ; Gorgone‐Barbosa et al ., , b ). The fire ecology of invasive species should therefore be carefully considered as part of fire management (Gorgone‐Barbosa, ) in tropical grassland restoration.…”

Loss of biotic resistance and high propagule pressure promote invasive grass‐fire cycles

“…There are likely several mechanisms by which fire increased cheatgrass invasion by more than an order of magnitude in our experimental plots (Figure ). It is likely that the most important was the loss of the native plant community, which can create biotic resistance to invasion through competition for space and soil resources (Chambers et al, ; Melgoza et al, ; Prevey et al ) and lead to more ideal germination conditions for invasion (Gorgone‐Barbosa, Pivello, Baeza, & Fidelis, ). Fire also causes physical disruption of soil crusts, which are known to inhibit the germination success of invasive grasses (Kaltenecker, Wicklow‐Howard, & Pellant, ) and results in nutrient pulses that promote invasive grass growth (Esque et al, ).…”

Controlling the invaderUrochloa decumbens: Subsidies for ecological restoration in subtropical Campos grassland

“…For example, invasion of Cerrado neotropical savanna by the grass Urochloa brizantha alters frequency and intensity of natural fires, which in turn benefits the invaders U. brizantha and Urochloa decumbens , i.e. creates positive feedbacks that aggravate the problem (Gorgone‐Barbosa, Pivello, Baeza, & Fidelis, ; Gorgone‐Barbosa et al., ). Moreover, eradication treatments normally create conditions for a new invasion by the same or another exotic species (D'Antonio & Meyerson, ; Davis, Grime, & Thompson, ).…”