Biomass allocation and leaf phenology of native and non-native grasses in a subtropical dry forest in Puerto Rico

Background/Question/Methods

Invasive non-native grasses capable of increasing the frequency, spread and intensity of anthropogenic fires have invaded subtropical and tropical dry forests worldwide.  Since many native dry forest tree species are susceptible to fire, this can result in decline of forest cover and loss of native biodiversity.  In some dry forest landscapes, native grasses can also be an important component of the ecosystem.  Little is known about how the ecological characteristics of these native grasses compare to those of invasive non-native grasses.   Within a tropical dry forest preserve in Puerto Rico (Guánica Commonwealth Forest), we asked whether three grass species: Uniola virgata (native), Cenchrus ciliaris (non-native) and Megathyrsus maximus (non-native) differed in their patterns of biomass allocation and leaf phenology.  We harvested randomly selected clumps of each species to measure aboveground and belowground dry weight as well as rooting depth.  For another set of randomly selected clumps, we measured live leaf moisture every 3 weeks for a period of 9 months, which included the transition from wet season to dry season.  

Results/Conclusions

Biomass allocation and rooting depth differed among species.  Native Uniola tended to allocate a larger proportion of biomass aboveground, while also tending to root more deeply than did the two non-native species.  Non-native Cenchrus and Megathyrsus tended to allocate more biomass belowground but did not root as deeply.  Furthermore, patterns to the change in live leaf moisture during the transition from wet to dry season differed among species.  Non-native grasses showed a more rapid and pronounced loss of leaf moisture during the dry season.  These patterns suggest different ecological strategies for native and non-native grasses within this dry forest ecosystem.  These results have implications for tropical dry forest restoration and suggest that non-native grasses are likely to be barriers to native woody species regeneration, while native grasses may produce microsite conditions that facilitate native woody species.