Fire and Grazing Impacts on Plant Diversity and Alien Plant Invasions in the Southern Sierra Nevada

Abstract

Patterns of native and alien plant diversity in response to disturbance were examined along an elevational gradient in blue oak savanna, chaparral, and coniferous forests. Total species richness, alien species richness, and alien cover declined with elevation, at scales from 1 to 1000 m 2 . We found no support for the hypothesis that community diversity inhibits alien invasion. At the 1-m 2 point scale, where we would expect competitive interactions between the largely herbaceous flora to be most intense, alien species richness as well as alien cover increased with increasing native species richness in all communities. This suggests that aliens are limited not by the number of native competitors, but by resources that affect establishment of both natives and aliens.Blue oak savannas were heavily dominated by alien species and consistently had more alien than native species at the 1-m 2 scale. All of these aliens are annuals, and it is widely thought that they have displaced native bunchgrasses. If true, this means that aliens have greatly increased species richness. Alternatively, there is a rich regional flora of native annual forbs that could have dominated these grasslands prior to displacement by alien grasses. On our sites, livestock grazing increased the number of alien species and alien cover only slightly over that of sites free of livestock grazing for more than a century, indicating some level of permanency to this invasion.In chaparral, both diversity and aliens increased markedly several years after fire. Invasive species are rare in undisturbed shrublands, and alien propagules fail to survive the natural crown fires in these ecosystems. Thus, aliens necessarily must colonize after fire and, as a consequence, time since fire is an important determinant of invasive presence. Blue oak savannas are an important propagule source for alien species because they maintain permanent populations of all alien species encountered in postfire chaparral, and because the vegetation mosaic in this region places them in proximity to chaparral. The speed at which alien propagules reach a burned site and the speed at which the shrublands return to their former closed-canopy condition determine alien invasion. Frequent burning of this vegetation alters the balance in favor of alien invasion.In the higher-elevation coniferous forests, species diversity was a function of fire severity and time since fire. High-intensity fires create gaps that decrease canopy coverage and increase light levels and nutrients for an ephemeral successional flora. Few species have persistent seed banks, so the time since fire is an important determinant of colonization success. There was a highly significant interaction between fire severity and time since fire for understory cover, species richness, and alien richness and cover. Understory was sparse in the first year after fire, particularly in low-severity burns, and increased substantially several years after fire, particularly on high-severity burns. Both fire severity and time since fire ...

Frequent burning promotes invasions of alien plants into a mesic African savanna

“…Finally, reduced competition from dominant resident species may act in concert with increased availability of resources and bare soil to enhance the invadability of frequently burnt mesic savannas by alien forbs. In the blue oak savannas of the southern Sierra Nevada, alien forb response to fire was also linked to increased post-fire availability of bare patches and resources such as light, which are critical for alien recruitment and growth (Keeley et al 2003). The same mechanism may be operating in mesic savannas, which are frequently burnt.…”

Dinâmica da comunidade arbórea em uma floresta estacional semidecidual sob queimadas recorrentes

“…A recuperação da vegetação após o fogo pode variar dependendo da intensidade e duração do incêndio (Keeley et al 2003). Em alguns casos, os incêndios podem inibir o ciclo de regeneração da floresta (Kennard et al 2002;Laurance 2003;Otterstrom & Schwartz 2006), enquanto em outros esses distúrbios, quando em intensidade intermediária, podem ser importantes para a manutenção de um grande número de espécies (Hubbell et al 1999), uma vez que provoca a reversão das comunidades a estados sucessionais iniciais, nos quais espécies previamente excluídas por competição, voltam a se estabelecer (Sheil & Burslem 2003).…”

Fire Probability, Fuel Treatment Effectiveness and Ecological Tradeoffs in Western U.S. Public Forests

“…For instance, non-native vegetation can be influenced by high fire severity [44] and some fuel treatments [10], especially if the treatments are repeated [9].…”