Fire and competition in a southern California grassland: impacts on the rare forb Erodium macrophyllum

Summary

Study 1

A study was undertaken to investigate how prescribed spring burns and non-native species in a southern California grassland impact the grassland plant community, specifically examining how these factors impacted the rare forb, large-leaved filaree Erodium macrophyllum. In association with this, a glasshouse study was undertaken to assess the effects of competion from the native purple needlegrass Nassella pulchra and non-native wild oat Avena fatua on Erodium macrophyllum growth.

The glasshouse experiment was conducted at the University of California (Riverside).

A seven-factor randomized block design, replicated five times, was used. Treatments were three different densities of A.fatua and N.pulchra around a single E.macrophyllum seedling, with E.macrophyllum growing alone (as a control). Densities were two, four or six 'competitors' surrounding an E.macrophyllum seedling.

On 20 May 2002, seedlings were transplanted into soil-filled plastic pots (30 cm diameter, 28 cm deep) approximately 4 days after germinating. A E.macrophyllum seedling was planted into the centre of the pot with the appropriate density of competitors, equi-distant from each other. The pots were watered every other day. On 2 July 2002 height and width (widest point) of all E.macrophyllum plants were measured and above-ground biomass havested, dried and weighed.

All densities of A.fatua decreased average E. macrophyllum dry biomass (2 Avena plants: 10 g; 4 plants: 5 g; 6 plants: 25 g) compared with controls E. macrophyllum (average 34 g). In contrast, there was no difference in biomass between E.macrophyllum growing with N.pulchra at any of the three densities. E. macrophyllum growing with six A.fatua plants per pot resulted in the largest height-width ratio (c.1.4). No other competitors at the three densities trialled had an effect on this variable.

This study showed that the non-native A.fatua competitively suppresses E. macrophyllum, indicating that poor performance of E. macrophyllum in a field experiment in an associated field experiment was propbaly due to exotic grass competition.

 

Study 2 

This study investigated how prescribed spring burns and exotic plant species in a southern California grassland impact the native grassland community, focussing on how burns and competition affect the rare forb, large-leaved filaree Erodium macrophyllum. Plant group (i.e. native grasses, native forbs, exotic grasses and exotic forbs) response to burning and weeding was monitored.

Study site: The study was carried out at Santa Rosa Plateau Ecological Reserve, California, southwest USA within an area of grassland (with native and non-native species) and coastal sage scrub.

Experimental design: A split-plot design was used (subplots replicated five times within each 40 × 40 m main plot). Main plots were burned or unburned and subplots were weeded or unweeded (50 replicates). In each burned area 10, 1 × 0.5 m subplots was established;  weeding treatment was randomly assigned to five in each burned or non-burned plot. Subplots were chosen on the basis that they contained at least three adults  purple needlegrass Nassella pulchra plants (a native perennial bunchgrass) and high non-native grass cover. During November 2000 exotics were manually weeded in assigned subplots (and subsequently through the growing season as needed). Prevalent species removed were compact brome Bromus madritensis, soft brome B. hordeaceous, oats Avena sp., rat's-tail fescue Vulpia myuros and long-beaked storksbill Erodium botrys. In early January 2001 (before onset of winter rains) 200 E.macrophyllum seeds were broadcast into each of the 100 subplots. These were monitored weekly until May (end of the growing season), germinating E.macrophyllum were marked with a numbered wooden toothpick.

During 2001-02, 10 randomly selected E.macrophyllum plants per plot were marked (as large recruitment reulted in higher densities making it impractical to follow every one) and these were monitored every 10–14 days. Total numbers of emerged seedlings were counted.

Burning: Burning took place on 13 June 2001 (when many exotic grass seeds had not dispersed but E.macrophyllum had senesced).

Monitoring: Marked E. macrophyllum plants were monitored and at the end of the growing season (2001 and 2002), their mature fruits were counted to assess fecundity. During the 2002 growing season, N. pulchra seedling density in all 100 subplots was estimated in three 10 × 10 cm quadrats. Percentage cover of native grasses, native forbs, exotic grasses and exotic forbs in each subplot was estimated, as well as E.macrophyllum, in March 2002.

Weeding of exotic grasses increased E.macrophyllum seedling emergence, survival and fecundity in both years. Burned plots had less recruitment of E.macrophyllum but more establishment of native grass seedlings. During the second season (2001-02) there were still high densities of exotics germinating in the plots. At the end of this second season (June 2002), burned plots had less cover of exotic and native grasses but more cover of exotic forbs. Nevertheless, E.macrophyllum plants in burned plots had greater fecundity than in non-burned plots, suggesting that exotic grasses are more competitive than exotic forbs.

Conclusions:  Fire altered plant competition dynamics and it was apparent that exotic grasses strongly competed with E.macrophyllum. Benefits of prescribed spring burns will probably outweigh costs of decreased E.macrophyllum establishment. Such burns may enhance native grass recruitment and although exotic forbs are still abundant, they are less competitive compared with exotic grasses dominating non-burned areas.


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