Road to recovery: Introduction of two rare vernal pool grasses, Greene's tuctoria (Tuctoria greenei) and Colusa grass (Neostapfia colusana)

Vernal pool habitats have been significantly reduced by conversion to incompatible agriculture and urbanization. As a result, a number of vernal pool dependent species have become rare, including Colusa grass (Neostapfia colusana) and Greene’s tuctoria (Tuctoria greenei). This study examined the potential for introductions of the rare grasses into vernal pool habitats. To this end, four study sites were established, two sites for each species – one introduction site with restored or created vernal pools and one reference site with existing populations of the rare grass. The year prior to introductions, pool hydrology and reference populations were monitored and mapped to inform introduction success. Using seed and inflorescence packets, the species were introduced into the restored/created pools and, for comparison, reintroduction into the reference pools. For Greene’s tuctoria, germination and survivorship to reproduction occurred at both introduction and reintroduction pools. The introduction pools had significantly higher average percent germination (60%) than the reintroduction pools (35%), which may be a result of disturbance to the germination and early seedling stage at the reintroduction pools. Plants from seed packets had significantly higher vigor but showed a trend towards lower reproductive output compared to the plants growing from the higher-density inflorescence packets. In the second year, despite relatively low rainfall and only partial pool filling, the introduction pools supported over 2,000 second generation Greene’s tuctoria plants. For Colusa grass, germination occurred at both the introduction pools (13%) and reintroduction pools (23%); however only one plant survived to reproduce at the introduction pools while reintroduction pools had 40% survivorship to reproduction. Soil testing suggested that the low survivorship may be due to elevated salinity at the introduction pools. The differences between species and packet methods illuminated different paths to potential success in introducing new populations. The results of this research are imperative in informing recovery efforts for Colusa grass and Greene’s tuctoria populations as well as for other rare vernal pool plants.