Effect of planting season, bunchgrass species, and neighbor control on the success of transplants for grassland restoration

Summary

Within some North American montane forests, due to wildfire suppression, plant communities have changed from relatively open conifer stands with an understory of bunchgrasses, to closed canopy forest with an understorey of shade-tolerant and fire-sensitive species. The retention and restoration of montane grasslands is important in terms of conservation but recovery may be hampered by poor seed source and weed competition. This study assessed the use of native bunchgrass transplants (bluebunch wheatgrass Pseudoroegneria spicata and spreading needlegrass Achnatherum richardsonii), for restoration within thinned montane forest in British Columbia, western Canada.

Study area: Trials were conducted in the Rocky Mountain Trench. Average annual precipitation is 385 mm, peaking in May and June. The summer prior to planting (2000) was unusually dry (45% of normal precipitation, May-September) as was that after planting (2001) (35% of normal precipitation).

Three forest blocks were selected with similar vegetation, being dominated by Douglas fir Pseudotsuga menziesii with an understory of pine grass Calamagrostis rubescens. Trees were thinned in May-June 2000 from around 504 to 243 stems/ha, to reinstate the former natural open structure. Historically, these stands were maintained at low tree density through frequent wildfire and were dominated by bunchgrasses e.g. bluebunch wheatgrass P.spicata and spreading needlegrass A.richardsonii.

Experimental design: A total of 304 bunchgrass plugs were grown from local seed. Within each block, plugs were planted 60 cm apart in rows in autumn (8-9 October 2000) or spring (8-11 May 2001). Half the plants of each species at each planting date had neighboring pine grass controlled using a glyphosate solution (7g/L) applied by wiping onto pine grass shoots within 30 cm of the transplants with a sponge. Pine grass around the autumn transplants were treated a second time during spring planting. The treatments in each planting season were:

1) A.richardsonii with pine grass reduced by herbicide application;
2) A.richardsonii with pine grass untreated;
3) P.spicata with pine grass reduced by herbicide application;
4) P.spicata with pine grass untreated.


Autumn transplants: During October 2000, 40 plugs of each species were randomly selected and planted at each of the three blocks. P.spicata transplants averaged 1.5 cm² in basal area, 27 cm in height, and had 9 tillers/plug. A. richardsonii transplants averaged 1.3 cm² in basal area, 8.1 cm in height, and 13 tillers/plug.

Spring transplants: In May 2001, 16 plugs of each species were planted at each of the two blocks. P.spicata transplants averaged 2.1 cm² in basal area, 22.5 cm in height, and 4 tillers/plug. A.richardsonii transplants averaged 4.9 cm² in basal area, 20.3 cm in height, and 13 tillers/plug. Sample sizes were reduced in spring due to a limited supply of plugs.

In September 2001, all transplants were measured for survival, evidence of grazing, and the number of tillers and inflorescences.

Effectiveness of herbicide control of pine grass: Herbicide treatment significantly reduced pine grass (47% less cover compared to untreated plots) in September 2001. Pine grass control was more effective in spring (average 11% cover) compared to autumn (average 16%).

Bunchgrass survival: Limited grazing occurred on the plugs, with the proportion grazed during 2001 varying from 2 to 15%. A greater proportion of P.spicata transplants (74%) survived than A.richardsonii (30%). Season of planting clearly affected survival. P.spicata survival was greater when planted in the autumn (81%) compared to the spring (44%). The opposite was evident for A.richardsonii, with survival of spring transplants (68%) greater than autumn transplants (44%). Pine grass control increased the overall survival of both bunchgrasses by around 7%. Initial size of transplants affected survival, with a trend for larger transplants of both bunchgrasses having a greater likelihood of survival.

Bunchgrass growth: A.richardsonii transplants surviving autumn planting lost fewer tillers compared to those surviving spring planting. In contrast, P.spicata spring transplants lost fewer tillers than those from the previous autumn. Pine grass control resulted in a positive response in the number of tillers for A.richardsonii transplants surviving from the autumn and P.spicata transplants surviving from spring. Tiller growth in all other treatments was negligible or negative, with no improvement resulting from pine grass control.

P.spicata plugs lost a greater number of tillers (−34%) versus A.richardsonii plugs (+6%). A.richardsonii outperformed P.spicata in all three blocks (the differences being 14, 16, 66%). Inflorescence data were too variable to detect significant differences among treatments, but all plugs producing seed heads during the study were planted in the autumn (32 of 240), and almost all flowering were P.spicata (30 of 32).

Conclusions: In this study, using bunchgrass transplants appears to be a suitable method to assist bunchgrass restoration within recently thinned montane forest. Both trial species, P.spicata and A.richardsonii, established despite drought conditions. Larger transplants survived better than smaller ones. Overall, A.richardsonii outperformed P.spicata (massively so in one of the three study blocks). Survival was enhanced by reduction of pine grass, but with limited benefits for tiller growth.

Whilst these results indicate that bunchgrass transplants can be used to aid grassland restoration, large-scale use may be cost prohibitive. Longer-term investigation is required to assess establishment beyond 1-year.


Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.blackwell-synergy.com/journal.asp?ref=1061-2971