Buried but unsafe – defoliation depletes the underground storage organ (USO) of the mesic grassland geophyte, Hypoxis hemerocallidea

Mesic grasslands in South Africa (> 650 mm a-1 MAP) are rich in herbaceous forbs, which outnumber grass species by more than 5 to 1. Many of these forbs have underground storage units (USOs), such as thickened rootstocks, rhizomes, bulbs, or corms, that provide resources (non-structural carbohydrates, minerals, and water) enabling them to resprout after dry, frosty winters, and fire. However, despite their extensive biomass and reserves ostensibly protected underground, geophytic mesic grassland forbs can be severely depleted or extirpated by chronic trampling and grazing of their aerial parts by livestock. This study examined a possible explanation for forb demise in overgrazed grassland by investigating, in a pot trial, whether the growth of forbs and the size of their USOs are negatively affected by simulated green leaf loss. In a 2x2 factorial (clipped vs. unclipped x spring regrowth in the dark vs. light), five replicate plants of Hypoxis hemerocallidea, a common mesic grassland forb that resprouts from a corm, were subject to six severe (clipped to 80 mm) defoliations during the growing season and regrown in spring under full or restricted light to measure stored reserve contribution to regrowth. Defoliated plants were resilient to defoliation during the growing season, matching the total biomass production of unclipped plants, though cutting reduced the number of leaves by ~60% and flowers by almost 85%. Spring regrowth on stored reserves equalled that from reserves plus concurrent photosynthesis, indicating the value of USOs for regrowth. However, there was a marked carry-over effect of previous season defoliation, resulting in a one-third reduction in shoot growth and 40% fewer inflorescence in spring. Crucially, corm mass was more than halved by clipping. Above-ground spring growth was linearly related to corm mass. It was concluded that buried stored reserves are not protected by recurrent disturbance to aerial plant parts and that continued diminishment of USOs under chronic disturbance by overgrazing or frequent mowing would weaken and likely eventually kill plants, reducing forb species richness. Lenient management by infrequent summer mowing or grazing at moderate stocking rates combined with periodic rotational full season resting and dormant-season burning is recommended to maintain the USOs and vigour of forbs in mesic grassland.

Is a long hygroscopic awn an advantage for Themeda triandra in drier areas?

Themeda triandra has bigeniculate hygroscopic lemma seed awns that twist when wet and drying, thereby transporting the caryopsis across the soil surface into suitable germination microsites. The prediction that awns would be longer in drier grassland and have greater motility to enable them to move quickly and further to find scarce germination sites was tested in KwaZulu-Natal. Awns (n = 100) were collected from 16 sites across a mean annual precipitation gradient (575-1223 mm), ranging from 271-2097 m a.s.l. The daily movement of hydrated long and short awns (n = 10) across blotting paper was tracked for five days, and the rotational speed of anchored awns was measured. Awn length varied considerably (mean: 41.4-63.2 mm; sd: 3.44-8.99) but tended to increase (r = 0.426, p = 0.099) not decline, with increasing MAP. Awn length was unrelated to elevation, temperature and aridity indices. Long awns rotated at the same rate (2 min 48 sec) but moved twice as fast (46.3 vs. 22.1 mm day-1) and much further (maximum: 82.1 vs. 38.6 mm day-1) than short awns. Whether moisture limits awn development, the benefit of longer awns to negotiate densely tufted mesic grassland, and the multifunctionality of awns require investigation.

Testing for consistency in the impacts of a burrowing ecosystem engineer on soil and vegetation characteristics across biomes

The impacts of ecosystem engineers may be expected to vary along environmental gradients. Due to some resources being more limited in arid than in mesic environments, disturbances created by burrowing mammals are expected to have a greater ameliorating effect in arid environments, with larger differences in microhabitat conditions expected between burrows and undisturbed areas. The aim of this study was to test if the impacts of a medium-sized burrowing mammal, the aardvark, on soil properties (soil temperature, moisture and compaction) and vegetation characteristics (plant cover, species richness and species composition) are consistent across three biomes that differ strongly in annual rainfall. Burrowing affected soil and vegetation attributes, but the direction and magnitude of these biogeomorphological impacts were not consistent across the different biomes. For example, plant species composition was altered by burrowing in the arid scrubland and in the mesic grassland, but not in the semi-arid savannah. Contrary to expectations, the difference in the impacts of burrowing between biomes were not related to rainfall, with burrowing having strong, albeit different, impacts in both the arid scrubland and the mesic grassland, but weaker effects in the semi-arid savannah. It appears, therefore, that the impacts of these biogeomorphic agents may be site-specific and that it may be difficult to predict variation in their biotic and abiotic effects across environmental gradients. As a result, forecasting the impacts of ecosystem engineers under different conditions remains a challenge to management, restoration and conservation strategies related to these types of species.