Functional morphology and productivity of a tussock grassland in the Bolivian Altiplano

Tropical and subtropical high elevation grasslands are generally dominated by tall tussock grasses, a life form that seems to dominate in year round cold climates but otherwise quite different soil moisture regimes, from very wet (New Guinea, New Zealand, Ecuador) to rather dry, even semi-arid, as is the case in the NW-Argentinan and Bolivian Altiplano. The biomass production of these vast areas is largely unknown, since the classical harvesting technique cannot be applied in perennial vegetation without affecting growth. Given the steady increase in land use intensity, such information is needed to estimate the carrying capacity of these vast rangelands. In this thesis, I developed the needed non-destructive tools and applied them for a 30-month productivity analysis in the Bolivian Altiplano. The work was conducted in Sajama National Park at 4250 m elevation. The monthly mean temperature in the study area varies between 10.7 (December) and 2.3 (July), and annual precipitation averages at ca. 350 mm, most of this falling as rain between December and March (the austral summer). The study plant, Festuca orthophylla, is a tall (up to 1 m, mostly around 60 cm) tussock forming grass that represents more than 90% of all biomass in many parts of the Altiplano, including the study area. Forming clones of initially compact, but later fragmented shape, persisting many decades, this species is characteristic for the appearance of the semi-arid, Andean landscape over thousands of square kilometers at elevations between 3600 and 4600 m a.s.l. As a first step, I analysed the clonal structure, the morphology and biomass allocation in representative tussocks (Chapter 2). The core of the theses is related to the tussock biomass production using a demographic approach and land cover data (Chapter 3), followed by an assessment of seasonal leaf dynamics (Chapter 4).
Plant growth is driven by the rate of photosynthetic uptake of carbon, the loss of carbon and by allocation of photoassimilates to certain plant compartments, which leads to particular morphologies. Plant performance, vitality and persistence of a plant are affected by this partitioning process and vice versa. Under harsh climatic conditions such as cold temperature and seasonal drought, perennial plants invest more in below-ground than above-ground structures. As shown in Chapter 2, Festuca orthophylla does not match to this ‘rule’. The shallow rooting system represents only 21% of total biomass. The tussock base (root-stocks, tiller meristems and the network of branching below-ground shoots), comprises 28% of the total biomass. Although partly below the soil surface, much of this biomass compartment is functionally above-ground (belonging to shoots). With their below-ground position, tiller meristems are protected against grazing and trampling by camelids as well as against fire and freezing. Fifty one percent of the biomass is above-ground (live leaves and inflorescences). In terms of phytomass (including attached necromass), 75% is above-ground. On average, a tussock consists of 3200 tightly packed tillers (56% are live). Tillers emerge predominantly intravaginally (i.e. within the leaf sheath of an existing mother tiller), resulting in dense canopies with strong self-shading: eighty percent of green foliage experience less than 50% of the incident light. The most important Altiplano plant species thus, exerts morphological traits in favour of protection and survival rather than productivity.
As mentioned above, primary production cannot be measured by repeated harvesting in conditions of continuous growth without interfering with growth. With a non-destructive method, based on the number of leaves produced per ramet per time, length to weight ratios of leaves, ramet density within tussocks, birth, growth and death of leaves in ramets and land cover by tussocks, we estimated the primary production per total land area. Festuca orthophylla yielded an annual biomass production of ca. 1 kg m -2 in both, a relatively dry and a normal year, hence the difference in precipitation (327 mm vs. 384 mm) had no effect. Surprisingly, fencing alone had only moderate effects on productivity at the given stocking rate and natural growth conditions. The addition of dung in fenced areas increased the biomass production in comparison to unfenced areas where dung has been distributed. Within fenced subplots, clipping reduced the biomass production in the first dry season, but increased the biomass production in the subsequent rainy season. Biomass production after fire was near to zero, and fire was the treatment which had the greatest benefit of fencing. New leaves emerged throughout the year, hence, there was no period of complete inactivity despite the 7–8 months rainless period. Ramet mortality (associated with ramet turnover and, thus, new ramet production) was higher during the rainy season, and fencing reduced mortality during the dry season, but not during the rainy season. Fencing also had no effect on flowering, but clipping and fire suppressed flowering. New root arrival in in-growth cores made up ca. 19% ± 5 of the original root biomass in bore hole. Llama dung addition had no effect on new root arrival, but fire decreased relative root production in in-growth cores. The lateral roots of Festuca orthophylla tussocks growing into the large inter-tussock area compensate for the low rates of precipitation (roots spheres are 6 times larger than tussock canopy area. Continuous new leaf production despite strong moisture seasonality explains year-round grazing by camelids in this semi-arid high mountain ecosystem.
A 188 days census of Festuca orthophylla leaves provided clear evidence for leaf production in both the rainy and the dry season, a mean leaf duration of 141 days for peripheral tillers and 169 days for central tillers, and no compensatory growth response. Leaves grow slower and reach shorter length during the dry season compared to the rainy season, and peripheral tillers are shorter but more vigorous than central tillers (lower drought effect). Overall, this study suggests a nearly 2-fold replacement of foliage per year. The significant contribution of dry season leaf growth to annual production confirms earlier data from ramet demography and regrowth (after cutting) studies, and is most likely related to the 6 times greater soil occupancy by roots than by the leaf canopy. Wide spacing of tussocks and a large root sphere mitigate the impact of periodic drought on tissue formation providing year-round forage for llamas.
In conclusion, our data provide a quantitative characterisation of the architecture and dry matter investment of this dominant Altiplano species, the first year-round productivity estimation for a high-elevation tropical, grassland, and a detailed assessment of leaf dynamics for the rainy and the dry season. In a number of ways the traits exhibited, contrast Festuca orthophylla from other, non-woody, high elevation taxa. In particular, the foliage of these tussocks operates at temperature close to that of the free atmosphere, while at the same time, providing shelter to below-ground shoot meristems. The large amount of dead plant material constrains photosynthetic light interception, and reflects slow rates of decomposition, a likely trade-off of generally poor nutrional quality (Patty et al., 2010), which, in turn, relates to the heavy herbivory pressure. The rates of biomass accumulation per unit of tussock area are quite high, much higher than one would expect in such a semi-arid rangeland. The most plausible explanation is that these tussocks are utilizing a far greater land area for water and nutrient acquisition than represented by their projected canopy area. The space in-between tussocks is, thus, a most likely mechanism explaining these high rates of productivity. The degree of land cover has already been shown to be a most critical mechanism to cope with water shortage in such high elevation grasslands (Geyger, 1985). Given that tussock root spheres do not overlap but rather leave unexplored inter-tussock space, there seems some leeway for increasing tussock density without losing the advantage of wide spacing in terms of water and nutrient relations. It remains to be explored how large the actual root-covered land area is. It seems tussock density could be enhanced by 30 to 50% before root spheres start to overlap. Current grazing pressure prevents tussock recruitment, so natural tussock mortality or man-made mortality by misuse of fire is currently not compensated by natural recruitment. Hence, in addition to llama dung distribution, grazing management with mobile fences seems like an additional management option to retain higher camelid stocks without destructive consequences, From our unquantified observations new tussock establishment occurs already in the first year of fencing. Thus, periodic camelid exclusion could assist in increasing tussock density and, thus, productivity per land area. There is no indication that low temperature per se has a major impact on productivity at these high elevations. We observed no compensatory growth in Festuca orthophylla in response to foliage removal. Nevertheless, we believe that clipping was a stimulus sensed by the tillers since it delayed the onset of leaf senescence, although not overall longevity. Continuation of leaf elongation during the long dry season is in line with a previous analysis of ramet dynamics and re-growth and underpins the extremely efficient ways of moisture and space utilization in these tussocks. Wide spacing permits greater moisture availability per tussock and, thus, permits year-round production of new foliage (fodder) in this semi-arid high altitude ecosystem.