ecosystem services to and from north american arid ... - Conabio

IV SIMPOSIO INTERNACIONAL DE PASTIZALES
San Luis Potosí, S.L.P. México; 22 al 24 de agosto de 2007
Artículos in extenso
Área temática: Productividad
seriously compromised. Each time a pasture is scanned for NDVI measurements, there is a
new set of conditions influencing the balance among growing and senescent material. Annual
bromes, for instance, have been observed to vary in the time at which they start growing in the
spring from year to year; these annual species along with tall fescue are the only bright green
forage growing mixed at the canopy level or underneath the brown bermudagrass. Nevertheless,
the appearance of the canopy in the pasture changes depending on how much bermudagrass
grew after grazing the previous season and the condition for the cool-season annuals and for tall
fescue to grow in the spring.
The presence of variable senescent vegetation in time and space was reported by Curran
(1983), when he indicates “As vegetation senesces, the near-infrared leaf reflectance does not
significantly decreases. However, the breakdown of plant pigments causes a rise in red
reflectance. Therefore if the amount of senescent vegetation in a canopy increases, the positive
relation between near-infrared reflectance and green leaf area index will probably remain
unchanged whereas the relation between red reflectance and green leaf area index will weaken
and probably disappear (Curran, 1980c). This is a problem in semi-natural vegetation,
particularly grasslands, where there is some senescent vegetation in the canopy throughout the
year”.
Longer duration experiments and the use of other vegetative indices may yield more promising
results than NDVI on the stability of the relationship with forage yield in mixed pastures. Babar et
al., (2006) found that near infrared based indices highly correlated with wheat biomass at late
developmental stages (heading and grain filling), i.e., when canopy had already reached
maximum biomass production. This may be indicative that near infrared based indices, instead
of indices intended to measure canopy photosynthetic area could perform better than NDVI in
estimating differences in biomass forage yields in mixed pastures.
Because of its short duration and because of restricted to only one location, we recognize the
results of this study are not conclusive. However, we expect that at least it can serve as
background information to plan more conclusive research about the potential of NDVI to relate to
forage yield in mixed, terraced pastures. Given the large influence of the variation caused by
terraces in soil fertility [Santillano-Cázares et al. (in review)] and forage yields [Santillano-
Cázares et al. (unpublished data)], we would suggest to take into account microrelief points as
part of the treatment structure and its interactions with fertilizers and seasons.
CONCLUSIONS
These results suggest that the utility of NDVI is severely limited as a tool to indirectly estimate
forage yield in mixed pastures. Observations made in this study agree with previous reports that
NDVI functions independently of forage biomass production at relatively low and relatively high
levels. In addition, the species present in mixed pastures vary in the proportion to the total
forage produced and in greenness over time due to changing environmental conditions. These
two problems in mixed pastures minimize NDVI’s ability to relate to forage biomass. Normalized
difference vegetation index does not work accurately in relating with forage biomass on mixed
pastures because frequently a substantial fraction of the yield is composed by non green forage
and NDVI was designed to relate with green biomass and before full canopy closure.
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