RESUMO
The nutrient demands of regrowing tropical forests are partly satisfied by nitrogen-fixing legume trees, but our understanding of the abundance of those species is biased towards wet tropical regions. Here we show how the abundance of Leguminosae is affected by both recovery from disturbance and large-scale rainfall gradients through a synthesis of forest inventory plots from a network of 42 Neotropical forest chronosequences. During the first three decades of natural forest regeneration, legume basal area is twice as high in dry compared with wet secondary forests. The tremendous ecological success of legumes in recently disturbed, water-limited forests is likely to be related to both their reduced leaflet size and ability to fix N2, which together enhance legume drought tolerance and water-use efficiency. Earth system models should incorporate these large-scale successional and climatic patterns of legume dominance to provide more accurate estimates of the maximum potential for natural nitrogen fixation across tropical forests.
Assuntos
Fabaceae/crescimento & desenvolvimento , Florestas , Chuva , Árvores/crescimento & desenvolvimento , América Central , Densidade Demográfica , Porto Rico , América do SulRESUMO
Vegetation indices are useful tools to remotely estimate several important parameters related to ecosystem functioning. However, improving and validating estimations for a wide range of vegetation types are necessary. In this study, we provide a methodology for the estimation of the leaf area index (LAI) in a tropical dry forest (TDF) using the light diffusion through the canopy as a function of the successional stage. For this purpose, we estimated the K coefficient, a parameter that relates the normalized difference vegetation index (NDVI) to LAI, based on photosynthetically active radiation (PAR) and solar radiation. The study was conducted in the Mata Seca State Park, in southeastern Brazil, from 2012 to 2013. We defined four successional stages (very early, early, intermediate, and late) and established one optical phenology tower at one plot of 20 × 20 m per stage. Towers measured the incoming and reflected solar radiation and PAR for NDVI calculation. For each plot, we established 24 points for LAI sampling through hemispherical photographs. Because leaf cover is highly seasonal in TDFs, we determined ΔK (leaf growth phase) and Kmax (leaf maturity phase). We detected a strong correlation between NDVI and LAI, which is necessary for a reliable determination of the K coefficient. Both NDVI and LAI varied significantly between successional stages, indicating sensitivity to structural changes in forest regeneration. Furthermore, the K values differed between successional stages and correlated significantly with other environmental variables such as air temperature and humidity, fraction of absorbed PAR, and soil moisture. Thus, we established a model based on spectral properties of the vegetation coupled with biophysical characteristics in a TDF that makes possible to estimate LAI from NDVI values. The application of the K coefficient can improve remote estimations of forest primary productivity and gases and energy exchanges between vegetation and atmosphere. This model can be applied to distinguish different successional stages of TDFs, supporting environmental monitoring and conservation policies towards this biome.
Assuntos
Ecossistema , Folhas de Planta , Clima Tropical , Brasil , Monitoramento Ambiental , Florestas , Estações do AnoRESUMO
BACKGROUND: The red gum lerp psyllid, Glycaspis brimblecombei Moore (Hemiptera: Psyllidae), an eucalypt insect pest from Australia, was reported in Brazil in 2003. This study evaluated damage patterns of this pest on Eucalyptus camaldulensis Dehn (Myrtaceae) and its hybrids E. urophylla X E. camaldulensis (urocam) and E. urophylla X E. grandis (urograndis). In addition, parasitism rates of Psyllaephagus bliteus Riek (Hymenoptera: Encyrtidae) on G. brimblecombei collected on different eucalypt genotypes are reported. METHODS: Plantation plots of three eucalypt genotypes were evaluated over one year. The eucalypt leaves were collected and examined for attack by G. brimblecombei. Nymph parasitism of G. brimblecombei by P. bliteus was recorded. RESULTS: Damage by G. brimblecombei was lower on the hybrid genotypes and on the adaxial surface of the eucalypt leaves. G. brimblecombei egg and nymph density were negatively correlated with monthly rainfall. Nymph parasitism of G. brimblecombei by P. bliteus was low (2.9%) independent of genotype and did not vary throughout the year. DISCUSSION: Our data indicate the use of less susceptible eucalypt genotypes (e.g., hybrids) as an alternative to G. brimblecombei management. Because of the current low mortality rates for G. brimblecombei resulting from P. bliteus parasitism, biological control with this natural enemy is not recommended as a management strategy for G. brimblecombei.