ABSTRACT
Recientemente se ha abordado la delimitación de parches de vegetación a través de clasificaciones orientadas a objetos con datos multiespectrales. En esta investigación se presenta una propuesta de caracterización de selvas secundarias basada en matrices de semejanza de composición específica, estructura de la vegetación arbórea e índices de diversidad, partiendo de una clasificación orientada a objetos de imágenes multiespectrales. El área de estudio corresponde a una selva mediana subperennifolia con antecedente de aprovechamiento agrícola bajo el sistema de roza tumba y quema (Ac: hace 8-23 años), aprovechamiento forestal selectivo (Fs: hace 43-53 años), y aprovechamiento forestal selectivo e incendio forestal (Fc: hace 21-28 años). Se delinearon tres parches de vegetación de acuerdo al historial de uso, y se analizaron las diferencias entre las medias de las métricas de campo de cada parche con un análisis de varianza multivariado permutacional (P < 0.001). Con un modelo logístico multinomial se identificaron, de entre más de 20 covariables, la riqueza de especies, densidad de tallos por hectárea, y el primer eje de variación del ordenamiento NMDS de la composición específica, como los atributos de la vegetación más distintivos. El parche Fc, presenta los mayores valores en diversidad de Shannon-Wiener, riqueza de especies, y densidad de tallos mayores a 7.5 cm de diámetro. El Fs tiene los mayores promedios de biomasa, área basal y altura individual, y el Ac tiene los menores valores de todas las métricas anteriores. Las especies de mayor Índice de Valor de Importancia fueron: Ac; Bursera simaruba y Psidia psipula, Fs: Gymnanthes lucida y Manilkara zapota, Fc: G. lucida y B. simaruba.
A proposal for characterizing habitat of forests, obtained from an object-oriented classification of RapidEye multiespectral imagery, based on dissimilarity matrices of vegetation structure, species diversity and composition is presented. The study area is a forested landscape mosaic after slash and burn agriculture (Ac: 8-23 years ago), selective logging (Fs: 43-53 years ago), and selective logging and forest fire (Fc: 21-28 years ago). The site is located in the central part of Quintana Roo, México, where three vegetation patches were delineated according to remotely sensed multiespectral imagery. Mean differences between vegetation structure properties of each vegetation patch were obtained through a permutational multivariate analysis of variance (P < 0.001). Species richness, stem density per hectare, and the axis-1 scores of the non-metric multidimensional scaling ordination of specific composition were identified as the vegetation attributes more relevant to differentiate the vegetation patches by a multinomial logistic model. Fc vegetation patch is characterized by the greatest mean values on Shannon-Wiener index, species richness, and stem density. The Fs has the greatest mean values of canopy height, basal area, and biomass at 80 percentile, and the Ac vegetation patch has the lowest values of all mentioned metrics. The species with the greatest relative importance value were: Ac: Bursera simaruba and Psidia psipula, Fs: Gymnanthes lucida and Manilkara zapota, Fc: G. lucida and B. simaruba. The uncertainty associated with the metrics assessed by vegetation patch was smaller than the uncertainty of the whole area, because of the efficient variability aggregation of the field data. We conclude that multiespectral information is a reliable tool for distinguishing vegetation patches with specific features, as stem density, specific composition, and species richness.
ABSTRACT
The rarity of nitrogen (N)-fixing trees in frequently N-limited higher-latitude (here, > 35°) forests is a central biogeochemical paradox. One hypothesis for their rarity is that evolutionary constraints limit N-fixing tree diversity, preventing N-fixing species from filling available niches in higher-latitude forests. Here, we test this hypothesis using data from the USA and Mexico. N-fixing trees comprise only a slightly smaller fraction of taxa at higher vs. lower latitudes (8% vs. 11% of genera), despite 11-fold lower abundance (1.2% vs. 12.7% of basal area). Furthermore, N-fixing trees are abundant but belong to few species on tropical islands, suggesting that low absolute diversity does not limit their abundance. Rhizobial taxa dominate N-fixing tree richness at lower latitudes, whereas actinorhizal species do at higher latitudes. Our results suggest that low diversity does not explain N-fixing trees' rarity in higher-latitude forests. Therefore, N limitation in higher-latitude forests likely results from ecological constraints on N fixation.
Subject(s)
Nitrogen , Trees , Forests , Nitrogen Fixation , North AmericaABSTRACT
Symbiotic nitrogen (N)-fixing trees can drive N and carbon cycling and thus are critical components of future climate projections. Despite detailed understanding of how climate influences N-fixation enzyme activity and physiology, comparatively little is known about how climate influences N-fixing tree abundance. Here, we used forest inventory data from the USA and Mexico (>125,000 plots) along with climate data to address two questions: (1) How does the abundance distribution of N-fixing trees (rhizobial, actinorhizal, and both types together) vary with mean annual temperature (MAT) and precipitation (MAP)? (2) How will changing climate shift the abundance distribution of N-fixing trees? We found that rhizobial N-fixing trees were nearly absent below 15°C MAT, but above 15°C MAT, they increased in abundance as temperature rose. We found no evidence for a hump-shaped response to temperature throughout the range of our data. Rhizobial trees were more abundant in dry than in wet ecosystems. By contrast, actinorhizal trees peaked in abundance at 5-10°C MAT and were least abundant in areas with intermediate precipitation. Next, we used a climate-envelope approach to project how N-fixing tree relative abundance might change in the future. The climate-envelope projection showed that rhizobial N-fixing trees will likely become more abundant in many areas by 2080, particularly in the southern USA and western Mexico, due primarily to rising temperatures. Projections for actinorhizal N-fixing trees were more nuanced due to their nonmonotonic dependence on temperature and precipitation. Overall, the dominant trend is that warming will increase N-fixing tree abundance in much of the USA and Mexico, with large increases up to 40° North latitude. The quantitative link we provide between climate and N-fixing tree abundance can help improve the representation of symbiotic N fixation in Earth System Models.
Subject(s)
Climate Change , Forests , Nitrogen Fixation , Trees/physiology , Mexico , Population Dynamics , Rain , Symbiosis , Temperature , United StatesABSTRACT
The rarity of symbiotic nitrogen-fixing trees in higher-latitude compared to lower-latitude forests is paradoxical because higher-latitude soils are relatively N poor. Using national-scale forest inventories from the United States and Mexico, we show that the latitudinal abundance distribution of N-fixing trees (more than 10 times less abundant poleward of 35 degrees N) coincides with a latitudinal transition in symbiotic N-fixation type: rhizobial N-fixing trees (which are typically facultative, regulating fixation to meet nutritional demand) dominate equatorward of 35 degrees N, whereas actinorhizal N-fixing trees (typically obligate, maintaining fixation regardless of soil nutrition) dominate to the north. We then use theoretical and statistical models to show that a latitudinal shift in N-fixation strategy (facultative vs. obligate) near 35 degrees N can explain the observed change in N-fixing tree abundance, even if N availability is lower at higher latitudes, because facultative fixation leads to much higher landscape-scale N-fixing tree abundance than obligate fixation.
Subject(s)
Nitrogen Fixation/physiology , Nitrogen/metabolism , Trees/physiology , Demography , Mycorrhizae/physiology , Soil , Trees/microbiologyABSTRACT
We evaluated the effect of soil "scarification" and vegetation clearing treatments on the natural regeneration and initial development of Tabebuia rosea (Bertold) DC. seedlings in a moderate sized semideciduous tropical forest subjected to wood harvesting on the coast of Jalisco, Mexico. The treatments were applied under "seed" trees, and the number of germinated seedlings and their development were evaluated for nine months. Soil "scarification" promoted seed germination and initial seedling development, while the control of the competing vegetation increased the seedling growth and reduced their mortality. These results should be taken into account for the natural regeneration of this species, after clearing, to improve wood production, and should be incorporated into the silvicultural techniques currently developed in the region.
Evaluamos el efecto de los tratamientos de remoción de suelo y roza de vegetación sobre el establecimiento y desarrollo inicial de la regeneración natural de Tabebuia rosea (Bertold) DC. en una selva mediana subcaducifolia sometida a aprovechamiento maderable en la costa de Jalisco, México. Los tratamientos se aplicaron bajo la copa de árboles semilleros y se evaluó el número de plántulas emergidas y su desarrollo durante nueve meses. El tratamiento de remoción de suelo favoreció la emergencia de plántulas y su desarrollo inicial, mientras que el control de la vegetación competidora promovió un mayor crecimiento y menores porcentajes de mortalidad. Estos aspectos deben tomarse en cuenta para la regeneración natural de la especie después de la aplicación de las cortas con fines de aprovechamiento maderable y se sugiere incorporarlos a las técnicas silvícolas que actualmente se desarrollan en la zona.
Subject(s)
Tropical Climate , Seedlings/physiology , Soil , Tabebuia/physiology , Trees , Mexico , Seedlings/growth & development , Tabebuia/growth & developmentABSTRACT
We evaluated the effect of soil "scarification" and vegetation clearing treatments on the natural regeneration and initial development of Tabebuia rosea (Bertold) DC. seedlings in a moderate sized semideciduous tropical forest subjected to wood harvesting on the coast of Jalisco, Mexico. The treatments were applied under "seed" trees, and the number of germinated seedlings and their development were evaluated for nine months. Soil "scarification" promoted seed germination and initial seedling development, while the control of the competing vegetation increased the seedling growth and reduced their mortality. These results should be taken into account for the natural regeneration of this species, after clearing, to improve wood production, and should be incorporated into the silvicultural techniques currently developed in the region.
