In human-modified tropical landscapes (HMLs) the conservation of biodiversity, functions and services of forest ecosystems depends on persistence of old growth forest remnants, forest regeneration in abandoned agricultural fields, and restoration of degraded lands. Understanding the impacts of agricultural land uses (ALUs) on forest regeneration is critical for biodiversity conservation in HMLs. Here, we develop a conceptual framework that considers the availability of propagules and the environment prevailing after field abandonment as two major determinants of forest regeneration in HMLs. The framework proposes that regeneration potential decreases with size, duration and severity of agricultural disturbance, reducing propagule availability and creating ill-suited environmental conditions for regeneration. We used studies from Southern Mexico to assess this framework. First, we identify regeneration bottlenecks that trees face during transit from seed to follow-up life stages, using demographic analysis of dominant pioneer species in recently abandoned fields. Then, we explore effects of ALUs on forest regeneration at the field and landscape scales, addressing major legacies. Finally, we integrate agricultural disturbance with landscape composition to predict attributes of successful second growth forests in HMLs, and provide indicators useful to select tree native species for active restoration. An indicator of disturbance inflicted by ALUs, based on farmers' information, predicted better regeneration potential than measurements of soil and microclimate conditions at time of abandonment. Cover of cattle pastures in the landscape was a stronger indicator of forest regenerating attributes than cover of old growth forest remnants. To conclude, we offer recommendations to promote forest regeneration and biodiversity conservation in HMLs.Abstract in Spanish is available with online material.
Quercus mulleri es un encino microendémico de la Sierra Sur de Oaxaca y se encuentra dentro de la Lista Roja de Especies amenazadas de la UICN como “en peligro crítico”, sin embargo, debido a la falta de información actual sobre la especie, no se conoce el estado de conservación de sus poblaciones y, por lo tanto, no se ha podido asignar una categoría adecuada de riesgo. El estudio se realizó con el objetivo de analizar abundancia, distribución y diversidad genética de la especie, para proponer estrategias de conservación adecuadas. Los individuos localizados fueron georreferenciados, se les midió la altura y el diámetro a la altura del pecho para clasificarlos en cinco clases de tamaños; la diversidad genética se analizó empleando cinco regiones de microsatélites de la serie quru-GA. Se relocalizó la especie, encontrando que está restringida a una pequeña región de la Sierra Sur, donde se encuentra de manera fragmentada y aislada geográficamente. Sus poblaciones mostraron baja frecuencia de individuos por clase de tamaño (6.13 ± 5.6). Los valores de diversidad alélica, empleando cinco regiones de microsatélites de la serie quru-GA, fueron bajos (AT=22 y Ao=4.4) y los de diversidad genética fueron moderados (Ho=0.54), lo que sugiere que la población atravesó por un cuello de botella. Este trabajo representa el primer reporte de Q. mulleri después de más de 60 años de su última clasificación taxonómica, y los resultados indican que Q. mulleri es una especie vulnerable, dado que en su zona de distribución existe un proceso de pérdida de hábitat que, junto con la fragmentación de su población, ponen en riesgo la permanencia de la especie, por lo que se recomienda incluirla en la Norma Oficial Mexicana 059 como “Especie en Peligro de Extinción”.
Forest regrowth is key to achieve restoration commitments, but a general lack of understanding when it occurs and how long secondary forests persist hampers effective upscaling. We quantified spatiotemporal forest dynamics in a recently colonized agricultural frontier in southern Mexico, and tested how temporal variation in climate, and cross-community variation in land ownership, land quality and accessibility affect forest disturbance, regrowth and secondary forest persistence. We consistently found more forest loss than regrowth, resulting in a net decrease of 45% forest cover in the study region. Secondary forest cover remained relatively constant while secondary forest persistence increased, suggesting that farmers are moving away from shifting cultivation. Temporal variation in disturbance was explained by annual variation in climatic variables and key policy and market interventions. We found large differences in forest characteristics across communities, and these were explained by differences in land ownership and soil quality. Forests were better conserved on communal land, while secondary forest was more persistent when farms were larger and soil quality is better. At the pixel-level both old forest and secondary forests were better represented on low-quality lands indicating agricultural concentration on productive land. Both old forest and secondary forest were less common close to the main road, where secondary forests were also less persistent. We demonstrate the suitability of timeseries analyses to quantify forest disturbance and regrowth and we analyse drivers across time and space. Communities differ in forest dynamics, indicating different possibilities, needs and interests. We warrant that stimulating private land ownership may cause remaining forest patches to be lost and that conservation initiatives should benefit the whole community. Forest regrowth competes with agricultural production and ensuring farmers have access to restoration benefits is key to restoration success.
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