Resilience and alternative stable states after desert wildfires

Improving models of community change is a fundamental goal in ecology and has renewed importance during global change and increasing human disturbance of the biosphere. Using the Mojave Desert (southwestern USA) as a model system, invaded by non-native plants and subject to wildfire disturbances, we examined models of resilience, alternative stable states, and convergent-divergent trajectories for 36 years of plant community change after 31 wildfires in communities dominated by the native shrubs Larrea tridentata or Coleogyne ramosissima. Perennial species richness on average was fully resilient within 23 years after disturbance in both community types. Perennial cover was fully resilient within 25 years in the Larrea community, but recovery was projected to require 52 years in the Coleogyne community. Species composition shifts were persistent, and in the Coleogyne community, the projected compositional recovery time of 550 years and increasing resembled a deflected trajectory toward potential alternative states. Disturbed sites contained a perennial species composition of predominately short-statured forbs, subshrubs, and grasses, contrasting with the larger-statured shrub and tree structure of undisturbed sites. Auxiliary datasets characterizing species recruitment, annual plants including non-native grasses, biocrust communities, and soils showed persistent differences between disturbed and undisturbed sites consistent with positive feedbacks potentially contributing to alternative stable states. Resprouting produced limited resilience for the large shrubs Larrea tridentata and Yucca spp. important to population persistence, but did not forestall long-term reduced abundance of the species. The non-native annual grass Bromus rubens increased on disturbed sites over time, suggesting persistently abundant non-native plant fuels and reburn potential. Biocrust cover on disturbed sites was half and species richness a third of amounts on undisturbed sites. Soil nitrogen was 30% greater on disturbed sites and no significant trend was evident for it to decline on even the oldest burns. Disturbed desert plant communities simultaneously supported all three models of resilience, alternative stable states, and convergent-divergent trajectories among community measures (e.g., species richness, composition), timeframes since disturbance, and spatial resolutions. Accommodating expression within ecosystems of multiple models, including those opposing each other, may help broaden theoretical models of ecosystem change.

优化群落变化模型是生态学的核心研究目标之一，在全球变化与人类对生物圈的干扰日益加剧的当下，其重要性愈发凸显。本研究以受非本土植物（non-native plants）入侵且遭受野火干扰（wildfire disturbances）的美国西南部莫哈韦沙漠（Mojave Desert）为模式系统，针对以本土灌木三齿拉雷亚（Larrea tridentata）或黑丛木（Coleogyne ramosissima）为优势种的群落，基于31场野火发生后36年间的植物群落变化观测数据，检验了恢复力（resilience）、替代稳定态（alternative stable states）以及趋异-趋同轨迹（convergent-divergent trajectories）的相关理论模型。
两类优势群落的多年生植物物种丰富度在干扰后23年内即可实现完全恢复。三齿拉雷亚群落的多年生植物盖度在干扰后25年内即可完全恢复，而黑丛木群落的多年生盖度恢复则预计需要52年。物种组成的变化呈现持续性特征；在黑丛木群落中，预计物种组成恢复时间长达550年且仍在延长，其群落轨迹偏向于向潜在替代稳定态发生偏转。
受干扰样地的多年生植物群落以低矮的阔叶草本（forbs）、亚灌木（subshrubs）和禾草为主，与未受干扰样地以高大灌木和乔木为主的群落结构形成鲜明对比。表征物种更新（species recruitment）、包括非本土禾草在内的一年生植物、生物结皮群落（biocrust communities）以及土壤特征的辅助数据集显示，受干扰与未受干扰样地间存在持续性差异，这与可能推动替代稳定态形成的正反馈（positive feedbacks）机制相符。
对于对种群存续至关重要的大型灌木三齿拉雷亚与丝兰属（Yucca spp.）植物而言，萌蘖更新（resprouting）仅能带来有限的恢复能力，且未能阻止该类物种在长期内的丰度下降。非本土一年生禾草红雀麦（Bromus rubens）在受干扰样地的丰度随时间推移持续上升，这表明非本土植物形成的可燃物持续充足，存在复燃潜力。
受干扰样地的生物结皮盖度仅为未受干扰样地的一半，物种丰富度则仅为其三分之一。受干扰样地的土壤氮含量较未受干扰样地高出30%，即便在最古老的火烧样地中，也未出现土壤氮含量显著下降的趋势。
受干扰的荒漠植物群落在群落指标（如物种丰富度、物种组成）、干扰后时间跨度以及空间分辨率（spatial resolutions）等维度上，同时契合恢复力、替代稳定态与趋异-趋同轨迹这三类模型。在生态系统中兼容多类模型（包括相互对立的模型）的表现形式，有助于拓展生态系统变化的理论模型体系。