Forests are critical to water resources, but high evapotranspiration (ET) can reduce water yield. Thinning and prescribed fire reduce forest density and often reduce ET, promoting higher water yield. However, results from such treatments have been inconsistent, possibly because of unknown interactions among individual ET components. We compare water budget components of longleaf pine (Pinus palustris Mill.) woodlands with frequent prescribed fire to the water budget components of fire-excluded stands. We hypothesized that fire exclusion would result in higher ET due to increased midstory transpiration (E_t) and interception (E_i), and higher evaporation from litter (I_litter). Reference plots were burned every two years while treatment plots had fire excluded for 15–20 years. Fire treatments were repeated in two sites representing a soil moisture gradient, noted as mesic and xeric. We measured woody E_t using sap flux, and we modeled groundcover E_t using physiological models. We measured E_i of canopy and groundcover layers, modeled E_s litter biomass, and constructed a total component-based water budget for each site and treatment. Compared with reference plots, midstory E_t was 300%–800% higher in fire exclusion plots. Groundcover E_t was ~80% less than reference treatments, countering the effects of midstory growth on total ET. Stand E_i followed similar trends, with groundcover E_i in reference plots countering the effects of midstory and litter E_i in fire exclusion plots. As expected, total ET in the xeric site was 18% higher in fire exclusion plots. However, ET in the mesic site was 16% lower in the fire exclusion plots due to high groundcover E_t and E_i in reference plots. Thus, our results show that fire exclusion changes total forest ET, but the size and direction of the effect vary depending on the balance between midstory and groundcover transpiration and interception. These results highlight the importance of groundcover in ecosystem function in low-density forests and may help explain inconsistent results from studies of water yields following thinning and fire. While prescribed fire is a valuable tool in forest management, we suggest that the effects of fire on ET are complex and require careful accounting of all water fluxes within a forest ecosystem.

中文翻译：

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防火改变森林蒸散量：长叶松林地水收支综合分析


森林对水资源至关重要，但高蒸散量 （ET） 会降低产水量。间伐和计划烧除会降低森林密度，通常会减少 ET，从而提高产水量。然而，这种治疗的结果并不一致，可能是因为单个 ET 成分之间的相互作用未知。我们将经常计划用火的长叶松 （Pinus palustris Mill.） 林地的水预算成分与防火林分的水预算成分进行了比较。我们假设防火将导致更高的 ET，因为层中蒸腾 （E_t） 和拦截 （E） 增加，以及凋落物蒸发量增加 （I_凋落物）。参考地块每两年焚烧一次，而处理地块则排除了 15-20 年的火灾。在代表土壤湿度梯度的两个地点重复火处理，标记为中湿和干燥。我们使用树液通量测量了木本 E_t，并使用生理模型对地被植物 E_t 进行建模。我们测量了冠层和地被层的 E，模拟了 E_s 凋落物生物量，并为每个地点和处理构建了基于总成分的水预算。与参考样地相比，防火样地的中层 E_t 高出 300%–800%。地被 E_t 比参考处理低 ~80%，抵消了林中生长对总 ET 的影响。Stand E 遵循类似的趋势，参考样地中的地被 E 抵消了防火样地中层和凋落物 E 的影响。 正如预期的那样，在防火图中，干燥场地的总 ET 高出 18%。然而，由于参考地块中的地被 E_t 和 E 较高，在防火样地中，中湿地点的 ET 降低了 16%。因此，我们的结果表明，防火改变了森林总 ET，但影响的大小和方向取决于林中和地被蒸腾和拦截之间的平衡。这些结果突出了地被植物在低密度森林生态系统功能中的重要性，并可能有助于解释间伐和火灾后产水量研究的不一致结果。虽然计划烧除是森林管理中的宝贵工具，但我们认为烧除对 ET 的影响很复杂，需要仔细考虑森林生态系统中的所有水通量。