Can prescribed fires restore C4 grasslands invaded by a C3 woody species and a co-dominant C3 grass species?

Prescribed fire is used to reduce woody plant invasion and restore herbaceous production and diversity in grasslands and savannas worldwide. Here we determined if a concentrated series of repeated-winter, repeated-summer, or alternate-season (winter and summer) fires in a short timeframe (“transition fires”) could catalyze the restoration of C4 perennial grasses in Southern Great Plains, USA grasslands that had become dominated by a fire-tolerant C3 woody N2-fixer (honey mesquite, Prosopis glandulosa) and a C3 perennial bunchgrass (Texas wintergrass, Nassella leucotricha). We applied transition fires over a 5-year span, and maintenance fires on a portion of each plot 7 or 8 years later. We measured herbaceous standing biomass and cover and soil variables (soil organic C, N, δ13C and δ15N) in unburned, transition-burned and maintenance-burned treatments. Greater δ13C at 10-20 (-17 ‰) than 0-10 (-20 ‰) cm depth increment confirmed that vegetation was historically mostly C4 grassland that shifted towards C3 dominance. Transition treatments with summer fire were most effective at top-killing mesquite, but no treatments root-killed >3%. Regrowth of top-killed mesquite was similar in all treatments and reached pre-fire height by 9 to 10 years post-fire. Herbaceous production and cover responses showed that: (1) alternate-season transition fires increased C4 mid-grass, but did not change Texas wintergrass, (2) repeated-summer fires reduced Texas wintergrass, but did not change C4 mid-grass, and (3) repeated-winter fires did not change C4 mid-grass or Texas wintergrass compared to the unburned control. All maintenance fires stimulated Texas wintergrass biomass and cover, thus eliminating the reduction of Texas wintergrass caused by repeated-summer transition fires. There were no long-term effects of transition fires on soil C, N, δ13C or δ15N. Results advance our understanding of the expectations and limitations of prescribed fire in shifting a woodland alternate state toward what was historically a fire supported C4 grassland/savanna.

Methods
Measurements of fire, plant and soil responses in remote field study site

计划性火烧（Prescribed fire）被广泛应用于全球草原与稀树草原生境，以减少木本植物入侵，恢复草本植物的生产力与物种多样性。本研究旨在探究：在短时间内集中开展一系列重复冬季火烧、重复夏季火烧或交替季节（冬夏交替）火烧（即“过渡火烧（transition fires）”），能否促进美国南部大平原草原的碳四（C4）多年生草本恢复——该区域当前已被耐火型碳三（C3）木本固氮植物（牧豆树，*Prosopis glandulosa*）与碳三（C3）多年生丛生草本德克萨斯羽茅（*Nassella leucotricha*）占据主导。

我们在5年周期内实施过渡火烧，并在7至8年后对每个样地的部分区域开展维持火烧。我们对未火烧、过渡火烧与维持火烧三种处理样地的草本现存生物量、盖度以及土壤指标（土壤有机碳、全氮、δ¹³C与δ¹⁵N）进行了测定。10-20 cm土层的δ¹³C值（-17‰）高于0-10 cm土层（-20‰），这证实该区域历史上以碳四草本草原为主，后逐渐转变为碳三植物主导的生境。

夏季火烧过渡处理对牧豆树的地上部分灭杀效果最佳，但所有处理均未实现超过3%的植株地下部分灭杀。所有处理中，被地上部分灭杀的牧豆树的恢复情况无显著差异，且在火烧后9至10年即可恢复至火烧前高度。

草本生产力与盖度的响应结果显示：（1）交替季节过渡火烧可提升碳四中型草本的占比，但对德克萨斯羽茅无显著影响；（2）重复夏季火烧可降低德克萨斯羽茅的占比，但对碳四中型草本无显著影响；（3）与未火烧对照组相比，重复冬季火烧对碳四中型草本与德克萨斯羽茅均无显著影响。所有维持火烧处理均会促进德克萨斯羽茅的生物量与盖度，因此抵消了重复夏季过渡火烧对德克萨斯羽茅的抑制效果。

过渡火烧对土壤有机碳、全氮、δ¹³C与δ¹⁵N均无长期影响。本研究结果加深了我们对计划性火烧的应用预期与局限性的认知，可为将木本植被主导的交替生境恢复至历史上依赖火烧的碳四草原/稀树草原提供参考。

研究方法
野外偏远样地的火烧、植物与土壤响应测定