Gap Partitioning Among Maples at Harvard Forest 1986-1989

We measured shoot architecture, photosynthesis, survival and growth by seedlings of three shade-tolerant species of maple (Acer pensylvanicum, A. rubrum, A. saccharum) in an experimental test of the gap partitioning hypothesis. Trees were felled to create a total of six cleared, elliptical canopy gaps of two sizes (8m x 12m, 75m2; 16m x 24m, 300m2). Naturally-established, undamaged, unbranched seedlings (15-30 cm tall, 10-20+ years old) of the three study species (2160 total, 720 per species) were transplanted into five plot locations (center and NW, NE, SW, and SE gap edges) within all six gaps and matching understory sites one year before gap creation. All plots were weeded regularly and spaded annually along the edges to remove above and below-ground competition. Measurements of microclimates and non-competitive seedling responses were made over one year before and two years following gap release. Architectural variation increased greatly over the two-year period. Striped maple (A. pensylvanicum) and red maple (A. rubrum) increased branch numbers, leaf numbers, and total leaf areas in gaps, especially large gaps, while sugar maple (A. saccharum) showed much smaller changes. Red maple tended to increase the number of leaves while leaf size decreased; striped maple increased leaf number but held leaf size constant. Diurnal patterns of photosynthesis differed within and between gap and understory sites. Red maple showed higher photosynthetic rates per unit leaf area than striped and sugar maple in all site/plot combinations except the large gap south plots, where striped maple exceeded red maple. Estimated diurnal shoot-level assimilation differentiated species more than unit area assimilation rates, and also altered the rank order of performance, with striped maple above red maple above sugar maple in all microsites except the large gap north. Population-level assimilation versus irradiance response curves exhibited a similar pattern, with red maple dominating unit area rates in most plot microsites. In contrast, shoot assimilation curves showed striped maple above red maple above sugar maple in all microsites except the large gap north, where red maple exceeded striped maple. Architectural variation among these species interacted with leaf-level assimilation rates to produce some differences among these species in shoot-level assimilation across the gap-understory microclimatic gradient. Because survival and growth patterns are usually correlated with differences in whole-plant carbon assimilation, the results suggested that the potential for gap partitioning among the three maple species. Red maple survived better overall across the study due to greater persistence in the north and center plots of large gaps. The small gaps and understories showed no differences among the species. Survival rates exceeded 80% in most sites and plots, with low values (30-56%) only in the exposed plots of large gaps. There were no relationships between post-gap survival and previous age, height, or basal diameter. By the end of two years of gap release, both gap sizes induced greater distinctions among the species in all growth variables than the understory. Striped maple exhibited greater leader extension, absolute stem height, net height change, absolute basal diameter, and net basal diameter change than red maple and sugar maple (in that order) in nearly all sites and plots. The exception was large gap center and north plots, where red maple equaled or exceeded striped maple in net basal diameter change but not net height increase. Sugar maple was the least responsive of the species to the gap-understory gradient. As with survival, there were no predictable relationships between pre-gap age or size and post-gap growth. A considerable amount of leader (mainstem terminus) damage occurred during the study due to unknown causes. Architectural and growth analyses were done separately on undamaged seedlings versus those that had experienced leader damage and recovery. For all species combined, survival decreased while the frequency of leader damage among survivors increased across the gradient of microsite exposure. Red maple showed the highest survival (65-93%) but also very high leader damage (80-97%). Striped maple showed fairly high survival (81-93%) in all but the most exposed microsites (24-36%) and had the lowest leader damage overall (17-44%). Sugar maple was intermediate for both survival (25-86%) and leader damage (55-96%). Growth differed significantly among sites and species. Both intact and damaged plants showed greater growth in gaps than in understory, particularly in large gaps. For most growth variables in most microsites, striped maple equalled or exceeded red maple which equaled or exceeded sugar maple when plants were intact, but red maple equaled or exceeded striped maple which exceeded sugar maple when damaged. Species differences in growth varied among sites, with large gaps produced more pronounced effects than small gaps and understory for both intact and damaged plants. Growth recovery was inversely related to leader damage frequency among species, and thus at least partially offset the effects of damage on net growth across the populations. Photosynthetic performance paralleled growth by these species across the gradient, particularly for shoot assimilation. When growth variables were plotted against irradiance and temperature measured at seedling plot positions, there were consistent and clear distinctions among species across the gap-understory gradient, providing limited evidence for gap partitioning in our system for undamaged plants. Striped maple appears to be a superior generalist, red maple is a weaker generalist, and sugar maple shows the poorest performance in a manner that is nearly insensitive to the gap-understory gradient. Leader damage in the understory prior to gap formation would reinforce this pattern of relative performance by favoring striped maple. However, damaged red maple seedlings show a decisive advantage in recovery and regrowth in the large gap centers, where the probability of a juvenile trees capturing canopy gap space is highest.

本研究针对林隙分异假说（gap partitioning hypothesis）开展受控实验，测定了3种耐阴槭树（Acer pensylvanicum、A. rubrum、A. saccharum）实生苗的枝条构型、光合特性、存活率及生长状况。
研究人员通过砍伐林木，共设置6个清理出的椭圆形林隙（canopy gap），分为两种规格：8m×12m（面积75m²）、16m×24m（面积300m²）。
在林隙形成前1年，将3种研究物种的自然萌发、无损伤、无分枝实生苗（株高15~30 cm，树龄10~20+年，总计2160株，每个物种720株）移栽至全部6个林隙内的5个样点（林隙中心及西北、东北、西南、东南边缘），以及与之匹配的林下生境。所有样地均定期除草，并每年沿边缘翻土以去除地上与地下竞争植株。
在林隙形成前1年及形成后2年期间，测定了微气候及非竞争性幼苗响应指标。两年观测周期内，枝条构型的变异程度显著增大。
条纹槭（striped maple, A. pensylvanicum）与红花槭（red maple, A. rubrum）在林隙（尤其是大林隙）中枝条数、叶片数及总叶面积均显著提升，而糖槭（sugar maple, A. saccharum）的变化幅度则小得多。红花槭的叶片数量有所增加但叶面积减小，条纹槭则叶片数量增加且叶面积保持稳定。
不同林隙与林下样点间及内部的光合日变化模式存在显著差异。除大林隙南样点外，所有样点组合中红花槭的单位叶面积光合速率（photosynthetic rate per unit leaf area）均高于条纹槭与糖槭，而大林隙南样点中条纹槭的光合速率超过红花槭。
基于估算的枝条水平日同化量（diurnal shoot-level assimilation），物种间的性能差异较单位叶面积同化量更为显著，且改变了性能排序：除大林隙北样点外，所有微生境（microsite）中性能排序均为条纹槭＞红花槭＞糖槭。
种群水平的光合-光照响应曲线呈现相似模式：多数样点微生境中红花槭的单位叶面积同化速率占优；与之相反，枝条水平同化曲线显示，除大林隙北样点（红花槭超过条纹槭）外，所有微生境中性能排序均为条纹槭＞红花槭＞糖槭。
物种间的构型差异与叶水平光合速率共同作用，使得3种槭树在林隙-林下的微气候梯度下的枝条水平同化量存在差异。由于存活率与生长模式通常与整株碳同化量差异相关，本研究结果表明3种槭树具备林隙分异的潜力。
整体而言，红花槭在本研究中的存活率更高，因其在大林隙的北样点与中心样点中持久性更强。小林隙与林下生境中各物种间无显著差异。多数样点的存活率超过80%，仅大林隙的暴露样点存活率较低（30%~56%）。林隙形成后的存活率与幼苗前期树龄、株高或基径无显著相关性。
在林隙形成后2年结束时，两种林隙规格均使得各物种间的所有生长变量差异较林下生境更为显著。几乎所有样点与样地中，条纹槭的主茎延伸量、绝对茎高、净高度变化量、绝对基径及净基径变化量均高于红花槭与糖槭（排序依次降低），仅大林隙中心与北样点例外：此处红花槭的净基径变化量等于或高于条纹槭，但净高度增加量未超过后者。
糖槭对林隙-林下梯度的响应性最弱。与存活率一致，林隙形成前的树龄或株大小与形成后的生长无显著可预测的相关性。
本研究期间，因不明原因出现了大量主茎顶端损伤（leader damage）。研究分别针对未受损幼苗与遭受主茎顶端损伤并完成恢复的幼苗开展构型与生长分析。
综合所有物种来看，随着微生境暴露程度的梯度升高，存活率下降而存活个体的主茎损伤频率上升。红花槭存活率最高（65%~93%）但主茎损伤率也极高（80%~97%）。除最暴露的微生境外，条纹槭的存活率较高（81%~93%），且整体主茎损伤率最低（17%~44%）。糖槭的存活率（25%~86%）与主茎损伤率（55%~96%）均处于中间水平。
不同样点与物种间的生长存在显著差异。无论植株是否受损，林隙中的生长量均高于林下生境，尤其在大林隙中更为明显。在多数微生境的多数生长变量中，未受损植株的性能排序为条纹槭≥红花槭≥糖槭，而受损植株则为红花槭≥条纹槭≥糖槭。
不同物种的生长差异因样点而异，大林隙对受损与未受损植株的效应均较小林隙与林下生境更为显著。生长恢复速率与物种的主茎损伤频率呈负相关关系，因此至少部分抵消了种群间损伤对净生长的影响。
本研究中各物种的光合性能随梯度变化的模式与生长趋势一致，尤其在枝条水平同化量方面。当以幼苗样点位置测得的光照与温度为横坐标绘制生长变量时，林隙-林下梯度下各物种间的差异清晰且一致，为未受损植株的林隙分异现象提供了有限的实证支持。
条纹槭似乎是更优秀的广适性物种，红花槭为较弱的广适性物种，而糖槭的性能最差，且几乎不受林隙-林下梯度的影响。林隙形成前林下发生的主茎顶端损伤会进一步强化这种相对性能模式，使条纹槭更具优势。然而，受损的红花槭实生苗在大林隙中心的恢复与再生生长中具有决定性优势，而此处正是幼树获取林隙冠层空间概率最高的区域。