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.

本研究针对林隙分区假说开展实验检验，测定了3种耐阴槭树（条纹槭（Acer pensylvanicum）、红槭（A. rubrum）、糖槭（A. saccharum））实生苗的枝条构型、光合作用、存活率及生长状况。研究人员通过砍伐林木，共设置6处清理后的椭圆形林冠林隙，分为两种规格：8m×12m（面积75m²）、16m×24m（面积300m²）。在所有6处林隙及匹配的林下生境中，选取自然定植、无损伤、无分枝的实生苗（株高15~30cm，树龄10~20年以上），每个研究物种各720株，总计2160株，于林隙形成前1年移栽至5个样点位置（林隙中心及西北、东北、西南、东南林隙边缘）。所有样地均定期除草，并每年沿边缘翻土以移除地上及地下竞争植株。在林隙形成前1年及形成后2年期间，对微气候及非竞争性实生苗响应进行测定。两年间结构变异程度显著升高。条纹槭与红槭的分枝数、叶片数及总叶面积在林隙中显著增加，尤其在大林隙中；而糖槭的变化幅度则小得多。红槭的叶片数趋于增加，但叶面积有所减小；条纹槭的叶片数增加，但叶面积保持稳定。不同林隙及林下生境内部和之间的光合作用日变化模式存在差异。除大林隙南部样点（此处条纹槭的光合速率超过红槭）外，在所有样点/样地组合中，红槭的单位叶面积光合速率均高于条纹槭与糖槭。基于枝条水平的日间总同化量较单位面积同化率更能区分物种，且改变了性能排名：除大林隙北部样点外，所有微生境中的性能排名均为条纹槭＞红槭＞糖槭。种群水平的同化量-光照响应曲线呈现相似模式：多数样点微生境中，红槭的单位面积光合速率占优。与之相反，枝条水平的同化曲线显示，除大林隙北部样点（此处红槭超过条纹槭）外，所有微生境中的排名均为条纹槭＞红槭＞糖槭。这些物种间的结构变异与叶片水平光合速率相互作用，使得跨林隙-林下微气候梯度的枝条水平同化量存在物种间差异。由于存活率与生长模式通常与整株碳同化的差异相关，本研究结果表明3种槭树间存在林隙分区的潜力。整体而言，红槭在本研究中的存活率更高，因其在大林隙的北部及中心样点中持久性更强。小林隙及林下生境中未出现物种间的差异。多数样点及样地的存活率超过80%，仅大林隙的裸露样点存活率较低（30%~56%）。林隙形成后的存活率与实生苗先前的树龄、株高或基径无相关性。林隙形成释放效应2年后，相较于林下生境，两种林隙规格均使得各物种在所有生长变量上的差异更为显著。几乎所有样点及样地中，条纹槭的主茎延伸长度、绝对株高、净株高变化量、绝对基径及净基径变化量均优于红槭与糖槭（按此顺序）。例外情况为大林隙中心及北部样点，此处红槭的净基径变化量等于或超过条纹槭，但净株高增加量未超过。糖槭对林隙-林下梯度的响应性最差。与存活率情况一致，林隙形成前的树龄或大小与林隙形成后的生长无可预测的相关性。研究期间，因未知原因出现了大量主茎顶梢（主干顶端）损伤情况。研究分别对未受损伤的实生苗及遭受顶梢损伤并已恢复的实生苗进行结构与生长分析。综合所有物种来看，随着微生境暴露梯度升高，存活率下降，而存活个体中的顶梢损伤频率上升。红槭的存活率最高（65%~93%），但顶梢损伤率也极高（80%~97%）。除最暴露的微生境（存活率24%~36%）外，条纹槭的存活率较高（81%~93%），且整体顶梢损伤率最低（17%~44%）。糖槭的存活率（25%~86%）与顶梢损伤率（55%~96%）均处于中间水平。不同样点及物种间的生长存在显著差异。无论完整植株还是受损植株，林隙中的生长均优于林下生境，尤其在大林隙中。多数微生境的多数生长变量中，完整植株的性能排名为条纹槭≥红槭≥糖槭；而受损植株的排名则为红槭≥条纹槭≥糖槭。物种间的生长差异因样点而异，无论完整植株还是受损植株，大林隙产生的效应均较小林隙及林下生境更为显著。生长恢复率与物种间的顶梢损伤频率呈负相关，因此在种群水平上至少部分抵消了损伤对净生长的影响。这些物种跨梯度的光合表现与生长情况相符，尤其在枝条同化量方面。当以实生苗样点位置测得的光照与温度为横轴绘制生长变量时，跨林隙-林下梯度的物种间差异清晰且一致，为未受损伤植株的林隙分区现象提供了有限的证据。条纹槭似乎是更优秀的泛化物种，红槭为较弱的泛化物种，而糖槭的表现最差，且其性能几乎不受林隙-林下梯度的影响。林隙形成前林下发生的顶梢损伤会通过偏向条纹槭而强化这种相对性能模式。然而，受损的红槭实生苗在大林隙中心（幼树获取林隙空间概率最高的区域）的恢复与再生长具有决定性优势。