Humanshave long utilized resources from all forest biomes, but the most indelible anthropogenic signature has been the expanse of human populations in temperate forests. The purpose of this review is ...to bring into focus the diverse forests of the temperate region of the biosphere, including those of hardwood, conifer and mixed dominance, with a particular emphasis on crucial challenges for the future of these forested areas. Implicit in the term ‘temperate’ is that the predominant climate of these forest regions has distinct cyclic, seasonal changes involving periods of growth and dormancy. The specific temporal patterns of seasonal change, however, display an impressive variability among temperate forest regions. In addition to the more apparent current anthropogenic disturbances of temperate forests, such as forest management and conversion to agriculture, human alteration of temperate forests is actually an ancient phenomenon, going as far back as 7000 yr before present (BP). As deep-seated as these past legacies are for temperate forests, all current and future perturbations, including timber harvesting, excess nitrogen deposition, altered species’ phenologies, and increasing frequency of drought and fire, must be viewed through the lens of climate change.
Despite a growing awareness that the herbaceous layer serves a special role in maintaining the structure and function of forests, this stratum remains an underappreciated aspect of forest ecosystems. ...In this article I review and synthesize information concerning the herb layer's structure, composition, and dynamics to emphasize its role as an integral component of forest ecosystems. Because species diversity is highest in the herb layer among all forest strata, forest biodiversity is largely a function of the herb-layer community. Competitive interactions within the herb layer can determine the initial success of plants occupying higher strata, including the regeneration of dominant overstory tree species. Furthermore, the herb layer and the overstory can become linked through parallel responses to similar environmental gradients. These relationships between strata vary both spatially and temporally. Because the herb layer responds sensitively to disturbance across broad spatial and temporal scales, its dynamics can provide important information regarding the site characteristics of forests, including patterns of past land-use practices. Thus, the herb layer has a significance that belies its diminutive stature.
Elevated anthropogenic nitrogen (N) deposition has greatly altered terrestrial ecosystem functioning, threatening ecosystem health via acidification and eutrophication in temperate and boreal forests ...across the northern hemisphere. However, response of forest soil acidification to N deposition has been less studied in humid tropics compared to other forest types. This study was designed to explore impacts of long‐term N deposition on soil acidification processes in tropical forests. We have established a long‐term N‐deposition experiment in an N‐rich lowland tropical forest of Southern China since 2002 with N addition as NH₄NO₃ of 0, 50, 100 and 150 kg N ha⁻¹ yr⁻¹. We measured soil acidification status and element leaching in soil drainage solution after 6‐year N addition. Results showed that our study site has been experiencing serious soil acidification and was quite acid‐sensitive showing high acidification (pH₍H₂O₎<4.0), negative water‐extracted acid neutralizing capacity (ANC) and low base saturation (BS,< 8%) throughout soil profiles. Long‐term N addition significantly accelerated soil acidification, leading to depleted base cations and decreased BS, and further lowered ANC. However, N addition did not alter exchangeable Al³⁺, but increased cation exchange capacity (CEC). Nitrogen addition‐induced increase in SOC is suggested to contribute to both higher CEC and lower pH. We further found that increased N addition greatly decreased soil solution pH at 20 cm depth, but not at 40 cm. Furthermore, there was no evidence that Al³⁺ was leaching out from the deeper soils. These unique responses in tropical climate likely resulted from: exchangeable H⁺ dominating changes of soil cation pool, an exhausted base cation pool, N‐addition stimulating SOC production, and N saturation. Our results suggest that long‐term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future.
Over the last decade, the field of plant ecology has significantly developed and expanded, especially in research concerning the herb layer and ground vegetation of forests. This revised second ...edition accounts for that growth, presenting research that approaches the ecology of the herb layer of forests from a variety of disciplines and perspectives. The book synthesizes the research of top ecologists and biologists on herbaceous layer structure, composition, and dynamics of a variety of forest ecosystem types in eastern North America. The 2003 first edition of this book was praised for containing the most extensive listing of herb-layer literature in existence. This second edition brings this material up to date, revised to include current research and data. The book incorporates quantitative data to support analyses that was previously unavailable during the publication of the first edition. Also featured are six entirely new chapters, focused on the response of the herbaceous layer to a wide variety of natural and anthropogenic disturbances.
1 This review brings into focus what is known about the response of the herbaceous layer of forest ecosystems to increasing nitrogen deposition. The emphasis on forests in general is important for ...two reasons. First, forests often occupy areas receiving high rates of atmospheric deposition of N. Second, compared with herb-dominated communities, about which much is known regarding response to excess N, forests generally display greater biological and structural complexity. The more specific focus on the herbaceous layer - here defined as all vascular (herbaceous and woody) plants$\leq 1 m$in height - is warranted because most of the species diversity of forests occurs in the herb layer. 2 Most responses of forest ecosystems to N saturation, defined here as ecosystem supply of N exceeding ecosystem demand for N, can be characterized by two complementary hypotheses, each emphasizing different facets of ecosystem structure and function. One focuses on ecosystem processes, such as net primary productivity, whereas the other addresses seasonal patterns of concentrations of NO3
-in stream water. Although neither hypothesis considers the effects of chronically elevated N deposition on forest herbs, both share a prediction - a dramatic increase in availability of NO3
-- that is relevant to forest herb response. Such a shift towards NO3
-dominance has important implications for changes in herbaceous layer dynamics. For example, increases in soil NO3
-pools can increase invasibility by exotic species. Also, because preferential use of NH4
+vs. NO3
-can be highly species-specific among forest herbs, predominance of NO3
-availability can bring about further changes in herb layer species composition. Also important to forest herbs is the tendency for enhanced mobility of NO3
-to increase mobility and decrease availability of essential cations, such as Ca2+. 3 The response of plant species of the herbaceous layer of forest ecosystems to excess N availability can arise from N-mediated changes in several processes. Here I identify six - competition, herbivory, mycorrhizal infection, disease, species invasions and exotic earthworm activity - and discuss how they respond to excess N and how this response affects the herb layer of forests. With the exception of the activity of exotic earthworms (which is correlated with, but not necessarily caused by, high N deposition), there is compelling evidence that all processes respond sensitively to N deposition and that this response generally leads to drastic shifts in species composition and decreases in biodiversity of forest herb communities. 4 A hypothesis is suggested - the nitrogen homogeneity hypothesis - that predicts loss of biodiversity in forest ecosystems experiencing chronically elevated N deposition. This hypothesis is based on the decrease in spatial heterogeneity of N availability that is typically high in forest soils under N-limited conditions.
China is experiencing a high level of atmospheric nitrogen (N) deposition, which greatly affects the soil carbon (C) dynamics in terrestrial ecosystems. Soil aggregation contributes to the stability ...of soil structure and to soil C sequestration. Although many studies have reported the effects of N enrichment on bulk soil C dynamics, the underlying mechanisms explaining how soil aggregates respond to N enrichment remain unclear. Here, we used a meta‐analysis of data from 76N manipulation experiments in terrestrial ecosystems in China to assess the effects of N enrichment on soil aggregation and its sequestration of C. On average, N enrichment significantly increased the mean weight diameter of soil aggregates by 10%. The proportion of macroaggregates and silt‐clay fraction were significantly increased (6%) and decreased (9%) by N enrichment, respectively. A greater response of macroaggregate C (+15%) than of bulk soil C (+5%) to N enrichment was detected across all ecosystems. However, N enrichment had minor effects on microaggregate C and silt‐clay C. The magnitude of N enrichment effect on soil aggregation varied with ecosystem type and fertilization regime. Additionally, soil pH declined consistently and was correlated with soil aggregate C. Overall, our meta‐analysis suggests that N enrichment promotes particulate organic C accumulation via increasing macroaggregate C and acidifying soils. In contrast, increases in soil aggregation could inhibit microbially mediated breakdown of soil organic matter, causing minimal change in mineral‐associated organic C. Our findings highlight that atmospheric N deposition may enhance the formation of soil aggregates and their sequestration of C in terrestrial ecosystems in China.
Nitrogen enrichment substantially stimulates soil aggregation and promotes macroaggregation, which protects particulate organic carbon against microbial decomposition and limits the accumulation of mineral‐associated organic carbon in terrestrial ecosystems. Nitrogen enrichment enhances soil organic carbon storage via increasing macroaggregate carbon and acidifying soils. The response of soil carbon pool to nitrogen enrichment depends on the combined effects of individual aggregate carbon, and generally shows a slight increase by nitrogen enrichment.
Anthropogenic nitrogen (N) deposition has accelerated terrestrial N cycling at regional and global scales, causing nutrient imbalance in many natural and seminatural ecosystems. How added N affects ...ecosystems where N is already abundant, and how plants acclimate to chronic N deposition in such circumstances, remains poorly understood. Here, we conducted an experiment employing a decade of N additions to examine ecosystem responses and plant acclimation to added N in an N-rich tropical forest. We found that N additions accelerated soil acidification and reduced biologically available cations (especially Ca and Mg) in soils, but plants maintained foliar nutrient supply at least in part by increasing transpiration while decreasing soil water leaching below the rooting zone. We suggest a hypothesis that cation-deficient plants can adjust to elevated N deposition by increasing transpiration and thereby maintaining nutrient balance. This result suggests that long-term elevated N deposition can alter hydrological cycling in N-rich forest ecosystems.
Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, ...there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO
and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.
Elevated nitrogen (N) deposition in humid tropical regions may exacerbate phosphorus (P) deficiency in forests on highly weathered soils. However, it is not clear how P availability affects soil ...microbes and soil carbon (C), or how P processes interact with N deposition in tropical forests. We examined the effects of N and P additions on soil microbes and soil C pools in a N-saturated old-growth tropical forest in southern China to test the hypotheses that (1) N and P addition will have opposing effects on soil microbial biomass and activity, (2) N and P addition will alter the composition of the microbial community, (3) the addition of N and P will have interactive effects on soil microbes and (4) addition-mediated changes in microbial communities would feed back on soil C pools. Phospholipid fatty acid (PLFA) analysis was used to quantify the soil microbial community following four treatments: Control, N addition (15 g N m(-2) yr(-1)), P addition (15 g P m(-2) yr(-1)), and N&P addition (15 g N m(-2) yr(-1) plus 15 g P m(-2) yr(-1)). These were applied from 2007 to 2011. Whereas additions of P increased soil microbial biomass, additions of N reduced soil microbial biomass. These effects, however, were transient, disappearing over longer periods. Moreover, N additions significantly increased relative abundance of fungal PLFAs and P additions significantly increased relative abundance of arbuscular mycorrhizal (AM) fungi PLFAs. Nitrogen addition had a negative effect on light fraction C, but no effect on heavy fraction C and total soil C. In contrast, P addition significantly decreased both light fraction C and total soil C. However, there were no interactions between N addition and P addition on soil microbes. Our results suggest that these nutrients are not co-limiting, and that P rather than N is limiting in this tropical forest.
Historical increases in emissions and atmospheric deposition of oxidized and reduced nitrogen (N) provided the impetus for extensive, global-scale research investigating the effects of excess N in ...terrestrial and aquatic ecosystems, with several regions within the Eastern Deciduous Forest of the United States found to be susceptible to negative effects of excess N. The Clean Air Act and associated rules have led to decreases in emissions and deposition of oxidized N, especially in eastern U.S., representing a research challenge and opportunity for ecosystem ecologists and biogeochemists. The purpose of this paper is to predict changes in the structure and function of North American forest ecosystems in a future of decreased N deposition. Hysteresis is a property of a system wherein output is not a strict function of corresponding input, incorporating lag, delay, or history dependence, particularly when the response to decreasing input is different from the response to increasing input. We suggest a conceptual hysteretic model predicting varying lag times in recovery of soil acidification, plant biodiversity, soil microbial communities, forest carbon (C) and N cycling, and surface water chemistry toward pre-N impact conditions. Nearly all of these can potentially respond strongly to reductions in N deposition. Most responses are expected to show some degree of hysteresis, with the greatest delays in response occurring in processes most tightly linked to “slow pools” of N in wood and soil organic matter. Because experimental studies of declines in N loads in forests of North America are lacking and because of the expected hysteresis, it is difficult to generalize from experimental results to patterns expected from declining N deposition. These will likely be long-term phenomena, difficult to distinguish from other, concurrent environmental changes, including elevated atmospheric CO2, climate change, reductions in acidity, invasions of new species, and long-term vegetation responses to past disturbance.
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•Past work on nitrogen (N) effects on ecosystems arose from increased N deposition.•Many forest ecosystems of the eastern U.S. have been sensitive to excess N inputs.•Federal laws have resulted in lower deposition of oxidized N in the eastern U.S.•A hysteretic model is proposed for forest recovery from decreased N deposition.•N-impacted forests should show time lags recovering toward pre-impact conditions.
Nitrogen (N)-impacted forest ecosystems are predicted to exhibit a time lag of varying duration toward recovery initiated by current declines in atmospheric N deposition.
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