The frequently observed forest decline in water‐limited regions may be associated with impaired tree hydraulics, but the precise physiological mechanisms remain poorly understood. We compared ...hydraulic architecture of Mongolian pine (Pinus sylvestris var. mongolica) trees of different size classes from a plantation and a natural forest site to test whether greater hydraulic limitation with increasing size plays an important role in tree decline observed in the more water‐limited plantation site. We found that trees from plantations overall showed significantly lower stem hydraulic efficiency. More importantly, plantation‐grown trees showed significant declines in stem hydraulic conductivity and hydraulic safety margins as well as syndromes of stronger drought stress with increasing size, whereas no such trends were observed at the natural forest site. Most notably, the leaf to sapwood area ratio (LA/SA) showed a strong linear decline with increasing tree size at the plantation site. Although compensatory adjustments in LA/SA may mitigate the effect of increased water stress in larger trees, they may result in greater risk of carbon imbalance, eventually limiting tree growth at the plantation site. Our results provide a potential mechanistic explanation for the widespread decline of Mongolian pine trees in plantations of Northern China.
Hydraulic failure has been proposed as an important reason causing tree die‐off in vast areas of water‐limited land across the globe, especially under the influence of climate change, but the underlying mechanisms remain poorly understood. Decline has been more frequently observed in larger trees, and significant changes in hydraulic architecture are also commonly found with increasing tree size, suggesting a mechanistic linkage between tree decline and hydraulic limitations mediated by variation in tree size. Here, we compared hydraulic architecture of Pinus sylvestris var. mongolica trees between a plantation and a natural forest site across different tree size classes to test whether greater hydraulic limitation in larger trees is responsible for tree decline in the more water‐limited plantation site. We observed clear contrasts in hydraulic‐related characteristics between trees of the 2 sites and found clear trends of change in these traits with increasing tree size at the plantation site but not at the natural forest site, which provide a potential mechanistic explanation for the widespread decline of this species in plantations of Northern China.
Aims The changes of nutrient limitation status for tree growth across a plantation chronosequence have great implications for plantation management. The underlying mechanisms for such a shift, ...however, have seldom been addressed. While plant nutrient use strategies would change in response to soil nutrient alteration, they may also create feedback on soil nutrient dynamics and thus plant nutrient limitation status. Methods We examined soil and foliar nutrients of larch (Larix kaempferi), the dominant timber species in Northeast China, across a plantation chronosequence. Results Total soil N increased but total soil P decreased across the chronosequence. Similarly, N concentrations in the green leaves were positively correlated, and P concentrations were negatively correlated with stand age. Foliar N:P ratios, N and P resorption efficiencies and PRE:NRE were positively correlated with stand age, indicating the shift from N-limitation to P-limitation across the chronosequence. P concentration in senesced leaves decreased and N:P ratios increased across the chronosequence, which has implications for decomposition and nutrient release. Conclusions Nutrient resorption, soil pH, biomass P sequestration and imbalanced inputs of N and P would contribute to the occurrence of P-limitation with increased stand age. Furthermore, adaptive fertilization management strategies should consider the shift of nutrient limitation patterns across the chronosequence.
To assess the impact of conversion of native forests to monocultural larch plantations on soil chemical properties, we compared the total and various fractions of soil phosphorus (P) and acid ...phosphatase activity (APA) between natural secondary forests (NSF) and
Larix olgensis plantations (LOP) on a montane forest site in eastern Liaoning Province, Northeast China. We found that the concentrations of total P (TP), inorganic P, and iron-bound P (Fe-P) were significantly higher, and the concentrations of microbial biomass P (MBP), sodium bicarbonate-extractable organic P (NaHCO
3-Po), and APA were significantly lower, in the LOP stands than in the NSF stands; whilst organic P, sodium bicarbonate-extractable inorganic P (NaHCO
3-Pi), aluminum-bound P (Al-P) and calcium-bound P (Ca-P) were comparable between the two forest types. Our study also showed that the ratios of MBP/TP, NaHCO
3-Pi/TP, NaHCO
3-Po/TP, and APA significantly varied with time during the growing season. Moreover, the concentrations of NaHCO
3-Pi, NaHCO
3-Po, and MBP had significant (
P
<
0.01) and positive linear relationships with APA. Overall, results from this study suggest that conversion of native forests to larch plantations in the region is more likely to cause compositional change in soil P than to result in reduction in overall P availability.
In the headwater catchments of the Hun River, Northeast China, secondary forests (SF) have been replaced by plantations since the 1960s. Concern has been growing over this loss and the decline in ...water quality caused by the plantations. To test the effects of plantations on water quality, we selected two separate catchments covered by SF and
Pinus koraiensis
plantations (KP) to monitor physical and chemical properties of various hydrological variables including throughfall, stemflow, through-litterfall and runoff (flowing out of outlets of the catchments). The physical properties of water declined after water flowed through the two catchments as compared with rainwater. The pH of runoff in both catchments also dramatically decreased. The concentrations of Cl
−
, NO
3
−
and NH
4
+
in the runoff from the two catchments were similar (concentrations of Cl
−
and NH
4
+
in both catchments were similar to those in rainwater). Total P concentration in runoff of the SF catchment was higher than that of the KP catchment (P concentrations in both catchments were also higher than in rainwater) because P concentrations in litter and soil of the SF catchment were higher than those in the KP catchment. In summary, the rainwater became acidic in both catchments, but the responses of most water quality variables were similar in the two catchments, suggesting that appropriate ratios of KP in SF are feasible for secondary forest recovery and for preserving water quality (KP did not cause a decline in quality) in the headstream regions in Northeast of China.
Promoting the seed regeneration potential of secondary forests undergoing gap disturbances is an important approach for achieving forest restoration and sustainable management. Seedling recruitment ...from seed banks strongly determines the seed regeneration potential, but the process is poorly understood in the gaps of secondary forests. The objectives of the present study were to evaluate the effects of gap size, seed availability, and environmental conditions on the seed regeneration potential in temperate secondary forests. It was found that gap formation could favor the invasion of more varieties of species in seed banks, but it also could speed up the turnover rate of seed banks leading to lower seed densities. Seeds of the dominant species, Fraxinus rhynchophylla, were transient in soil and there was a minor and discontinuous contribution of the seed bank to its seedling emergence. For Quercus mongolica, emerging seedling number was positively correlated with seed density in gaps (R = 0.32, P<0.01), especially in medium and small gaps (<500 m(2)). Furthermore, under canopies, there was a positive correlation between seedling number and seed density of Acer mono (R = 0.43, P<0.01). Gap formation could promote seedling emergence of two gap-dependent species (i.e., Q. mongolica and A. mono), but the contribution of seed banks to seedlings was below 10% after gap creation. Soil moisture and temperature were the restrictive factors controlling the seedling emergence from seeds in gaps and under canopies, respectively. Thus, the regeneration potential from seed banks is limited after gap formation.
Background and aims Conversion of natural forests to plantations often results in a considerable loss of plant species and thus likely a reduction in quantity and quality of plant debris entering the ...soil. Larch plantation is widespread in northeastern China, but its ecological impacts receive little attention. This study aimed to assess soil quality under larch stands against the secondary forest stands using a suite of soil chemical and microbiological properties. Methods Four pairs of larch plantations and secondary forests were randomly selected from a mountainous area and mineral soils of top 15 cm were collected from each field. Results Soil carbon (C) and nitrogen (N) concentrations, microbial biomass, C and N mineralization and the activities of hydrolytic enzymes were significantly lower in the larch plantations than those in the secondary forests. However, light fraction C as a proportion of soil C was greater in the larch plantations, suggesting less accumulation and stabilization of soil C to heavy fraction in the larch plantations compared to the secondary forests. We also used δ¹⁵ N records in light and heavy fractions to derive the relative stability of soil C and found that soil C stability was lower in the larch plantations. This was supported by Fourier transform infrared spectroscopy analysis because carboxylate stretching, which might result from microbial oxidation, was less abundant in the larch plantations. Conclusions The differences in soil organic matter quality between the larch plantations and the secondary forests were reliably reflected in soil microbial properties and microbially-mediated processes. Our results indicated that the larch plantations reduced soil quality as well as nutrient cycling rate.
It is desirable and necessary to preserve the continuity of a coastal forest through reasonable management because it can provide many shelter benefits through altering the wind behavior along the ...shore. Thinning is an undoubtedly important measure for the continuity of forests as it provides suitable conditions for natural regeneration; however, thinning increases the risk of wind damage immediately after thinning in the coastal areas. Therefore, few thinning study related to regeneration in a coastal forest has been made. In order to test whether coastal forest of Japanese black pine (
Pinus thunbergii Parl.) requires a specific gap size created by thinning for regeneration and to compare seedling establishment among four thinning treatments, observations of emergence, survival and establishment of
P. thunbergii seedlings, together with soil water content, litter, wind and light regime were made. The observations were conducted over four growing seasons in three sizes of circular gaps (the gap diameter to stand height ratios for the four gap sizes were 0.5, 1.0, 1.5 and 0.0, the control) corresponding to the four thinning treatments in Niigata shore, Japan. Results indicated that density of seedlings older than 1 year increased with gap size or canopy openness (OP). Seedling establishment was greater in 1.5 gap sizes than in any other gap sizes, while seedlings peaked near the west and north edges of the gaps but not in the gap centers exposed to direct solar radiation. Seedling growth in 1.5 gap sizes was also significantly higher than that in any others. A tendency of seedling height increasing from east to west edge and from south to north edge across the gap was observed. Only 1- or 2-year-old seedlings occurred in gap sizes of 0.0 and 0.5, therefore establishment in both gap sizes was considered as failing. The results imply that although
P. thunbergii seeds can germinate in small gaps, even in under canopy, the seedlings are unable to survive. The seedlings apparently require a minimum gap size ≥1.0, or OP >30% in order to survive, and may require at least gap size ≥1.5, or OP >40% for further development into sapling. These results can be explained by the changes of microclimates, i.e. increase of light, soil water and airflow exchange, decrease of litter and canopy cover, and alleviation of the competitions for water in gaps created by thinning. Therefore, thinning strategy, especially patch-pattern thinning is potentially a viable silvicultural measure in management for the coastal pine forest. These results provide references for establishment and management of coastal
P. thunbergii forests.
•Hydraulic limitation underlies the crown dieback of Populus trees in windbreaks.•Higher branches show symptoms of greater drought stress.•Higher branches have lower stem hydraulic conductivity and ...increased embolism risk.•Higher branches show higher leaf and stem soluble sugar contents.
Populus trees play an important role in windbreak construction in northern China, but widespread decline and mortality of Populus windbreaks have occurred in recent years. The phenomenon of die-backs from the top of Populus trees downwards is commonly found in declining windbreaks preceding tree deaths, particularly in water-limited areas. To provide a mechanistic explanation for such top-down diebacks, we analyzed the variation in xylem hydraulics, leaf water relations, and tissue non-structural carbohydrate contents (NSC) along the vertical axes of Populus pseudo-simonii trees, i.e. one of the most commonly used tree species for creating windbreaks in northern China. Consistent changes were observed along the height gradient of the tree crown, i.e. with the height increase branches showed a clear trend of decrease in hydraulic efficiency and leaves showed symptoms of greater drought stress. The stem hydraulic conductivity standardized using whole cross-sectional wood area decreased from 4.66 at the bottom to 1.69 kg m−1 s−1 MPa−1 at the top of the tree crown. Such a change in hydraulic efficiency was accompanied by significant increase in the degree of xylem embolism (i.e. from < 10% at the bottom to about 25% at the top of the tree crown) and an overall reduction in hydraulic safety margin with branch height increase. Lower xylem hydraulic efficiency and symptoms of less favorable water status, however, did not result in reduced contents of tissue NSC in branches of higher positions. Rather, the stem and leaf soluble sugar contents both showed significant positive correlations (P < 0.01, linear regressions) with sampling heights and the starch contents largely remained stable along the height gradient. Our results highlight that greater risk of hydraulic failure rather than carbon depletion is underlying the commonly observed crown dieback in Populus windbreaks.
The Siberian elm (Ulmus pumila L.) is one of the most commonly found tree species in arid areas of northeast Asia. To understand the morphological and physiological characteristics of Siberian elms ...in arid regions, we analyzed leaves from seven study sites (five arid or semi-arid and two mesic) in China, Mongolia and the Republic of Korea, which covered a wide range of average annual precipitation (232 mm*year-1 to 1304 mm*year-1) under various aridity indexes (AI) and four different microenvironments: sand dune, steppe, riverside and forest. The traits of Siberian elms varied widely along different annual precipitation (P) and AI gradients. Tree height (H), leaf size (LS) and stomatal area per unit leaf area (AS/AL) decreased with increasing AI, whereas leaf mass per unit leaf area (LMA) and water-use efficiency (WUE) increased significantly. In addition, trees at the five arid sites showed significant differences in LS, LMA and AS/AL but not in H and WUE. Thus, our study indicated that indigenous Siberian elm trees in arid areas have substantially altered their morphological and physiological characteristics to avoid heat stress and increase water conservation in comparison to mesic areas. However, their changes differed depending on the surrounding microenvironment even in arid areas. Trees in sand dunes had a smaller LS, higher LMA, thicker leaf cuticle layer and higher stomatal density and AS than those in steppes and near a riverside.
Purpose
Conversion of secondary forests to larch plantations results in a reduction in quantity and quality of plant litters entering the soil and may reduce the soil nutrient cycling rate. As a ...major process influencing soil nutrient cycling and biochemical properties, leaf litter decomposition has received little attention in temperate secondary forest systems. This study aimed to assess the decomposition of tree litters of varying quality and its impacts on the chemical and biological properties of soils obtained from a temperate secondary forest in a short-term experiment.
Materials and methods
In this study, impact on soil biochemical properties of tree litter originating from five dominant tree species in Northeast China (
Quercus mongolica
,
Juglans mandshurica
,
Fraxinus rhynchophylla
,
Fraxinus mandshurica
, and
Acer mono
) were tested. A 42-day laboratory decomposition experiment was conducted to determine the short-term influences of the decomposition of the five tree species leaf litters on secondary forest soil carbon (C) decomposition, nitrogen (N) mineralization, soil microbial biomass, soluble organic compounds in microbial biomass, enzyme activities, soluble organic C, and pH.
Results and discussion
Carbon decomposition, equivalent to 29–41 % of leaf litter C, occurred during the incubation.
F. mandshurica
and
A. mono
leaf litters decomposed faster than those of
Q. mongolica
,
J. mandshurica
, and
F. rhynchophylla
due to their low lignin concentration and lignin-to-N ratio and high labile C pool. In contrast,
F. mandshurica
and
A. mono
leaf litters immobilized less N than the other three leaf litters, which affected N availability. Soil amended with
F. mandshurica
leaf litter had higher microbial biomass C than the other four leaf litters, indicating that
F. mandshurica
could increase soil nutrient cycling. The five leaf litter types all increased activities of exoglucanase, ß-glucosidase, and N-acetyl-ß-glucosaminidase, but there were no effects on phenol oxidase activity.
Conclusions
This study suggested that litter of
F. mandshurica
incorporated in soils tends to decompose faster than litter of other species and increases the microbial biomass.
F. mandshurica
is, therefore, more advantageous than the other studied tree species for increasing nutrient cycling in a temperate secondary forest soil in this short-term laboratory incubation experiment. Future studies should consider the interactions among soil types, different climates, and long-term decomposition of leaf litters.
PDF
