Experimental manipulation of forest ecosystems: lessons from large roof experiments

Pores;~;ology

Management Forest Ecology and Management 101 (1998) 339-352

Experimental manipulation of forest ecosystems: lessons from large roof experiments P. Gundersen a,* , A.W. Boxman b, N. Lamersdorf ‘, F. Moldan d, B.R. Andersen a a Danish b Department

Forest

and Landscape

Research

of Ecology, Section Environmental ’ Institute of Soil Science and Forest d Swedish

Environmental

Biology, Nutrition, Research

Institute,

H@rsholm

Kongeoej

II,

DK-2970

Horsholm,

Denmark

Uniuersi@ of Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen, Netherlands Gijttingen University, Biisgenweg 2, D-37077 GBttingen, Germany Institute (IVL). Bon 47086, S-402 58, Gb’teborg, Sweden

Accepted 11 March 1997

Abstract Environmental impacts on forest ecosystems can be studied by manipulating energy, water, and element input or by changing the internal element cycling. In practice, the intended manipulations in a complex ecosystem such as a forest are followed by unintended manipulations of other factors that may cause artifacts in the experiment. The character and extent of such unintended changes were assessed in five major roof manipulation studies in coniferous forests in Europe. In all five cases the roofs were placed beneath the canopy 2-5 m above the ground and designed to study the response to reduced N and S deposition and effects of drought. Photosynthetic light was reduced 15-50% below the roofs and might have contributed to an observed decrease in forest floor moss cover. Soil temperature differences were up to +OS”C, colder than outside during summer and warmer during winter. Climatic differences were least at the smallest roofs. The sprinkling system was the most critical component in the experimental design. The sprinklers could not reproduce the temporal and spatial variability of natural rain; event size and rain intensity increased, and the number of rain events decreased. It proved particularly difficult to reproduce small rain events. The stemflow proportion of the water input was increased by sprinkling. Observed decreases of litter decomposition and mineralisation under some of the roofs were probably caused by a reduced moisture content of the surface litter due to the differences from natural rain. Exclusion of throughfall by the roof disturbed the internal cycle of nutrients leached from the canopy (Ca, K, Mg) or present in suspended material (N, P, Mg). The circulation of these elements had to be restored by addition or recycling of suspended matter. The unintended changes probably delayed the soil response to reduced acidity input and accelerated the decline of nitrate leaching in response to reduced N input. 0 1998 Elsevier Science B.V. Keywords: Acidification; Artifacts; Climate effects: Coniferous forest; Experimental manipulation; Nitrate leaching; Nutrient cycling; Roof; Spatial variability; Throughfall

1. Introduction

* Corresponding author. Tel.: +45-45-763200; 763233; e-mail: [email protected]. 0378-l 127/98/$19.00 PII

SO378-1127(97)00148-5

fax: +45-45.

Ecosystem manipulations performed as large scale field experiments are an important research tool in the study of environmental impacts (Beier and Rasmussen, 1994; Jenkins et al., 1995; Carpenter et al.,

0 1998 Elsevier Science B.V. All rights reserved.

1995). Manipulations of terrestrial ecosystems may be separated into four groups according to the manipulated medium (energy, water. element input and element cycling) (Fig. 1). An important restriction on the manipulation approach is that manipulation of one medium will often cause unintended manipulations of other media. For instance, a ‘soil warming’ experiment will not only change the heat balance of the soil but also the water balance due to increased evaporation. Studies of organic matter accumulation and turnover (C-storage) by removing and/or adding litterfall input will also change the nutrient input, and root trenching changes the water balance. In several experiments roofs have been constructed below the canopy with the aim of studying the biological and biogeochemical response of forest stands or catchments to environmental changes such as reduced atmospheric deposition (clean rain) and drought. Major roof experiments in mature forests have been conducted at some of the EXMAN and NITREX sites (Wright and Rasmussen, 1998). Introducing a roof in a forest is a significant manipulation that will interact with all four media. A range of unintended manipulations of processes cannot be avoided. It is important to assess the character and extent of the unintended manipulations and to evaluate if these will cause artifacts to the main experiment. In this paper we compare the available information from 5 roof experiments on the unintended differences between natural and roof manipulated plots (hereafter called ‘roof effect’) and discuss the

possible artifacts that may have developed from these differences. The objectives are (i> to evaluate the importance of the unintended changes; (ii) to bring together the experience from the ongoing roof experiments that may be useful in the design of future roof projects and for the interpretation of results from the existing roofs: and (iii) to discuss the possible implication for other types of manipulation experiments.

2. Experimental

design of the roofs

Transparent roofs supported by wooden beam structures were constructed 2-S m above the forest floor but beneath the canopies in five mature coniferous forests (Table 1). At four sites, Klosterhede (KH), Speuld (SP), Ysselsteyn (YS) and Soiling (SL), the roofs were established in plantation stands, whereas at GBrdsjiin (GD) a whole natural spruce forest catchment was covered by several overlapping roof sections. In the plantations the roof-covered area was separated into two or three sub-plots (100-300 m’) that received different treatments (Tabfe 1). Similar control plots were established outside the roofs. At GD a nearby catchment served as a control. The constructions had no walls in order to allow good ventilation. Around each stem penetrating the roofs a hole was left open to let the trees move in strong winds. At the KH, SP and YS sites flexible plastic tents covered the holes around the stems. Irrigation systems with storage tank, pump and sprinklers were installed below the roof to supply

Eeosystcm M En*qq

- Climate

- Ecosystemwarming - Soil warming - I&zeasedUV-B

watm

- k&

- Drought - Irrigation

mts - laput

- N or S reduction - N or S addition - CO, inmme - Liming/fertilizing Fig. I. Examples of manipulated media.

manipulationsused

in studies

of environmental

impacts

on terrestrial

ecosystems;

organised

according

to the

“A manual

[mm/h]

character

Roof

control

/drmgh?:

collected throughfall stored temporarily in a stainless steel tank.Orher: deionized groundwater from a distant well stored and transported to the site in a tank. Gundersen et al.. 1995

6 (constant)

Drought (roof control, I st year) - Clean rain - fertilisation + irrigation 24 centrifugal sprinklers placed I .3 m above ground in a 6 X 6 m grid that allow some overlap of the sprinkler distributions. Sprinkler droplet size: large. Regulated manually, sprinkling of ca. IO-15 mm for each IO-15 mm of accumulated throughfall

(YS)

below

Boxman

6 Deionized

(see also Fig. 6).

et al.. 1995, 1997a

tap water

25 centrifugal sprinklers placed 75 cm above ground in a 5 X 5 m grid that allow some overlap of the sprinkler distributions. Droplet size: medium. Automated” , sprinkling approximates natural rain real-time. Small events < 2 mm do not activate the system. Storage and sprinkling capacity is too low to reproduce heavy rain storms

- Roof control - Clean rain

the Netherlands SP: 1989- 1994YS: 1989-ongoing SP: Douglas fir, 40 yearsYS: Scats pine. 55 years 14X28 m, separated in 2 plots (IOX 10 m), leaving 2 m buffer zones 2-3 m above ground. PVC cover sheets supported by a light beam construction

Speuld (SP) and Ysselstcyn

The roofs were constructed

system like at KH was used the first 2 years of treatment

reference

operated

Design details,

Sprinkling capacity Source of water

of sprinkling

system

Sprinkling istics

Timing

applied

Treatments

-

48X25 m, separated in 3 plots (10X15 m), leaving 3-5 m buffer zones 3-5 m above ground, PVC cover sheets, strong supporting construction shadow area ca. 15%

Roof and plot size

Roof construction

Norway

Forest stand and age

spruce. 75 years

Denmark 1987-1995

(KH)

Klosterhede

Country Years in operation

design of the five European roof projects. roofs are technically identical

Site

Table 1 Details of the experimental The Speuld and Ysselsteyn to allow (GD)

manipulation

Moldan

et al., in press

1.9 to 2.8 Deionized lake water

Regulated manually. sprinkling a few days after each (or series of) natural rain event(s). Precipitation as snow was not simulated. the equivalent water was sprinkled in the subsequent snow melt.

275 centrifugal sprinklers placed 0.3 to 1 m above ground in a 5 X5 m grid. Sprinkler droplet size: small

- Clean rain

2-4 m above ground, poiycarbonate cover sheets. strong supporting construction

Norway spruce, 80-100 years natural forest catchment 7000 m’ in 8 irregular sections covering a 6300 m’ catchment

Sweden l990-ongoing

Glrdsjiin

the canopy (SL)

each 300

Automated, sprinkling approximates natural rain real-time. Small events < I mm do not activate the system. No simulation of snow fall/cover, but resprinkling of melting water from snow cover of the roofs 4 (constant) Roof conrrol: collected and prefiltered (3.50/50 pm) throughfall water. Drought plot: collected, prefiltered and stored throughfall water (in 1994 + 30 mm tap water) Bredemeier et al.. 1993, 1997

3-3.5 m above ground, highly transparent polycarbonate plates, very strong supporting construction - Roof control - Clean rain - Drought 68 centrifugal/half-centrifugal sprinklers per plot, 3 m above ground. Sprinkler droplet size: small (mist).

roof plots,

spruce, 64 years

3 separate m’

Norway

Germany 1989-ongoing

Soiling

of water- and solute input to the soil.

342 Table 2 Qualitative and quantitative differences (unintended) between natural forest plots and roof plots at five sites with permanent roofs below the canopy. The temperature differences shown are based on monthly means. More details may be found in Gundersen et al. (19%). Boxman CI al. (1997a) and Moldan et al. (in press) Factor/process

Change from natural plots + root plots (specification and qualitative description)

Light (PAR. % of control) Air temperature. > 0.5 m above ground [“Cl

Significant reduction Winter months, variable difference

Klosterhede

(KH)

Ysselsteyn (YS) dnd SpeuldtSPI

G%dsjBn

- 3s to -so

- 15

+0.1

i I (maxt

ND rir 0.1

+ 1 (maxI

+ 0.3 (max 0.4)

+0.1

-t 0.6 to t I .?

Increase in spring/’ summer months Increase during winter

iO.1

tmax

+0.2

tmax 05)

Decrease during summer Long term difference Short-term changes when rain and sprinkling is out of phase

- 0.3 (max 0.6) no change +s-10

-0.

Event size [mm]

Decreasing, dew and and small events disappear Increasing

several events --f one event < 3 + IO-IS

Rain intensity

Variable

‘! --f 6

events < 2 mm disappear > 7 mm no change ‘! 4 6”

yes

minimal

std: I30 + 280

ND

Mineral soil temperature. IO cm depth [“C] Relative

Water

humidity

[%I

I

Soiling

NJ

- IO to - 20 ri

- 0.5 no change & 10

k.3

inpztt

Event frequency

Droplet

[mm/h]

size

Timing Spatial

pattern of

throughfall

amount

StemfIow (“/c of throughfall amount) Water amount t% of throughfall amount

Element

+0.X)

1GD)

+ constant (high)

Variable + constant (sprinkler dependent) Changed: e.g. water input on dry days Canopy cover related

change

several events -+ one event

‘I -+ 2.6 ( 1.9--1.X)

yes: no bnow cover

ND

minimal change, hut no snow covet minimal change at central \ampling subplot

distribution --j overlapping sprinkler distributions increasing variability Dramatic increase due to stem ‘catch’ For treatments where throughfall water was used for sprinkling water was lost due to limited storage capacity and overflow in the pipe system at heavy rain storms

(Fig. la Fig. 4b Fig. Sa)

Canopy cover related distribution -j constant Canopy cover related distribution --) proportional to sprinkling water distribution, higher stemtlow

Fig. 4c -+ Fig. 4d

ND

ND

ND

Fig. Sh

ND

ND

ND

< 0.5 + 7 Drought:

ND

not changed

- 10

itpa

Spatial pattern of concentrations Spatial pattern of element of fluxes

P. Gundersen

et al. /Forest

Ecology

and Management

343

101(19981339-352

Table 2 (continued) Factor/process

Change from natural plots + roof plots (specification and qualitative description)

Element cycling Suspended matter in throughfall

Roof control/drought: losses due to sedimentation in storage tanks and by overflow in heavy rain. Clean rain (all sires): missing (- 100%) Dissolved organic Clean rain: almost absent matter in throughfail Other plots: transformation during storage Canopy exchange Disturbed, can be restored by addition Continuous + a few events. Litterfall timing Decomposition and loss of nutrients in the collection period Spatial pattern Canopy cover related of litterfall distribution + ‘uniform‘ Derived biological differences Ground vegetation Decline of vegetation cover

Litter

decomposition

Needle litter at surface (litterbags). Reduced weight loss compared to ambiemt plots)

Klosterhede

Drought:

(KH)

Ysselsteyn (YS) and Speuld (SP)

ND

Roof control:

Gardsjon

(GD)

ND

Solling

(SL)

Roof control/ drought: loose 10 - 20% of total input See Table 3.

Clean rain - 100% Drought:ND

ND

Clean rain: - 80%

Clean ruin: - 65%

Missing first 2 years + 4 events/yr

added

added

added

+ 4-6 events/yr

-+ 3 events/yr snow cover on

+ monthly,

+ uniform

+ uniform

+ uniform

+ uniform

Decline of plants and mosses 70% + %20 moss cover - 30%

Decline of mosses

ND

Decline of mosses and grass cover

reduction of 1st year decompositiond

ND

ND

except

the roofs

“At ciean conditions. bThe pumps and sprinkling system has a ‘window’ of intensities extremely high flow, water is lost from the gutter system. ‘Central 3 X 3 m subplot for soil water sampling and soil moisture dData from Koopmans et al., 1997. ND = no data.

either cleaned water or to resprinkle throughfall collected on the roof. Water for the ‘clean rain’ treatments was treated in a de-ionising unit and a mixing unit allowed application of dissolved salts and nutrients to mimic sea-salt deposition and canopy leaching. The design of the sprinkling systems differed among the sites (Table 1). Litterfall accumulated on the cover plates or in the draining pipe systems was

(6 mm/h

to ?), which

can reproduce

approximately

‘real-time’.

At

measurements.

collected regularly and redistributed on the plots below the roofs. At all sites, monitoring programs were set up to measure climatic and chemical differences between ambient controls and roofed plots/catchment. Detailed descriptions of the roof constructions, sprinkling systems and monitoring programs may be found in the references listed in Table 1.

3. Results The KH roof was the first roof to be constructed and a thorough analysis of the ‘roof effects’ was undertaken. Thus the most detailed data and evaluations are available from the KH roof (Gundersen et al., 1995; Gundersen, 1995). Some of the early experiences from constructing and running the KH roof were used to adjust the design of the other roofs. Table 2 displays a summary of the observed physical and chemical differences between natural plots and roof plots. Some of these differences arc discussed in more detail in the following. 3.1. Climatic ejtkts Construction of a roof below the canopy causes a reduction of the light reaching the forest floor due to reflection from the cover plates and shadow effects from the supporting beam construction. In a mature coniferous forest photosynthetic active radiation (PAR) at the forest floor is already reduced to < S10% of PAR irradiation above the canopy. Comparative measurements of PAR irradiation with or without roof showed a light reduction of up to 50% at the forest floor, highest at the centre of the KH roof (Table 2). Accumulation of needle litter and dust, as well as algae growth on the cover plates may have contributed to the light reduction, since sweeping and cleaning of the cover plates was only carried out a few times a year.

Due to the reduced irradiation under the roofs, a temperature decrease was expected in the growing season compared to outside the roof. whereas an increase was expected in the dormant season due to reduced heat radiation. The observed soil temperature differences confirmed the expected seasonal pattern (Table 2 and Fig. 2). The soil temperature increases under the roofs were 0.1 to 0,6”C during the winter months, and the decrease5 were 0. I :o 03°C during summer months (Table 71. To some extent. the air temperature differences followed the inverse seasonal pattern of the soil temperature differences (Table 2) but the air temperature differences were small. The maximum heating observed- under the KH roof during the warm and sunny summers of 1991 and 1992 was + 0.6 to 10.8”C. The ‘roof effect’ on light and temperature tends to decrease with the size of the roof. At small roofs heat and light transfer from the sides may reduce the ‘roof effect’. The differences were smallest at the SP and YS roofs which, besides being small. had a supporting construction with low shadow effect.

A critical component in the experimental set-up was the sprinkling system. Due to technical constraints it was impossibIe to simulate the temporal and spatial variability of throughfall. The most significant change in water input was probably the loss of small rain events and the increased ‘rain intensity’

Fig. 2. + Seasonal variation of soil temperature differences (“C) be:ween a roof plot and a control plot at 10 cm depth below the surface [mean monthly difference: roof - control) at the Klosterhede site. The soil temperature differences were almost equal at IO and 40 cm depth. The larger temperature decrease observed during the summers of 1991 and 1992 was probabIy related to the warm and sunny summel climate these years. The light reduction had more influence on the energy balance than in previous summers where the energy input frem light was lower. Redrawn from Gundersen et al. (1995).

P. Gundersrn

et cd. / Forest

Ecology

Fig. 3. The relative contribution of natural rainfall volume caused by rains with different rainfall intensities at Gtrdsjiin. The sprinkling system under the roof operated at intensities indicated by the horizontal line (the median, 2.6 mm/h, of the sprinkling intensities is marked by a dot). About 55% of the throughfall volume came with a rain intensity lower than the sprinkling intensity. The low intensity rain ( < 2.6 mm/h) occurred in about 90% of the rain hours. Redrawn from Moldan et al. (in press).

produced by the sprinkling systems (Table 2). The sprinkling system at KH, which was dependent on transport of water from a distant well, caused the largest difference from the natural rain giving lo- 15 mm showers at a constant intensity of 6 mm h- ’ . At KH (and probably at all sites) 60% of all rain events were < 2 mm and a rain intensity of > 6 mm hP ’ only occurred for 10% of the rain volume (Gundersen, 1995). Even though the sprinkling systems were better at the other sites, wetting of the forest floor under the roofs occurred in a strongly reduced number of hours. At GD, where the sprinkling intensity was the lowest (2.6 mm/h) (Fig. 3) the number of rain hours under the roof was reduced by about 75%. The difference in spatial patterns of throughfall and sprinkling water was only studied for Norway spruce at the KH site (Fig. 4), but comparable patterns should be expected for other species of conifers at the other sites. In spruce forests the throughfall amount decreases close to the stems and decreases with tree size (Fig. 4a) due to interception of water from the canopy cover (Beier et al., 1993). The spatial pattern of throughfall water amount was changed under the roof to a pattern created by overlapping circular distributions from the sprinklers (Fig. 4b). The sprinklers caused a ‘catch effect’ on the exposed sides of stems and a ‘shadow effect’

and

Manugement

101 (19%)

339-352

345

behind stems. Most of the stems were hit by water from more than one sprinkler, possibly increasing the net catch effect and reducing the net shadow effect. The resulting water distribution was clearly more variable than in the natural situation (Fig. 5a). Model calculations for the KH roof showed that 7% of the sprinkled water was caught by stems and appeared as stemflow. This amount of stemflow will be constant in all sprinkling events whereas natural stemflow only occurs in heavy rainstorms and in total only as < 0.5% of a yearly throughfall amount at KH and 0.5% at GD. When part of the sprinkling water appears as stemflow the areas between the trees obtain less water than intended (up to S-10%) and the soil will tend to be drier than outside. At KH, the total water input was slightly increased from 1992 and onwards to compensate for the ‘loss by stem catch’, so that the average water input between the stems was comparable to the ambient situation. Thus, the total water input exceeded the natural input, but the increase only occurred near the stems on a few percent of the area of the plot. At SL a small central sub-plot for soil water sampling was set up to match the ambient situation as much as possible with respect to total water and element input to compensate for these problems. 3.3. Element input Because of water evaporation and element leaching in the canopy and increased dry deposition in the upper part of the crown, concentrations of all elements exhibit a radial pattern with high concentrations near the stems. Concentration differences of a factor 5 to 10 depending on distance from stems and a factor 2 to 3 depending on tree size have been observed in spruce stands (Beier et al., 1993). In Fig. 4c an image of the spatial pattern of element concentrations in throughfall at KH is shown. Element fluxes were less variable than concentrations, since throughfall water flux has the inverse distribution of the concentrations. However, the element fluxes were still a factor 2 to 3 higher near the stems compared to between the stems (Beier et al., 1993). This distinct spatial variability in throughfall concentrations was lost in the manipulated plots where concentrations did not vary (Fig. 4d). The spatial vari-

P

P. Gundersen

et al./ Forest

Ecology

and Management

a)

100

101 (1998)

339-352

b)

300

500

700 mm/year

900

25

1100

75

125 175 kg Nrdhdyear

Fig. 5. The calculated frequency distribution of (a) water amount and (b) element flux (Na used as an example) Klosterhede based on the spatial distributions of throughfall and sprinkling water shown in Fig. 4. The increased input under the roof was confirmed by point measurements (n = 69) (Gundersen et al.. 1995).

ability in element fluxes under the roof was thus determined by the distribution of the water flux alone (Fig. 5b). This is the reverse of the natural situation where the highest element fluxes (which appear near the stems) occur with the lowest water fluxes. Further, the increased ‘stemflow’ from sprinkling contained a considerable fraction of the element input (at KH roof 7% of the total input), although it was not chemically concentrated as natural stemflow. In the ambient plots at KH soil water concentrations of all elements, except inorganic N, exhibited a distribution pattern similar to that found in throughfall (higher close to the stem and under large trees) (Gundersen, 1993). Under the roof, where input concentrations were constant on each plot, the concentration gradients changed within the first months of the roof treatments, and high soil water concentrations near the stems were no longer found (Gundersen et al., 1995).

225

275

in the ‘clean rain’ plot at spatial variability of water

mistake on the ‘clean rain’ plot at KH. During the first two years of roof treatment canopy leaching of Ca, Mg, and K was not replaced by addition to the sprinkling water. In this period K was rapidly depleted from the soil solution and the concentrations decreased to near the detection limit (Gundersen et al., 1995). The effects on Ca and Mg concentrations were small. Removal of water at drought plots may also remove a significant amount of canopy leached K, since canopy leaching of K is substantial during the summer when the needles are active. This loss amounted to ca. 20% of the K flux over the 5 years where summer droughts were applied at KH. Apart from the nutrient ions, throughfall also contains suspended matter and dissolved organic compounds, including organic N leached from needles, bark and epiphytic micro-organisms. The artificial throughfall sprinkled under the roofs did not contain organic compounds or suspended matter (except for a minor contribution from the lake water source at GD). At SL stored and filtered throughfall

3.4. Element cycling Removal of throughfall by a roof cuts off the internal cycling of nutrients with throughfall. The canopy leaching of Mg, Ca and especially K is substantial and this element cycling needs to be restored by additions to the ‘clean rain’ treatments. Details and considerations on the compositions of ‘clean rain’ used at the sites are found in the references listed in Table 1. The importance of internal cycling by canopy leaching was illustrated by a

Table 3 Estimated decrease in nutrient input (kgha- ’ yr- ‘) due to sedimentation and filtration (350 pm filter) processes at the Solling roof experiments (roof control, drought) in 1992. Numbers in brackets are the percentage decrease compared to the input (throughfall + litterfall) on an ambient control plot (Lamersdorf and Blanck, 1995) Plot

Org. mat.

N

P

K

Ca

Mg

Roof control Drought

352(13) 475(17)

1004) 14f21)

0.4(26) 0.8(48)

0.8(3) 1.3(3)

0.6(3) 0.9(4)

0.6tll) 0.9(17)

water collected on the roof was used as water source for the different roof treatments (Table I ). This experimental set up made it possible to measure the flux of elements from suspended matter in throughfall (Table 3). It appeared that this material contributed a considerable fraction of the nutrient input to the soil (Table 3), especially the input of P. The nutrient composition of the material was comparable to that found in the very fine litter particles and the C/N and C/P ratios (I I - 14 and 132-358, respectively) were much lower than found in needle litter (38 and 674, respectively) (Lamersdorf and Blanck, 1995). This nutrient input to the soil (which is not included in conventional litter and throughfall sampling performed in forests) may be an important nutrient input to the microbial community in the soil. The litterfall was redistributed under the roof to maintain the internal cycling of nutrients with litterfall. However, some readily available nutrients and soluble organic compounds may have leached from the litter by throughfall or may have been released by decomposition during the collection period. In this way it was lost from the system in roof runoff (Table 3). 3.5. Derived biological @ects The reduced light penetration to the forest floor has affected the ground floor vegetation (Table 2). At KH, YS and SL the roof treatment reduced the plant and moss cover. The decline of mosses may partly be explained by the temporal change of the water input, since the lack of all smaller rain events may harm the growth of mosses. Walking within the plots during sampling may also have contributed to the effects on ground flora, especially at YS where the sampling space was limited by the small plot size. A 30% decrease in weight loss was observed in litterbags under the KH roof, irrespective of the treatment (Hansen et al., 1995). Similar but less pronounced roof effects were observed at SP and YS within the first year of decomposition (Koopmans et al.. 1997). We suspect that this decrease in decomposition was caused by the change in water input. The lack of smaller rain events and the reduced duration of the rain events may have reduced the moisture content of the surface litter under the roof. Lack of

readily available dissolved C. N and micro-nutrients in artificial throughfall and the slight decrease in soil temperature during the summer may also affect the turnover of organic matter. Studies of decomposition in deeper litter (F and L layers) at KH. GD and YS showed no significant effect of the roof (Boxman et al., 1998b).

4. Discussion

When planning the roof experiments, concern was placed on the possible warming caused by a greenhouse-like construction in the forest. Although the light penetration was reduced considerably the in&tence on the energy balance was limited as illustrated by the relative small temperature differences (Table 2. Fig. 2). The light reduction under the roofs was probably the most important climatic change, which caused some decline in the ground vegetation, Small roofs and light supporting constructions may reduce the climatic problems. Changes in the ground vegetation will probably not affect nutrient cycling over the course of the experiment since the ground vegetation biomass is small in these forests. The most critical component in the roof experiments was the sprinkling system. Centrifugal sprinklers created a different and more variable temporal and spatial distribution of water than outside the roof (Figs. 4 and 5). In new experiments, a careful testing of different sprinkler types is recommended. The effect of different position5 in relation to stems and different water pressure should be tested in order to optimise the distribution. Many small sprinklers or tube sprinklers (Emmett et al., 1995) placed between trees may be a more optimal solution. Sprinklers operating at the lowest possible intensity should be used in order to simulate a larger proportion of the small rain events. The overall importance of the change in spatial variability by introducing a sprinkling system is difficult to evaluate. The effect may depend on the scale and duration of the experiment. Since root turnover in forests is relatively rapid, the root distribution may quickly adjust to the new water distribution from the sprinklers. On the scale of a root

P. Gundersen

et al. /Forest

Ecology

system of a tree, a plot or a catchment the difference from the natural situation may thus be small. However, at each sampling point for, e.g., soil water, the difference will be very important, especially since only a few samplers usually are used to represent a plot or a catchment. Investigations in the roof covered catchment at GD indicated that there were no changes in the hydrological behaviour of the catchment after construction of the roof (Moldan et al., in press). With respect to sampling design sprinkling has the advantage over natural rain in that the spatial distribution of both water and elements will be constant in time and therefore easy to measure just in one sprinkling event. Quantifying these constant differences in input above each soil water sampler is important for the interpretation of the results, especially when the number of samplers is low. The change in spatial variability and the possible changes in the transition phase underline the importance of establishing pre-treatment data on soil water chemistry etc. from at least one year before construction of a roof. Treatment effects and roof effects may easily be masked by pre-treatment differences between individual samplers. 4.2.

Importance

of roof

and Management

101 (1998)

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339-352

clean rain treatments (Boxman et al., 1995, Boxman et al., 1998a, Bredemeier et al., 19981, but the same was observed on roof control treatments. At YS and SP, marked differences in soil water chemistry emerged between the roof control and the ambient control on both sites. Especially nitrate concentrations at the roof control treatments were reduced to levels comparable to the clean rain treatments (Fig. 6). Loss of throughfall water due to storage capacity limitations and denitrification of nitrate during stor-

600 400 200

b) 3000

/

2500 i 2000 4

artifacts

The integrated effect of the unintended changes is difficult to assess. However, we need to evaluate if the extent of the roof effects is important in relation to the main aims of the roof projects which were to show the recovery of forest ecosystem after a decrease in air pollutant load and effects of summer drought. Beier et al. (1995) calibrated the chemical process model MAGIC with data from the ambient control treatments and predicted the soil solution response to reduced acid load at KH and GD. The observed responses followed the model predictions fairly well indicating a minor importance of the roof effects in this respect. If anything, the recovery was probably delayed by the roof effects (Moldan et al., in press), due to reduced base cation inputs (missing organic matter, Table 3) and reduced decomposition and thus release of base cations from organic matter. At nitrogen saturated sites (YS, SP, SL), prompt decreases in nitrate leaching were observed at the

10001 500

:

04, Jan-89

I1 Jan-00

/, Jan-91

I, Jan.92 Date

I, Jaw93

,4 Jadkl

Jan-95

Fig. 6. Nitrate concentration in soil water at (al 10 cm and (bl 90 cm depth in ambient control, roof control and clean rain roofplots at Ysselsteyn. The roof treatments started early 1989 and the soil water samplers were installed soon after. Thus the very first results in 1989 are uncertain. The arrows mark a change of the sprinkling system from a manually operated system (weekly sprinklings of the accumulated and stored (roof control) throughfall amount) to an automated system that approximates natural rain more closely (Table 1). In the period 1989-91 a strong ‘roof effect’ emerged where the nitrate concentration at the roof control followed the pattern of the clean rain plot at both depths. Soon after the improvement of the sprinkling system late 1991, concentrations at the roof control approached those of the ambient control, especially at 10 cm depth. Furthermore, the roof control also started to follow the seasonal pattern observed at the ambient control. The same type of ‘roof effect’ and improvement after change of the sprinkling system was observed at Speuld (Boxman et al., 19951.

age had decreased the water and element input to the roof control compared with the ambient control. To solve this problem the sprinkling system was rebuilt in 1992 to reduce storage time and approach real-time watering. After this technical improvement the soil water chemistry developed more in parallel to the ambient plot but still with lower nitrate concentrations (Fig. 6). The reasons for these differences could be a decrease in decomposition and mineralisation due to drier conditions caused by the increased stemflow component and the loss of small rain events (< 2 mm). After the improvement of the sprinkling system at YS and SP, soil moisture contents of the organic layers under the roofs were most of the time still lower than outside the roofs (Koopmans et al.. 1995). Occasionally, a decrease in nitrification and mineralisation was observed as a ‘roof effect’ (Koopmans et al., 1995). probably due to the moisture difference. At SL, differences in soil water nitrate concentrations were observed between the roof control and the ambient control similar to those found at YS and SP. Lamersdorf and Blanck (1995) suspected that this roof effect could be caused by the exclusion of particulate organic material due to sedimentation and filtering of throughfall before resprinkling (Table 3). From the summer of 1994 this material was redistributed to the soil of the roof plots. and in response the nitrate concentrations increased at the roof plots in 1995 (Lamersdorf et al.. 1998). Thus, the roof effects have accelerated the decline of nitrate leaching at the N saturated sites YS, SP and SL. The very prompt decrease of nitrate leaching after starting the clean rain treatments at these sites may mainly be caused by the roof effects. However, the increase of nitrate leaching at the roof control treatments after experimental improvements showed that nitrate leaching is quite responsive to environmental changes and that in the longer term the clean rain treatments did decrease nitrate leaching. The experience from YS, SP and SL shows the benefits of roof controls, especially for the evaluation of possible artifacts. At GD. a control roof was not built mainly due to the high cost of repeating the experiment on the catchment scale. This is only feasible at the catchment scale if very small catchments can be defined as in the roof experiment in a sub-alpine forest at Risdalsheia. Norway (Wright et

al.. 1988). Coordination between the roof experiments within the NITREX and EXMAN projects made it possible to share experience of having roofs of different size and construction. roof controls. and whole catchment cover. The evaluation of the experimental results benefited from this.

Several of the problems may be generally valid for (forest) ecosystem manipulations. An ecosystem manipulation is an experiment rather than an exact scenario of future environmental conditions. Therefore, manipulation experiments have certain inherent limitations in simulating natural ecosystems. Sprinkling systems are critical in forest manipulations experiments, not only because of the different spatial and temporal distribution of inputs, but also because element fluxes are proportional to water fluxes, which is not always the case in natural systems. The sampling design in manipulated plots should be made according to the new spatial and temporal distribution of input and not just resemble the design in the natural plots. Pre-treatment data on point measurements are necessary to distinguish treatment effects and unintended effects from the natural variability. Manipulation control treatments are important. Addition experiments are presumably the most straightforward type of ecosystem manipulation. However, addition by means of a sprinkling system also poses some restrictions to this type of manipu!ation. Some differences between JOOf plots and nalilral plots could be eliminated by building the roofs above the canopy as it was done at Risdajsheia, NOJWCIY (Wright et al.. 1988). This requires a compromise on tree size, which often may not be feasible with the research objectives. Also in this type of roof experiment problems related to sprinkling still remain. In drought experiments and possibty other types of experiments, more simple constructions with temporary roof covers during the intended drought period could be attractive. Temporary roofs of varying design were used at Skogaby, Sweden (Nilsson and Wiklund, 1992). Ballyhooly, Ireland (Ryan et al.. 199% and Hiiglewald, Germany (Lamersdorf et al., 1998). The disruption of the internal nutrient cycles with throughfall is an important restriction in this

P. Gundersen

et al. / Forest

Ecology

design. The nutrient loss has to be compensated by addition, if the drought treatment is repeated. Use of environmental gradients and changes of such gradients may be an alternative to the costly and technically demanding manipulations. Godt (1995) gives an example in which the creation of a forest gap was used to study effects of soil warming within the stand next to the gap. The important restriction on this type of manipulation is the possible below ground litter input of dead roots from the cut trees and missing water and element uptake by the same roots if these contributions cannot be quantified. The (roof) manipulation concept is a valuable experimental tool despite the difficulties and limitations discussed above. The results obtained constitute important knowledge for improving and validating mathematical models for forested ecosystems and for evaluating forest management and pollutant emission politics. Even the results from the manipulation control treatments (control roofs), where several small changes are integrated, may comprise a demanding test of the models. Ecosystem manipulations have advantages over traditional controlled laboratory experiments, and at the same time the difficulty that they involve the complexity of natural ecosystems. The unintended manipulations in the roof experiments have created a new understanding of the ecosystems and have helped focus on important processes at the ecosystem scale, such as the importance of rain event frequency for litter decomposition, the rapid internal cycling of K, and the considerable nutrient input to the soil from suspended material in throughfall. Despite the inevitable drawbacks in the experimental design of roof projects the main experimental results on recovery from reduced pollutant load are still justified. The drawbacks are small relative to the gains, especially when the alternative is to experiment on plants in controlled greenhouse environments where even more drawbacks can be expected.

Acknowledgements The roof projects were funded in part by the Commission of European Communities (EXMAN STEP-CT90-0038, EV5V-CT920091 and EVSVCT940429; NITREX EV5V-CT930264 and EVSV-

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CT9404361 and a long list of national funding agencies. We thank Ingvar Andersson, Claus Beier, Kevin Bishop, Kai Blanck, Michael Bredemeier, Harrie van Dijk, Karin Hansen, Hans Hultberg, Lennart Rasmussen, Jan Roelofs, G.-A. Wiedey, Dick Wright and the anonymous referees for their contributions.

References Beier, C.. Rasmussen, L., 1994. Effects of whole-ecosystem manipulations on ecosystem internal processes. TREE 9,218-223. Beier, C., Hansen, K., Gundersen, P.. 1993. Spatial variability of throughfall fluxes in a spruce forest. Environ, Pollut. 81. 251-261. Beier, C., Hultberg, H., Moldan, F., Wright, R.F., 1995. MAGIC applied to roof experiments (Risdalsheia, N., Gtidsjon, S.. Klosterhede, D.K.) to evaluate the rate of reversibility of acidification following experimentally reduced acid deposition. Water Air Soil Pollut. 85, 1745-1751. Boxman, A.W., van Dam, D., van Dijk, H.F.G., Hogervorst, R.F.. Koopmans. C.J., 1995. Ecosystem responses to reduced nitrogen and sulphur inputs into two coniferous forest stands in the Netherlands. For. Ecol. Manage. 71, 7-29. Boxman, A.W., van der Ven, P.J.M., Roelofs, J.G.M., 1998a. Ecosystem recovery after a decrease in nitrogen input to a Scats pine stand at Ysselsteyn, the Netherlands. For. Ecol. Manage. 101 (l-3), 155-163. Boxman, A.W., Blanck. K., Brandrud, T.E., Emmett, B.A., Gundersen, P.. Hogervorst, R.F., Kjonaas, O.J., Persson, H.A., 1998b. Vegetation and soil biota response to experimentallychanged nitrogen inputs in coniferous ecosystems of the NITREX project. For. Ecol. Manage. 101 (1-3). 65-79. Bredemeier, M.. Blanck. K.. Wiedey, G.A., 1993. Experimentelle Manipulation des Wasser- und Stofthaushalts in einem Fichtenwald - das Dachprojekt im Solling. Forstarchiv 14 164). 154-158. Bredemeier. M., Blanck, K., Lamersdorf, N.P., Wiedey, G.A., 1998. The Solling roof experiments - site characteristics. experiments and results. For. Ecol. Manage. 101 (l-3), 281293. Carpenter, S.R., Chisholm, S.W., Krebs, C.J., Schindler, D.W., Wright, R.F., 1995. Ecosystem experiments. Science 269, 324-227. Emmett, B.A., Brittian, A., Hughes, S., GGrres. J., Kennedy. V.. Norris, D., Rafarel, R.. Reynolds, B., Stevens. P.A., 1995. Nitrogen addition (NaNO, and NH,NO,J at Aber forest, Wales: I. Response of throughfall and soil water chemistry. For. Ecol. Manage. 71. 45-59. Godt, J., 1995. Effects of climatic change on element budgets in an N-saturated beech stand. In: Jenkins, A., Ferrier. R., Kirby, C. (Eds.). Ecosystem Manipulation Experiments: Scientific Approaches, Experimental Design, and Relevant Results, Ecosystems Research Report No. 20, European Commission, Luxembourg, pp. 314-322.

Gundersen, P., 1993. Spatial variability of soil water chemistry in a spruce forest. In: Cerny, J. (Ed.), Abstracts from BIOGEOMON and Workshop on Integrated Monitoring. Czech Geological Survey, Prague, pp. 108-109. Gundersen, P., 1995. Unintended differences between natural and manipulated forest plots. In: Jenkins, A.. Ferrier, R., Kirby, C. (Eds.), Ecosystem Manipulation Experiments: Scientific Approaches, Experimental Design, and Relevant Results. Ecosysterns Research Report No. 20. European Commission. Luxembourg, pp. 335-343. Gundersen, P., Andersen, B.R.. Beier, C., Rasmussen, L., 1995. Experimental manipulation of water and nutrient input to a Norway spruce plantation at Klosterhede, Denmark: I) Unintentional induced physical and chemical changes. Plant Soil 168-169, 601-611. Hansen, K., Beier, C., Gundersen, P., Rasmussen, L., 1995. Experimental manipulation of water and nutrient input to ;t Norway spruce plantation in Klosterhede, Denmark: 3) Effects on throughfall. soil water chemistry and decomposition. Plant Soil 168-169. 623-632. Jenkins, A., Ferrier. R., Kirby, C. (Eds.), 1995. Ecosystem Manipulation Experiments: Scientific Approaches. Experimental Design, and Relevant Results. Ecosystems Research Report No. 20, European Commission. Luxembourg, 373 pp. Koopmans, C.J.. Lubrecht, W.C., Tietema, A.. 1995. Nitrogen transformations in two nitrogen saturated forest ecosystems subjected to an experimental decrease in nitrogen deposition. Plant Soil 175, 205-218. Koopmans. C.J., Tietema, A.. Verstraten. J.M.. 1097. The impact of experimentally reduced nitrogen deposition on litter decomposition in two nitrogen saturated forest ecosystems. Soil Biol. Biochem. (in press).

Moldan, F., Andersson, I.. Bishop. K., H&berg. H.. in press. Catchment scale acidification reversal experiment at Ctdsjb;n, SW Sweden - Assessment of the experimental design: In: Hultberg, H.. Skeffington, R. (Eds.). Experimental Reversal ot Acid Rain Effects: The GIrdsjiin Roof Project. Wiley. Lamersdorf, N.P.. Blanck, K.. 1995. Evaluation