Transient effects of nitrogen addition and rainfall suppression on Vachellia karroo growth under grass competition in a southern African savanna

Figures & data

Table 1. Effects of N addition, rainfall suppression, grass competition and their interaction on height increment of Vachellia karroo saplings.

	Start of experiment		1st season		2nd season		3rd season		4th season
Effect	F	P	F	P	F	P	F	P	F	P
Intercept	9923.54	0.000	1993.85	0.000	3586.89	0.000	4811.70	0.000	4816.85	0.000
Fertiliser (F)	0.110	0.742	0.214	0.647	0.333	0.568	0.052	0.821	0.045	0.833
Rainfall (R)	3.638	0.066	4.012	0.055	5.171	0.031	3.306	0.079	3.142	0.087
Grass (G)	0.010	0.922	0.210	0.650	1.576	0.219	2.192	0.149	2.404	0.132
F*R	0.273	0.605	1.609	0.215	6.369	0.017	3.006	0.094	3.216	0.083
F*G	2.873	0.101	1.342	0.256	1.081	0.307	0.614	0.439	0.601	0.444
R*G	0.010	0.921	0.155	0.696	0.005	0.946	0.002	0.964	0.043	0.837
F*R*G	0.002	0.963	0.104	0.749	0.008	0.931	0.060	0.808	0.071	0.792

Figure 1. Effects of N fertilisation, rainfall suppression and grass competition on the mean log height of Vachellia karroo saplings after (i) first growing season, (ii) second growing season, (iii) third growing season and (iv) fourth growing season C = control, F = N addition, G = grass competition and R = rainfall suppression. Error plots represent log mean ±95% confidence interval.

Table 2. Results of the post-hoc analysis based on the least significant difference test on tree height growth in the second season.

	C	G	Rl	F	R &G	F R	F G	F R G
C
G
R
F		●
RG				●
F R				●
F & G		●			●	●
F, R G				●		●

Notes: The black dots indicate treatments that significantly differ from each other.

Figure 2. Effects of N fertilisation, rainfall suppression and grass competition on diameter of Vachellia karroo saplings after (a) first growing season, (b) second growing season, (c) third growing season and (d) fourth growing season. C = control, F = N addition, G = grass competition and R = rainfall suppression. Error plots represent mean ± CI.

Table 3. Effects of N addition, rainfall suppression, grass competition and their interaction on stem diameter growth of Vachellia karroo saplings.

	Start of experiment		1st growing season		2nd growing season		3rd growing season		4th growing season
Effect	F	P	F	P	F	P	F	P	F	P
Intercept	374.27	0.000	331.60	0.000	290.20	0.000	383.40	0.000	404.8	0.000
Fertiliser (F)	0.202	0.656	0.022	0.885	1.298	0.264	1.694	0.203	2.331	0.138
Rainfall (R)	0.965	0.334	0.094	0.762	0.305	0.585	0.179	0.675	0.051	0.822
Grass (G)	0.017	0.898	1.264	0.270	0.571	0.456	0.194	0.662	0.378	0.543
F*R	0.137	0.714	4.213	0.049	3.202	0.084	2.414	0.131	1.510	0.229
F*G	0.340	0.565	0.812	0.375	0.386	0.539	0.589	0.449	0.886	0.355
R*G	0.721	0.403	0.463	0.501	1.075	0.308	0.699	0.410	0.737	0.398
F*R*G	0.157	0.695	0.007	0.934	0.028	0.867	0.054	0.819	0.007	0.936